IC-R75 AM & SAM mod
Contents
1. s manual Reconnect all cables to the radio 10 Enjoy the new AM and S AM performance of the R75 If you are not satisfied with the results the mod can be easily reverted by de soldering the wires from pins 3 and 4 and soldering the wire from pin 23 to ground Some thoughts The effect of the synchronous detector is most noticeable on stations with selective fading 1 e when the audio is distorted during the fade You can check the audio sample provided separately Also it is much more obvious with narrow 9 MHz filter 2 4 kHz I wonder what genius decided to prohibit 455 kHz filter selection for the S AM mode There is no LOCK output on the IC so it is not possible to make the synchronous detector work as in other receivers the audio to be from the envelope detector until the synch 1s locked and after that to switch to synch audio This could have eliminated the whines It will be good if one can choose between fast and slow PLL speeds 1 e be able to connect and disconnect R1 Firs I was thinking of using the LOCK button on the front panel for 5 this This way after tuning in a station one can push the LOCK button and switch to slow synch The button will also lock the tuning knob so no way to de tune and hear whistle Unfortunately this is not very easy modification not to say impossible Another option is to mount a switch on the back panel of the radio or as a remote switch with cable For more info2. VERY slow thus leaving only the receiver s AGC in charge In parallel to the capacitor connected to pin 4 AGC of IC2001 10 uF an external capacitor is added C1 220 uF Synchronous detector As implemented the stereo AM IC is put into tuning mode pin 23 BLEND connected to ground This puts the internal PLL in FAST lock mode practically meaning that the output signals from the quasi synchronous AM detector used for AM and the in phase synchronous detector used for S AM will be identical In order to make the S AM mode work as it should some hysteresis has to be added to the PLL This is done by connecting a capacitor C2 between pin 23 BLEND and ground as illustrated in l the Motorola s IC datasheet For the purpose of synchronous AM detection my experimentation showed that larger value of the capacitor makes for more stable operation A value of 100 uF worked best in the datasheet a value of 20 uF is shown but it is for the case when the IC is used for stereo AM demodulation 1 e with strong stable MW stations PLL lock de lock speed My experience showed that the capacitor C2 alone is not enough to make the synchronous detector stable In deep fades it still loses lock In order to improve the stability a small positive bias is added to pin 23 BLEND through a resistor R1 from pin 3 REF 3 0V This makes the synchronous detector rock stable It 1s capable to fly over a complete carrier absence of about 2 seconds
3. IC R75 AM amp S AM mod Ver 1 1 WARNING The operations described below are not as easy to perform as they may seem There is a real danger of seriously damaging your radio Only persons with experience in such procedures should try to perform the modifications It is better to have a radio with reduced performance than a not working one DISCLAIMER The information in this document is provided as is The author is not responsible for any damage or malfunction of the receiver caused by or during the application of the modifications described below 1 Introduction This modification addresses the not so good AM performance and the not working synchronous AM detector of the IC R75 The modification consists of three parts AGC synchronous detector and PLL lock de lock speed The modification could be performed partially according to your tastes for example only AGC or AGC synch detector Below is a schematic of the modification pin 3 Ref 3 0V pin4 AGC pin 23 Blend AGC For the detection in both AM and S AM modes a stereo AM IC is used Motorola MC13022A IC2001 in the R75 schematic This IC has its own AGC system which goes on top of the receiver s AGC As implemented the AGC of the IC 1s very fast thus defeating the slow AGC of the receiver The result is very unpleasant sound in AM mode especially for rapidly fading stations The fix provided here increases the time constant of the IC s AGC making it
4. The price for this stability however is slower locking speed which means about 1 or 2 seconds whine after switching to S AM mode Also tuning around in S AM mode is not very pleasant a lot of whines I use AM mode while tuning and when I m on a station of interest I switch to S AM If the stability is important to you I recommend you to also perform this part of the mod 2 Materials and precautions You will need A low power soldering iron lt 30W Thin insulated wire Two electrolytic capacitors 100 uF and 220 uF both 15V or more Some resistors 10 kQ 2 2 kQ and a 10 kQ pot if you like to experiment with the PLL lock de lock speed A small type screwdriver I used one with 1 mm blade width Precautions READ THIS You ll need some soldering experience don t try to perform the mod if you don t feel confident with the soldering iron STRONGLY RECOMMENDED If you decide to perform the modification do some training before working on the R75 Take a PCB from scrapped electronics VCR TV phone etc with surface mount ICs on it Practice soldering and pin lifting on the ICs Only after you feel confident about your skills go to R75 In order to avoid static discharge ground yourself and the soldering iron Wind the bare end of a wire on your wrist or to a metal bracelet which you ll wear and connect the other end of the wire through 1 MQ resistor to ground Wind one or two ti
