Valvestate VS265 Hum, RF, Voltages

[bobloblaws]

Hey guys, I'm working on a Marshall VS 265, stereo power amps with chorus which has potentially a number of issues (different symptoms at any rate). It has a steady background hum that is not affected by any of the controls. I'm thinking maybe filter caps but I haven't gotten to swapping them out. If I plug a guitar cable in there is additional hum and RF (local country station) especially if I turn the volume pot to zero or turn up the reverb pot. In this scenario holding the guitar cable and moving it around affects the amount of noise and the "reception", like the cable is an antenna. I've looked at all 3 circuit boards for bad solder joints and re-soldered a couple that seemed dodgy but it seems to have been of no consequence as the symptoms remain.

I replaced the 12AX7 with a Chinese tube and that got rid of some of the background hum, but not all, sounds like the part that got eliminated is of a lower frequency. Heater voltage looks a bit high, more like 7.3VAC on pins 4 and 6 as opposed to the 6.3 that I would expect. Does that make a difference?

For the HT voltage, if I measure AC volts with both DMM probes on either side of the xfrmr tap it measures 235 VAC, but if I measure AC wrt ground I get 132V on one side and 125VAC on the other. Is this a problem? F1 and D2, D3, D4, and D5 seem OK.

According to the schematic I should have +15V and -15V at the outputs of REG1 and REG2 but in fact I'm seeing +12V and -12V. On the input of REG1 and REG2 I would have expected an approximate mirror image as well but in fact REG1 is +21V and REG2 is -28V. Meanwhile on the other side of R21 and R20 I have +40V and -40V, respectively. Both resistors appear to be in spec. The two regulators and two resistors get pretty hot. REG1 in particular shows signs that it's been cooking pretty good. Should I be looking at he caps in that section?

Well, that's what I'm facing, any suggestions are greatly appreciated, as always.

- Bobby

[g1]

Those are 12V regulators, with ground pins going to zero volts, so the output should be +/-12V. That 15V is a schematic error/typo.


schematic attached

[g1]

I misread your info about the resistors feeding the regulators so edited that out of above.
If there is stuff running off the +12 only, that would create an imbalance feeding the regulators, and would be normal. Otherwise, there is probably a fault on the +12V line somewhere.

[bobloblaws]

Those are 12V regulators, with ground pins going to zero volts, so the output should be +/-12V. That 15V is a schematic error/typo.


schematic attached

Thanks for attaching the schematic, I forgot that.

[g1]

Looks like you have about an extra 40mA flowing through R21 compared to R20.
TR1,2,3, & 4 on preamp pcb all run off the +12V rail. I doubt they are responsible for the whole difference, but you can check. Measure voltage across their emitter or collector resistors and calculate the current through each of them.

[bobloblaws]

Looks like you have about an extra 40mA flowing through R21 compared to R20.
TR1,2,3, & 4 on preamp pcb all run off the +12V rail. I doubt they are responsible for the whole difference, but you can check. Measure voltage across their emitter or collector resistors and calculate the current through each of them.

I hope I measured across the correct resistors, here's what I got:

TR1 -> R32(1.5K) -> 6.24V -> 4mA

TR2 -> R45(47K) -> 0V

TR3 -> R34(47K) -> 0.83V -> (effectively zero current)

TR4 -> R38(10K) -> 0V

[g1]

So nothing there that would account for the difference. Do any of the IC's feel particularly hot? Are the zeners ZD2 & ZD3 giving around +/-6V ? (shown to left of Reg1 & 2).
If no obvious heat anywhere you can isolate various parts of the amp by disconnecting connectors carrying the +/-12V.

[bobloblaws]

So nothing there that would account for the difference. Do any of the IC's feel particularly hot? Are the zeners ZD2 & ZD3 giving around +/-6V ? (shown to left of Reg1 & 2).
If no obvious heat anywhere you can isolate various parts of the amp by disconnecting connectors carrying the +/-12V.

The IC's don't feel hot and the zeners are right on the money. The only connector carrying that voltage is the one going to the daughter board. I measured with it connected and diconnected, for the former there is a 37mA discrepency, for the latter there is still a 28mA discrepency.

[g1]

The MN3007 & 3101 on the daughterboard are running on the + supply only, so I think that would account for the 10mA difference for the daughterboard.
I guess the next thing you could try is to check for any DC on output pins of op-amps on preamp board.

[Axtman]

I recently repaired a Marshall VS65R amplifier that had a hum. All that was wrong with it was cracked solder joints on PCB mounted filter capacitors. Have you checked those?

[bobloblaws]

I recently repaired a Marshall VS65R amplifier that had a hum. All that was wrong with it was cracked solder joints on PCB mounted filter capacitors. Have you checked those?

Yes I have, but it wouldn't hurt to double check that. I appreciate the input, thanks.

[bobloblaws]

The MN3007 & 3101 on the daughterboard are running on the + supply only, so I think that would account for the 10mA difference for the daughterboard.
I guess the next thing you could try is to check for any DC on output pins of op-amps on preamp board.

I checked the output pins on the op-amps and what I found that is potentially of note is 0.241 VDC on pin 7 of IC7 and on pin 5 of IC2 there is 0.034. Would either of those be of any consequence?

[Jon Snell]

A slight DC off set is normal. When op amps fail, it is usually catastrophic and the output goes to a rail voltage with no AC output.
Don't allow yourself to get confused between DC and Hum. DC is not hum!

Remove C6 (the power amp input) if the hum stops then the hum is coming from a previous stage, if the hum is still there, it is power amplifier/ground born.

Work backwards, it is easier and more logical as you can hear what is happening.

[Jon Snell]

C3 is the right hand amplifier.
If the hum is in both power amplifiers with C3 and C6 removed, check the main smoothing.
I have had one repair with one of the 2k2uF capacitors open circuit. C9 I think. Replace both if this is the case.

[bobloblaws]

A slight DC off set is normal. When op amps fail, it is usually catastrophic and the output goes to a rail voltage with no AC output.
Don't allow yourself to get confused between DC and Hum. DC is not hum!

Remove C6 (the power amp input) if the hum stops then the hum is coming from a previous stage, if the hum is still there, it is power amplifier/ground born.

Work backwards, it is easier and more logical as you can hear what is happening.

Thanks Jon, and I will definitely try your suggestions tomorrow.

I have a feeling I'm dealing with separate issues in addition to some background hum as described in post #1. For example, do you have any opinion on what might be going on with the significant hum and RF that shows up when I turn the clean channel volume right down to zero? If the the volume is on say 1 or higher it works relatively normal but when I turn up either of the reverb pots I get the same symptoms as turning the volume pot to zero. I can't help thinking this has a separate cause than the steady background hum.