VR-002 - RDF Products
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1. F REPLACEMENTS FOR C1 amp C12 10 UF 25 V G REPLACEMENT FOR C6 100 UF 50 SRESIS TORS SB cane ested cent SECTION VI OVER VOLTAGE amp REVERSE POLARITY PROTECTION ZENER sestacne secu SECTION VII SWITCHING TRANSISTORS Q1 amp Q2 SECTION Vill PARTS VENDORS eed LIST OF ILLUSTRATIONS Figure 1 Heathkit HP 13B DC Power Supply Schematic Figure 2 Full Wave Voltage Doubler w 1N4007 Diodes amp Equalizing Resistors Figure Electrolytic Capacitors Then amp NOW Figure 4 Chassis Photos W anmotations 2 2 ccccceceeeceeeeeeeeeseeeeeeeeecteeeeseeeeeneeecees Figure 5 Replacement Timing Capacitor C1 Figure 6 Voltage Across 220 UF Figure 7 Voltage Across 660 UF total Figure 8 Voltage Across 4 420 UF total Figure 9 Over Voltage and Reverse Polarity Protection Circuitry SECTION2. BW Limit 20MHz Volts Div Probe 10 Voltage Invert CH1 2 00 VEy CHI 560m 3 461 22kH Figure 6 Voltage Across C6 220 uF As per this photo the magnitude of the switching spikes is 3 VPP which is very large for a 12 VDC input Clearly the large ripple current resulting from these spikes was responsible for the capacitor overheating To improve things two additional 220 uF 50 V capacitors were added in parallel with C6 for a total of 660 uF The result of this additional capacitance is shown in Figure 7 Coupling BW Limit 20MHz 4 Volts Div Probe 10 Voltage Invert CH1 2 00VEy CH1 Z 640m 3 45401kH Figure 7 Voltage Across C6 660 uF total As per this photo including the additional 440 uF of capacitance cut the switching spike magnitude in half to approximately 1 5 VPP Also the additional capacitance eliminated the heating problem as a result of the fact that this additional capacitance reduced ripple voltage and distributed the lesser resulting ripple current through three rather than one capacitors 13 of 20 RDF Products Vancouver Washington USA Tek J B CHI BY Lirit 20MHz Ww 14 wr en v Probe 10 Voltage Invert CH1 200 8 CH 0 M 250 CH1 Z 320m 1 11 07 39 24 4484 Figure 8 Voltage Across C6 4 420 uF total Although this was a significant improvement it seemed that this ripple magnitude was s
3. familiar with all necessary safety precautions Those unfamiliar with these safety precautions or inexperienced in working on equipment containing high voltages should not open the unit HONORARIUM This paper is written in honor of CWO James D Henson U S Army retired who served this country over a period of 30 years 1941 1971 that included three wars CWO Henson built this HP 13 power supply in 1964 and had it in his possession until 2011 CWO Henson is still an active amateur radio operator operating as W5BVN out of Killeen Texas 1 of 20 RDF Products Vancouver Washington USA SECTION Il SUMMARY CIRCUIT DESCRIPTION Referring to the schematic of Figure 1 the HP 13 essentially is a transistorized inverter power supply that accepts 12 VDC typically from an automotive electrical system steps up the resulting AC voltage via a multi secondary power transformer and then rectifies and filters the various secondary voltages to obtain the desired DC outputs The rated input voltage range is 12 0 to 14 5 VDC negative ground The rated maximum input current is 25 amperes at full load The HP 13 supplies three separate unregulated DC voltage outputs in addition to a pass thru 12 VDC filament voltage output These DC output voltages are as follows High Voltage The high voltage output is generated by a full wave voltage doubler Its rated output is 800 VDC no load and 750 VDC 250 mA Low Voltage The low voltage output i
4. near their maximum ratings 3 Temperature Rating The original electrolytic capacitors employed in the HP 13 and most non military electronic equipment built in the 1960s are rated for operation up to 85 C Keeping in mind that the service life of electrolytic capacitors is typically in the order of thousands of hours and that this service life degrades at higher temperatures recommend that premium grade capacitors rated for operation up to 105 C be used for longest service life 4 ESR Equivalent Series Resistance Since electrolytic capacitors dissipate a certain percentage of the charge applied to them into heat they are characterized as having an 8 of 20 RDF Products Vancouver Washington USA equivalent series resistance This ESR is frequency sensitive but is mostly constant over the frequency range of interest under 1 000 Hz for high voltage power supplies Low ESRs are desirable for good performance and long life 5 AC Ripple Current Electrolytic capacitors employed as power supply filters are subject to AC ripple current as a consequence of the fact that there is always some AC ripple voltage across them Since this ripple current flows through the capacitor ESR heat is generated that shortens capacitor life It is therefore important that electrolytic capacitors be selected both for high ripple current capacity and low ESR C REPLACEMENTS FOR C3 C4 C8 C9 amp C10 20 uF 450 V C3 C4 C8 C9 and C10 ar
