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My first SS repair, I need some help

24.127.181.152

henrybasstardo

I'm fixing a Denon PMA-520. I think that's the model but not sure it's not in front of me.

It had a bad output transistor.

I ordered 4 new output transistors. It's prudent right? just thinking outloud. Can I just change the bad one out? Do they wear out?

I am changing the 4 output caps to slightly higher capacitance and voltage ratings. Probably not necessary but I hope it gives it another life.

I plan to rebias once I power it on. It has four pots and I need to trace them in the schematic but I assume they are for bias and dc offset.

If all go well and no fires start and for some reason I can get all the voltages right, fingers crossed, do I run it in and then rebias? How do you break in SS?

I also need the white insulation/ heat / gasket type stuff for between the transistors and heat sinks. Where can I get it? I spoke to the tech at digikey ordering the parts and told him I need that goo but I got something else. Can I use rtv black? lol

Thanks for all the help.

I don't like fires.

Cheers

Follow Ups:

RE: My first SS repair, I need some help - mikee55 04:58:35 01/19/11 (0)
http://sound.westhost.com/heatsinks.htm

Get that heatsink polished, that will require less thermal compound. Be good to the bias, don't turn it up.

Separate power supply for driver stage and output stage? New smoothies?
Post probably too late.
Mike
"Yeah, but it was fused!"
RE: My first SS repair, I need some help - pictureguy 19:29:08 05/19/10 (3)
Got heatsink compound for the transistors? NOT RTV.
Are the output transistors PNP / NPN pairs? If so, are they 'matched'?

Some transistors MAY have a mica washer under them as well.

Ever done anything like this before?

Too much is never enough
- RE: My first SS repair, I need some help - henrybasstardo 19:41:48 05/19/10 (2)
  Thanks pictureguy.
  
  I have the mica insulators. Do I still need the heatsink compound? Use both?
  
  My first time.
  
  I have to keep taking firsts.
  
  I appreciate the help
  
  You are right on the transistors, 2NPN and 2PNP and I have them. Cool electrostatic bags. Don't know what you mean by matched? Could I have requested that? Do they match them like vacuum tubes?
  
  The Denon is layed out so well I'm diving in.
  
  Thanks again.
  
  Cheers
  - clarification - henrybasstardo 19:48:39 05/19/10 (1)
    I got the mica insulators but my gut says I do not use them on these transistors. The one I pulled out did not have them. Just compound between the transistor body and the heatsink.
    
    I'll get under the hood tomorrow
    - RE: clarification - pictureguy 21:17:47 05/19/10 (0)
      Than put it back EXACTLY as found.
      No Mica insulator, apparently.
      
      The heatsink compound use is NOT a case of 'The Bigger the Blob, the Better the Job'. !!
      use with moderation.
      
      Transistor matching? Gain and Threshold (turnon voltage) would be my guess. If you wanted to get weird, you could match for RDon. The resistance of the device when ON. Other parameters?
      I don't know.
      I've seen cost advertised for such matching and it isn't much.....It can be done easily using a special o-scope setup called a curve tracer.
      
      I haven't fixed anything in 25 years. Spent all my time in process, not maintenance of equipment.
      
      You may want to buy a tech a cup of coffee and ask for some tips:
      
      Too much is never enough

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