Murphy a252 vhf question

[pwdrive]

The capacitance of those two capacitors is chosen so as to tune out any leakage inductance from the transformer so as to keep the antenna input impedance resistive.

Lawrence.

[Mike76]

Any idea why touching on one of them made such a difference in the volume level?

[pwdrive]

No idea off hand but sometimes tin whiskers can give similar symptoms, quite rare though, I've also had things like a lazy oscillator coming back to life through general prodding around, other things to check for are dry joints and poor connections including valve/valve sockets and chassis grounding connections, what's the VHF antenna like?

Lawrence.

[Refugee]

Does it matter what end of the capacitor is touched?
Is there just a wire between the coil and the capacitor?

[Mike76]

I've got it plugged into an external one so it's good. The effect of increased volume is had with the lightest touch with anything metal on the capacitor leg and seems most pronounced wit it grounded to the chassis through another capacitor. I need to get my signal generator working and align everything properly as I just tweaked it all without it until I got the best reception. I've no idea where it is on the scale as it's not attached to the chassis at the moment.  I'm just glad it's working really.  I am just learning and went about it the wrong way really.  I think I should have started by trying to align it first and seen where that got me, that way I guess it would have pointed me towards any problem areas. That and checking all the voltages at the valve bases. You live and learn.
Thanks for your suggestions,

Mike

[Mike76]

The coil is connected to a tagstrip right next to it directly, it also has wires to the aerial socket and then the capacitors soldered across it. I will have to look again to see which leg it is, but does seem to be the one leg yes.
Mike

[Murphyv310]

By touching the cap you are effectively changing the capacitance of the circuit. I'd say there is alignment issues. Does the effect occur across all the band or just at one end?

[pwdrive]

Is there continuity between the output side of C1 and C2 and chassis?

The center tap between L1 and chassis is via the AM antenna coupling coils as the VHF antenna is designed to be used as an AM antenna as well in the absence of an external AM antenna connected to the AM antenna socket.

Lawrence.

[Murphyv310]

Hi.
I'd disagree with aligning the set firstly, if the cores and trimmers have not been tampered with, most likely you have a fault than an alignment issue. Aligning a set with leaky caps and off value resistors just mean you'll need to do it all again after replacing faulty components and that includes valves as well.
One thing to note is when replacing decoupling caps and especially in an FM front end is to use capacitors of similar size and tolerance but also to fit them as near to the original position as possible and always use the same earthing point. Failure to do this can give rise to instability, wrong tuning, low gain and other maladies.
Good luck with the set, a great performer once completed.

[Refugee]

Does it have the same effect when the end of the wire where it connects to the coil is touched.
Check the wire with a low ohms range too.