Univox Echo EM-200

[dmp]

Trying to repair one of these that had all the pots pulled before I got it.
I can't read the value of the mixing pot in the mid left that connects the upper signal path with the lower echo path. 100 KB?
Any ideas?

 

[PRR]

> 100 KB?

Seems good to me. Each half of that is similar to the 68K and 39K around it. Exact value not critical, it's just to get a smooth variation without excess loss.

 

[dmp]

Thanks. I put in the new pots  and it's working. Luckily, the tape parts were  all healthy. I replaced all the electrolytics while I was in there.
The echo sounds really good, but somewhere the high frequencies are being lost in the dry signal. The circuit is pretty simple for the dry signal (comes in top left, goes through a few amp stages, output is in the middle).  I attached the upper left of the schematic highlinghting this area.
There is a suspicious 0.68 uf cap to ground after the second opamp stage. It seems like that would filter out high frequencies depending on the output impedance of the opamp? I'm not sure why a cap to ground would be in that spot after a op amp?
Also is the 100p cap on the output amp a little large?

 

[JohnRoberts]

Thanks. I put in the new pots  and it's working. Luckily, the tape parts were  all healthy. I replaced all the electrolytics while I was in there.
The echo sounds really good, but somewhere the high frequencies are being lost in the dry signal. The circuit is pretty simple for the dry signal (comes in top left, goes through a few amp stages, output is in the middle).  I attached the upper left of the schematic highlinghting this area.
There is a suspicious 0.68 uf cap to ground after the second opamp stage. It seems like that would filter out high frequencies depending on the output impedance of the opamp? I'm not sure why a cap to ground would be in that spot after a op amp?

"Suspicious" is being kind...  that looks like an amateurish attempt to clean up oscillation. Remove that cap and look at more conventional approaches to stabilize and problems (PS decoupling, feedback cap, etc).

Also is the 100p cap on the output amp a little large?

I can't read the resistor value... you can calculate the pole pretty easily.  f-3dB= 1/(2 x pi x R x C)

JR

 

[dmp]

I think it is 220k, which gives a -3dB point at 7kHz. Maybe that is fine for guitar.
I'll pull out the .68uf and see what happens. There's a similar placed  cap sitting on the output jack that I can't read the value of. I might try it without that one too.
I already bumped up all the PSU filtering caps.