5. protection diode can best be explained by examining an actual circuit where it has been successfully employed Figure 9 is a simplified schematic of the input power circuitry of the RDF Products DFP 1000A DF bearing processor the predecessor to the current DFP 1000B The following detailed explanation of this circuit is an excerpt with some minor paraphrasing from the DFP 1000A Service manual The DFP 1000A contains robust power input protection circuitry to protect the unit from the inadvertent application of reverse polarity DC input power and input voltages substantially exceeding the maximum 16 volt DC input voltage rating The essential elements of this circuitry are illustrated in Figure 9 Referring to Figure 9 11 16 VDC input power is applied from the external power source to the DFP 1000A rear panel power connector This DC power is then applied through the 2 5 ampere rear panel fuse and then to the front panel ON OFF switch The shunt 18 volt power Zener diode D9 located on the Front Panel Board works in conjunction with the fuse to provide both reverse polarity and over voltage protection If reverse polarity power is applied D9 forward biases and clamps the input voltage at approximately 1 VDC The resulting high input current almost immediately blows the fuse and thus protects the subsequent circuitry If over voltage is applied if the DFP 1000A is inadvertently powered from a 28 VDC aircraft power source for example
6. the 1970s list the SP838 Based on an internet search of various Heathkit users groups this transistor appears to be a 45 volt 25 ampere 100 watt PNP germanium type that has been out of production for many years This is not surprising given that germanium transistors as far as know are no longer manufactured and have long been supplanted by more reliable and temperature stable silicon types One of the postings found suggested the NTE179 and the ECG197 as generic replacements although the poster stated that he had not actually tried them Also these transistors are very expensive which is likely an indication that they are no longer in production and that the remaining stocks are nearly depleted Another posting claimed success using a 2N6330 silicon transistor although it reported occasional inverter mis starts where the inverter would not oscillate at power up The reported remedy was to switch the HP 13 off and then back on again This issue suggests that it may be necessary to use a matched transistor pair for the HP 13 to start reliably Unfortunately the 2N6330 is not available from my primary semiconductor suppliers Mouser Electronics and Digi Key Corp and is likely an obsolete or hard to find part ultimately decided just to leave the original transistors in place since they appeared to work properly My primary concern was that attempting a substitution might risk damaging the power transformer which would force me
7. the SB 200 requires a 2 400 VDC plate supply however eight diodes are employed in each leg to obtain the necessary PIV rating However no shunt voltage equalizing resistors are employed The author of this paper tactfully addresses this questionable Heathkit design practice as remarkable but then goes on to say that with modern diodes this practice is acceptable Once again have to offer an amicable disagreement To get back to the basics on this subject the salient issue is the silicon rectifier diode back resistance If we place two 1 000 PIV diodes in series to effectively increase the total PIV rating to 2 000 volts our implicit assumption is that both diodes have nearly the same back resistance If this is not the case there will be voltage hogging To explain if one of the diodes has for example twice the back resistance of the other then that diode with the higher back resistance will drop 2 3 rather than 7 of the total reverse voltage With simple arithmetic we can see that the total effective PIV rating of these two series diodes would then be only 1 500 rather than 2 000 volts i e with 1 500 reverse volts applied across the series pair the higher back resistance diode will see its maximum rated 1 000 PIV To resolve this matter we need a better understanding of the characteristics of diode back resistance Unfortunately diode back resistance is not directly specified in the 1N4007 diode data sheet The dio
8. to scrap the whole power supply since it would be very difficult to find a replacement transformer from any source other than another HP 13 Even so see no reason why the two germanium transistors could not be replaced with modern silicon types This task would of course have to be done carefully and with the full expectation that some circuit redesign might be necessary Such an effort would be a worthy follow on project 19 of 20 RDF Products Vancouver Washington USA SECTION Vill PARTS VENDORS As discussed above purchased the necessary parts from Digi Key Corp since they have an excellent selection of capacitors Other parts vendors with good parts selections and reasonable minimum order requirements are Jameco Electronics and Mouser Electronics Contact information is as follows Digi Key Corp www digikey com Jameco Electronics www jameco com Mouser Electronics www mouser com lt gt Post Note received a message from reader Burkhard Riemer in Germany who has also reconditioned an HP 13 Burkhard advises that a European source for capacitors and other components can be found at the following link http www die wuestens de dindex htm katalog htm 20 of 20 RDF Products Vancouver Washington USA
