Learning to debug Frequency Central XVCO2 build

[DanaFo]

Hey,
I'm currently at the point where I've got a trimmer that doesn't seem to be doing its intended job, it's a 1k trimmer (I replaced it to make sure it wasn't the issue). I'm supposed to get 0.63V when calibrated but the range I get is like 0.59-0.66 thereabouts.

I noticed what might be a burnt pad on the 15k resistor:

Rick from Frequency Central was kind enough to reply, showed what the circuit was supposed to connect and suggested the following fix (2nd image):

So I did this and didn't see any change, I tried a few variations like soldering the 15k to the left/collector (bypassing the bad pad). But all I got was that the range moved up like 0.05V upon testing.

I didn't feel any excessive heat from the ICs, there was no smell or magic smoke but I feel they should be tested (just so I can learn how to test them). The caps all look ok (don't worry I replaced the damage cap in the photos).

To test ICs: I just can't make heads or tails of the datasheet regarding what to do with a DMM on it. Should I do a continuity test from the power header pins (-12, +12, gnd) to (vcc-,vcc+,???) respectively?

Otherwise, I'm not really sure how to proceed other than tear it all down, re-test the components and give it another go. I'd honestly rather just buy a fresh pcb.

Thanks!

[guest]

I'm supposed to get 0.63V when calibrated but the range I get is like 0.59-0.66 thereabouts.

im a bit confused, as 0.63V falls within that range. what is the problem your seeing?

[emmaker]

470pf cap looks burnt too. Don't know if it's the lighting, from a soldering iron or toasted.

Jay S.

[DanaFo]

I'm supposed to get 0.63V when calibrated but the range I get is like 0.59-0.66 thereabouts.

im a bit confused, as 0.63V falls within that range. what is the problem your seeing?

Ah sorry I meant 0.659 to 0.660, I am able to debug English and numbers somewhat

[DanaFo]

470pf cap looks burnt too. Don't know if it's the lighting, from a soldering iron or toasted.

Jay S.

Yep I know replaced. Values were the same before and after anyway thereabouts.

[guest]

is everything else working like it should?

[guest]

also, which trimmer is it? what does it control/whats its label? im guessing this is the temp setting for the heater core.

[DanaFo]

also, which trimmer is it? what does it control/whats its label? im guessing this is the temp setting for the heater core.

Ah sorry I only said 1k trimmer, it's the heater trimmer.

[DanaFo]

is everything else working like it should?

I'm not sure lol, I did the continuity check on the main chip via the header pins before powering it up. Part of this post is to ask what I should be testing and how. I have a decent DMM and a dso 138 oscilloscope.

Mainly I'm trying to diagnose what is wrong to cause the trimmer test value to be out of whack. Like the trimmer doesn't really do anything from all the way CCW to a little past center on the CW half.

[guest]

measure on the trimmer directly - does the middle wiper pin change voltage as you turn it?

give the module a listen - does it make the right sounds, does it generally track 1V/oct?

[DanaFo]

measure on the trimmer directly - does the middle wiper pin change voltage as you turn it?

give the module a listen - does it make the right sounds, does it generally track 1V/oct?

to test the trimmer - one probe on ground and one on middle pin? should the module be plugged in?

[guest]

yes, that is all correct.

[DanaFo]

measure on the trimmer directly - does the middle wiper pin change voltage as you turn it?

give the module a listen - does it make the right sounds, does it generally track 1V/oct?

it goes from 518 mv to 692mv it seems

[DanaFo]

measure on the trimmer directly - does the middle wiper pin change voltage as you turn it?

give the module a listen - does it make the right sounds, does it generally track 1V/oct?

it's not outputting anything I recognize as 'ok', kind of like deep clicks

[tassie tiger]

I've built two of these and they worked straight off the bat. I'd carefully check your resistor / capacitor values, and that you've got the transistors in the right spots (e.g. BC547's in 'n', 2N5485 in 'f') before you start again.
By the looks of picture #2 you've also got some long ends on resistor leads (after trimming) - just make sure none of these are shorting anything else.

[guest]

and if you want to post pictures of the boards, we might be able to spot something. also, what are you using for your 3046?

[DanaFo]

and if you want to post pictures of the boards, we might be able to spot something. also, what are you using for your 3046?

I'm using an AS3046. There's a wire loop I haven't trimmed yet to support pad damage on that yellow cap, continuity seems fine on it.

Right now there's a 15k resistor soldered to the collector of the 547 to try to bypass the burnt pad. I tried all the transistor legs with no dice.

[guest]

ok, there is something wonky besides for the trimpot issue, as that shouldnt effect it making sound at all. did you test it after soldering that resistor down where it should go?

[DanaFo]

ok, there is something wonky besides for the trimpot issue, as that shouldnt effect it making sound at all. did you test it after soldering that resistor down where it should go?

I have tested the trimpot with the 15k resistor in its proper position, and soldered trying each leg of the 547 transistor with no clear effect.

I'm trying to figure out if the ics are busted but this is really hard to figure out using google.

[medbot]

it's not outputting anything I recognize as 'ok', kind of like deep clicks

Do the clicks go faster or slower when you change the tuning/octave? VCOs can sound like clicks if they are tuned below audio rate, so if the clicks change rate with the controls maybe you've got a component or two wrong and it's dropped your tuning super low?

These VCOs should be super close to the Roland System 100m VCOs, and those schematics are easy to find if you're looking for a reference.

[guest]

one quick thing to check, is measuring the voltages on the 3406. all of the Vbe drops should be 0.6V.

[guest]

you can also apply an external CV to see if you can get the oscillator to go to higher frequencies.

[DanaFo]

one quick thing to check, is measuring the voltages on the 3406. all of the Vbe drops should be 0.6V.

cheers for that, just about to give it a go!
and yes I'll check out the cv modulation

Thanks!

[DanaFo]

one quick thing to check, is measuring the voltages on the 3406. all of the Vbe drops should be 0.6V.

so I powered it up, put the ground of the DMM on the free ground header pin of the flying bus, and checked the DC voltages on the 3046 pins (inserted chip) - only pins 6 and 7 made any sense for a vbe of around 0.8v Pin 6 was 9.7v and pin 7 8.9v. 3046 spec seems to indicate between 0.715 and 0.8v vbe but I do'nt know if I'm reading this chart right:

All of my measurements here by pin:
1.8 mV
227 mV
-387.3 mV
1.2 mV
-0.7 mV
9.7 V
8.9 V
12.07V
0.984V
1.2 mV
3.4 mV
1.03V
1.03V
1.03V

[guest]

ok, that actually doesnt seem too bad. did you try an extrnal CV? it looks like its beeing driven really hard into the subaudio range. pin2 should go +/-100mV.

the difference between the base and emitter voltages is about 0.6-0.9 on most of them.