Shure M67 repair...

[tonedude]

I bought a broken M67 mixer from Ebay. After replacing q12 and q13 with 2N3904 and q14 with an 2N3906, I got sound out of it, but it is rather distorted.

According to service manual:
Q12 is supposed to be a TIS97
Q13 an TIS92, and...
Q14 an TIS93

Q13 and Q14 is also suposed to be a matched npn/pnp pair.

- What do you suggest that I use as a replacement for those TIS transistors?
- And do 13 and 14 need to be matched? If so, how to match the replacement pair?

 

[rackmonkey]

What led you to replacing all the transistors? the chances that all of them need to be replaced are slim to none, unless the mixer was the victim of a lightning surge or something. Even that might not have been enough to fry all the transistors. Seems like an awful lot of work for a shotgun approach that may not even fix the problem.

You got no sound at all initially, from any of the channels? What troubleshooting steps did you take up to now? If i were in your shoes, I’d be using a signal tracer to work my way through the audio path, once I ruled out power supply issues.

 

[tonedude]

No sound at all initially. Dead silent.

All the "channels" worked, but they all have to go through the stage consisting of Q12-Q14, which is the "driver circuit" for the output transformer. And from there it was all silence...

PSU is fine.

I quickly narrowed it down to the OT driver.

I changed the transistors one by one, starting with Q12. And finally there was sound, but way more distortion than 1%...

 

[rackmonkey]

TIS92and TIS93 in my books show 2N4401 and 2N4403 as equivalent, so anything similar to those common BJTs should work. You may even have some of those around if you’re like many of us here, as they were common in so many older audio circuits.

 

[heatwalk]

Might be a silly question for you but I've dealt with a couple of these units (and have one on the bench currently with issues I asked about on here recently) and their distortion can vary pretty dramatically depending on the gain of the master and what kind of levels youre sending into it. What kind of gain staging and levels are you sending through it and metering now that you have signal passing? Also, using the mic or line out?

 

[tonedude]

I have two of those and I fixed the faulty one by reusing the tis97 transistor I removed first. What was in the other two spots wasn't that important, kind of strange.

I know how to use them so it wasn't a user error... the distorting one had tremendous amounts of distortion, no mater what.

Thanks for helping.

 

[heatwalk]

Hm. Well Im not super experienced but curious to see how it resolves since its equipment Im familiar with. Is the signal prior to those driver transistors distorted or does the distortion occur in that area where sound initially wasnt passing?

 

[ricardo]

The first thing to check is always the DC voltages. These are the voltages circled on the schematic.

Voltages in the rect. boxes are AC voltages for some input at some point. It's not clear from what you posted what the input is.