9. D9 clamps the input voltage at 18 VDC Once again the resulting high input current almost immediately blows the fuse and thus protects the subsequent circuitry Although D9 is a heavy duty power Zener diode capable of dissipating up to 5 watts its ability to survive reverse polarity and over voltage power inputs depends upon the circumstances It has been our experience that D9 is sufficiently robust to survive the application of reverse polarity power The large inrush of current blows the 2 5 ampere fuse before the junction temperature of D9 rises to the point where the diode junction fuses although it is possible that this might not be the case if the current capacity of the power source is sufficiently high If D9 survives as is usually the case it is necessary to replace only the 2 5 ampere fuse 16 of 20 RDF Products Vancouver Washington USA In cases where over voltage is applied however it has been our experience that D9 s junction typically fuses before the 2 5 ampere fuse blows although it is possible that D9 can survive if the external power source has a sufficiently low current limiting threshold resulting in both a blown fuse and D9 presenting a direct short circuit as a result of its fused junction In this case it is necessary to replace both the 2 5 ampere fuse and D9 Even so the input power protection circuitry has still functioned effectively preventing major DFP 1000A damage at the expense of requiring replacemen
10. INTRODUCTION OVERVIEW amp HONORARIUM This paper is a vintage radio technical document that describes the reconditioning modification and modernization of a Heathkit HP 13 DC power supply This unit was designed by Heathkit back in the 1960s to provide the necessary supply voltages for mobile 12 VDC operation of Heathkit SB and HW series HF amateur transceivers and other equipment procured this unit on Ebay and found it to be in fair condition Although the unit was functional and structurally sound it required significant reconditioning modification and modernization to restore it to like new operational condition This paper describes this process in detail along with the necessary supporting technical background Although this project did not pose any major technical challenges and was not particularly difficult it is my hope that the enhanced technical detail provided will be helpful to other vintage radio afficionados interested in refurbishing the HP 13 and other power supplies with similar issues Also visit my N6DC vintage radio website at www rdfproducts com N6DC Vintage Radio htm for possible revisions to this paper as well as other vintage radio technical articles DANGER The Heathkit HP 13 DC power supply contains dangerous high voltages that can be lethal if contacted Those intending to work on this power supply should be well versed in working on equipment with high voltages present and be completely
11. RDF PRODUCTS Vancouver Washington USA 98682 Tel 1 360 253 2181 Fax 1 360 635 4615 E Mail mail rdfproducts com Website www rdfproducts com VR 002 Vintage Radio Application Note RECONDITIONING AND MODERNIZING THE HEATHKIT HP 13 DC POWER SUPPLY Rev A03 03 14 vr_apl_002 By Alex J Burwasser Original Writing August 2011 TABLE OF CONTENTS SECTION I INTRODUCTION OVERVIEW amp HONORARIAM SECTION II SUMMARY CIRCUIT DESCRIPTION SECTION III CLEANING amp COSMETICS SECTION IV SILICON RECTIFIER DIODE REPLACEMENT _ B PLACING DIODES IN 8 SERIES RECTIFIER DIODE REVERSE VOLTAGE ISSUES D EQUALIZING SERIES RECTIFIER DIODE REVERSE VOLTAGE SECTION V ELECTROLYTIC CAPACITOR REPLACEMENT OVERVIEW soe iat bocce B IMPORTANT CHARACTERISTICS OF ELECTROLYTIC CAPACITORS C REPLACEMENTS FOR C4 C8 C9 amp C10 20 UF 450 D REPLACEMENTS FOR 11 amp C11b 20 UF 150 E MOUNTING C4 C8 C9 C10 11 amp 11
12. SIS GROUND lt gt INDICATES VOLTAGE READING POWER PLUG TERMINALS VIEWED FROM INSIDE OF CHASSIS LETTER NUMBER DESIGNATIONS SHOWN ON CIRCUIT BOARD X RAY VIEW ORDER BY PART NUMBER FROM HEATH COMPANY 250 VDC Low 300 V DC HIGH LUG 6 AND OR LUG 7 MAY HAVE 12 V DC WITH RESPECT TO GROUND DEPENDING UPON THE SPECIFIC CIRCUITRY ANO CONNECTIONS OF THE EQUIPMENT USED WITH THE POWER SUPPLY SCHEMATIC OF THE HEATHKIT TRANSISTORIZED DC POWER SUPPLY MODEL 13 Do gt oe 6 A CIRCUIT BREAKER Figure 1 Heathkit HP 13 DC Power Supply Schematic 3 of 20 RDF Products Vancouver Washington USA SECTION Ill CLEANING amp COSMETICS As stated above the HP 13 procured on Ebay was functional and structurally sound but somewhat shop worn and aged Even so there was little repair work that needed to be done aside from replacing missing hardware retightening some nuts and conducting a general clean up Since the power output cable was found to be functional and in good condition it was left as is Fortunately this unit was assembled very well and required no rework to correct for assembly errors In general however workmanship for Heathkits must be checked carefully given that the assembly skill of the builder can vary over a wide range In addition to inspecting solder joints all Heathkits should be inspected for solder balls solder splashes and any other debris tha
13. are much smaller than the originals the bigger problem is that they are radial lead units whereas the originals are axial lead units Since modern capacitors are designed mostly for PC board mounting it is increasingly difficult to find axial lead capacitors gt ame p38 CN 01 d 9 5 ouy 78F 973 119 9 F rol JEEE Figure 4 Chassis Photo C3 C4 C8 C9 C10 C11a C11b amp D1 D4 replacements annotated 11 of 20 RDF Products Vancouver Washington USA Since these capacitors are small enough to be mounted upright mounted them as pictured in Figure 4 Although the capacitor leads were long enough to reach the original component PC board mounting holes these capacitors were not well secured to the board using this unorthodox mounting technique To correct this issue staked these capacitors down to the board using RTV RTV is a form of silicone sealant used Permatex Part 66B which is non corrosive and available in many hardware and auto supply stores but many other types are equally suitable After allowing several hours to cure the RTV secured these capacitors firmly to the PC board F REPLACEMENTS FOR C1 amp C12 10 uF 25 V As per the schematic of Figure 1 C1 and C12 both 10 uF 25 V are the inverter timing capacitors that are used to establish the 1 500 Hz nominal switching frequency Although it was not clear to me what the ripple current requiremen