5. es the second part of the mod If you do not like to perform the PLL speed mod go to step 9 8 The last part of the modification is the PLL speed mod Solder a wire to pin 3 REF 3 0V same procedure as in step 2 See the picture to the right the yellow wire goes to pin 3 Now there are two options Use the resistor value I found works best Experiment and find the value which suits your taste most If you choose to use my value then take a Ps j iNi iii a x aE J l ee ee a i aa ee os we ij i 10 kQ resistor solder it in series with M q PE EN E Del mest ocr T ty 2 2 kQ one total 12 2 kQ Solder on end of a Oe Te i te oy ni ran sawia a eunuchs i the resistor series to the wire from pin 3 Solder the other end of the resistors to the joint between the wire from pin 23 and the positive leg of the 100 uF capacitor C2 The joint will then be wire from pin 23 resistors from pin 3 positive leg of 100 UF capacitor C2 If you want to experiment then use a series of 10 kQ resistor and 10 kQ pot Connect the cables to the receiver and turn it on as it is open Tune around and adjust the pot for the best compromise PLL lock speed stability Disconnect the resistor series and measure the resistance this will be your value for R1 9 Wrap the capacitors and the resistors with tape and stick them to the back of the front panel shield see picture Put back the top cover as described in the user
6. ker hole is in the bottom left corner see the picture the blue wire goes to pin 4 If you are not successful wait for about 30 seconds before trying again in order the IC to cool down When there 1s little solder on the pin align the solder soaked end of the wire with it vertically see picture and apply the tip of the soldering iron on top of the wire for several seconds As soon as the solder melts remove the iron Don t try to make a nice looking solder joint if there is no solder bridge to neighbor pins and the wire stays connected leave it like this Be careful not to pull the wire as this may brake the joint 3 Scrap a small patch of the insulation covering of the PCB near the screw in the bottom left corner see FF the picture the gray wire goes there Solder another wire there this is ground Take the 220 uF capacitor and shorten for 1 2 seconds both legs together in order to discharge the cap if there is i Ai a a i nie of Pe OETI FON some charge in it Cut the legs to about 1 cm length juudi Thread pieces of heat shrink or pieces of insulation from thick wire on both wires ground and pin 4 SRI 7 Solder the negative leg of the cap to the wire which goes to round then the positive leg to the wire which goes to pin 4 of the IC Be careful with the polarity of the capacitor ul i 4 That finishes the first part of the mod If you would like to stop here no synchronous detector and PLL speed m
7. mes the bare end of a wire on the metal part of the soldering iron close to the handle and ground the other end of the wire at the same ground as your wrist ground Ground the R75 as described in the user s manual to the same ground Disconnect all cables except for the grounding from the IC R75 before opening and working on it Check everything 3 times before going to action Be careful with the pin numbers on the IC Check the polarity of the electrolytic capacitors Read the whole procedure before starting the application 3 Modification instructions 1 Disconnect all cables except for the grounding from the IC R75 and open the top cover as described in the user s manual You ll be working in the area encircled with yellow on the picture the receiver front panel facing you This is the MC13022A stereo AM IC 2 The first part of the mod is the AGC mod It involves soldering a 220 uF capacitor 15V or more between pin 4 AGC and ground C1 i 4 a Remove about 2 mm of the insulation from both ends of a thin insulated copper wire about 15 cm long Soak the bare copper with solder Clean the Me tip of the soldering iron with wet cotton cloth for EF example and take with it very little solder Put SB some solder on the upper part of pin 4 AGC by mete ag come age touching it with the iron for about 3 4 sec Pin 4 is OMS 9 9 po ua the 4 pin from left on the lower side of the IC shee when the mar
8. ods cover the legs of the capacitor with the insulation and go to step 9 5 The second part of the mod is for the synchronous goca detector The most tricky part is the peeling off of Ea S E the pin 23 BLEND of the stereo AM IC Pve remediate het a wih Py done that with a small screwdriver type with 4 blade width 1 mm Check the pin number pin 23 is the 6 pin from left on the top side of the IC when the marker hole is in the lower left corner see the first picture on the right the magenta rae Wy wire goes to pin 23 Take the screwdriver and Di 931 4 lie ray i Coy reji J place it from one side behind and below the pin 1 see the second picture on the right By gently pushing and slowly turning the screwdriver the pin comes off easily Gently lift the pin up to have about 0 5 mm between it and the PCB I first tried US F to cut the trace on the PCB but there is also a trace coming from below the IC so this 1s not working A pene Y 6 Solder one end of a wire to the lifted pin 23 same soldering procedure as in step 2 Take a 100 uF electrolytic capacitor 15V or more shorten the legs for a short time to discharge it cut the legs to iii 1 cm length Solder the positive leg to the wire coming from pin 23 put some insulation piece on the wire before soldering it to the cap Solder the negative leg together with the negative leg of the 220 uF cap to the ground wire 7 That finish
9. rmation check the R75 mailing list at http www egroups com group icomr75 Questions and suggestions are welcome E mail me at rado nips ac jp Rado December 1 2000
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