Teisco Checkmate 25 Solid State Low Signal

[misterc57]

Bought a few amps yesterday. This Teisco was part of the bundle.

After a bunch of jumpers inserted, the amp powers up and works, however the signal being passed can barely be heard. I read another article by galaxiex with a similar description and replacing the first coupling cap C1 was the first big step in getting a strong signal.

I have not found a schematic. I lifted the preamp board and found where the input signal enters the board (yellowish wire).

In first pic it is the 3rd wire down from upper left corner, next to the orange cap,
In second pic upper left, the wire enters the board at label "IN".

The yellowish wire comes in but I see no connection or trace to another component which leaves me scratching my head. What am I missing?

Have about 43V at the first power supply cap and 16V at the preamp board.

Thank you.

[Jon Snell]

No saying this will cure all but this is where I would start.
Replace all components I marked with a X.
Then see what you have, hopefully a fully working amplifer again.
Observe the polarities!!!

[misterc57]

All those orange ones are 10uF electro being used as coupling caps. In another article these were replaced with 1uF film caps.

How is the audio signal affected by using 1uF instead of 10uF?

I should also mention I have a lot of new 10uF but no 1 uF

Thank you

[g1]

Looks to me like the yellow wire connects to the orange cap. The other side of that cap goes to the base of the transistor.

[misterc57]

Looks to me like the yellow wire connects to the orange cap. The other side of that cap goes to the base of the transistor.

I have 340K R between those two points. And there is no trace between them.

Wait, you are right. I was reading the wrong side of the cap

[misterc57]

Any recommendations on what value to use to replace the coupling caps (currently all 10 uF electrolytic)? Should I stay with 10uF?

Thank you

[The Dude]

I see no reason to redesign the circuit.

[galaxiex]

I posted a schem over on SSGuitar.

I changed all those 10uf coupling caps to to 1uf film caps.
It works fine. Don't be scared.

[misterc57]

I posted a schem over on SSGuitar.

I changed all those 10uf coupling caps to to 1uf film caps.
It works fine. Don't be scared.

I see no pics in that thread. Do I need to be a member to see them? Thank you

[galaxiex]

Yes, need to be a member.
It’s free.

[misterc57]

Progress.

Replaced the (11) 10 uF with new 10uF, and a pair of 33uF in parallel (one on top and bottom of board) with a 100uF.

I hear an impressive guitar signal now, but in the background are random crackling noises that rise with the VOL level. Any suggestions on this?

Have not changed any filter caps yet.

New pic of preamp board with the updated caps and also a pic for heehaws of what I started with.

Thank you

[misterc57]

Problem now is hiss and static/scratchy noises bleeding through.

Here are some readings I took.

No DC at output.
19 VDC at preamp main filter cap with no measurable AC.
16 VDC at power board with no measurable AC.
43 VDC at first supply cap with no measurable AC.

2.5 VDC at trem pot. Should not be any DC?

Amp has two Sylvania output transistors. TO-3, 185, ECG104.
If I have the pinout correct it is Base (yellow wire), Emitter (green wire), Collector (red wire)
Reading different DCV. This cannot be good or right. Yet the amp is amplifying. Readings taken at idle.

DCV at first

B 16.4
E 16.5
C 0 (after pulling up the amp board I can see this is connected to ground, hence the 0 VDC).

DCV at second

B 42.7
E 42.9
C 16.9

[Jon Snell]

The 10u coupling capacitors are the correct value to use. There is a coupling transformer in use and you will see weird voltages.
The hiss could be noisy old transistors ... not sure what you can do about that as they will be obsolete and possibly germanium types.

[misterc57]

Yes, need to be a member.
It’s free.

I am now a member. Galaxiex, that amp is a beauty! Did or do you have any hiss or noise issues?

[Helmholtz]

Freeze spray can help to localize noisy components.

Any reason to not post the schematic here?