Can someone help me diagnose the cause of a too-low voltage in an FM tuner section?

[haskellbob]

On the attached schematic, can someone tell me what might be the cause for the voltage at C being too low?

There are two pictures of the schematic; at the lower right-hand corner of "New Image" you can see the point "C" coming out of the power supply. In "New Image b" you can see where point "C" connects into the FM tuner circuit (which is not working). Transistor voltages for Q4, Q5 and Q6, that should be 7.3 are down around 4.3.

So this is not an "independent" voltage; when I isolate that juncture of 100mf capacitor and 1800 ohm resistor at "C" by the power supply (disconnecting it from the tuner circuit), I get like 17 volts; when I connect the two "C"'s it drops to 4.5. I would have thought (being electronically naïve) that the voltage at "C" would be constant no matter what was connected to it, as it's a power supply source. But NO...

Transistors Q 4,5 and 6 test good. Could there be a bad transistor in the tuner circuit - or what?

I believe that the speaker leads were shorted at one point, which caused transistor Q106, one of the output transistors, to fry. But how could that have caused voltages at "C" to drop?

I'd appreciate any help. I just don't have the expertise or experience, and I'm baffled.

Thanks,

Bob H.

 

[Chippie]

Disconnect R9 and then R11 in turn...Does the voltage return to the right level?

Which ever does then that is the area to concentrate on...Look for faulty/leaky capacitors as a first...

 

[ericgibbs]

hi Bob.
I would expect the 'C' to rise to 35V with the 'C's disconnected.

Check the voltage at 'A' should be 35V

EDIT:
if its 17V at 'A' my best guess would be its a blown rectifier.

The other point might be the transformer has been set for 240Vac [UK] and you are running from 120Vac [USA]

 

[unclejed613]

Q1 being shorted b-c might cause a big drop, but the cap in the p/s from "C" to ground vould cause problems too. the other transistors are isolated by higher resistances than Q1 is. what is the voltage at "C"? never mind i read the first post again....... sounds like your 35V rail is too low.........

 

[Chippie]

EDIT:
if its 17V at 'A' my best guess would be its a blown rectifier.

Quite possible if one of the o/p devices had failed as previously stated..

 

[haskellbob]

The 35 volts are there and steady as they should be. But I will look into everybody's suggestions and report back.

 

[haskellbob]

Oh yes... it is wired for 110V.

I removed the entire FM tuning unit and the voltage at C went up too high... I forget how much... like 11 or 12 volts....

Resistor R202, at the "C" junction, actually measures around 2300 ohms... (removed from circuit).

If Q1 is blown I'm in trouble, because it's wedged way down inside a tightly-packed tuner unit.

Maybe I should get a resistor closer to the specified 1800 Ω, and a good 100 mfd capacitor for the "C" point, and see what happens. Does that seem like a possible problem-removing action?

By the way, what's an "O/P" device?

Is it possible for an 1800 ohm resistor to have drifted over time to a higher resistance?

Thanks...

 

[ericgibbs]

The 35 volts are there and steady as they should be. But I will look into everybody's suggestions and report back.

hi,
If the 'C' at the psu is 17V when the 'C' output is disconnected and its 35V at 'A', then its either R202 gone 'high' or C203 is 'leaky'.

I would suggest you recheck the values you have posted.

 

[Chippie]

O/p device...I meant one of the power transistors in the audio section...Sorry...me being lazy.

Is it possible for an 1800 ohm resistor to have drifted over time to a higher resistance?

Yes its possible...

A good idea would be to replace the 1800 ohm resistor and the capacitor...

I removed the entire FM tuning unit and the voltage at C went up too high... I forget how much... like 11 or 12 volts....

So it looks like the power supply is ok...no blown diodes..

Back to R9 and R11 then...

What happens if you disconnect each in turn? Does the voltage at C come back up?

 

[haskellbob]

When i disconnected R9 the voltage went up to around 12V (which is still significantly higher than the 9.5 it's supposed to be), but when I left R9 connected but disconnected R11, the voltage dropped to .5V. There must be a short inside the tuner unit. Since I'm just playing around and this unit isn't too important to me - unless I'm wrong and the problem isn't inside the tuner unit (which is a tiny aluminum pre-wired box), I'm going to give up - at least in terms of getting the FM to work. Seems like I could get a monaural amplifier going - but who needs a monaural amplifier?

Thanks for your help, and let me know if I'm wrong. It seems like the short or whatever is inside the tuner unit and therefore practically inaccessible...