14. as being operated far in excess of its ripple current rating Also even though Heathkit chose a 50 V capacitor in most mobile 12 VDC applications it is safe to use 25 V capacitors This is especially the case if a power Zener diode is employed as an over voltage clamp This useful input power protection technique is discussed in detail in Section VI 14 of 20 RDF Products Vancouver Washington USA RESISTORS Although a discussion of the HP 13 resistors might seem outside the purview of this discussion of its electrolytic capacitors replacing the capacitors also presents an opportunity tp check these resistors More specifically significant disassembly of the HP 13 is required to replace its capacitors As a result most of the resistors will have at least one lead disconnected during this process so it is easy to check their values with an ohmmeter Although resistors are not usually considered as high maintenance components prone to failure the resistors used in the HP 13 are all power resistors and have thus been subjected to thermal stress Also these resistors are all carbon composition types which are significantly less stable in value than the more modern carbon film types The resistors in my HP 13 all measured on the high side of their nominal values in some cases having drifted outside the 10 tolerance specification However the values had not changed enough to warrant replacing any of them The high voltage supply bl
15. average forward current rating of 750 mA Although these are old diodes they are fully adequate for this application Even so being 40 years old and with newer and better diodes available at very low cost it seemed prudent to replace them Although there many modern silicon rectifier diodes available to replace the 132071 the 1N4007 is a particularly good choice in that it is widely used readily available and very inexpensive Rated at 1 000 PIV with a maximum average forward current rating of 1 0 A the 1N4007 is superior to the 132071 in all respects Being very similar in size to the 132071 the 1N4007 is thus an excellent choice for a fit form function replacement for the 1N2071 The 1N4007 can be purchased in small quantities from Digi Key Corp Jameco Electronics and Mouser Electronics purchased mine from Digi Key Corp P N 1N4007FSCT ND along with replacement electrolytic capacitors as discussed in Section V B PLACING DIODES IN SERIES As per the schematic of Figure 1 the full wave voltage doubler employs series diode strings D1 D2 and D3 D4 in each leg to increase the PIV ratings To put some numbers on this the required rectifier PIV rating for a full wave voltage doubler is given by the following equation PIV 2 82 Erms 1 where PIV is the maximum peak inverse voltage and Erms is the power transformer RMS secondary voltage Although Equation 1 is valid only for a sinusoidal waveform rather than the square
16. de specification most closely related to back resistance is the reverse leakage current The typical reverse current for a 1N4007 at its maximum 1 000 volts reverse voltage at 25 C is given as 0 05 microamperes which corresponds to a 20 000 megohm back resistance However this same data sheet lists the maximum reverse current for this same diode as 10 microamperes which corresponds to a far lower 100 megohm back resistance This in itself is a huge variation but it even gets worse At 100 C the typical and maximum numbers are given as 1 0 and 50 microamperes respectively corresponding to back resistances of 1 000 megohms and 20 megohms respectively Clearly diode back resistance is an uncontrolled parameter that is all over the place varying both from unit to unit and over temperature Given the above numbers it seems amazing that Heathkit s practice of placing these diodes in series without shunt voltage equalizing resistors could ever have worked Equally clearly the problem cannot be solved simply by substituting modern diodes since back resistance is still an uncontrolled and unspecified diode characteristic have to think that what pulled Heathkit through on this aside from a generous dose of luck was that most of the diodes supplied with their kits must have come from the same batch 6 of 20 RDF Products Vancouver Washington USA EQUALIZING SERIES RECTIFIER DIODE REVERSE VOLTAGE The issu
17. e the five 20 uF 450 V electrolytic capacitors mounted in a row at the rear most section of the HP 13 circuit board adjacent to the 2 watt resistors As per the schematic of Figure 1 C3 C4 are used in the high voltage supply while C8 C10 are used in the low voltage supply Since the high voltage supply imposes the most demanding requirements on these capacitors we will focus primarily on C3 C4 A vendor search to locate 20 uF 450 V replacement electrolytic capacitor was unsuccessful However a premium grade 47 uF 450 V manufactured by Panasonic P N EEU EE2W470 was available from Digi Key Corp P N 13677 ND The relevant specifications for this capacitor are as follows Temperature Range 25 C to 105 C Rated ESR 120 Hz 6 77 ohms Rated Ripple Current 120 Hz 105 C 1 2 amperes The rated service life is dependent upon a number of variables but was very favorable compared to other capacitors that were also considered To determine the suitability of this capacitor for the application at hand we must first calculate the overall impedance of this capacitor which comprises the 6 77 ohm ESR in series with the capacitive reactance at the 3 000 Hz ripple frequency i e twice the rated inverter 1 500 Hz switching frequency for the full wave voltage doubler First we compute the capacitive reactance 1 2 x 1 X F x C 3 1 13 ohms where Xc is the capacitive reactance Tr is 3 14159 F is 3 000 Hz and C is 47 uF Nex
