1970s Peavey Bass 300 - popping/shorting when load connected

[Drummer4gc]

I'm working on a Peavey Bass 300 from the early 70s. It seems stable when turned on and no load is connected - on a bulb limiter, I've got a stable dim glow that doesn't change. However, when I plug in a speaker, I get nasty loud popping that corresponds to the amp's "on" light dimming. When turned on and connected to a bulb limiter, the bulb glows brightly as the speaker pops, dims in between pops. Unplug the speaker, bulb stays dim again. Same effect if I connect a dummy load instead of a speaker (just can't hear the pops).

All the power transistors check fine out of circuit, and I've replaced the mica insulators and grease. I know the electrolytics are all way past their lifespan and will be replaced at some point. I'm curious where I should start looking to diagnose this and would love any thoughts and ideas - thanks!

Matt

[Jazz P Bass]

You have to narrow down the issue to preamp, power amp or power supply.
I see a 'mute' switch where the signal goes in to the power amp.
Throw the switch to mute the preamp. if it still pops then you have a power amp/ supply problem

[Helmholtz]

First thing with SS amp problems is checking for DCV at the speaker terminals.
If you see DCV you have a power amp problem and you shouldn't connect speakers until the problem is solved.

[g1]

So the bulb goes dim between pops?
The bulb lighting up is normal for any kind of sound, whether it be a pop or playing a note through the amp. (try playing through it and you will see)
It doesn't light with no speaker because the amp isn't doing any real work, so no current draw through the bulb.
You have a popping noise to cure, but the bulb isn't really telling you anything in particular to worry about.

[Drummer4gc]

Thanks for the observations, some responses:

-It's definitely a power amp/power supply issue. Disconnecting the preamp connections to the power amp makes no different in symptoms.
-With no load, there is about ~0.03v at the speaker terminals, which I don't think is an issue. It's difficult to measure DC when a load is connected since it gets so poppy, but I am pretty sure I did read many volts of DC present, intermittently, with load connected.
-As I've played with it more, it always remains stable with no load. With a load connected, I've seen it remain stable (no bulb lighting and no popping), intermittent but frequent popping/lighting, and straight up glowing bulb with obvious DC on the test speaker (cone extended). I haven't been able to determine any pattern or reason for the different states. Moving the board around, chopsticking, input signal or not, doesn't seem to have a predictable impact on the situation.

[Enzo]

OK, unstable under load. But is the amp blowing fuses? I mean, is the bulb necessary at this point? Many amps are unstable on the bulb.

[Jon Snell]

This design of amplifier taken from the original RCA 70W amp, when the HT drops below 32volts or so, the amplifier goes unstable because it cannot maintain the current drive from each HT rail. That is what happens will low mains voltage, remove your mains bulb, measure the HT and load the amplifier with a power resistor of approximately 8 Ohms in value, that avoids damaging the speakers if there is another problem.
You will find the amplifier should now be stable. If it isn't, replacing the capacitor 50u bootstrap and 25u AC feedback decoupling will fix it. If it is the main smoothing, it will sound odd with mains ripple but still work.

[J M Fahey]

Some amps wake up stupid through bulb limiter and load applied.
1) turn amp ON, WITH limiter, NO load.
What happens?

2) after it turned on "normally", connect load. What happens?

3) if 1 and 2 normal, connect amp straight to mains, no load, what happens?

4) if 3 normal, repeat turning it ON, WITH speaker connected. What happens?

As you see, we go step by step, checking each one.

What jon snell said, by the way, only using more words.

[Drummer4gc]

Updates:

1) turn amp ON, WITH limiter, NO load. What happens? Not much. Amp turns on, limiter bulb turns on then glows dim and stays dim.

2) after it turned on "normally", connect load. What happens? Plugged into 8R dummy load, not much for maybe 15 seconds. Then, bulb starts to flash brightly, then more brightly more often until I turn it off. Plugged into 4 ohm speaker, lots of crackles then big pops associated with dimming amp power light and bright bulb limiter, more quickly than with the 8R load.

3) if 1 and 2 normal, connect amp straight to mains, no load, what happens? Did this anyways. Same answer as #1.

4) if 3 normal, repeat turning it ON, WITH speaker connected. What happens? Same answer as #2.

I did these tests after replacing the capacitors Jon suggested - hoping I interpreted correctly that the "50u bootstrap" is the one connected to the 1.8k/1.8k divider in the power amp and the "25u AC feedback decoupling" is the cap tied to the 10k resistor at the bottom of the schematic.

OH - I also realized yesterday that the 24v to the preamp was reading more like 32v. There is a trimpot on this amp located where the 1k resistor is on the schematic in the power section, and I was able to adjust it to bring the voltage down to 24v. However, this amp is full of 25v spec'd caps, so it's possible some/many/all are damaged. I intend to replace them, but am hoping to figure out this fault without shotgun replacing components.

Matt

[g1]

Sounds to me like a transistor breaking down, resulting in DC at the output. Problem is that it will probably read fine on diode check when it is unpowered. Because it is load sensitive, output transistors are suspect.
Do you have any freeze spray?

[Drummer4gc]

Do you have any freeze spray?

I can turn a can of dust-off upside down, if that works? Not sure how to use freeze spray in diagnosing a problem though, can you please give me some tips?

[g1]

The amp seems to be ok when first powered up with no load. So once you narrow it down to a suspect area, it's likely that any suspected semi-conductor would test ok out of circuit as it would be cold and unloaded.
So rather than testing the part out of circuit, you would get the unit into fault mode and give the suspect a shot of spray. If it responds, chances are that the suspect part has a fault that is heat related, and should be replaced.

[Jazz P Bass]

It seems to me that the OP wants to know what is simply wrong by the symptoms.
While that may be a good approach to find where in the amp the problem is occiring, that does not mean that tests will not be needed.
I think it would be best to start testing the amp.
Start at the power supply.

[Drummer4gc]

Hey, I’m all good to conduct tests!

The power supply appears all right to me. Puts out a steady +\- 42VDC to the power amp, and I dialed in ~24VDC to the preamp with the trimpot I mentioned earlier.

When the speaker pops, it corresponds with significant intermittent DC on the speaker out, up to about -18VDC. Back down to 0v or close between pops. No fuses blowing. Couldn’t get any reliable difference with freeze spray, although I did notice some of the power transistors getting warm (not hot).

could one or more of the power transistors be breaking down? Worth replacing them all, or is there more zeroing in here?

Appreciate the advice and guidance!

[TomCarlos]

Drummer4gc:

If you replace the output transistors, they should be of the same type. We frown on mix-n-match. As per G-1's recommendation, it would be prudent to try and identify the culprit.

It just so happens that I have two 70484140 transistors that I salvaged from an old Peavey amp. So if you want them, ping me via the private message with your contact info.

Tom