My Scope Says Yes but My Ears Say No..

[valis]

Greetings all,

I'm still having troubles with my MFOS VCO Rev. 2006 and it's starting to drive me a little nutty. I'll try to organize this post as best I can so it is easy to understand..

Backstory: This VCO used to work but then I rehoused it in another panel and accidentally plugged the -15v into Ground and vice versa. Now it no longer works. I actually have another MFOS VCO (Rev. 2008) which does work so I can compare a little.

Symptoms: As I'm looking through my 'scope (albeit I'm not the most skilled scope user) I see the correct outputs but only hear white noise. For example, if I look at my banana jack output for my sine wave, I see a 6.6v p-p sine wave but all I hear from that output is white noise. My triangle looks like a triangle, My ramp looks like a ramp but all I actually hear is white noise. I do not see a square wave output though and hear nothing. This is all starting to drive me crazy. Why would this happen?

I also get the correct wave output at U2- Pin 1 and U4- Pin 2 (see schematic).

What I've done so far: After I plugged the VCO in wrong I wasn't getting any output from U2. I replaced U2 and got an output but some of the other chips seemed to act a bit wonky so I replaced every chip in this VCO except the SSM. Now everything looks fine but all I hear is white noise:

I've also:

- Checked the voltage and ground at each IC. All test good.

- Triple checked my wiring.

- Triple checked my voltage levels and voltage continuity. All seems OK.

- Checked functionality of pots. All seem to work okay.

- Changed all ICs except for the SSM chip.

- Checked all AC signal test points as indicated on the schematic. All looks okay.

- I am using TL082s and TL084s instead of TL072s and TL074s but it worked earlier when I did this.

- Before I replaced the TL084, and after I plugged the module in wrong, the chip was getting real hot. This stopped after I replaced the TL084.

- Replaced C7 & C10

- Double checked that the banana jack output is going to a 1/4" plug (tip to banana and sleeve to PSU ground).

- I tried listening to the module through my amp, mixer, headphones, etc.


So, I can't, for the life of me, figure out what is going on. Why would I see a 6.6v p-p sine wave (or ramp, etc.) but just hear noise? My sine is at about 485hz. What am I doing wrong?

Help would be appreciated as I've been sinking way too much of my free time into trying to figure out what is going on..

[prgdeltablues]

Sounds to me like the problem is not with your VCO circuit but with the connections to audio. Have you tried putting the scope probes across the 1/4" jack to see whether you get the signal or just noise there? It could be that the ground point you have connected the scope to is not connected to the ground you're using at the 1/4 output? Another thought is that most headphones are stereo jacks (TRS), but it sounds from your description as if you are using a mono jack socket?

You could try 'sniffing' with your amp at points where you know from the scope the signal is present (See Ray Wilson's 'Guitar Amp signal tracer' under 'Hot Tips').

Persevere!

Peter

[kkissinger]

As you adjust the frequency control on your front panel does the output frequency vary?

If "yes", then make sure that the frequency is in the audio range. If you still hear only noise then check your audio interface.

If "no", then suspect one of your chips is damaged and is pulling down your power supply -- the waveform would be the result of the filter capacitor charging and discharging.

[valis]

Thank you everyone for your replies. My apologies for cross-posting this over at muff.

http://www.muffwiggler.com/forum/topic-21793.html

I've been trying to figure this out on my days off but with no luck. I'm really trying not to let it drive me nutty..

Here's where I stand..

[valis]

... So, I pushed down on the SSM2210 chip and it gave me a little tone, not a lot but a little. The SSM2210 was the one chip I wasn't sure how to read. I'm not getting any real voltages at any of the pins of the chip. Should I? What do you think the chances of a dead SSM2210 causing the problems are?

[prgdeltablues]

So the connections out are ok. And if you're getting a sine wave out at 6V P-P there can't be much wrong. 487Hz should be audible, but if kc means kilo-cycles (per second) that's 121kHZ, which would of course be inaudible. I'd try setting both the trimmers at roughly their half-way points - they adjust the V/Oct tracking, which isn't critical at this stage! You should then get about 20mv per 1V of CV at pin 7 of the SSM2210. If you read anything much higher that could be forcing the frequency out of the audible range. Another thought is to replace C6 - the timing cap. If that has got overheated, who knows what value it will have (I would have asked if you had used the correct value - 1nF (or 1000pF on the schematic) but as the VCO was working fine before I assume that isn't the problem.)

Peter