Bill Joerger
From:
Sunset Beach, NC USA
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Posted 9 Oct 2011 10:42 am |
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Most of the time an electronics tech. might suspect a shorted electrolytic capacitor relative to the power supply voltage or worst a problem power transformer if fuses were also blowing out.
However, my experience in two such incidences has shown it to be a shorted transistor in one of the six power output transistors easily assessable on the outside back of the chassis. Each is covered with a black plastic cap held by one screw. After removing the cap the transistor is held by the remaining screw. Beneath each transistor is a teflon insulating gasket. It’s imperative that this gasket not be destroyed and certain to be reinstalled. To forget it, is to really invite a disaster.
The original NPN transistors used were Motorola SJ6357. You may search far and wide to still find any available. In my recent search, I found them to be out of date and not as efficient or reliable as it’s newer replacement Motorola MJ15003.
These are available through Mouser Electronics and can be order off their web site for as little as $3.50 each.
To be sure that one of these old transistors is faulty, you will need an Ohm meter set to Rx1k to test each one individually after removal.
Holding the (red) positive lead on the metal case, continuity should occur only when you touch the negative(black)lead to one of the post terminals. This would be the base post and labeled with a B if labeled at all. The other post would be the emitter labeled E and that should appear open.
Switching leads holding the negative (black) lead to the case, no continuity should appear when touching either post with the positive (red) lead. This test would indicate a working transistor.
A bad or shorted transistor will show continuity or short circuit no matter which way the leads are placed. Should you find one that‘s bad, I would suggest you replace all six with the newer MJ15003. For $21 plus postage you’ll be up to date with a more reliable replacement and save yourself a lot of future grief. You may even discover your amp to be surprisingly quieter.
If you don’t have the meter to go through the above test, you can’t go wrong by just replacing all six anyway. Keeping an extra on hand might not be a bad idea either.
Remounting the transistors will require that you pull the chassis enough as to hold the mounting wafer firmly to the chassis from behind with your finger. Just don’t forget that teflon insulator.
As many will testify, the Peavey Session 400 was and is still a very good Steelman’s amp. It’s just been found to be testy and discarded but for it’s weak link in these old less reliable power output transistors.
I hope this discourse has been of some help before you go trashing that old hummer. -BJ
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Gibson Hound Dog, Aria Pro II Bass, Tatey Classic guitar, Peavey Basic 112 Bass amp.
Last edited by Bill Joerger on 11 Oct 2011 6:28 am; edited 1 time in total |
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Mike Schwartzman
From:
Maryland, USA
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Posted 9 Oct 2011 1:30 pm
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| Quote: |
I hope this discourse has been of some help before you go trashing that old hummer. -BJ
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It certainly has been helpful and I thank you for it. I was fortunate to get a 1977 model about 6 mos. ago. I am experienced in doing tube amp (but not Solid State) work, so I was able to do a nice recap and reverb repair myself thanks to the generous info here and elsewhere. It's a great sounding amp, but I was secretly wondering what to do should one (or more) of those OP transistors go south. Question answered...thanks Bill.
Slim
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Emmons Push Pull, BMI, Session 400, Home of the Slimcaster Tele. |
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