Home brew mixer adventure

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ultrajosh

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I bought a 16-channel "home-built" mixer off eBay for peanuts and I'm amazed to find that it is really well built with what appear to be quality components. Some wacky Canadian locked in a cabin for a long winter with a soldering iron, I guess. I'm hoping to use this mixer for recording and mixing with my Tascam MSR-16S.

The frame is made of wood and is well-shielded inside. There are Beyer transformers on each channel strip, and the boards appear to be from a small batch - some are un-marked and others are marked EK 75 and EK 76. (Could this have been a mail-order kit?). The back panel appears to be from another mixer and has all XLR inputs/outputs.

The strip faces are grey/blue and appear to be professionaly silk-screened in white; the knob marks in the master section all go up to 11! Is that unusual? Have I found Spinal Tap's actual mixer?

Each channel strip has a buss-assign toggle for each of the 8 busses, 4 channels of switchable EQ, a built-in compressor/limiter, an Echo/Return section, and then two mystery toggles and a mystery red button.

There's an 8 VU meter bridge that seems to be hooked to the busses, but there are also several knobs and buttons on the meter bridge that I haven't figured out, yet. There is a knob beneath each VU meter that seems to also control the volume of the channel.

There are separate power supplies for the board and for phantom power.

All 8 buss outputs DO put out signal. And the phantom power also seems to work. But the master section isn't working at all, no matter what buttons I push. The Stereo Outs in the back don't work, and only one of the 4 Monitor channels works.

I pulled the 3 cards that comprise the master section (two 8-knob single channel Monitor strips and one stereo Master strip). There are various chips missing from the Master/Monitor boards and also the channel strips. I assume this is to get everything out of the circuit except the preamps and the buss outputs so it can be used as a front end for a DAW or summing mixer. I want to use it for mixing and recording, so I want it all back to working as-designed and get me some stereo output.

All the channel strips have had the chips removed from the Pan and Echo sections. The remaining chips in the channel strip are all pairs of "AB TL071CP" (2 8-pin chips per socket, 7 sockets). Would I just use more of the same chip to fill in these empty sections of the channel strips or would they likely require something else?

The 2 Monitor strips (8 volume knobs, plus a master knob and a fader) are filled entirely with "ML741CS 7343" chips (2 8-pin chips per socket, 4 sockets per board) except one is missing from each one board next to the master volume knob. Ditto above, is it likely that this is the same as all the others?

The stereo Master strip has one 8-pin "777TC 7532" chip in the first 16-pin socket, and then the remaining 3 sockets are filled in with "AB TLO71CP"s (3 more sockets, 6 more chips). Is it reasonable to assume that I just need another "777TC 7532" in the top socket?

Should I replace all the chips while I'm at it? Some of them are kind of expensive from Mouser. Most of the chips in the board are marked "Malaysia". I've found similar chips on Digikey but I'm not sure if I need to find the exact same ICs (as-marked) or if they just need to be the same type.

Oh, also the power supply hums kind of loudly. Is this a simple fix like a cap job or more likely something major? Is the p/s a good thing to upgrade/replace?

I can post pics of this beast if anyone's interested. Thanks in advance for any and all help!
 
PICS! The enquiring public wants PICS!

Advice?

Sounds like a solid, well built board, so it sounds like you got it made.;)

Custom home-built board? Must be unique. :eek:
 
Yep - they're EPIs. I found them in someone's trash with the woofers gone - threw in some "audiophile" woofers (also trash-fished) as an experiment. They sound pretty good but kind of weird since the woofer is so detailed and focused (these came from mastering speakers). They're probably more of a mid-range type speaker. I'm using the EPIs' adjustable crossovers.

For mixing I have some Tannoy Series 6 speakers my neighbor loaned me.
 
Haven't looked at the pics yet, but without a circuit diagram it's a little hard to work out what's going on.