18. eeder resistors R3 R4 100k 2W are the most critical Although a modest upward change in their values is of no great concern it is important that their values track with each other reasonably well To explain if the value of R3 is significantly different than the value of R4 there will be an unequal voltage division across and C4 voltage hogging As a result voltage appearing across one of these two capacitors might come close to or exceed its 500 V rating Fortunately these resistor values were closely matched in my HP 13 Any resistor that is excessively out of tolerance i e by more than 20 of its nominal value should be replaced Similarly resistors that appear burned or cooked should also be replaced On the positive side resistors employed in extensively used 40 year old equipment are fully aged and will likely not drift in value much further 15 of 20 RDF Products Vancouver Washington USA SECTION OVER VOLTAGE amp REVERSE POLARITY PROTECTION ZENER An apparent HP 13 design shortcoming is that it has neither over voltage nor reverse polarity protection at its 12 VDC input This was remedied by adding a single Zener protection diode PWR ON OFF F4 ON FPB1 gt VOLTAGE REGULATORS 11 16V DC INPUT 2 5A REAR PANEL REAR PANEL E Figure 9 Over Voltage and Reverse Polarity Protection Circuitry D9 1N5355B 18 VOLTS ON FPB1 The operation of this
19. es associated with the highly variable back resistances of series diodes have been well known for many years along with a mature and time proven solution Essentially the remedy is to place shunt swamping resistors of the same value across each diode in the series string If this resistor value is much lower than the diode back resistance then the effective back resistance is essentially the value of this resistor and the voltage will divide nearly equally across each diode resistor shunt pair Although there is likely no truly optimum value for this shunt resistor the 1974 ARRL Radio Amateur s Handbook in its chapter on AC operated power supplies provides the following rule of thumb Rs PIV x 500 2 where Rs is the required value of shunt resistance and PIV is the diode rated peak inverse voltage Essentially then the author recommends using 500 ohms of resistance per diode PIV rating For a 1N4007 diode rated at 1 000 PIV the required resistor value would be 500 000 ohms A 470 000 ohm resistor is thus selected as a good match standard value Again referring to the HP 13 schematic of Figure 1 the 800 volt open circuit high voltage is dropped across four rectifier diodes If 470 000 ohm voltage equalizing resistors are placed across each diode then the voltage across each 470 000 ohm resistor is 800 4 200 volts The resulting power dissipation in each of these resistors can then be computed by Ohm s Law as E x E R 200
20. f we really wanted to finesse this we could buy a small quantity of nominal 14 volt 1N5351B and 15 volt 17 of 20 RDF Products Vancouver Washington USA 1N5352B Zeners and then select them by careful measurement to find one with forward voltage threshold very close to 14 5 volts 1N5355B Zener diodes are available from many sources including Digi Key Corp P N 1N5355BRLGOSCT ND This part is also available from Mouser Electronics P N 610 CZ5355B The 1N5351B and 1N5352B are also available from these same sources 18 of 20 RDF Products Vancouver Washington USA SECTION VII SWITCHING TRANSISTORS Q1 amp Q2 It would seem reasonable to want to replace the two power switching transistors 40 year old inverter power supply Unfortunately found this task to be far from straightforward and ultimately decided to leave the originals in service As per the schematic of Figure 1 Q1 and Q2 are identical PNP power switching transistors used in conjunction with T1 and various peripheral components to form a 1 500 Hz power oscillator Although the circuitry is conventional and straightforward the HP 13 assembly manual does not identify these transistors other than by the Heathkit house part number 417 60 An examination of the transistors themselves revealed only this same house part number Motorola s logo and the additional identifier SP838 1 None of my Motorola data manuals even the very old ones dating back to
