IC-781 Technical & M..
Contents
1. REG PCB The only cooling is by conduction from the surrounding metallic walls and by the local air turbulence produced by the blower It is like trying to cool a 3CX800 while covering and sealing its anode chimney exhaust air outlet This is why overheating may damage the REG PCB and the PCB itself may present burned areas and or burned PCB traces especially in the area close to R32 and Q7 Power resistor R32 can dissipate its rated power at 25 C but not at 70 C This overheating may also affect the electrolytic capacitors and even other small parts close to them Special attention needs to be paid to potentiometer R2 which is included in the switching supply feedback loop If R2 goes open or is damaged it may cause output overvoltage with possibly harmful consequences The REG PCB with most of the original parts on it A few of the overheated parts near R32 area were already removed po E Ky eae D TETY 33I 3 3 009 38 33 gt i cy ee i 1 i R i Burned PCB and copper traces ne ar the R32 Q7 and R26 some parts are missing here In the event that the power supply gets burned and if the TR unit seems ok then Restore the PCB s circuit connections as needed using the terminals of the newly installed parts if required Check and if needed replace Q7 R32 D5 R2 Replace the small electrolytics with new high quality parts these must be rated for 105 C If any sign2. Ti E a l m vy b l i i a pn E ptal i j iL f Fe r il li a 1 _ ie E z i i J eS a F i rT I j amp i E rl bn B ej i a T Po a s Li i a _ i H i ai r P E E i Pm j b J 4 4 i i ie ii i pa i Piste r has saii z PEEL ind i Sa Si Ai D er aj a AG MASTER 7 Late eid Gilan laut T Go Nol Remove a E y r i From Tech Support Area i a WE J Lh i F r e Cad aa r3 e e as l i tins Art gm 4 a oe Sii i ri Ml ae pe ae Sas L E 3 rf E F Ri I r ECO S a The CRT module diagram is not included in the service manual but the above is a low resolution copy which may be of some help many thanks to K2SB Some interesting and inexpensive small LCD modules accepting a CVBS input signal exist on the market some are used for surveillance systems They may worth a try for these radios but they need to be adapted The IC 781 REG power supply unit The REG board and especially its linear voltage regulators are dissipating a significant amount of heat While they are mounted on a big heatsink the entire REG PCB is sealed it is like being placed inside a hot oven The designers allowed for an air passage between the PA s blower air exhaust and the REG compartment but as the air can not go out of it unlike the PA it has no holes on the back cover like the PA does it is obvious that the air will not move much across the
3. adjustments as described in the Service Manual page 6 40 OUTPUT VOLTAGE and CURRENT PROTECTOR include a check of the APC circuit and PA current limiting APC SET page 6 26 and finally check for all the output voltages page 6 1 To avoid future overheating problems you may wish to install the Sherwood cooling improvement fan kit or a homemade equivalent http www sherweng com IC781Fan htm From Adam VA7OJ AB4OJ with thanks the 781 was intended for 1OOV 200V mains reducing the mains voltage when using the 110 220V mains will tend to lower the power dissipated by the linear regulator Q7 and will lower the generated heat also Please see http www qsl net icom Actually the manual specifies the power supply requirements as being 100 120V and 200 240V so a specific design problem is involved anyway The problem seems to be the R32 and Q7 coming defective because of poor cooling Once more caution High voltages are involved so please be careful The TR module Q1 Q3 heatsink and other parts are at a high potential with respect to chassis The main VFO optical encoders For both radios the VFO encoder knob had been rotating with some difficulty even when its adjustable brake was released This was caused by the old grease used in the shaft bearing This grease has solidified because of age It is inadvisable to use the radio with a stiff encoder shaft for any length of time as this will cause a rapidly increasing backlash The en
4. capacitors used in the DC DC converter on the REG PCB board may fail The usual symptom is no reception along with an abnormal S meter indication It is caused by an incorrect value of the negative output voltage C26 C29 C33 and possibly the other caps on the DC DC converter in the REG Unit may be affected They have to be replaced If possible replace all of the converter s electrolytics Use high quality 105 C ones The DC DC converter is shielded located on the REG PCB near the big filter capacitors and mains transformer under the RF TV Units module so this one has to be removed as well After replacing the capacitors you may add a cooling fan so as to avoid any future problems I have installed a temperature controlled 3 fan using two 82 ohm NTC thermistors in series and it s mounting device from an old computer case The mounting I have found fitted exactly under the back panel fins near the REG Unit There was not need to run wires inside the radio nor even to open it it can use the voltage available at the DC output connector on the back panel While the airflow is not very high it is adequate The cooling improvement is still significant The interior of the power supply is no longer hot and the noise is very low as the fan speed is limited by the two NTC s The fan can be removed in a few seconds if required e g for shipping IC 781 fan blower maintenance The IC 781 uses a radial fan squirrel cage blower for cooli
