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Author Topic: No AM on TS940  (Read 616 times)
G4IMS
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Posts: 2




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« on: January 22, 2013, 06:39:25 AM »

My TS940 works in all modes except that it won't transmit on AM.  I don't believe it has a fault but that this behaviour is caused by a modification to the Control Unit.  What I have found out is as follows.  The radio has a Receive voltage rail (RV) and a Transmit rail (TV).  These rails are enabled alternately with a small delay between them.  On my set, only on AM mode, TV is inhibited via logic circuitry on the Control Unit which pulls down input pin 11 on IC4.  On investigating why I could not control pin 11 via the transistor connected to it, (Q8) I found another transistor collector connected to it on the reverse side of the Control Unit PCB.   There a several modifications on the back of the PCB, some of which are recorded on the PCB layout in the Service Manual.  It’s not clear if any of these mods. are interrelated, so it's difficult to check them through for a diagnosis.  The modification I examined isn’t in the manual, or on any of the web sites I have looked up, but as the previous owner bought the set new and as far as I know didn’t modify it, the mod could well have been put on in the factory.  Does anyone recognise this mod?  or can think of any reason why AM transmit should have been inhibited?
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KE3WD
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Posts: 5694




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« Reply #1 on: January 22, 2013, 09:14:09 AM »

I doubt the extra transistor, etc. would be done to simply inhibit AM operation.  There are far more simple ways to go about doing that. 

I would try to DRAW the circuit as modified, using the service manual schematic as the starting point.  Double check my drawing against what is actually there, maybe tripe check it, then I'd be able to figure out what the additional components are doing and thus be able to troubleshoot. 

Typically the insertion of another transistor stage after the production fact would likely be done to add a stage of buffering in order to prevent the following transistor from failing in the field.  Sometimes that can be due to switching transients that may exceed the original transistor's ratings, things like that. 

And don't overlook Ye Olde Dry Jointes when it is a 940.  Sometimes just resoldering an area, resoldering the pin connectors to the board, removing/replacing interconnects to clean them, can solve 940 problems such as this one. 

Finally, there is a 940 group on Yahoo where you can find a wealth of info on these fine transceivers, matter of fact I'd start there, for there is likely someone or more who already know the situation under the board - and more. 


73
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KE3WD
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Posts: 5694




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« Reply #2 on: January 22, 2013, 09:15:24 AM »

BTW -  take a look at L-100.  If the wax originally around it is melted, suspect that coil.  Replacement is the way to go there if found. 


73
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G4IMS
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Posts: 2




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« Reply #3 on: January 22, 2013, 12:18:13 PM »

Thanks for the advice, and I have now joined the 940 Yahoo group.  I have examined the component layouts and the complete Parts List for the TS940 but so far have not found L-100; can you tell me where it is?
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KE3WD
Member

Posts: 5694




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« Reply #4 on: January 22, 2013, 12:56:59 PM »

Its been awhile since I serviced one, so please forgive the lack of board number or nomenclature, but the board is under the bottom panel on the LH side of the rig when rig is in operating position and facing you.  The coil is rather large and located near center of board.  Easy to spot as it is potted in some kind of wax that may be beeswax involved, it is beige.  If that wax is found to be melted away from the coil form proper, suspect that coil.  The very first one I encountered like that measured fine as to continuity on the ohmmeter, even sowed signs of adjustability, finally, ordering and replacing it because I'd tried just about everything else did the trick dontcha know. 

This is a shot in the dark, but worth mentioning because of your reported symptoms, with this rig the problem could also easily be elsewhere.  But I learned about the 940 that following all that switching stuff from transistor to transistor was almost never where the problem lay. 

If you have the Service Manual, and you should, scroll down or page back to the near end for the Parts List, find L100 (darn, it might be L101...) and the locator designation will be there. 


73
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