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Topic: Session 400 woes. fixedđ |
Ken Fox
From: Nashville GA USA
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Posted 2 Sep 2023 4:58 am
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Took on my last solid state amp repair for a friend in Louisiana
We talked about the amp for several years. I avoided working on it as it had been thru a really reputable shop. It came back and same problem cropped up.
Low level noise on the output. Sometime worse
Well I was on my way back from Texas and picked up the amp
Ran clean for about first 24 hours then it started up. Now very consistent
Observations
Noise come from the power amp board
Previous repairs:
All electrolytic caps on board replaced
Power transistors and drivers replaced
2 each 5 watt resistors replaced
MOV on power line input
I noted that with a scope I could see the noise on the bias line but not before that area of the amp
At suggestion on previous shop and talking with Peavey repair I replaced the 8 each 10 watt resistors. No luck. I pulled the outputs and tested then replaced the insulators with a much better brand
Tested the drivers. Replaced all diodes in the bias line. Replaced the diff transistors with a matched pair and fold back transistors. 2 each 2 watt 22 ohm resistors. Reflowed every solder joint and cleaned board
No luck. Then it reminded me of a Musicman RD 50 That drove me near insanity with a similar noise. Ended up being a bad power transformer that was arching inside
I think thatâs where Iâm at with this old Session
Now to find a transformer
This has become basically a charity repair. Hoping to find a transformer at a good price to help out my friend
Last edited by Ken Fox on 23 Sep 2023 10:42 am; edited 1 time in total |
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Steven Black
From: Gahanna, Ohio, USA
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Posted 2 Sep 2023 5:44 am NS 400 reply
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Good Luck with that, I had trouble with my Randall RG300 the transformer broke down, finding one was impossible since Randall sold out. |
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Doug Earnest
From: Branson, MO USA
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Posted 2 Sep 2023 6:57 am
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Could we finally be starting to see the death of these great old Peavey amps due to the effects of time? The first Sessions are 50 years old, after all. Can it be reasonable to expect to be able to keep these old amps working perfectly for reasonable cost? I know very little about electronics.
I took an LTD in for repairs three different times recently. It would work great and then poof, some other problem would arise. Then a buddy of mine took it and had one of the great technicians work it a couple of times, same story.
I guess just keep buying working units for $300 and put them in the corner when they quit. It doesn't take much to rack up a $150 repair bill. These amps have served us very well. Thanks again Hartley! It doubt there will be any mass produced amplifiers built this well again.
Heck I guess us guys that have used these amps since they came out are pretty close to done, too. |
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Raybob Bowman
From: S. Lake Tahoe, CA, USA
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Posted 2 Sep 2023 8:42 am
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If it has been over 20 years on the filter cap replacement, I'd suspect filter caps again. _________________ Sierra U12 4+5 / 1933 Dobro / homemade Tele B-bender |
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Donny Hinson
From: Glen Burnie, Md. U.S.A.
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Posted 2 Sep 2023 11:52 am
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Possibly a bad master-volume control? Have you checked the pot for leaking DC voltage somewhere?
It may be time to break out a can of freeze-mist! |
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Jack Hanson
From: San Luis Valley, USA
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Posted 2 Sep 2023 12:32 pm
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Doug Earnest wrote: |
I guess us guys that have used these amps since they came out are pretty close to done, too. |
Ain't that the truth!
I bought this one new in '77, and the only time I've had the back panel off was after its original JBL blew, and I stuffed a Neo mag Black Widow in there. Sounds like a million bucks (for now), and still looks purty good.
I found this one a couple years back for dirt cheap at a big Denver music store, and it sounds fantastic pushing a TT-12 in a small cabinet.
Wouldn't be surprised if they both outlast me. |
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Henry Brooks
From: Los Gatos, California, USA
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Posted 2 Sep 2023 3:25 pm
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You should be able to test the transformer by disconnecting the secondary from the amp and use a 200 watt ligth bulb as a load and bring the transformer primary up to 120 volts slowly with a Variac to see if it starts arcing? Also, there are a couple of diodes one on the + 50 volts and the other on the - 50 volts that connect to the output of the amp. Perhap one of them is bad and letting power supply ripple get on the output.