5. dim picture was easy to fix by adjusting the abovementioned pots Do not worry much about the burns if they are not very bad and if the image is still ok You may not need to replace the CRT immediately unless the need exists for a very bright and high contrast image Replacement may be worthwhile if you intend to keep the radio for a long time or if you wish to avoid unpleasant surprises in the future I do not recommend the crude method of rejuvenating the CRT cathode emission by applying a higher CRT filament voltage it may not last for a long time Bad solder joints on the module board or at the CRT socket board may cause intermittent display problems Check especially for the soldering on the terminals of the big coils If the power transistor fails don t hesitate to replace it with a newer and slightly better part I have used the 2SC4106 with great success it runs relatively cool and is problem free The most and only difficult part to obtain in case of a problem is the fly back transformer If you need to repair the module outside the radio you may use a separate 12V supply and an external CVBS video signal I have successfully used an old computer having CVBS output for this area cs aS ral a at i d i F in n P Fi u sq timai Lf H i ord wo o f e z n 1 ale i ae LEI l um UHD iE gt l B fei La leva Tr e i j a Ls d EET dais T T Po 7 H a dai aie at
6. of damage is visible at the C8 C9 C13 C15 C16 C20 to C23 electrolytics these may be replaced if spares are available use the highest quality parts obtainable Remember that the power supply must deliver 500W DC output and the caps must also be of 105 C type 5 Check the soldering around R22 If the transceiver had unstable output power before this may be due to poor soldering of R22 which may cause abnormal and intermittent PA current limiting Check feedback resistors R2 and R12 which adjust the output voltage and also R6 which adjusts the short circuit high current limit if any doubt replace R2 R6 and R12 Check the protective snubber R C circuits R26 C28 R25 C27 R24 C17 and all the Semiconductor devices Secure L4 L5 L6 L7 C7 and the other large parts on the board using hot melt glue and check the soldering on the back of the board for cracks or other problems Don t hesitate to make use of better and more trustworthy spare parts it is not necessary to look for the 20 year old authentic originals I have found Q7 shorted and R32 interrupted and overheated The 2SD1406 used for Q7 has 20W dissipation and maximum collector current 3A I have used a 70W 8A TIP132 Darlington and insulating mica foil instead of the 2SD1906 Don t use much higher current power bipolar transistors as they usually have very low amplification beta and D5 may not withstand the higher base current required Don t f
7. RT module top cover The ATU preset pots PCB must also be removed These adjustments are located on the CRT module PCB close to its right side The pots have common sense labels from L to R H HOLD 1s the adjustment for the Horizontal Hold H synchronization H PHASE adjust the Horizontal Phase the position of the picture in the H direction V HOLD adjust the Vertical Hold V synchronization V LINEAR adjust the linearity of the picture in the V direction V SIZE adjust the vertical size of the picture SUB BRI and BRIGHT are the ones adjusting the brightness of the picture adjusting them CW will increase the brightness of the CRT displayed picture Note don t worry if the CRT PCB seems to have slightly different parts on it I had taken the above pictures after completing the entire work which included preventive replacement of some capacitors as I wanted that the possible new owner to have no problem regarding it There are another two but less obvious adjustments one is the same type of small pot which adjusts the Contrast CONT the other one is a larger pot including its shaft which adjusts the focus of the picture they are located on the same PCB just between the fuse and the CRT flyback transformer Contrast and Focus adjustments If the CRT picture is dim try to adjust the Contrast pot first and then the Brightness pots When adjusting them do not exaggerate Don t set so high that the background normally bl
