Hoo boy.

[Sunnbobb]

I picked up a pretty nice PL400 and ran it through it's paces with a DBT and variac. It had a missing right supply fuse, I put one in. Everything tested fine. I was able to adjust the bias down from 450mv to 360mv, and the DC offset was close to nil.
However when I hooked up speakers, the DBT lit up immediately. i took the driver transistors out and the DBT returned to normal. I then took out all the output transistors and placed the drivers back in. Again, without a load, the DBT was fine, but with speakers it lit up again. I took the drivers out of column 2 and four, and again, the DBT was fine. I replaced the drivers with know good drivers, and the problem remained. I tested Q6 and they are fine.

Now, for the "Hoo boy" part. Here is a picture of what I found when I opened up the strange looking heat shrinked item between the Caps. That is one creative Bridge rectifier. I order new caps and Bridge rectifier from Joe, and plan to install them before trying to source out this issue.

Do these amps ever have a single repeatable problem? Everyone I've worked on has a different odd issue. LOL.

 

[WOPL Sniffer]

Did the PO have the correct outputs and in the correct positions? Some guys like to monkey with stuff and end up installing incorrect parts.

 

[WOPL Sniffer]

Ya gotta love the smoked wires going to the bridge rectifier, are the green DC wires from it to the bulk caps intact? One looks disconnected. Could be a trick of the light too.

 

[Sunnbobb]

Ya gotta love the smoked wires going to the bridge rectifier, are the green DC wires from it to the bulk caps intact? One looks disconnected. Could be a trick of the light too.

Yeah, it's connected. Every test I've run so far shows everything to be in proper spec. Just this issue of the DBT when speakers are connected.. Drives me nuts. I want to dial that before I install a WOPL setup. Kinda want to know if I have a backplane or PCB issue, or both LOL

 

[WOPL Sniffer]

If you are going to do a full WOPL, why waste time and money getting a 40 year old dog running?

 

[Sunnbobb]

If you are going to do a full WOPL, why waste time and money getting a 40 year old dog running?

That certainly is a good question. I'll chalk it up to my insanity.

 

[WOPL Sniffer]

YOU have come to the right place.

 

[wattsabundant]

OK, if I have it right, the amp was started up with a speaker connected and the series light bulb on the AC input. The bulb went full brightness. The reason is operator error. When the amp comes up on the bulb there is DC offset until everything stabilizes. The speaker is a low resistance load which prevents the amp from starting up. Catch 22.

When using the series bulb you start the amp with no load attached. Once stable, the bulb dims, and no DC offset, you switch the amp to full line voltage and then connect a speaker. Another option is my relay board does does the time delay for you.

When the amp is turned on normally with out the light bulb, there is a thump/pop which is the DC offset until the amps stabilizes. My shameless plug for the relay board resolves this annoyance.

That discreet bridge rectifier is a cluster f__k. Buy the correct one and snubber board from Joe.

 

[Sunnbobb]

OK, if I have it right, the amp was started up with a speaker connected and the series light bulb on the AC input. The bulb went full brightness. The reason is operator error. When the amp comes up on the bulb there is DC offset until everything stabilizes. The speaker is a low resistance load which prevents the amp from starting up. Catch 22.

When using the series bulb you start the amp with no load attached. Once stable, the bulb dims, and no DC offset, you switch the amp to full line voltage and then connect a speaker. Another option is my relay board does does the time delay for you.

Brilliant! I should have known this. Thanks for the tip.