HELP! Rescuing Dad's old PL400 S2 (pics)

[gadget73]

do you have anything you can feed straight into the amp? An ipod or a cd player or something would do it. Just need something to test whether you get sound out of the amplifier itself.

 

[lightfall]

Do I want to do that if I am getting a short indication on the DBT? I could, but I was worried if I should even power up again. I can't figure what would have changed that made it trip the DBT. I quickly tested all the casings of the OT and drivers to chassis but that was OK. I guess I'll pull all the OT again or something next and look for visible damage.

 

[laatsch55]

No speakers hooked up with the DBT in line right? Measure DC offset first. That gives a pretty good indication on what's going on first.

 

[laatsch55]

Then bias.....

 

[lightfall]

I want to check bias and offset with the unit turned on w the DBT? It won't fry anything worse?

 

[laatsch55]

Sure won't..

 

[lightfall]

OK. I just wanted to make sure before proceeding on that step as I have been advised in the past to immediately shut down in the dim bulb lights up and does not fade to a glow, I will test those when I get home then but I did not want to keep it on without confirming it was OK since the DBT indicated the short. Another 12 hour shift here...

 

[laatsch55]

You checked the OTs for shorts and had none correct ?

 

[lightfall]

I only checked the chassis to the OT cases, which revealed nothing. I have not had a chance to pull the OTs and test their transistor function yet. I probably won't get the time to do that until friday or so.

 

[laatsch55]

Do the standard short test before you remove them.

 

[lightfall]

Yea, the standard short test where I touch the casing of the OT or driver with one probe and the other on the metal chassis - that test was negative for shorts on all OT/drivers. I have not pulled the OT or drivers yet.

 

[laatsch55]

That was the outside short test, that only checks for collector to ground shorts. Now you need to go inside and check base,emitter junctions...

 

[lightfall]

I plan on doing that next chance I get I couldn't do it last night, I didn't have time to let the caps bleed and sit down with it. In the mean time I am just speculating, sorry to waste your time musing about incomplete troubleshooting info.

 

[laatsch55]

OK....

 

[lightfall]

Well guys this is certainly interesting, I am writing this message from my phone which is playing music direct into the amp which is powering two small bookshelf speakers seemingly perfectly.

I got home and brought the unit down to my workbench in the basement and plugged into the DBT. To my surprise the DBT did NOT indicate a short. I thought hmm...I checked the offset and it was registering 0mv on both channels. That seemed unusual so I hooked up the bookshelfs and my phone's music output directly into the amp sure sure enough it is behaving quite well.

So I presume this is either a problem with the preamp or with line levels at that which a preamp would output. What do you think?

Wow this is crisp, I am in love.

 

[lightfall]

The preamp does not set off the DBT so I am going to start reading the service manual more in depth on that unit to see what the deal is there.

I should also investigate the outlet I was using upstairs somehow.

 

[laatsch55]

Good news.....

 

[gadget73]

Very possible the preamp is wonky. You might want to try checking for DC voltage on the output of the preamp. It should not have any, but I have seen that before on things that weren't working correctly.


Have you cleaned the controls on the preamp? If not, give that a shot.


This sounds odd, but maybe try checking for AC voltage between the case of the amplifier and ground, and the case of the preamp and ground. Perhaps even the case of the preamp to the case of the amp with no cables between them. You should not see anything significant, but if there is a component that is leaking line voltage to the case, it could do some funky stuff when you connect the leaker to a non-leaker. I don't know if the 400 or the 2000 has a line to chassis capacitor, but I have seen those things go bad in a lot of gear before. Usually happens after a lightning strike. The holes blown in the side of the cap and the char marks on the chassis tend to be an indicator, but they don't always blow up like that.

 

[kevin]

We'll.... Is the amp fixed ????? Was it bad Ot's that was the problem???????

 

[lightfall]

I haven't done a full recheck of everything, particularly the AMP, but there were a few problems. Some of my emitter resistors will need to be replaced though they are functioning. That was not the problem, but rather a solution.

There were two OTs that were bad, that was definitely the problem. There were also two weird OTs made by GE that were not on the PL spec list which certainly did not help anything.

The infinity RS3A speakers probably caused the problem somehow back in the day under load.

The preamp has an issue, though I am not sure what. There is no short circuit on the preamp. There are dirty switches that need more cleaning. There was a broken RCA jack that I fixed.

The AMP APPEARS to be OK, since it plays beautiful music when I hook it up direct to my mp3 player and a set of bookshelf speakers, so I have to assume the reason for the weirdness during initial testing was the preamp. Though the AMP testing + on the DBT and then working ok after the fact will require further investigation.