8. RT burns Just look below for burns on the CRT s bottom edge see how the upper parts of the highlighted menu labels were shifted to the right side and the normal rectangular menu labels became similar to a flag as if caused by a horizontal sync problem The cause of the problem was C11 a 100uF electrolytic on the CRTC module C11 had to be replaced As the CRTC module is hard to remove from the Digital board I had to cut extract the body of C11 while leaving its terminals still attached to the PCB Then carefully free its terminals from the PCB by heating them with a soldering iron from the component side I have used a small 100uF cap a slightly larger value one will do also which was installed horizontally over the passive components near C2 with its terminals bent down and soldered at the original C11 holes from the more accessible component side of the PCB This avoided the need for removing the entire CRTC module a difficult and time consuming exercise Don t forget use a good quality capacitor and you may not have any further problems in this area N li M MAMAN ui E E en itr as ETT gt S video dd The opened CRTC module C11 is the blue cap to the right of the CRTC module The IC R9000 power supply REG Unit heat and DC to DC converter problem The IC R9000 power supply REG Unit also generates a large amount of heat Because of this and also because of their low quality the electrolytic
9. Some technical and maintenance issues regarding the C 781 and IC R9000 by Traian Belinas YO9OFZS When Icom released their C 781 HF transceiver and IC R9000 receiver these were the best radios available to the amateurs Even now more than 15 years later the 781 and R9000 are considered as very capable radios which can still compete with the more modern ones at least regarding some of their characteristics They were at the top of the wish list for such a long time for many of us contemplating the radio magazines ads brochures and catalogs However as for any such product they are not perfect and some users are not aware of this I have owned maintained and done some repairs on both the IC 781 and IC R9000 here are some technical related observations and my experiences regarding them I don t like to mention this but have to specify Don t forget that what you are doing is at your own risk and that injury or expensive damage may be caused by improper work by accident or by the consequences of wrong information or misunderstandings for which I am not responsible If required please seek further assistance from a qualified trained person Pay attention to the dangerous high voltages that may exist in some areas especially in the REG and CRT Units The CRT display module The CRT module displays most of the information and it is the most important user interface It is actually a video monitor accepting a standard 1V
10. ack becomes very bright amber because using it in this way for a long time will shorten the CRT life as explained previously and the very high contrast may tend to a faster appearance of the burns on the screen If the image does not have very good focus quality not crisp not sharp you may need to adjust the focus But if the focus is fine so you can see separate the individual scanning lines and dots when looking very close to the CRT center image and if the image is not very foggy unfocused at the CRT corners then you may leave it as it is as adjusting it while the module is still inside the radio is difficult There is a special dedicated hole for this adjustment but it is located on the CRT module back panel inaccessible while the module is still inside the radio If adjusting the internal contrast and brightness doesn t give results then the CRT may be bad soft because of long term use and may need replacement If to obtain a decent brightness and contrast you have to turn up these adjustments very much near to max and the focus is bad and impossible to improve then you may also have to replace the CRT In the event of CRT replacement will have to readjust all of the four pots of course For both of the radios I had IC 781 and IC R9000 the CRTs had burns and were dim they are normal for these old devices except for the few which have very low hours on them and or were used at low brightness contrast settings The
