I'm still being bugged by the problem that first brought me to this forum months ago , and that I thought I had solved with advice from here.

My Philips 123 slowly fades in volume over an hour or two, but snaps back to normal volume if I switch power off and on, or sometimes if there is a spike in the power such as a fridge turning on or off elsewhere in the house, or sometimes if I inject a bit of static to the signal by touching the antenna wire.

I've resoldered just about every connection and replaced most caps (everything in the HT zone). I've substituted valves around and not affected this particular problem. The symptom doesn't seem to me like a dry solder joint or socket contact, because the fade is gradual and it is corrected without any physical movement, just by an electrical pulse.

It's hard to see it as heat-related, because it corrects without any heat change.

It's not dirt in the volume control pot - that's clean and smooth.

Logically, it has to be either

a) something reducing the signal by increasing resistance in the path, by decreasing detection, or by increasing leakage to ground, or
b) something reducing amplification in a way that corrects itself when getting a jolt in signal or power.

But I'm not an engineer so my logic may not be relevant.

Any suggestions where I can look next?

Maven

Dodgy IF transformer?

Mica caps???

I actually have a 123L. Do beware of the power transformer in those & some others with that transformer. I have seen a few with the windings slippiing out of the wrap.

That sort of thing can be the realm of a Mica cap. Normally one with HV DC on it, albeit I have had them failed all over the show.

It might pay to use an analogue meter to monitor the volts on B+. If it rises a valve is losing conduction.

The other thing is a "charge discharge" fault where something charges to a certain voltage, then lets go & starts all over again. Open circuit grid leak resistors have been known to cause this issue.

Marc

Thanks for these pointers. I'm working through them.

Opening the thing up again, I found I had missed a 10μF 40v axial electrolytic (C25) between the power transformer and ground. I've replaced that and things seem to have improved, but I will need to run for a few hours to know if the problem is fully solved.

Meantime I'll look for replacements for the four micas not yet replaced. Three are in the tone circuit, so could they be causing this volume issue?

There's no sign of any deformity in the main transformer and voltages seem stable.

I sense myself drifting toward replacing every capacitor and resistor in the circuit... but I'll change things one at a time to try to find out exactly what has caused this fade and snap.

Maven

It can only be one on the tone. If it happens on all of them, that is not it.

More likely to be one on the detector, or one with HV DC on it.

If it is going of f the station then look at the ones on the oscillator coil.

Marc

My AWA had similar issues & I found if I wobbled or tapped a valve it would come back up, cleaning the valve socket with modified micro scrub & retensioning the contacts made it go away, can't remeber exactly which valve, but it wasn't the rectifier or power out.
Just another item to throw out there, sometimes its the simpler things....
Regards

[I]My AWA had similar issues...[/I]

As has an AWA radiogram of mine. It will start and stop of its own accord but I've narrowed it down to a valve with either a dicky socket or a broken wire in its base. It's a ten valve chassis so unfortunately I need to lift some weights and warm myself up for the job of extracting the chassis again.