70's home-built mixer - please help me id missing chips!

[ultrajosh]

I bought a home-built mixer on eBay, and it's pretty cool, built in the 70's. However, all of the channel strips are missing a few chips from the same 3 sockets. They seem to be associated with the Pan and Echo controls and the Solo/(pfl?) button. The rest of the strip (eq, compressor, buss assign) is populated with TL071's. Currently I can only use the top portion of the strips and send things out through the busses, but I want to use it as a full featured mixer (e.g. pan, stereo buss, etc).

http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 001.jpg

What is the best way to figure out what is supposed to go in here? What family of chips am I looking for?

http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 046.jpg
http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 045.jpg

Here's 1/2 of the monitor section:

http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 042.jpg
http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 047.jpg

- just stick in another 741?

Here's the master section:

http://www.ultrajosh.com/images/HomeBrewMixer/HomeBrewMixer 043.jpg

Any help hugely appreciated and I can some take better pics later...

 

[Netlist]

Hi ultrajosh and welcome,

It's hard to tell but I would take the risk with 741's or whatever might be in there in channel 16.
It's out now anyway and usually less used.

Nice website you have there. A lot on one page but informative and funny altogether.

/Hugo

 

[moamps]

First check that pins "7" ( and "4") are connected with one another.

 

[Frank Berry]

ultrajosh,

I don't think that it's a homebuilt mixer. It's probably a prototype of some product that was never produced.

As for the IC's. In the 1970's, the 1458 IC's were very popular.
What are the other IC's in nearby sockets?

This should give you a clue as to what goes into the empty sockets.

 

[ultrajosh]

The master channel is all 741's.

I originally tried TL071's in the empty sockets of the channel strips and got a high pitched rising squealing sound out of the stereo mains so I took 'em all out again.

If pins 7 and 4 are connected as in your diagram (haven't checked, yet) what would that indicate?

 

[moamps]

If pins 7 and 4 are connected as in your diagram (haven't checked, yet) what would that indicate?

You can insert then single opamps in that sockets (071, 741, 5534 etc.)
If "7" from 071 is connected to "8" on the empty socket, you should use double opams. Please, take better quality pictures of empty sockets area from the component and the track side.

Regards,
Milan

 

[ultrajosh]

Thanks so much - I will take better pics tomorrow and post them...

 

[ultrajosh]

Better Pics

http://www.ultrajosh.com/tubeamp/Proto_Mixer/

 

[ultrajosh]

So there's (-) on pin 4 and (+) on pin 7 (if I'm looking at it right) on all the empty sockets, so it should take more 071's.

So maybe the feedback/squealing (oscillation?) is coming from the Master section? Or could that be caused by bad/fried chips somewhere?

 

[moamps]

So there's (-) on pin 4 and (+) on pin 7 (if I'm looking at it right) on all the empty sockets, so it should take more 071's.

You are right.

So maybe the feedback/squealing (oscillation?) is coming from the Master section?

Did you check the group outputs? If the oscillation is there as well, the input module is faulty. I suggest you pull out all input modules except the one with the 071s inserted. Then you can check the signal routing to groups, pfl and master. When you make the console work with one input module, you can fix all the others easily.

Regards,
Milan

 

[ultrajosh]

progress!

I did some experimenting and found that if I leave the socket that's 2nd from the bottom empty, it works fine - pan works, solo button works, etc.

If I put chips into that socket I get all kinds of squealing and feedback when I adjust the pan pot. It seems to be associated with the echo send, echo pan controls, so no biggie, I just left them out and reassembled the board and now I'm in business with a stereo output.

Any idea why including those chips would cause the feedback problem? The power supplies look to be all in the same places... Maybe it's the echo part of the Master section?

Also, I have to keep the main stereo output set VERY low (like -45 db) or it hisses a lot. If I keep it down low and crank the individual channels I get a clean sound. But it seems weird that I would have to keep the mains so low to avoid hiss.