11. another of my article on the TS950 ATU mod as well as much other great information I would like to take this opportunity to thank to all the contributors to www mods dk 10 Other observations After removing the C 781 RF Filter Unit module and before putting it back check for these 3 items 1 The coaxial plugs on the cables to the RF unit may slip out from their mating jacks so you may heave to reconnect them You may secure them using small cable ties 2 Take care that the wires near the blower are not so close to its squirrel cage rotor as to risk blocking it Also ensure that wires are not pinched and damaged by the larger parts 3 Most important Ensure that the small coaxial cable from the PA output to the Filter Unit input is secure If it gets disconnected or loose the MRF422 finals may be destroyed at the first transmission The reason is simple The IC 781 includes PA protection for high forward and reflected power high SWR and high collector current Note that the VSWR bridge is located on the Filter Unit and in the event that the small coaxial cable is disconnected from its jack then the protection will not be in line and so will not act The PA will see infinite impedance at its output which may result in destruction of the power devices General rules for less experienced operators Pay attention to the usual safety ESD conditions etc and use the correct tools Always check for possible mistakes such as cables pinched
12. between large or metallic parts disconnected jacks connectors and so on Work seriously and cleanly restore everything precisely as it was before you started the work tighten the cables and wire harness using wire ties etc Keep an eye on the aesthetic part of the job Don t try to do too many things repairs mods at once Do them one at a time only and check after each of the completed jobs for correct operation carefully note errors or malfunctions By doing so you may avoid a lot of trouble Try to use good quality parts from trustworthy sources Don t forget that low quality or counterfeit parts may exist while the seller may not even know what he is offering for sale Don t be afraid to ask for some help from a knowledgeable ham when things get tough if he is a real ham he will be glad to help you as you are in such cases Now the final recommendation if you can t assume the risk then don t do it The IC 781 User and Service Manuals can be found and downloaded from the http www mods dk or from http www marcucc1 it english download index htm websites thanks again to the respective OMs You may also want looking at http www gqsl net ab4oj icom icom html it contains useful information on Icom radios thanks to Adam Farson VA7OJ AB4OJ also for his help on this article There is a lot of work and effort involved in this project but in my opinion it is best that the information be shared among radio amateurs I h
13. coders must be removed and disassembled To do this remove the VFO knob unplug the encoder s connectors and then unscrew the screws holding the encoder to the front panel This will allow removal of the encoder Take care while opening the encoder unit as it is a precision device involving small parts Take special care in handling the thin slotted disk Be careful when disassembling the more complex C R9000 encoder which also includes the switch stepping mechanism Clean the dirty shaft and the sleeve bearing remove the existing grease using isopropyl alcohol and then re lubricate shaft and bearing To reduce any backlash lubricate with grease rather than oil Don t use much grease a small quantity will suffice Avoid getting lubricant on the optical parts clean them if needed Also lubricate the small Teflon and metallic washers and take care when reassembling the encoder The lubricated encoder will rotate easily and will feel like a new one The CRTC CRT Controller Unit The IC 781 and IC R9000 both use the same CRT module similar DDS modules and the same CRT Controller Over time the image may become distorted with the vertical lines bent and the picture shifted to the right side for some areas especially for the larger bright areas of the picture This occurred in both radios I had it seems that the R9000 was presenting this behavior for a long time before acquired it as the distorted image became permanent because of the C
14. composite video signal CVBS as its input signal It is designed for the US and Japanese video TV standard NTSC so the specified vertical sync frequency is 60 Hz not 50 Hz as it is here in Europe As it is used to display a reduced number of particular pictures or video patterns which are selected by the F1 to F6 function keys long term continuous display of these images may affect the CRT and produce so called CRT screen burns This may happen and it is a common problem for any CRT used to display the same picture or pattern for a very long time Even the expensive plasma TVs and displays may be affected by this This is why it is a good practice to periodically change the patterns displayed on a CRT or plasma display while this is simple for a computer display it is inapplicable to the IC 781 or IC R9000 Thus after so many years of use most of them are presenting this CRT screen burn problem It is as if the picture pattern has been imprinted on the CRT screen For the affected areas the picture brightness and CRT phosphor efficiency is lower This is permanent damage and cannot be corrected or avoided but fortunately this kind of problem is not really catastrophic this type of CRT may still function for many years after the first visible appearance of the burns even if it may be uncomfortable to some If needed an external video monitor accepting NTSC CVBS input may be also used or NTSC TV You may see below how these burns
