Golborne Vintage Radio

Quick question about this record player, perhaps someone more learned about valve amps can shed some light on this curiosity.

The schematic is available in a trader sheet, just google the search term HMV2304 and it will pop straight up as a PDF.

Ok, so it all works, but there's a slight mains hum which I'm trying to eliminate. If you turn the volume pot to zero the noise goes. If you disconnect everything before the volume pot then the noise remains the same, so the noise is somehow introduced in the circuit around V3a (the pre-amp).

The curiosity is that when I measure the voltage on the Grid of V3a with respect to ground, I get a NEGATIVE voltage. Bare in mind that there is no negative (WRT to mains neutral/earth) voltage anywhere in this set. The voltage decreases (increases negatively?) from 0 to about -2v as the valve warms up, if you disconnect from the mains the voltage very slowly drops back to zero. My thoughts were that the AC heater chain (this is the most earthy valve in the chain) was somehow acting like a diode on the grid and causing a negative voltage on this pin, but if that were the case then the voltage would disappear the instant that power was removed.

Very odd indeed.

I've also checked the main rectifier valve and the mains filter cap (by substitution) and the noise is exactly the same.

Can anyone help?

2 volts -ve seems a bit high to me for V3a grid, is the 10 meg the correct value?

Lawrence.

Hi Lawrence,

Checked that resistor and it's reading about 10.5 Meg0hm. The cap connected to it seems good too.

The bit that confuses me is that it's -ve at all, i know the grid should be -ve WRT to the Cathode, but this is WRT to the chassis. The cathode is at +15v or so, the Grid is -2.

Cheers, Dave.

If the cathode of V3a is +15 volts I would measure the resistance between cathode and chassis.

Lawrence.

Hi Lawrence, I'll check that tomorrow, have you got any idea where the negative voltage can come from though?

Thanks, Dave.

Is the volume control ganged with the on off switch?

Have you traced out the cathode resistor chassis connection?
It could go to a bias resistor in the common to the rectifier arrangement.

The manual is easy to find as per Dave's instructions Ref. The amp has NFB and the PSU is a simple half wave type. Amps like this can be prone to AC pickup and its known that leakage from the on off switch to volume control if combined can give hum at anything other than minimum volume. Id remove the mains lead connections from the switch if combined and hard wire them temporarily away from the switch and see if the hum goes. I'd also get the hum issue resolved and then see if the discrepancy in bias has changed.
Also check the VC is earthed properly and there is no hum loops, the triode is very high impedance so very easy to induce pick up.

So the chassis is 2v + WRT the grid is another way of looking at it.

(21-10-2018, 07:57 PM)davegsm82 Wrote: [ -> ]Hi Lawrence, I'll check that tomorrow, have you got any idea where the negative voltage can come from though?

Thanks, Dave.

Under normal conditions the bias for the V3a will be due to grid current bias so far as I can make out, and the voltage developed across the cathode resistor would be minimal, wouldn't have thought more than around 0.2 volts or so across the cathode resistor, the grid current for the grid current bias will be tiny, hence the high value grid resistor that is used.

I've known hum to be caused by PCB leakage, often if it's been subjected to heat and discolouration has occurred.

Lawrence.