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

[laatsch55]

Test the can lookin transistors on the driver board. Check the schematic and parts locator to find d9, d10.

 

[lightfall]

I am looking up the schematic now and testing those parts as I type.

Current shopping list (feel free to add suggestions you think I should do):

AGX 5 fuses (supply)
AGX 8 fuses (supply)
AGC 10 fuses (line)

.125 AGC fuses (llittelfuse USA 312 3AG 1/8A 250v) (preamp)

Mica insulators and heatsink compound for output transistors
Output transistors (x12) MJ 21196

Emitter resistors (not sure which ones I need)

Deoxit d100 and d5

“push button tone switch†for the 2000 series 2 preamp

 

[lightfall]

Well my brain is squirming, and I spent a good half hour trying to find D9/10 on the main PCB, but I am guessing these are actually the green squares near the speaker output terminals? I get 1.39 on both and they look like they slowly climb from 1.3 for a few seconds until they kind of settle on 1.39 in diode mode.

I see two 1" cans that are silver on the bottom half of the main PCB and two 1cm black can things on the upper half and I am testing those now.

 

[lightfall]

in diode mode if I touch the two prongs coming out the foil side of the PCB beneath the two large silver can shaped attachments (capacitors?) I get 1.5 one way and 1.8 the other way. I am probably testing the wrong thing the wrong way.

 

[lightfall]

or if the small black cylinder more in the middle of each channel that looks like the attached picture is D9/10 then my readings are Left .562/ Right.57

 

[gadget73]

Check the 7.5k resistors on the driver board. Those are the two dark brown ones. The ones on my 700 and my friend's were both far off spec. All the rest of the resistors on mine were fine, many of his were off value and one or two were open. I have no idea if his were off value because of the failure, if they contributed, or if its totally unrelated. I'd probably just meter them all out and order any that don't measure up. You've got the same driver board my amp and my friend's amp has. I might replace the 4 small capacitors on the board too. They may not be bad, but they're old and new ones are cheap enough.

I think there are pics in the service manual of the PL36 driver board that shows which part lives where. If not, its in the PL 700-II manual.

There are several can like transistors on the driver board. Not totally sure which ones Lee means, but there are a series of 4 near the bottom. Those are the pre-driver transistors. Two are NPN, two are PNP. Looks like you are missing a heatsink from one of those. The PNP transistors will test backwards compared to the NPN transistors. Just reverse the meter leads.

 

[lightfall]

With the meter in 20k mode the left 7.5k resistor shows 8.2 and the right 8.15. Am I doing that right, and is that considered within spec?

 

[laatsch55]

Yes, and your diodes are good also...

 

[laatsch55]

in diode mode if I touch the two prongs coming out the foil side of the PCB beneath the two large silver can shaped attachments (capacitors?) I get 1.5 one way and 1.8 the other way. I am probably testing the wrong thing the wrong way.

Correct..

 

[gadget73]

With the meter in 20k mode the left 7.5k resistor shows 8.2 and the right 8.15. Am I doing that right, and is that considered within spec?

barely in spec, but in spec. Its 10%, which allows up to 8250 ohms before being out. You're right at the top end of that range. If you're putting in an order, I'd stick a pair of resistors on the order. They're cheap. Not doing it won't likely hurt anything though.

 

[lightfall]

So what should I do next? I am working on testing those four cans on the main board now. What do you suppose caused those output transistors to fail? Am I going to replace them only to get hosed? I am really fearful of that, naturally. And what of that blown preamp fuse? Do you think a power surge or something blew both fuses, but more power go through to the amp and caused those output transistors to fail? Or did it come from upstream somehow within the amp?

 

[laatsch55]

Once more, one thing at a time. Outputs of that era blew quite a bit. Overdriving, overheating, who nows, it happened a lot.

Don't worry about what you need to order yet!! Let's get through the test phase then we'll lead you through the ordering process. I suck at this remote help thing and if you start jumping around and doing things I'm not aware of I am going to assume things I shouldn't be.

All transistors on the board can be tested in circuit for shorts or opens. Do that next, DON"T WORRY ABOUT THE CAPACITORS YET....they rarely fail ..,..and if an electro has failed there is no question it has. A ceramic (the little orange ones) I have never seen fail.

Test ALL TRANSISTORS on the board. Check the schematic for NPN, PNP pol;arity and test accordingly. Check every Diode on the board. Only one (d13) will show conductivity both ways, the rest only a forward bias. They test the same as a Bi-Polar Junction transistor which is what you have been tyesting...

 

[lightfall]

Lee you don't suck at the remote help thing, I have been hanging on your every word! I am just keeping a running tally as I go of everything I need thus far. I had off today so I had lots of time to sit around and study this thing.

What exactly does the term "electro" refer to?

I will continue testing all of the transistors in the unit. I have had some trouble correlating the schematic with the diagram outline in terms of identifying parts since I am bad at reading the schematic it is hard to tell since I do not have the symbols memorized etc. I will report back with any suspected faults. I will also check the diodes. I am also a visual learner, so I am trying to catch up with all of these terms and so forth.

Where can I reference what readings I should get from all of the smaller resistors on the PCB? I want to make sure those are all within 10%. I noticed I could only read the 7.5k when I set the multimeter to 20k since that value was higher than 7.5k and I guess my meter only tests in such large increments in a base 2 for some reason (the more expensive units must have base 10 readings).

 

[lightfall]

on the layout diagram I see some numbers inside of the drawings, some of them will explicitly say 1.8k whereas others do not. I will proceed by referencing each piece, adjusting my multimeter, and seeing if I can make sense of it. My goal is within 10% for each but I am worried about my multimeter working in increments of 2 that is pretty confusing I must say.

 

[laatsch55]

Ok, I'm sorry , I tend to assume you know more than you do. "electro" refers to an electrolytic capacitor. Usually aluminum cased. The power supply caps are electros, the 4 -47uf caps on the board are electros....Is there a parts locator in the service manual like this one??

 

[laatsch55]

Don't worry about resistors yet. You can do that when you get bored waiting for parts. The 7.5k's always get replaced on a PL36 board..They form part of the low voltage supply for the board. What we need now is to test the transistors on the board along with the diodes. Don't wqorry about the op-amps yet. That's much later..

 

[lightfall]

Mine had a schematic but no part diagram quite like that. Those diagrams, and the manual, while I can see how they are regarded as quite good, are not as helpful to a total beginner because it is full of a lot of assumed knowledge but hey that is part of the mystery I am unraveling.

I will add a couple of 7.5ks to replace on my "to do list"

Heh, op-amp, whatever that is. I wish there was a picture diagram for this thing that said "an op amp is this, and it does this, and looks like this and it is on the board here and you test it by doing this, your results should be this." Now THAT would be nice for the newbie! Opamp amplifies the electricity, got it but I will worry about that later.

For now, transistors and diodes!

 

[laatsch55]

You are just scratching the surface of what you should know. It's daunting at times....remember---Google is your friend...

 

[lightfall]

I have been googling furiously. Daunting is right, I have wanted to quit this about 10x over but it is fascinating and the guidance keeps me on track. Without a background in electronic education I have to do a lot of cross referencing to make sense of things.

 

[laatsch55]

Me too, I didn't know shit 5 years ago...