15. may look this example is from an IC R9000 The CRT below had maybe 10 years of use most at a government agency before I got it so it may had intensive use and even now after I have sold it it is still functioning at the new owner s QTH without these burns causing much trouble This is also because the CRT itself is a relatively good quality one made by Toshiba The CRT module is made by Nanao The CRTs are vacuum tubes so they are actually consumables This is true not only for the cathode system regarding its electron emission capability but also for the CRT s screen surface phosphor The burns are the result of the consumable phosphor As the sum of these two characteristics the CRT s brightness contrast and focus quality may get low after extended use This is why the allowed CRT brightness and contrast will be reduced after extended use Note The use of the CRT depends not only on duration of usage but also on the brightness and contrast of the displayed picture The higher they are set the faster the CRT will be used up This is why the IC 781 User Manual specifies Great intensity will shorten the life of the CRT display page 12 The low brightness of the old used CRT s may be improved by using the internal adjustments of the CRT module You have to open the CRT module top cover to gain access to them To do this you need to open the transceiver case then to take out the six screws holding the C
16. ng the big PA and REG heatsink It is located under the RF Filter Units module at the rear left side of the radio and 1s attached to the big heatsink with screws This fan may need lubrication Its motor may have ball bearings not requiring lubrication but the other end of the squirrel cage at the right side looking from the front it uses a sleeve bearing The RF Filter module has to be removed to gain access to the fan Putting the radio on its left side may help to apply a small drop of oil to the sleeve bearing using a syringe If the fan and especially its squirrel cage rotor are very dirty and dusty then the fan should be completely removed and dismantled and the rotor cleaned Clean the heatsink fins as well The fan will not get much quieter it is really noisy but at least it will work better and last longer Fa a ai i IC 781 heatsink and fan removed The fan is sitting on its motor side and the sleeve bearing is at the top part of the fan s case The backup battery Each radio uses two batteries one for the RAM memory and one for the clock If the batteries are dead you may replace them with the more common CR2032 type if you also install the required sockets This will facilitate their future replacement without the need for soldering IC 781 ALC problem This one is described on the www mods dk webpage so there is no need for detailing it here Please visit www mods dk You will also find on this site
17. ope this will be of help to those interested in servicing these fine radios Good luck and enjoy your hobby All the best amp 73 Traian Belinas YO9FZS yo9tzs office deck ro Bucharest May 2005 11
18. orget to use thermal compound insulating washers and mica foil where needed D2 D3 Q2 Q3 Before assembling the PCB you may check the uPC1042 IC it has to start oscillating and verify the 9V and 5V regulator functionality after connecting an external current limited 15V PS at C13 observe polarity Re install the PCB inside its compartment only if all of these are OK As a precaution you may insert a 3 15A fuse in the 280V line you may also leave it in circuit The power supply cannot be verified by simply applying the rectified filtered 280V DC power it needs a start voltage STAR from the transceiver When applied this voltage is only momentarily delivered to IC1 by the Q6 circuitry with R27 R28 C29 as the timing network Once IC1 has started it will continue power itself from the 15V output via D1 If you want to repair or verify the entire REG module on the table or without the outputs connected to the radio then you have to supply the needed starting voltage from an external source In this case you should also use some light loads for the 5V and 33V rails and a 230V 100W lightbulb or two in series as the current limiting device in the 280V line The repaired and re installed board there is an added 3 15 amp fuse in the 280V input line R2 was replaced with a high quality sealed multi turn pot mounted on one of the 33V line electrolytics After finishing the work if everything checks out then proceed to the REG
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