Soundcraft 600 Restoration

Founded in 1973, Soundcraft first made its mark with the launch of the Series 1, the first mixing console in a flightcase.

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Re: Soundcraft 600 Restoration

Postby Gonecat » Thu Dec 25, 2014 9:02 pm

Happy Holidays to you all!

I self-Santa'd this year and got a scope. Tek T935A, 35mhz dual trace with delayed sweep.
Good timing since the mixer's right master channel has gone out on me!

When i hit the desk's oscillator it is near silent on the right, same on headphones and the control room outs. I only really hear it when i drive it so hard it sounds like a high-pass filter is opening up.

It seems like it'd be near impossible to work on the master section without IDC extension cables.
I'll have to double check, but they looked like standard 40pins.
Anyone got a source for these that they like?
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Re: Soundcraft 600 Restoration

Postby GJC Designs » Fri Dec 26, 2014 2:25 am

S600 bus ribbon is 34 way.

Remove a couple of channels or groups to free-up the bus ribbon, then move the master module along a couple of connectors. This negates the need for an extender ribbon.

The high-pass filter type of sound is likely to be crosstalk, not real signal. Provable by panning fully towards the failed output, whereupon the signal disappears.
Does the insert send work? If yes, then the mix amp and insert driver are OK. Take the insert pre/post jumpers into account as these will include or exclude the fader in the path you are testing.
Gareth
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Re: Soundcraft 600 Restoration

Postby Gonecat » Fri Dec 26, 2014 7:55 pm

Hi Gareth,

Thanks a lot for the info and the tip on avoiding extension cables (means i can dig in on it now!)

I am hearing that squealing with it panned hard-right (the bad side), but at super high gain only. It just suddenly 'slides in' - sounds just like opening the filter up on a synth.

I ran a tape return thru a tie line out to some outboard, it is sending out fine.

By the way, do you usually use an audio probe or a scope to track down issues like this?

Cheers,
Steve
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Re: Soundcraft 600 Restoration

Postby Gonecat » Sat Dec 27, 2014 1:00 am

Ok, i started tracing from the oscillator circuit (around VR6) and got as far as P108 with the signal coming through.

I honestly can't tell from the schemo where to check next. I've never used a scope before nor worked on such a complex schematic and packed PC board! Whew! But, i'm diggin the learning curve.

I get a much diminished signal at P106 (same as at P6 and P8).

Nothing around IC3.

I'm working from this schematic, page 8:
http://www.soundcraft.com/downloads/fet ... des&id=666


Image
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Re: Soundcraft 600 Restoration

Postby GJC Designs » Sat Dec 27, 2014 12:23 pm

A couple of general pointers and suggestions on fault tracing:

Check op-amp outputs for DC. An audio op-amp will typically be within a few tens of mV near to 0V. Any huge excursion in the range of several volts points towards a failed chip. This will often have the output sitting at a voltage close to the power rail.

As a general rule of thumb, you will be able to scope a signal on an op-amp output and on the non-inverting input if it has a signal going into it (IC2 pin 5). You will not be able to measure a signal on the inverting input of an op-amp (IC2 pin 2).

Get a sensible level on your scope. Fill at least 70% of the screen with a clear sinewave (1kHz is good) measured at a known reference point (VR6 wiper, osc output).

As you have a stereo path with one side OK, comparing L & R channels should lead to the fault.
Make sure both sides are set the same with regard to jumpers.

Study the circuit, write a list or table of points at which to test, write down the results as you work through the list. This is a good way to reduce confusion and "circuit blindness". Discrepancies stand out quickly.

Start at the first points where there should be a measurable signal: IC1 pin 1 and IC3 pin 1.
Good signal on both? If yes, move on to IC2 pin 1 and IC4 pin 1.
Continue moving on through the circuit until the signal disappears.
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Re: Soundcraft 600 Restoration

Postby GJC Designs » Sat Dec 27, 2014 12:29 pm

Make sure both channels have the insert jumpers set as per the manual, then:
Are you getting equal signals on P9 & P109?
If yes, are you getting signals on the insert send jacks?
If yes, are you getting signals on P6 & P106 (insert returns).
If no, exercise the sockets with jack plugs, it may be a dirty break contact.
If yes, how about P1 and P101 (fader top terminals)?
........ and so on.......
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Re: Soundcraft 600 Restoration

Postby Gonecat » Sat Dec 27, 2014 6:51 pm

That was really great advice; and keeping the list of test points and measurements really helped in terms of staying on track and seeing differences between the channels.

During the process, I guess I re-seated an IC or cable and the right channel came back up.
However, it is around 3-5db lower than the left.

I do have TL072's on hand, maybe i'll just swap these out anyway.

Oddly enough, the oscillator level pot does not effect the volume now (it did before!). The rest of the oscillator functions are fine (on/off and 10k).

The readings I got:

___ Pin 1 / Pin 3 / Pin 5 / Pin 7
IC1: 8.35 / 1.51 / 1.25 / 7.72
IC3: 7.98 / 1.48 / 1.45 / 7.04

IC2: 3.00 / 1.53 / .09 / -1.75
IC4: 3.20 / 1.54 / .08 / -2.00

IC5: 1.95 / .01 / .01 / 1.15
IC14: 1.8 / .01 / .01 / 1.36
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Re: Soundcraft 600 Restoration

Postby Gonecat » Sat Dec 27, 2014 7:02 pm

(also have some NE5532's on the way)
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Re: Soundcraft 600 Restoration

Postby Gonecat » Sun Dec 28, 2014 8:52 pm

Went back to square one and made sure levels were good, patch cables, etc.
The results i took yesterday - totally erroneous.

I went ahead and switched out IC1 and IC3 for new TL072s.
(Tested at the 2000m setting of DCV on my DVM)

    IC1 -
    Pin 1: 22
    Pin 3: 18
    Pin 5: 19
    Pin 7: 28

    IC3 -
    Pin 1: 18
    Pin 3: 18
    Pin 5: 19
    Pin 7: 20

Scope shows hardly any waveform on IC3.
Voltages on TR1 and TR2 are about the same.


Made an audio-probe and found that the signal is already lower before it reaches R105 (above IC3 on the schematic).

I have not been able to find a good test point preceding that (between it and Con 1).

If anyone has an idea of where to go from here, I am all ears.
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Re: Soundcraft 600 Restoration

Postby Gonecat » Mon Dec 29, 2014 3:04 am

By the way, how can I test the cable?

Further confusing my understanding, i popped the 1st new TL072 in IC1 and IC3, and the damn thing actually worked great - L and R at same volume. I double checked everything i could to be sure of it.

Cleaned the fader, put it back together and SOB if the thing didn't go right back to its previous state - the right channel lower in signal. So, i figured it was the cable, but no jostling of it caused any change in the voltages or waveform. Then i opened it back up, put in a new TL072, no improvement.

In my very limited knowledge, I'm figuring the prob is somewhere between Con 1 and R105.

I say this because the jumpers 6-9 get normal signal as expected; and (i guess) the oscillator signal gets split at Con1 to the two different channels, and thus henceforth in the chain, there are Left and Right differences which i see as of R105 (and R5, respectively).

But i have not yet figured out how to test the components in-between these two points (Con1 and R105) with any self-revealing results.

I tried testing the little 'slit' plates on the side of the main connector (Con 1) and got mixed results based off of what i could read from the schematic and my newbie-dive-in-the-deep-end knowledge.


On to more important questions! Who has big plans for New Years? There are only 3 bars near where I live, and 2 of them are dangerous on any night of the year. Sometimes, not having options is ok.
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