Ampeg PB800 issue. Help me troubleshoot?

[Nev375]

Picked up an Ampeg PB800 last week from Craigslist.
(solid state, class D power amp I believe)


First couple days it worked fine, but I'm discovering an intermittent problem.

Sometimes, when I plug it in and power it up there's no output.
Sometimes, turning it off and then on again after checking speaker cables can trip the house breaker.

When it's powered on and no output, I can get both the peak and limit LEDs to clip, so that tells me the preamp stage is fine.

Once it is working after these issues have appeared and not knowing what I did to magically fix it, it seems to work fine, I can power it off and leave it plugged in and it comes right back on. But once I unplug everything and pack it up, god only knows if it work next time.

Anyone know what these symptoms point to?

[Jerrold Tiers]

it's hard to say.

One possibility is just a dirty contact in the FX loop jacks. Try cleaning them with contact cleaner, or just plugging into them a couple times.

Other than bad contacts there or elsewhere, I can't think of anything reasonable that would cause a no-sound..... and be intermittent with no other symptoms. I'd have to check the schematic and see if I can come up with other ideas, so try that one. This unit has no relay.



And the fact of it working OK after you have got it to work once sort of shows it isn't anything too big.... and suggests the bad contact idea, simply because the contact once made, will be good for a while....

[Nev375]

Would an intermittent shorted speaker cable cause this? I got it to work after plugging both cables into separate cabs, then removed the cable to the cab that wasn't working.

Seems to be working fine again.... for the moment.

[B-string]

Originally Posted by Nev375

Picked up an Ampeg PB800 last week from Craigslist.
(solid state, class D power amp I believe)


First couple days it worked fine, but I'm discovering an intermittent problem.

Sometimes, when I plug it in and power it up there's no output.
Sometimes, turning it off and then on again after checking speaker cables can trip the house breaker.
When it's powered on and no output, I can get both the peak and limit LEDs to clip, so that tells me the preamp stage is fine.

Once it is working after these issues have appeared and not knowing what I did to magically fix it, it seems to work fine, I can power it off and leave it plugged in and it comes right back on. But once I unplug everything and pack it up, god only knows if it work next time.

Anyone know what these symptoms point to?

This part is a bit of a worry.

[Stumbo]

Originally Posted by Nev375

Would an intermittent shorted speaker cable cause this?

Seems to me it could cause the problem. If the the amp works with an alternate cord, then that is the problem.

[Jerrold Tiers]

If it has a speaker cable short, typically you will hear that...... Class-D will often make some noise direct from the amp when the current limits are on. Typically a high pitched whine. Might not be audible as that case is reasonably closed-in. Not impossible, but usually an intermittent short won't get fixed for the whole night..... it will come back due to vibration. Not impossible, just not my first guess in this case.

Bad/loose connections somewhere seem most likely.

The surge as various circuits get voltage can and often does temporarily "fix" bad connections.... but with time they tend to get worse.

[estranged]

Hey guys...

I too have a PB800 with an issue...

intermittent - i'll get a pop thru the speakers - a little bit of DC crackling if you will - it only lasts about 3 seconds, tops - then goes away - comes back maybe every 10, 20, 30 minutes - no pattern to it. I called tech support and i did isolate that it is not coming from the preamp section.

Jerrold - any thoughts? (an HELLO again - i was over in inside sales from '98-'07 - now back again with the new SLM, but i work remotely from Eugene, OR!)

thanks!

aaron