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VCO gremlin after 4 months (BACK AGAIN)
MUFF WIGGLER Forum Index -> Oakley Sound Systems  
Author VCO gremlin after 4 months (BACK AGAIN)
Hi, I'm a bit premature in posting this but after working fine for months, all wave outputs on my vco now sound the same...suggesting I'm only hearing the core. I will go look at the schematics, and reseat some ICs, and hook up an oscope. I don't see any reason than a component failure perhaps on power up. Case has not been dropped or even moved much and I haven't been inside case since Jan.

Let's hope it's quick to figure out.
I'm struggling to get my head around what could cause that. Probably not the electronics on the VCO board. Which leaves some sort of weirdness with the socket board interconnections.

The scope will no doubt reveal the real culprit.

Ok Cheers. Ill start with looking at socket board connections.

I should really get around to hooking up scope to pc for pictures but it hasnt been a priority.
Anyways pulse and saw look as they should. triangle is like an inverted hockey stick ( or a forward slash truncated at top)

and sine is an exponential curve flavor of forward slash also truncated at top.
key discovery I do not have a triangle wave on pin 3 of U6 0nly a reverse ramp. The hocky stick distortion (clipping or stunted ramp ? ) appears first on pin 6 U6.

Whilst i might replace U6 just to be sure - believe the problem is upstream.
any tests I should do to confirm the triangle shaper ? Not sure what to look for next.

fyi details:
The voltages to the board test correct.
No ICs are running warm.
No change to waveshape when tapping ICs with finger.
Tweaking the main1 and main2 interconnects did nothing , the distorted waveforms for Sine & Tri are rock solid. Removing and reseating - ditto.

R81(2) has the distorted TRI (ramped hockey stick) upstream of the interconnect and TRI is upstream of the Sine circuit
so focus on TRI path and U6

Pin 3 input of U6 has a clean reverse ramp (expect a triangle??)
Pin 1 output of U6 has clean reverse ramp ( expect a ttiangle ??)
Pin 6 input of U6 has a hockey stick on ots side ___ .....(Not good)
R83(1) is 10.84V expected = 10V so in the ballpark
Ditto R69(2).

Hmm shouldnt i have a triangle wave not at ramp. at this point ?
Pin 3 of U6 should indeed be a triangle wave of around 2V peak to peak.

Remove U6, power up and test pin 3 again. Still a sawtooth? If so the problem is probably the SHP-T trimmer or Q5. If it is a triangle at pin 3 then U6 is probably stuffed.

U6 replaced - no effect.
SHP-T appears to have zero effect on shape but voltage on pin2 varies
Q5 has saw on emitter and collector and base
U5 pin1 & 2 has saw

Now is Q5 the culprit Ill swap and see.


Sine and Triangle are back ! Need to recalibrate a little

Thanks Tony.
thumbs up
Hi Tony, fyi the same problem has appeared again. I t replaced Q5 resolved it again but on a 30 minute listening, i can hear the breakdown of the triangle/sine wave to a distorted ramp as an occasional random glitch.

This suggests to me the transistor will likely break down again at some point but is not the real fault.

I tried flexing the pcb a tiny bit to see if i could induce the glitching. I did once or twice but not in a consistent fashion. so could be coincidence.

clutching at straws now.
You've probably got a bad solder joint in there somewhere. Since it's affecting the triangle output it would be wise to listen to the output and tap each component in the triangle shaper circuitry with the handle end of a small paint brush or screw driver. Especially the SHP-T trimmer. Mechanical parts are more likely to fail than passives, but a bad solder joint on any component can cause difficult to find intermittent problems.

If you have a problem somewhere but can't locate it exactly, then reflow all the solder pads in the tri shaper area. Reflow simply means reheating the joint and adding a little bit of fresh solder to each pad.

Thanks.ill do that.. Shp-T FYI is nearly full ccw to calibrate triangle.
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