Murphy a252 vhf question

[Mike76]

I have this set that the fm stopped working on, while I clipped a probe of a pc oscilloscope between one of the adjustable cored coils and the chassis it suddenly got reception with a lot of noise over it. There's no loose connection, i think was having the probe attached that did it. Can anyone suggest why this should be.

Thanks for any replies,

Mike

[pwdrive]

There's lot's of coils in that receiver.....

Lawrence.

[Refugee]

That may well be due to an open circuit grid resistor causing the valve to turn hard on.
Dab a 10 meg resistor on and if that works replace the factory fitted resistor and it should be all right again.

[Mike76]

As soon as I get a chance to I'll try that. Thanks for the advice and I'll let you know if that's the culprit.

Mike

[Mike76]

It was between c7 and L4 that I attached the probe, if that is of any help understanding why I got reception. The resistor tests ok when I checked it too.

[Murphyv310]

I very much doubt is a grid resistor, more likely a decoupling problem.
What test equipment do you have and do you have a circuit diagram and component layout?

[Refugee]

We can soon find out if it is decoupling or a resistor by adding a resistor or a 100pf capacitor.
I have not got a circuit diagram in front of me.

[Geordie McBoyne]

Is this the receiver that uses two separate triode valves in the VHF tuner? Types EC92 and 6L34.

Geordie McBoyne.

[Mike76]

It is that one yes. Well I replaced c6 which the service sheet says is critical, and it works. However whilst I poked around a tag strip that the aerial connects to I touched the leg of one of the two 22pf capacitors on there and got much more volume, then a bit more poking and it went dead again. It looks very degraded so I will replace it, but why did touching the leg give so much more volume. I'm wondering if this was always the problem as it went completely silent at one point.

[Mike76]

One more thing, it only happens when that leg of the capacitor is touched and not if you touch the aerial socket it is wired to? Why was 22pf chosen for those two capacitors, could I substitute them for a pair of a different value that I had or is it important that they are the same, i need which case I will get some.