Bassman style amp blowing rectifier diodes

[sluckey]

I don't think I can fit double the diodes

Pretty easy if you have a pcb or turret/eyelet board and will fit in the same space. Just unsolder one end of a diode and stand it up. Solder the new diode in the hole vacated by the original diode and stand it up too. Observe polarity. Now solder the two upright leads together. Repeat for remaining three diodes.

[BungleSim]

Here's a measurement of the cathode of D5 while the the standby switch is being closed. This was done with a 100X scope.

PXL_20210408_165712269.NIGHT.jpg

[pompeiisneaks]

I'd think seeing when it opens is more helpful, as the spikes likely come when an amps caps are pulling large inrush current to charge up, but maybe I've misunderstood the potential fault?

~Phil

[BungleSim]

I'd think seeing when it opens is more helpful, as the spikes likely come when an amps caps are pulling large inrush current to charge up, but maybe I've misunderstood the potential fault?

~Phil

But aren't the caps charging when the switch closes, not opens?

[pompeiisneaks]

I guess maybe the terminology is confusing me. Closed in my head meant 'shut off' The scope seems to show that the voltage dropped drastically, spiked a few times, then dropped again, like caps were discharging and then the voltage was gone. Since there's no mark of what the actual bottom and top voltages represent my 'guess' was that bottom was near 0v and top was near b+ (seeing 2v per division with a 100X probe I was guessing top was about 600VDC or just under and bottom was 0)

I'd think that turning the standby "ON" meaning turning the amp "ON" would be switch closed, because that means the circuit is connected, but seeing the graph confused me.

Where are you probing? Before or after the switch?

Basically, now I'm really lost... the voltage significantly drops after power on? or is this literally only dropping 8 volts and that's it?

~Phil

[BungleSim]

I guess maybe the terminology is confusing me. Closed in my head meant 'shut off' The scope seems to show that the voltage dropped drastically, spiked a few times, then dropped again, like caps were discharging and then the voltage was gone. Since there's no mark of what the actual bottom and top voltages represent my 'guess' was that bottom was near 0v and top was near b+ (seeing 2v per division with a 100X probe I was guessing top was about 600VDC or just under and bottom was 0)

I'd think that turning the standby "ON" meaning turning the amp "ON" would be switch closed, because that means the circuit is connected, but seeing the graph confused me.

Where are you probing? Before or after the switch?

Basically, now I'm really lost... the voltage significantly drops after power on? or is this literally only dropping 8 volts and that's it?

~Phil

Sorry, I guess there is a lot going on in that image to keep track of. Since it's a 100X probe, that means every voltage you're seeing on the screen should be multiplied by 100. So Cursor 1 is at -560V and Cursor 2 is at 328V, with a Delta of 888V. The mistake I made was measuring AC voltage rather than DC.

As for terminology, a closed switch is when terminals are connected and an open switch is when they are not. That's how I'll refer to the switches. I haven't referred to any terms like "in standby", "standby on/off" or anything like as that can get confusing.

I'm probing at the cathode of D5, referencing the schematic in the original post. Below is an updated measurement (DC volts this time) and I can see a 1280VDC spike, after which the voltage settles at 535VDC.

PXL_20210408_181530622.NIGHT.jpg

[pdf64]

Phil, I see your rationale, but I think it’s standard terminology for a switch, when ‘closed’, to allow current to pass, and for it to stop current flow when it’s ‘open’.
Other common switch terminology is ‘make’ and ‘break’, perhaps that’s a bit less open to misunderstanding?
Basically, in every way, standby is topsy turvey.

[pompeiisneaks]

Phil, I see your rationale, but I think it’s standard terminology for a switch, when ‘closed’, to allow current to pass, and for it to stop current flow when it’s ‘open’.
Other common switch terminology is ‘make’ and ‘break’, perhaps that’s a bit less open to misunderstanding?
Basically, in every way, standby is topsy turvey.

It seems I did understand it correctly, as I took 'closed' to mean closing the circuit and allowing it to work, but then why would voltage drop to zero when connecting the b+ before the switch to that after it? It should do some bouncing, but there's a massive drop in voltage that remains afterwards.... this seems odd/bad no?

Possibly I'm misreading that scope output, but it seems like it's losing massive voltage not the typical smaller drop I'm used to (say from 500VDC to 450VDC under load)

~Phil

[BungleSim]

Phil, I see your rationale, but I think it’s standard terminology for a switch, when ‘closed’, to allow current to pass, and for it to stop current flow when it’s ‘open’.
Other common switch terminology is ‘make’ and ‘break’, perhaps that’s a bit less open to misunderstanding?
Basically, in every way, standby is topsy turvey.

It seems I did understand it correctly, as I took 'closed' to mean closing the circuit and allowing it to work, but then why would voltage drop to zero when connecting the b+ before the switch to that after it? It should do some bouncing, but there's a massive drop in voltage that remains afterwards.... this seems odd/bad no?

Possibly I'm misreading that scope output, but it seems like it's losing massive voltage not the typical smaller drop I'm used to (say from 500VDC to 450VDC under load)

~Phil

The scope is showing 500us/div, so each division is only 500 microseconds and you aren't able to see the voltage rising and stabilizing at 535VDC.

Anyway, I have put a 4.7K resistor across the standby switch and I'm looking at the transients on the scope. To be continued...

For reasons not worth getting into as it'll only muddy up the thread, getting rid of the standby switch and doubling up on the diodes are not feasible solutions, unfortunately.

[pompeiisneaks]

Oh ok so it returns to normal outside the window. Makes more sense, should have seen the time scale... sorry hehe.

Those spikes do look pretty weird, are those what are called flyback spikes? if the peak there was 12.8 v that means it peaked at 1280VDC and those diodes are usually 1kV max... which means something is definitely causing some stress on those diodes.

Or is that some kind of arc across as the contacts get close, but before fully engaged?

~Phil

[pdf64]

...Anyway, I have put a 4.7K resistor across the standby switch ...

Merlin suggests 47k, but as your HT voltage is so high, I’d increase that, eg 100k.

[trobbins]

I use PSUD2 for quick power supply assessment. Knowing the primary and secondary winding DC resistances is part of that modelling. The amplifier loading info that is the basis for assessment is the idle current requirement, and a circuit schematic of the output stage and awareness of what the idle current could increase to under heavy signal.

The scope plots do look like contact bounce results in the voltage across the C24 ringing due to the transformer secondary leakage inductance wanting to keep pushing current through D5 and so C24 gets pushed high in an oscillatory manner (due to value of C24).

You could also try and use an alternative diode with a 2 or 3kV PIV rating, but again that would need some assessment of diode current requirement.

I'd have to check if the 1ms bounce of the contact was consistent with other switches - but that could be difficult as most such reporting is on relay contacts and not some large Standby switch. But certainly the switch contacts seems to be getting a good work out, which would I suggest be more onerous that it was rated for.

[BungleSim]

...Anyway, I have put a 4.7K resistor across the standby switch ...

Merlin suggests 47k, but as your HT voltage is so high, I’d increase that, eg 100k.

Thank you for pointing this out because I was just going to post how when the standby switch is opened the HT only drops to about 475V and doesn't go any lower... I will try 100K but I am skeptical about its effectiveness because even using the 4.7K resistor didn't mitigate the transients as much as I'd hoped it would. I imagine the 100K would be even less effective, but I'll find out today.

[pdf64]

The trickle charge bypass cap just acts to reduce the inrush current surge required to charge the reservoir cap, it can’t mitigate contact bounce.