Session 400 woes. fixed😃

Ken Fox · From: Nashville GA USA

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 that’s where I’m 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

Steven Black · From: Gahanna, Ohio, USA

Good Luck with that, I had trouble with my Randall RG300 the transformer broke down, finding one was impossible since Randall sold out.

Doug Earnest · From: Branson, MO USA

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.

Raybob Bowman · From: S. Lake Tahoe, CA, USA

If it has been over 20 years on the filter cap replacement, I'd suspect filter caps again.
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Donny Hinson · From: Glen Burnie, Md. U.S.A.

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! Winking

Jack Hanson · From: San Luis Valley, USA

Henry Brooks · From: Los Gatos, California, USA

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.

Ken Fox · From: Nashville GA USA

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 😀

Think I will try the idea of testing the secondary of the transformer on a light bulb. Sure worth a try

Larry Dering · From: Missouri, USA

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.

Ken Fox · From: Nashville GA USA

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 don’t like to give up the fight😀

Larry Dering · From: Missouri, USA

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.

Ken Fox · From: Nashville GA USA

Finally nailed it. Bad transistor in bias line

Larry Dering · From: Missouri, USA

Sweet victory. Glad you got it sorted out. You have patience and stamina to never surrender. Excellent work Ken.

David Higginbotham · From: Lake Charles, Louisiana, USA

We all had confidence you’d eventually find the issue and address it. We’ve watched you over the years tackle many challenges and persist until you find success. You’ve shown it’s not in your DNA to just give up on a challenge such as this! 🙂

George Biner · From: Los Angeles, CA

Good news -- those are complicated amps.
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David Weisenthal · From: Arizona, USA

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?

Ken Fox · From: Nashville GA USA

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.

David Weisenthal · From: Arizona, USA

Ok Ken, thanks for the info.

Tony Oresteen · From: Georgia, USA

That is a hard one. Glad you found it and let everyone know.

Thanks!
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Newnan, GA

Too many guitars, not enough time to play
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