broken micro synth

[9 volts]

Hey there, I'm trying to repair a eh micro synth that loses all signal when i turn the start frequency down below 9. I've checked the pots which seem alright and have checked power to the ic's and they all seem to be getting a similar amount. Any suggestions on what to do next?

[StephenGiles]

That might indicate a fault in the VCF. If you can test the CA3094 chips in something else, you could at least eliminate those. Are you sure that the wires to the sliders are soldered OK?

[9 volts]

HI there, I've rechecked the wires and the connections to the sliders and all seems fine.
If i have the following three sliders up full ( resonance, start frequency and end frequency) the sound definitely has a filtered (moogish) quality. altering the end frequency slider doesn't seem to affect the the sound and like i said on an earlier letter dropping the start frequency reduces the total signal at the same time (At about 8 the signal disappears completely). Unfortunately I don't have any thing else with  ca3094 chips in to try and test them........i made a eh small stone but used ca3080 chips.......Is there other way to check these chips.(actually the chips are eh1048 but from what i've read on this site they're the same as ca3080)

I've been wondering around the two boards with a audio probe but it hasn't really helped. This micro synth is the second edition (1978?) which uses one 2n5087 in the filter sweep circuitry.

Once again any help/guidance appreciated

[Paul Perry (Frostwave)]

Maybe a leaky capacitor? An audio ocillator can go bad (as in not oscillating at the low end) because of this.

[9 volts]

I've just found two 10 uf capacitors that have suspicious voltage readings. one had -9.5 on the neg leg and -9.2on the pos leg.
The other one had -9.5 on the neg leg and -8 on the other leg. From what I've read these readings could indicate that they are faulty yes?

[9 volts]

I've changed the two 10uf caps and this hasn't altered the situation. I've just measured the voltage on the pins on the 2n5087 transistor which on the schamatic is called 'the adaptive schmitt trigger' and recorded the following results.
e= 6.4v
b= 5.7v
c= 6.4v
If I audio probe around this it is just a blatty sound.
Once again appreciate any feedback.

[Mark Hammer]

Slider pots are the absolute worst for acquiring dirt and becoming unusable or at least in desparate need of serious rehabilitation.  If it is feasible to do so with being too invasive or destructive, I would recommend popping the offending pots open and giving the resistive strip a serious cleaning.  Either than or if if you have a can of compressed air giving them/it one serious blowjob.

I have found many devices respond very favourably to something I am fond of mentioning here, called Stabilant ( http://www.stabilant.com/ ).  This is "contact enhancer" that, unlike cleaners or lubricants, provides an electrically conductive polymer layer on whatever you apply it to.  It is chemically stable over very long periods, so one application will do nicely for years.  It has the pleasing property of essentially bridging the gaps that can sometimes form if a resistive strip is worn a bit inside a pot or if a wiper or switch contact has lost a bit of its spring and tension.

In yet another thrilling episode, this past weekend I opened up my beloved Luxman stereo amp that hadn't worked properly for a long time (we haven't had a functioing stereo in the house for over 4 years).  Initially, because the power-amp worked fine but the preamp didn't, I thought there was a fried transistor and I was going to have to hunt around for obscure japanese unit to repair it.  On a long shot, I applied some Stabilant to the contacts on the rotary selector switch.  Boom, contact made, and functionality restored.  I buy the stuff in little 0.5ml vials locally for 50 cents each.  This repair job used less than 1/20 of that.  Will it *always* cure the offending device?  Probably not, but at that price, how can you afford NOT to try?

[9 volts]

GOODS NEWS....... IT WORKS!
I cleaned both the potentiometers relating to the envelope (start and stop frequency) and that didn't solve the problem (but they were really dirty so it was worth doing).
Both these potentiometers lead into an rc4558 ic which I replaced and now its working! (much to the horror of my neighbours) Thanks for the replies, even if it doesn't nail the problem it encourages looking at the problem from a different angle with a fresh approach.