21. in that there would be no unwanted Zener forward conduction at the maximum specified 16 VDC input Also we validated this choice by confirming that the DFP 1000A B would not suffer damage from prolonged exposure to an 18 VDC input In the case of the HP 13 however the maximum rated input voltage is only 14 5 VDC so it would be reasonable to substitute a Zener with a lower forward voltage rating The 16 volt 1N5353B would be a good candidate and possibly even the 15 volt 1N5352B The downside of using a Zener with too low a forward voltage rating is that it can conduct and gulp large amounts of current at too low an input voltage As a case in point if the 15 volt 1N5352B is selected for the HP 13 a Zener at the bottom edge of the rated 5 tolerance would begin conducting at 14 25 VDC which would unfortunately be inside the HP 13 14 5 VDC maximum rated input voltage specification Ultimately this is a matter of deciding how far we want to go to protect the HP 13 As discussed in Section VII the germanium switching transistors Q1 amp Q2 would likely be very difficult to replace Given this reality the prudent decision might well be to use a lower voltage Zener particularly if the applied DC voltage is not likely to rise too close to the HP 13 14 5 VDC maximum rated input To further amplify on this a typical lead acid battery can reach a 14 5 terminal voltage under hard charging although lower terminal voltages are more typical I
22. longer any direct fit form function replacements for the original 1960s vintage electrolytic capacitors employed in the HP 13 Asa result adaptations must be made to accommodate the different sizes and footprints of modern electrolytic capacitors On the positive side modern electrolytic capacitors are much smaller better performing and more reliable than their 1960s vintage predecessors Since electrolytic capacitors have service lives that are much shorter than nearly all other components aside from vacuum tubes and dial lights chose premium grade replacements In the following paragraphs the important characteristics of high voltage electrolytic capacitors are discussed followed by identification of specific modern substitutes for the obsolete originals Suggestions are then made as to how to install these modern substitutes in the HP 13 B IMPORTANT CHARACTERISTICS OF ELECTROLYTIC CAPACITORS 1 Capacitance and Voltage Rating Replacement electrolytic capacitors should have capacitances and voltage ratings equal to or greater than the originals 2 Service Life The service life of electrolytic capacitors is typically in the order of thousands of hours which surprisingly is only somewhat better than that of vacuum receiving tubes Service life is highly dependent upon operating temperature applied voltage and applied ripple current When operated conservatively electrolytic capacitors last far longer than when they are operated
23. s generated by a full wave voltage doubler Its rated output is 310 VDC no load and 300 VDC 150 mA This low voltage output can be reduced for appropriate applications by selecting the lower voltage tap on the T1 power transformer secondary With the lower voltage tap selected the rated output is 265 VDC no load and 250 VDC 150 mA Bias Voltage An adjustable bias voltage output is generated by a half wave rectifier This is a negative voltage rated at 40 to 130 VDC no load depending upon the setting of R10 Bias Adj Current capacity is 20 mA at 130 VDC and 1 mA at 40 VDC Capacitive input filters are used for all three DC voltages The low voltage supply also employs an additional series filter choke L1 followed by an additional shunt filter capacitor for improved AC ripple reduction The inverter switching frequency is nominally 1 500 Hz with loud audible ringing Unlike modern inverters that employ switching frequencies above 20 000 Hz and are inaudible as a result inverters built back in the 1960s had to be designed using earlier generation power transistors that did not offer the enhanced high frequency performance available from modern power transistors As a result these inverters had to use much lower switching frequencies to operate efficiently 2 of 20 RDF Products Vancouver Washington USA NOTES ALL RESISTANCES IN OHMS 1000 1 000 000 ALL VOLTAGES MEASURED FROM INDICATED POINT TO CHAS
24. se harmonics also progressively diminishes as they increase in order To account for the additional ripple current contributed by these harmonics we can add 15 or so to the result of equation 5 but even with this addition the ripple current is still very comfortably within the capacitor 1 2 ampere rating Another imperfection associated with the above computations is that the rated 6 77 ohm capacitor ESR is specified for the 100 120 Hz ripple frequency typically associated with full wave rectifiers operated from standard 100 120 Hz AC mains rather than for the inverter 3 000 Hz ripple frequency It is therefore reasonable to expect that the ESR is different at 3 000 Hz as compared to 120 Hz Fortunately capacitor ripple current ratings tend to improve as a function of frequency so this issue actually works in favor of a margin of safety In reality the above computation overstates ripple current Ir since the substitution of the larger capacitor 47 uF versus 20 uF substantially reduces the ripple voltage Vr As a result the margin of safety is further improved Since ripple voltage Vr across C8 C9 C10 used in the low voltage supply is even lower this same capacitor can be used with an even greater margin of safety Figure 3 is a photo of the modern 47 uF 450 V Panasonic replacement capacitor bottom along with the original 20 uF 450 V General Instrument Corp unit top This picture is worth a thousand words on the topic of impro
