Quad FM3 issues

[johndoe]

On the off chance, check the jumper connection that connects to the two PCB's for dry joints, I've found 3 with odd function tuning lamps that were cured when the cracked joint was found.

[frsimen]

I expect that a low resistance somewhere on the positive supply has damaged the regulator transistor.

Try making some resistance measurements on the tuner's positive supply lines to see if there is something wrong. You may not find a short circuit, it's more likely to be a few 10s of ohms type problem.

Another area to look at is the lamp holders. I don't know how likely it is, but could one of the lamp holders have shorted to the chassis at some time, causing the regulator transistor to fail?

Where you have fitted replacement transistors, have the leads been cut short so that they cannot touch the chassis when the board is in place?

Paula

[stevehertz]

Originally Posted by PYE 405

Another thing to be careful of is the use of de-soldering tools. The common spring loaded plunger affair can jerk and cause a pad to lift off. Solder wick maybe better and with a swift action with a very hot iron, the solder can be removed without the pad.

Personally I would expect a spring loaded solder sucker to be less likely to lift a PCB pad than solder wick. With solder wick you're pressing down onto it and the PCB pad, directing a lot heat into the area for a few seconds maybe. With a solder sucker you apply the tip of the iron to the soldered joint and as soon as the solder melts, press the button. I don't think the mechanical effect of the tip momentarily dabbing down will cause the pad to come away, it's more like to do so as a result of excess heat. Just my view.

[mikes08]

Just to update found one of the power supply transistors had blown - TR102 so have ordered replacement and once fitted will check the +- supplies again mainly the positive which is only showing 3v at present. If that resolves that problem i can investigate the original problem - the lamp again but have already replaced the driver Tr's here. I think i may have touched something i shouldn't have which might have caused TR102 to blow. Today's project is to go through the tuner checking all the resistor values are correct and nothing amiss in the tuner section now though it was working as a receiver before i intervened.
Many thanks to Paula for her assistance here.
Mike

[mikes08]

Transistor TR102 arrived today and back in place now and 14v on both -+ rails.
Have temporarily disconnected the jumper wires to the tuner end at Paula's suggestion and will fit some 22ohm 1/4 watt resistors in line and connect up initially again at Paula's suggestion.
Have checked over the boards and checked resistor values and all seems ok - have also taken the lamps out for now.

[mikes08]

Up and running again now and full stereo reception but the right hand tuning lamp does not dim as tuning in and out just stays bright so some more investigation needed - i am sort back to where i was but with the lamp working although not correctly. I am going to measure readings against my working one and see if i can find any discrepancies.

[mikes08]

All good now..I was testing with no aerial in but as soon as I put it back into my system it is working fine.
Thanks to Paula and all who helped with suggestions.

[mikes08]

All working well still but just tinkering - oh dear - when i was repairing i noticed pre-sets RV2 & RV3 seemed a bit tired. I have replaced the RV3 now (10k) and adjusted but also want to replace RV2 which controls the brightness of the tuning lamps - it is currently 20k but i can only find 22k pre set from Cricklewood - can i substitute the 20k for the 22k and adjust the brightness accordingly?

[stevehertz]

Originally Posted by mikes08

All working well still but just tinkering - oh dear - when i was repairing i noticed pre-sets RV2 & RV3 seemed a bit tired. I have replaced the RV3 now (10k) and adjusted but also want to replace RV2 which controls the brightness of the tuning lamps - it is currently 20k but i can only find 22k pre set from Cricklewood - can i substitute the 20k for the 22k and adjust the brightness accordingly?

Yes, it won't make any difference; it's adjustable and 20k is no different to 22k in this application.

[mikes08]

Many thanks Steve.