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

[TomCarlos]

I am sure you probably checked this.... but....

The way you have the cable going from amp to speaker is suspicious. I have a speaker cable I use for testing. A 1/4 phono at one end, alligator clips at the other. I make sure I have a good connection at the speaker terminals. So check that cable getup. Make sure the alligator clip that is attached to the 1/4 phono is making a good connection.

And you are working with a known good speaker?

And you tried using the #2 output jack?

If you had a long test cable with alligators on all ends, you could tap off the PC board and thus bypass the output jacks.

As I posted earlier, I have extra output transistors I can donate to you cause.

It just sounds like an intermittent connection.

Tom

[Drummer4gc]

Good thought, probably makes sense to avoid sketchy testing connections when looking for an intermittent connection!

Lost the speaker cable and alligator clipped directly to grounded speaker jack and speaker output on the PC board. Used some additional alligator clips to ensure ground connections were solid. No difference. Slight crackles through all connection configurations, then sudden total DC on the speaker meant I needed to turn it off. Interesting, speaker cone sucked in this time instead of popped out...

[g1]

Interesting, speaker cone sucked in this time instead of popped out...

If you were just using clip leads, it's easy to get the + and - reversed on the speaker. That would reverse the push to a pull.

[Drummer4gc]

Will I damage anything by pulling a complimentary pair of transistors and running the amp? I have more and more of a feeling that I’m having a connection issue with one or more of the transistor sockets, and this could be a way or identifying a faulty socket.

[g1]

If you are not running any appreciable signal or power output it will be fine. I think you just need to listen to it at idle so no problem not having all the outputs installed.

[Drummer4gc]

Well, removing one pair of complimentary output transistors made no difference. Replacing those and removing the other pair made no difference. Removing both pairs at the same time made no difference.

After removing all six output/driver transistors, there is still some crackling. It seems generally quieter, I assume because the source of the noise is no longer being amplified. But the crackling/popping is definitely still there.

edit - nevermind, the crackling is just as bad with all 6 transistors removed.

[g1]

That kind of leads back to the 40409 & 40410 again.
How are you listening to the crackling with the outputs removed, do you have a speaker connected? Does it still quit doing it when you remove the load?

[Drummer4gc]

Yep, I have a speaker connected. Hard to tell if it’s crackling without the speaker connected but the DC voltages on the output seem to be jumping around a bit, the same as they do with a speaker connected.

[The Dude]

If you have a scope, unhook the speaker and use the scope to monitor the output.

[Drummer4gc]

Yep. Confirmed with scope, crackling/popping happens whether speaker is connected or not.

[The Dude]

Yes, I figured that. I was also trying to point out that using the scope instead of speaker is a way to monitor the noise without risking damage to the speaker or further damage to the amp.

[g1]

Well this is a little different, as originally it only happened when you had the speaker connected? Maybe the part is finally failing. Those smaller driver transistors won't be able to drive a load very hard.
There have been other threads about retro-fitting TO-220 case transistors into those 40409 & 40410 heatsinks. Maybe someone will be able to link such a thread.

[The Dude]

I'd start checking transistor voltages and look for a discrepancy. Maybe too little or too much B-E voltage somewhere. I know this is a lot of hunting, but what else ya gonna do?

[Drummer4gc]

Well this is a little different, as originally it only happened when you had the speaker connected? Maybe the part is finally failing. Those smaller driver transistors won't be able to drive a load very hard.
There have been other threads about retro-fitting TO-220 case transistors into those 40409 & 40410 heatsinks. Maybe someone will be able to link such a thread.

True, it doesn’t seem to be acting quite as wild with a load attached as when I started this thread, although I wasn’t looking for crackling on a scope with no speaker attached at that time, so this may have been happening.

I saw a thread where someone suggested MJE340/350 for replacements for the drivers, any thoughts?

before going there though, it would be nice to confirm that they’re the issue. I did previously remove and test them on a little 9v tester, where they looked fine.

Here are current voltages:

40409:

C 42.4
B 1.17
E .55


40410:

C -41.5
B -.7
E -.04

[Jon Snell]

The voltages look right and from my experience, if one has failed, the output will be DC at one of the rails. B E voltage of around 0.6v is correct.
Reading the posts, you seem to be grasping at straws and not fault finding at all.
Look at the circuit diagram, understand what to expect and test accordingly.
Noise as you have described is usually caused by the front end of the amplifier; the 2N4249s or the stressed driver 40409.
Don't forget, the more you replace/twiddle with, the more the possibility of a man made fault and they are the worst.

Keep logical and ignore the smoke and mirrors!