25. t the total impedance is found by vectorially adding Xc and the ESR as follows Z Square Root Xc x Xc ESR x ESR 4 Square Root 1 13 x 1 13 6 77 x 6 77 6 86 ohms 9 of 20 RDF Products Vancouver Washington USA According to the 13 specifications the high voltage ripple is rated at 1 under a 250 mA presumably typical load Since the high voltage DC output under a 250 mA load is specified as 750 volts 1 ripple corresponds to 7 5 volts RMS based on the commonly accepted definition of ripple percentage Given this to be the case and given that only half of this 7 5 volt RMS ripple voltage Vr will appear across each capacitor since C3 C4 are in series the approximate ripple current is as follows Ir Vr Z 5 3 25 6 86 0 474 amperes where Ir is the RMS ripple current Vr is the RMS ripple voltage and Z is the capacitor total impedance as defined in equation 4 Since 0 474 amperes is well below the 1 2 ampere capacitor ripple current rating we can expect long capacitor service life The reason that equation 5 is an approximation is that the ripple voltage has a sawtooth rather than sinusoidal characteristic Since a sawtooth waveform has odd order harmonics e g at 9 000 Hz 15 000 Hz 21 000 Hz etc these harmonics result in additional ripple current since the capacitive reactance Xc as defined in equation 3 progressively diminishes at these higher frequencies However the magnitude of the
26. t may be lodged in the chassis Of course all foreign objects should be removed as part of the cleaning process Upon completion of the reconditioning used cable ties to secure various wires and non corrosive silicone RTV to secure various components The HP 13 includes a power relay that is used to switch the 12 VDC input power on or off It was necessary to clean both sets of relay contacts these are wired in parallel as per the schematic of Figure 1 since they were very dirty did this by cutting up a 3 x 5 index card into narrow strips and then soaking the end of one of these strips with spray contact cleaner then ran this soaked strip through the relay contact manually pushing the relay contacts to their closed position to generate the friction necessary to clean the contacts as the strips were pulled through then repeated this process with a new strip until the strip pulled through the contacts clean Although the unit looked its age no cosmetic restoration effort was made aside from general cleaning Since my intent was to install this unit out of view from the operating console saw no reason to invest any effort into cosmetic restoration 4 of 20 RDF Products Vancouver Washington USA SECTION IV SILICON RECTIFIER DIODE REPLACEMENT A DIODE REPLACEMENT The HP 13 rectifiers 01 07 from Figure 1 are all 1960s vintage 132071 silicon diodes These diodes are rated at 600 PIV peak inverse voltage with a maximum
27. t of only two inexpensive components Since the HP 13 12 VDC input circuitry is protected by a 30 ampere circuit breaker rather than the faster acting 2 5 ampere fuse employed in the DFP 1000A it is probable that the 1N5355B Zener diode would not survive the application of reverse polarity power Even so this inexpensive and readily replaceable part would still protect the HP 13 power transformer and power transistors parts that would likely be irreplaceable or very hard to find at best An incidental benefit of this power protection Zener is that it limits input voltage spikes to 18 volts As a result the voltage rating of C6 need be no greater than 25 V for safe operation The 4 700 uF capacitor substituted for C6 as discussed above is rated at 35 V only because this 35 V capacitor has a higher ripple current rating than the comparable 25 V unit strongly recommend adding this inexpensive part to the HP 13 Referring to the HP 13 schematic of Figure 1 the 1N5355B cathode be connected to the positive terminal of i e at the power transformer center tap and the anode should be connected to ground A possible amendment to this technique would be to select a similar power Zener with a lower forward voltage rating In the case of the DFP 1000A B discussed above the maximum specified input voltage is 16 VDC Since the forward voltage tolerance for this Zener family is 5 we decided to use the 18 volt 1N5355B to be absolutely certa
28. till high especially considering the fact that there was no load on the power supply Also it seemed that further improvement would require a large amount of additional capacitance My next step then was to add eight 470 uF 25 V capacitors in parallel with the three 220 uF 50 V units already in place With this very large amount of capacitance the ripple voltage significantly diminished as illustrated in Figure 8 These additional 470 uF 25 V capacitors were used since they were already in stock These units are manufactured by Panasonic P N ECA 1EHG471 are available from Digi Key Corp P N P5543 ND These capacitors are visible in Figure 4 Note that the 470 uF 25 V capacitors are grouped into sets of four and bundled together using heat shrink tubing With the benefit of hindsight however itis clear that this piecemeal and incremental approach leaves much to be desired Subsequent to the photo shoot for Figure 4 these capacitors were all removed and replaced by a single 4 700 uF 35V electrolytic capacitor suitable for this application This capacitor is manufactured by Panasonic P N ECA1VHG472 and available from Digi Key Corp P N P5558 ND This unit has a very high ripple current rating 1 91 amperes In closing the original 100 uF 50 V capacitor used for C6 appears to have been very marginal for the intended application 100 uF is far to small to provide good ripple filtering and it is almost certain that this capacitor w
