Session 400 repair
Posted: 11 Nov 2023 8:46 am
I recently bought a Session 400 from a musician who got it out of a barn somewhere. He said he spent a lot of time cleaning it up, but it has a long way to go. Serial number dates to 1976. He had stacks of Peaveys, and said he would not get around to fixing this one. He plugged a guitar into it, and it played, tremelo and reverb was working, with a lot of scratchy static. He made full disclosure that two resistors were loose on the board when he opened it up.
I got it home and took the chassis out. I haven't traced the schematic yet, but a .33 ohm and a 1 ohm ceramic resistor were barely attached or loose from the amp board. I also found a 47 uf cap dangling by one lead on the other board. I don't know how this could have happened, but there it was. And it amazingly made noise! Any parts of the amp that were sheltered from mice and dirt appear pretty clean. The power transistors were numbered by sharpie, and I saw some previous solder repairs in a couple of places. The top of the reverb tank was nasty and rusted, but looked like new inside.
I am not a tech, but it appears that maybe the power transistors that are in the affected circuits are in parallel, which would explain the amp still working to an extent.
The amp is a twin speaker model, with two 12" speakers. The previous owner said one speaker was blown, so he replaced it with a Celestion G12 he had laying around. So it has definitely mismatched speakers. I read somewhere that the solid state amp was pretty forgiving with speaker loads. I was thinking about installing some switches so that I could switch from speaker A to speaker B to A and B for comparison listening, since it has two different speakers. I know you wouldn't want to switch them with the power on.
Is this a stupid idea?
I am making a shopping list for parts. What should I consider replacing while I am in there? As I said, I am not a tech, so I figured I would fix all the obvious issues, clean the pots and jacks, and see what I have at that point.
Favorite component suppliers?
Thanks in advance for any responses.
I got it home and took the chassis out. I haven't traced the schematic yet, but a .33 ohm and a 1 ohm ceramic resistor were barely attached or loose from the amp board. I also found a 47 uf cap dangling by one lead on the other board. I don't know how this could have happened, but there it was. And it amazingly made noise! Any parts of the amp that were sheltered from mice and dirt appear pretty clean. The power transistors were numbered by sharpie, and I saw some previous solder repairs in a couple of places. The top of the reverb tank was nasty and rusted, but looked like new inside.
I am not a tech, but it appears that maybe the power transistors that are in the affected circuits are in parallel, which would explain the amp still working to an extent.
The amp is a twin speaker model, with two 12" speakers. The previous owner said one speaker was blown, so he replaced it with a Celestion G12 he had laying around. So it has definitely mismatched speakers. I read somewhere that the solid state amp was pretty forgiving with speaker loads. I was thinking about installing some switches so that I could switch from speaker A to speaker B to A and B for comparison listening, since it has two different speakers. I know you wouldn't want to switch them with the power on.
Is this a stupid idea?
I am making a shopping list for parts. What should I consider replacing while I am in there? As I said, I am not a tech, so I figured I would fix all the obvious issues, clean the pots and jacks, and see what I have at that point.
Favorite component suppliers?
Thanks in advance for any responses.