Mixer fetish?

Nigel Goodwin

Super Moderator
Most Helpful Member
I think my friend Chris has got a mixer fetish

I've mentioned previously that the band I used to do the PA for has reformed for a reunion gig, and potentially other gigs - the reunion gig went 'OK'

Anyway, it's all been organised by the bass player Chris, you used to run the band, and has recently moved back to the UK from Croatia.

So he contacted me about equipment (he'd got nothing, not even a bass guitar), but I'd got a small PA, mixer/amp, speakers, mikes, leads etc. that I used for my daughters bands, so initially we practised with that. But he wanted bigger, better

First off he bought a cheap Soundcraft mixer off Ebay, and brought it to me fix - so I repaired that. Next, while I was out of action with my detached retina, he bought a slightly larger Yamaha mixer (presumably not as cheap, as it didn't need fixing) - and that was the mixer we used for the gig.

Since then he's bought a Studiomaster mixer, because it has 5 Aux outputs, and I've currently got that for repair. It had no PSU with it, but we've managed to source a plug, and I've ordered a couple of 24V SMPSU's from China, which I'll modify to give -17V and +17V as required, currently I'm running it from my dual workshop PSU. The Aux5 control and knob was also missing, but we've managed to source a suitable control.

I'm currently working on the right LED meter, which isn't working, while the outputs are - and the meters are actually driven off the top of the headphone volume pot - via a couple of opamp rectifiers, and three LM3900 comparators per channel.



Next he told me he'd put a bid in on a larger Studiomaster mixer, which was spares or repair, no-reserve, and had a starting price of £0.01 - so he put a cheap bid in, and eventually won it for £7.50 It was collection only, so he drove to Wales on Sunday to collect it - later on I got a text, saying he needed a hand getting it out of the car, but didn't ask for me to go and help. Then the next day he sent me a picture via text, I suspect it was somewhat larger than he thought.

I asked if he had tried it, but he hadn't at that time, and I've not heard anything since - however, if nothing else it's an awful lot of knobs for only £7.50

Needless to say, I'm concerned he's going to want to bring it me for repair, and I think we might struggle getting it to my attic!.

 
ClydeCrashKop said:
And you are the champion Mixer Fixer!
Click to expand...
I'm doing my best

The Chinese PSU's arrived this afternoon, so I modded those - the top resistor was 20K, the bottom was 2320 ohms (nice random value) - a bit of ohms law, and I worked out 39K in parallel with the 20K gives just under 17V. I've just connected it up, and it runs about +/-16.6V - which seems pretty decent to me.

I didn't want more, as where the power goes in one of the opamp chips (headphone driver) is fed via a ten ohm in each rail, with 16V decoupling capacitors on the rails!.
 
Some smps can feed significant noise into audio systems. You might want to use a center-tapped & rectified 12v transformer to get your +/- 17v. Our old ears may not hear the higher frequency noises from cheap Chinese SMPSs but some of the audience likely will. If you use a megahertz GaN SMPS, then your likely ok with noise-free power.
 

We'll see how it goes - SMPSU's are commonly used in 'later' mixers.
 
You're a legend Nigel. The amount of knowledge you have is unbelievable.

Make sure you pass it on.

Mike.
 
OK, I called to see the BIG mixer the other day, a lot of the problems might be that we can't work it

Anyway, I removed one of the extension modules (at the far end) and brought it home, I actually removed 17/18/19/20, simply because it had had two screws holding it in place.

BTW, for those who like bikes, I got him to fire up the Beta trials bike in the picture, took a bit of kicking over (it seems really high compression), and it sounded really throaty when it started

The grumpy looking person in the dressing gown is his wife Sue, who seems less than amused with it all!

Here are some quick pictures of the modules:

Looking at the second picture, the RH of the four jack sockets is a direct output, fed from the output of the slider, via a buffer amp, so by connecting that to an amp and providing 15-0-15 power to the red/black/green wires I should be able to easily test most of the board on the bench.

 
They appear to be single sided PWBs. I am correct?
And of course, 100% THT components.
Would you guess the manufacturing date? To me it looks like mid-1980s.
 
schmitt trigger said:
They appear to be single sided PWBs. I am correct?
And of course, 100% THT components.
Would you guess the manufacturing date? To me it looks like mid-1980s.
Click to expand...

Yes, all single sided, the IC's are even in sockets - each board has a sticky label, but no date - however, the rear of the front panel has a 'tested' label on it, date 16/8/16 (written in ballpoint) - which presumably is 16th Oct 2016?.

The service manual is dated 22/3/1995.

I wonder if perhaps this expansion module was added later?, but 19 years sounds a bit unlikely - so perhaps it was a repair label from 2016?.

Edit: Quick extra thought, the opamps on the boards are TI TL072CP date code 8745XR (so presumably week 45 1987?), and the smaller mixer, in the first picture in post #1, has the manual dated 26/2/1997
 
Last edited:
Here are the schematics.
 

Attachments

  • studiomaster_pro2_163_203_sch.pdf
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OK, got the expansion module powered up last night

The little PCB by the way is a 1KHz sinewave generator, using an 8 pin PIC16F18313 - I was getting tired of messing about with my function generator, and just wanted a simple 1KHz test signal. It's based on Roman Blacks excellent 16F628 design, changed to use XC8 (rather than MikroC) and a more modern device, using the internal oscillator instead of a crystal.

BTW, before anyone comments it says 12F1840 on the PCB, I did code for either, just minor tweaks, and pin compatible.

It's just a lookup table feeding a PWM module, and a low-pass filter on the output.

Not a bad sine on the scope

 
Well my mate rang me last night, and said "I've bought two more..." - before he could continue I said "mixers?"

But no, he's bought two more motor bikes - he'd already got a Beta trials bike (which was nice) and an old Mk1 BSA Bantam (123cc two stroke) - so he went out and bought a BSA C15 (250cc four stroke), and while there noticed a BSA Bantam under a cover, and asked if that was for sale - it was, so he bought that as well

However, he has sold the Beta apparently, pity, I'd have quite liked that
 
I was never struck on the Bantam, I just did not like two-stroke bikes. I had several Tiger Cubs (198cc ?) in the 70s, and a C15 for a while, though that was bought and sold again still in bits. I had a decent A7 (500cc twin) for a while though.
 
The obvious advantages of two strokes are more power from a smaller engine, and considerably lower revving than four stokes as well.

What I didn't know until fairly recently, was that the technology involved all came from Germany, where they were forced to give up their patents on two stroke engines in reparations after WWII.

The BSA Bantam was a direct result of that, as were all the super fast Japanese two strokes.


I too had a Tiger Cub at one time (I swapped a Philips cassette recorder for it with a friend ), and they were indeed 198cc.

I've always like two strokes, I've had (on the road) a Royal Enfield 250cc twin, a Suzuki 350cc twin, a Suzuki 125 twin, a Suzuki 380cc triple, a Yamaha 400cc single, and a Yamaha 175cc single.
 
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