29. ts were for these capacitors decided to once again go with premium grade replacements selected 10 uF 63 V capacitors manufactured by Panasonic P N EEU EB1J100 and available from Digi Key Corp P N P13138 ND As expected these capacitors are much smaller than the original units C1 is shown in Figure 5 just below 270 ohm resistor R1 staked down both C1 and C12 with RTV Although the nominal inverter switching frequency is 1 500 Hz the actual measured switching frequency with the original C1 C12 capacitors was near 1 600 Hz This frequency rose slightly to near 1 700 Hz with the replacement capacitors G REPLACEMENT FOR C6 100 uF 50 V Referring to the schematic of Figure 1 C6 is a large bypass capacitor intended to hold the 1 inverter transformer center tap at AC ground In conjunction with choke L2 it also serves to help filter inverter switching spikes from the 12 VDC input Since there was no question in my mind that C6 needed to be a high quality capacitor selected a premium grade 220 uF 50 V unit manufactured by Panasonic P N EEU EB1H221 and available from Digi Key Corp P N P13131 ND To my surprise however this capacitor became very warm to the touch when the power supply was powered up Probing this replacement capacitor with an oscilloscope the voltage across it appeared as in Figure 6 no load on power supply 12 of 20 RDF Products Vancouver Washington USA Coupling
30. vements in electrolytic capacitor technology since the 1960s Figure 3 Electrolytic Capacitors Then amp Now In general there is little or no downside to using replacement filter capacitors with higher capacitance than the originals provided that good quality units are used with adequate ripple current ratings as discussed above Although larger filter capacitors result in more surge current when the power supply is turned on modern silicon power diodes e g the 1N4007 have very high surge current ratings The advantage of higher capacitance 10 of 20 RDF Products Vancouver Washington USA units of course is lower ripple voltage REPLACEMENTS FOR 11 amp C11b 20 uF 150 V As per the schematic of Figure 1 C11 is a dual electrolytic type both sections 20 uF 150 V that is used to filter the rectified bias voltage 130 VDC nominal maximum Although the 47 uF 450 V capacitors used for the high and low voltage supplies as discussed above are higher rated than necessary for use in this bias supply both in terms of capacitance and voltage rating decided to use them since had them on hand and they were not very expensive E MOUNTING C4 C8 C9 C10 11 amp C11b Although the 47 uF 450 V capacitors selected to replace C3 C4 C8 C9 C10 offer great performance and reliability they did not fit the HP 13 PC board patterns that were made for these capacitors While the replacement capacitors
31. wave output of T1 using this equation builds in a margin of safety An easier way to estimate the required diode total PIV rating for each leg is to simply equate it to the no load DC output voltage which is rated at 800 VDC as per Section II Since this is far in excess of the 1N2071 600 PIV rating the Heathkit engineers correctly decided to employ two series diodes in each leg for a total nominal rectifier PIV rating of 1 200 volts Although at first glance it might seem that the 1 000 PIV rating of the 1N4007 replacement diodes would eliminate the need for two series diodes this would be marginal design practice that would leave this margin a little thin for conservative design standards Thus even with the higher 1N4007 PIV rating two series diodes should still be used in each voltage doubler leg 5 of 20 RDF Products Vancouver Washington USA SERIES RECTIFIER DIODE REVERSE VOLTAGE ISSUES Although have always given the Heathkit engineers high marks for their circuit designs have to offer an amicable criticism of their practice of omitting shunt voltage equalizing resistors in their series rectifier diode strings this topic is discussed in more depth in Section IV D below In a paper recently read on Heathkit upgrades for the SB 200 linear amplifier this same topic was addressed As it happens the SB 200 employs a similar full wave voltage doubler circuit that employs the same 132071 silicon rectifier diodes Since
32. x 200 470 000 0 085 watts Purely from the standpoint of power dissipation we could use 1 8 watt 5 carbon film resistors However resistors also have maximum voltage ratings and it would be unsafe to use a 1 8 watt resistor for this high voltage application 1 4 watt 5 carbon film resistors have arated maximum working voltage of 250 VDC which would probably be adequate However it is better to use watt 5 carbon film resistors with their higher rated maximum working voltage of 350 VDC The modified diode section of the full wave voltage doubler is illustrated in Figure 2 47 5 47 k 5x RED c2 RED gt Figure 2 Full Wave Voltage Doubler Modified to Include 1N4007 Silicon Diodes and Shunt Voltage Equalizing Resistors Figure 4 is a photo showing D1 D4 all 1N4007s mounted piggy back on their associated 470k 1 2W equalizing resistors located behind C3 and C4 7 of 20 RDF Products Vancouver Washington USA SECTION ELECTROLYTIC CAPACITOR REPLACEMENT A OVERVIEW Electrolytic capacitors especially high voltage ones are likely not to be reliable after 40 years of service Although the electrolytic capacitors in my HP 13 were functional decided to replace them for this reason Of all the HP 13 restoration tasks replacing the 10 electrolytic capacitors turned out to be the most difficult and time consuming Although high voltage electrolytic capacitors are still available there are no
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