And another question...

I have an Ensoniq SQ80 which appears to have one dodgy voice channel - every eighth note played is slightly lower volume than the others. This is not every eighth note along the keyboard - it is not a key scanning issue. It has eight identical voice circuits based around CEM3379 VCF/VCAs. The triggered voices are allocated to the CEM units cyclicly, one of them is not sounding as clearly as the other seven, thus the problem on every eighth note.

In order to identify the voice channel causing the issue and maybe be able to spot a physical problem (loose chip, dodgy-looking cap if I am lucky) I anticipate the need to "listen in" to the voice circuit board as notes are played.

So, is there a safe way to create an "audio probe", probably connected to my mixer and listened to via headphones. It is possible that the probe could touch a DC supply voltage, so I don't want to knacker mixer input channels. Looking online suggests a 0.1uF 50v DC blocking capacitor in line with the signal probe. Is that all there is to it? Also any suggestions to avoid thumps - I could run the probe via an Alesis 3630 compressor to maybe reduce initial clicks/thumps../ good/bad idea?

Your thoughts very welcome

Bill

Hmmm, just checked the Alesis 3630 spec (it's a while since I used it) and there is no "attack" control on the gate, so it probably can't be used to reduce attack thumps. Any better ideas?

The 3379 is long out of production and I very much doubt you will be able to get one if one is down.



You are just going to have to put an Oscar on the ins and outs (starting with pins 15/17) and see which chip is the offending beast and then go through the pins with your Oscar and find out what and where, who and why. Could just be a supply voltage. Dig gently!

Red, thanks for your thoughts. Yeah, if the 3379 has actually failed, I'm a bit stuffed, just hoping that it might be something around the chip. Haven't even opened the box yet to see if they are mounted in sockets - thought I would ask here what tools I should prepare before going in! I will certainly dig carefully.

Absolute worst case is that if I can't cure the problem, for a lot of chords, subject to the synth sound, it's not particularly noticeable. Also if a program is set to mono, provided that voice doesn't address the dodgy channel, it sounds fine however many notes are played, so repetetive lines will be OK. Looking on the bright side, it's seven monosynths going through curtis filters... but I would prefer a fully functioning polysynth.

Don't have an oscilloscope, if that is what you meant by an Oscar (presumably not the synth!)

Any other thoughts about audio probes welcome.

Yes, I meant an oscilloscope.

You could probably identify the offending chip by testing the output against chassis with a decent multimeter set to AC and a couple of volts. Once you know which one it is, test the surrounding stuff and compare to the others.

Also hit it (gently!) with the handle of a screwdriver to see if it is just a 'seating' problem. You could also try zapping it hard with freezer spray, though this has little effect on this type of chip (but as Alexander Pope said 'Hope springs eternal in the human breast!')

Quote The Red Bladder:

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Yes, I meant an oscilloscope.

(but as Alexander Pope said 'Hope springs eternal in the human breast!')

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+1 for most literary citation in an SoS thread in some while.

PS: still hoping for any stories you can tell about your time working in Conny Plank's studio. from one kraut-rock fan to another (am listening to Stephen Kaske's "Meditation - Mystic Spring" as i type this...)

cheers,

d

btw: to OP. personally, i'd suggest taking the Eno approach to the malfunction, and see if you can find interesting uses for the dropped 8th voice. i could see some cool effects in arpeggiations. just a thought ;-)

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"Patsy had the drug tolerance of Keith Richards and the moral rectitude of Brian Jones." - Dr. Walter Bishop, "Fringe"

Hi Bill,
Any 1/2 decent digital mmeter will read ac signals reliably above about 10mV (-40dBu) but they have a very high input Z and that can give weird readings. Also DMM "open" test leads are not best suited to the task.
Make up a probe, old Bic pencase and a needle and fit a 100k resistor across the input. Come out on a mtr or so of screened lead and the last 50mm terminate in 4mm plugs. Have about 100mm of earth lead and try to pickup earth as close to the measuring point as possible.
If you wanted to get fancy you could use an NE5534 as a unity gain buffer as well and drive cans via a 100R.

Dave.

Quote:

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personally, i'd suggest taking the Eno approach to the malfunction, and see if you can find interesting uses for the dropped 8th voice. i could see some cool effects in arpeggiations. just a thought ;-)

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the same things occured to me, it's not a fault, it's a characteristic... yeah, it's not impossible to live with, but I would still prefer to fix it if I can.

Quote:

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Make up a probe, old Bic pencase and a needle and fit a 100k resistor across the input.

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So this probe is to test for voltages, not to listen to a signal? But with a 5534, it could also be used to listen in? I might have an old headphone amp lying around... actually, that's just given me a thought, I could use any old hifi/cassette deck with a headphone output, rather than my trusty Mackie!

Any thoughts about that 0.1uF DC blocking capacitor? Is that a good idea?

Going back to the SQ80, found this pic of the CEM3379 board (not mine!), all in sockets

So there is the posibility that if I can find which voice is causing the problem (listening test), and if there are no obviously failed components, I could swap two chips and see if the problem follows it or not. If not, it isn't the chip, if so, it is... I read that you have to be extremely careful lifting chips from old sockets (done it on my own experiments before, never on a manufactured board) so that will be last resort...

Well I always trust meters more than ears Bill!

Yes the 0.1 can't hurt. With the suggested 100k that gives you a turnover frequency of about 20Hz, good enough for old synths?!

Re lifting old chips from old sockets. Not sure what problems you might mean, stuck pin perhaps?

Might be an idea to give them all a blast of Deoxit or similar and then drop a tiny bit of lubricant, electrolube, WD-40, down each pin and leave overnight? You never know! MIGHT fix the problem!!!

Dave.

Dave, just to clarify, when you say "fit a 100k resistor across the input" do you mean between signal and earth?

Bill

Quote BillB:

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Dave, just to clarify, when you say "fit a 100k resistor across the input" do you mean between signal and earth?

Bill

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Yes, 100k or thereabouts. It is a compromise between loading the circuit under scruitiny and the very high meter Z picking up all the **** in Creation. Handy dodge too if measuring unloaded mains traff windings, otherwise you get all sorts of weird results.
DVM's are great but I sometimes wish I had not busted my Avo 9!
Dave.

Regarding the probe:
I use a capacitor and a resistor in series as a probe. Say 1uf and 1k.
The capacitor blocks DC, the resistor blocks any high current so no damage is done. I use a couple of crocodile clips and an audio amp (I have also used a guitar amp) just clip the ground of the lead from the audio amp to the ground of the circuit, one end of the probe to the signal of the audio amp and probe away with the other end.

Re all this, a PC line-in is a perfectly adequate meter if you don't have big voltages. If the gadget in question runs off batteries, you're probably fine. I'd use the capacitor and resistor (as mentioned before) just in case, but otherwise go for it. And numerous freeware oscilloscope programs are out there which will display the results.

Quote DunkH:

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Regarding the probe:
I use a capacitor and a resistor in series as a probe. Say 1uf and 1k.
The capacitor blocks DC, the resistor blocks any high current so no damage is done. I use a couple of crocodile clips and an audio amp (I have also used a guitar amp) just clip the ground of the lead from the audio amp to the ground of the circuit, one end of the probe to the signal of the audio amp and probe away with the other end.

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Yep. I go along with this, but would make the point that the cap should be unpolarised (preferably a 'dry' cap).

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It wasn't me!
(Well, actually, it probably was)