Some thoughts:

1. Replacing the 741's with a cheap pin compatible drop-in like the TL071 woud improve performance and reduce noise.

2. The "7343" and "7532" are date codes meaning the 43rd week of 1973 and 32nd week of 1975 respectively. You've got yourself a funky 70's mixer!

3. The TL071's are fine, no need to replace them at the moment.

4. I have googled the 777 and haven't struck anything too useful; its referred to in some AD application notes as having similar circuitry to other op-amps like the good old 741, but as for pinouts etc I have no idea if the likes of a TL071 would drop in. I know that there were some weird and wonderful op-amps out there in the 70's that seem to have disappeared - a lot of ETI (Australian magazine) projects used a TDA220 (I think) which was a triple op-amp which needed some external biasing, and my dad built a mixer using the LM381 preamp chip in the mic section (hissy as anything). If you can ID power supply, input and output pins then you might be in luck.

5. In terms of the other empty sockets, a 741/TL071 pinout has power on pins 4 (negative) and 7 (positive). This means that on a split rail (often +/- 15 V, look for a 7815/7915 pair in the power supply) you'll have -15 V on pin 4 and + 15 V on pin 7. Alternatively some designs from the era used op-amps like this in unity gain sections (e.g. EQ) with AC coupling and biased the input, using something like a 24 V rail, so you'd see 0 V on pin 4, 24 V on pin 7 and about 12 V on pin 6. The ETI Master Mixer was designed like this.

If you're not sure about what the pins are, looking down in the IC with the notch at the top, pin one is the top left, pin 4 bottom left, pin 5 bottom right, pin 8 top right; e.g.

--u--
1| |8
2| |7
3| |6
4| |5
-----

Hope this is of some help to get you started. If you're ever in New Zealand I'd love to have a look, like 'getting my hands dirty' with this kind of thing.
I have access to some 70s/80s era documentation, will try to have a look through it in the weekend to see if I can find out info on the 777.

Cya
Andrew
 
Have had a look at the pictures now and things don't look too bad to me - they're single sided PCBs (makes the whole process of tracing what's happening a lot easier). Its a bit of a mess - looks like it's been hacked around a bit - but working out what's going on with the ICs etc and then methodically replacing resistors and caps once you know its worthwhile would produce a quite usable mixer.

How much do you know about electronics, circuits and op-amps? If you trace the circuit based on what's in there - don't even worry about values at the moment - it will make fault finding a lot easier. Start with tracing from each pin of the ICs and IC sockets, identify the rails etc and you'll be fine.

Cya
Andrew
 
Wow!

Thanks for all the help! I'm a semi-newbie to electronics - I've built some tube pre-amps from kits, a couple fx pedals, done some repairs and cap jobs and tweaked guitar amps. It's true that these boards are pretty easy to figure out since the traces are so big. I will put up some pics of the circuit side later today...

It's good to know that the 777 is something unusual because I spent a while on Google with little luck, and was thinking maybe the hits I found on the second number were meaningful. So that's a date code...

I suppose I can figure out what the pinouts are doing and hopefully find something close or find a supplier of wicked old ICs.

So how about all them 11's, eh?
 
hissy stereo outputs

Okay, I traced the pinouts and found they all had power on pins 7 and 4, and so I loaded the boards up with TL071's.

Everything works now except I had to pull the chips associated with the Echo/Send knobs because leaving them in caused a ton of feedback to come through the Stereo mains. So I have no Send circuit but everything else works.

Although I'm getting output from the stereo mains, it has A LOT of hiss. It also seems to have a lot of gain. If I leave the master fader at -45db the hiss isn't audible, but the mix is pretty quiet like that. Could it be that the stereo mains are meant to power unpowered speakers? They have XLR's - could I check this with my meter?

What's weird is that the 8 buss outs are super quiet, as are the 2 Monitor outs. It's just the Stereo outs that hiss. What could cause this? What should I look for?
 
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