HMV 2304 Racord player/Radio combo.

[Murphyv310]

PCB leakage is quite possible as Lawrence has said. The very high valve grid resistor is to keep the grid current low. UCL83's are prone to leaks but normally in the pentode section and mind the grid voltage will be damped somewhat by your DVM so could even be more negative in real life.
Is the audio distorted at all?

[Terry]

The cathode is at +15v or so ...

Are you sure you are measuring the voltage on the right pin? The anode voltage is stated as 105V and is fed from ~175V via a 100k resistor, so the anode (and therefore, being a triode, the cathode current) is around 700µA.

Thus the cathode voltage should be no more than 0.15V

Hmmm. 15V ... 0.15V (150mV)... seems a bit of a coincidence - are you certain you didn't misread the meter?

[Mark Hennessy]

First thing to do when you get a confusing voltage reading: use an oscilloscope

It's just about possible that there is an AC waveform at that point - perhaps hum or similar - and if the AC waveform is not symetrical either side of 0V, then there will be a DC offset that could easily be negative, depending on the waveshape.

With a x10 probe the 'scope will present a 10M resistance to that point, so shouldn't affect the signal too much. The capacitance will be around 10-20pF, give or take, so shouldn't affect an audio signal like hum significantly (if it's HF oscillation, that might be different). If nothing else, checking this might give you more information about where to go next...

[davegsm82]

Apologies for not getting back to you all on this problem, haven't had a chance to get this set on the bench again. Will try to get it looked at shortly as it's a mystery I'd like to get to the bottom of.

Cheers, Dave.

[davegsm82]

Just closing this one off as I have actually sold the unit now as I need to downsize my collection a little.

Looks like the UCL83 at the bottom of the heater chain was slightly leaky from Heater to Cathode, but also I was doing my testing on top of an ungrounded anti-static mat on the workbench, it was inducing a horrendous amount of noise which went away as soon as you moved away from the bench. The player ended up being largely hum-free after swapping out the UCL83.

Regards,

Dave.