First PL700 Check Out

[MarkWComer]

You do NOT need to apologize for posting pictures of PL gear on this board!

You have a lot of good iron there and the people on this board are just as interested as you are in getting them all in working condition.

And you will learn a lot in the process. You are what keeps this board going.

You fell into a great group here!

“learn a lot in the process...” True words there.

...and ask questions, even if you think they’re stoopid!

 

[silver_face]

great collection of amp's..!

 

[wattsabundant]

The first video showing the bulb stays bright may not be shorted outputs. It could be failed input transistors causing the -85VDC. It is safe to say there are not shorted ouputs on the + bank.

Attached are the manuals. I can't scream loud enough to read the 700B manual cover to cover before going any further. Especially the troubleshooting section that tells how to check all transistors, section 6-1.2. It is possible to find a shorted output without removing them by following the manual. The manual was written before digital meters. You can use the diode scale to check outputs.

With a 100 watt light bulb, a 700 will come up to 80-90 volts AC if it has a chance of working.

 

[Kma4444]

Thank you very much for that. I will read it probably a couple times before I proceed at all.
I used a 100 watt bulb. I'm trying to figure out what you mean by coming up to 80 to 90 volts. I'm presuming that's something I will learn when I do my reading homework.
Thanks again

 

[wattsabundant]

When using the bulb, I'm referring to the AC voltage at the power cord on the amp. A blown amp acts like a dead short. In that case, all of the AC voltage is on the light bulb and there is nothing left for the amp.

 

[Kma4444]

I gotcha, voltage drop. I didn't measure that but I will.

 

[Kma4444]

So here's my initial "plan". I am going to try and see if I can get #3 to operate since it did work before it blew the line fuse. I will go through the testing of the outputs and such and see what I come up with?? Then do a full WOPL on #1 as it was my dad's first one I think. Then probably do #3 WOPL after I get #1 done. Hopefully that sounds logical. Of course I will probably change my mind when I check out the 400s which is next on the to do list. I work in racing and business has been painfully slow since everything shut down. As usual, the first thing to stop when things go bad is folk's racing. Usually it's just because they have an income reduction so spending slows way down on "fun" stuff. This time of course it has been compounded by the lack of any racing going on at all. Oh well, it will likely improve.

Thanks again all. And the reading has been going pretty well. Just learning the lingo etc...

 

[Kma4444]

Here's some innards pics of #3. If my plan sounds more like folly then please don't hesitate to tell me so.

 

[laatsch55]

Is #3 a 700B?

 

[Kma4444]

It is

 

[Kma4444]

Is this a typical board in a 700B?

 

[laatsch55]

Yes, it's a PL 20 board. Its had extensive work. Someone has replaced the TO-92 Small signal transistors with TO-220's and the high voltage stage with TO -220's also.the hi voltage would have been TO-39's.

 

[laatsch55]

Those could be TO-126's too, hard to tell from the pic.

 

[Kma4444]

Copy that and thanks. I went through the service manual procedure for checking the output transistors and had one rail that had one shorted. That's all I did so far. I am pretty sure I got that right. I will double check tomorrow. Thanks again

 

[laatsch55]

You are welcome sir...

 

[wattsabundant]

Pull the failed output and proceed with checkout. You just can't test it at full power.

 

[Kma4444]

Thank you, I will do it.

 

[Kma4444]

With the one transistor removed the rest check ok. I will try and get another identical one from one of the other units and see what happens. If I am successful in finding one, how would I proceed to check out the amp? This is the one that I initially powered up with the variac then tried to play sound through. I only had one speaker hooked up and was using my phone as the music source. It seemed ok then as I turned the volume up a bit the line fuse blew.

 

[Kma4444]

As it happens, I don't have any On brand transistors. So I will have to get some.

 

[Kma4444]

Since I have been stymied on the 700 front I figured I would check out some 400s. First one passed the DBT and dcv checks. I hooked up my phone and a cheap speaker. I got distorted sound from the left and very low very distorted sound from the right.