Henry
P.S. It would be nice if the variac has output volt and current meters. |
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Ken Fox
From: Nashville GA USA
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Posted 2 Sep 2023 6:16 pm
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Thanks for the suggestions
I can disconnect the preamp with the connector. No difference. The problem is not in the preamp. The power supply diodes have been changed as well. The diodes on the outputs have been changed out yet. The noise can be seen on the bias line even at the fold- back transistors and at the outputs. No where prior to that
I run a current meter prior to my variac. That way I can watch current flow as voltage is increased. I also have dim bulb test circuit I use when being up amps after outputs are changed. Save a lot of grief and parts đ
Think I will try the idea of testing the secondary of the transformer on a light bulb. Sure worth a try |
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Larry Dering
From: Missouri, USA
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Posted 3 Sep 2023 5:50 am
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Ken. You have a real stinker there. I'm lost for ideas and following this thread to see what happens. If you can't fix it then I can't imagine how anyone can. Good luck. |
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Ken Fox
From: Nashville GA USA
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Posted 3 Sep 2023 7:21 am
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Hope to get back at it soon. I work on it between other projects
It has been subject to bad shipping sometime in the past. The corner of the chassis at the rear shows that and the transformer is bent vim relation to the chassis
That may or may not be the issue but us suspect
Like I said this is a charity case at this point. I could never charge for the time I am spending on it
I just donât like to give up the fightđ |
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Larry Dering
From: Missouri, USA
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Posted 3 Sep 2023 5:51 pm
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You sure don't. I'm confident you will uncover the issue and resolve it. And I appreciate your sharing the efforts with us. Always a good read and lesson. |
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Ken Fox
From: Nashville GA USA
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Posted 23 Sep 2023 10:44 am Done
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Finally nailed it. Bad transistor in bias line |
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Larry Dering
From: Missouri, USA
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Posted 23 Sep 2023 11:12 am
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Sweet victory. Glad you got it sorted out. You have patience and stamina to never surrender. Excellent work Ken. |
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David Higginbotham
From: Lake Charles, Louisiana, USA
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Posted 25 Sep 2023 5:38 am
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We all had confidence youâd eventually find the issue and address it. Weâve watched you over the years tackle many challenges and persist until you find success. Youâve shown itâs not in your DNA to just give up on a challenge such as this! đ |
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George Biner
From: Los Angeles, CA
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Posted 24 Oct 2023 8:57 am
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Good news -- those are complicated amps. _________________ Guacamole Mafia - acoustic harmony duo
Electrical engineer / amp tech in West Los Angeles -- I fix Peaveys
"Now there is a snappy sounding instrument. That f****r really sings.Â" - Jerry Garcia |
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David Weisenthal
From: Arizona, USA
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Posted 25 Oct 2023 8:49 am
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Very interesting article guys. I had a similar thing happen a number of years ago with the session 400. At very low volumes there was noise in the output that you could hear. It didn't seem affected by the amount of preamplification or any of the controls. I took the circuit board out, and it looked clean. tugged and tapped on all the components and cables and found nothing. Since there was some hum probably from electrolytics, I packed it up and sent it to Peavey and they did the standard caps and cleaning. They did a good job and did it fairly quickly. But the low noise problem remained and it's still there today, I wonder if it's the same issue. Not a hum, just like a bad connection or failing component. Ken, do you think a can of that freeze spray for testing Electronics would help diagnose it? Do you think it would have helped on the bias transistor? |
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Ken Fox
From: Nashville GA USA
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Posted 28 Oct 2023 10:19 am
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The noise I had from a bad transistor in the bias line was not affect by heat or cold
I replaced it and the two drive transistors as well. |
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David Weisenthal
From: Arizona, USA
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Posted 28 Oct 2023 6:23 pm
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Ok Ken, thanks for the info. |
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Tony Oresteen
From: Georgia, USA
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Posted 30 Nov 2023 2:46 pm
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That is a hard one. Glad you found it and let everyone know.
Thanks! _________________ Tony
Newnan, GA
Too many guitars, not enough time to play
'72 Sho-Bud 6139, '71 Marlen 210
'78 Fender Stringmaster Quad black
PedalMaster D8 |
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