After assembling the VCO without any issues I am running into a problem with the VCF. I have just finished tripple-checking resistor and capacitor values and the proper polarity of the latter ones. I hope somebody can point me in the right direction here. I cant detect any Signal on VR4, I am just picking some very faint mains hum:
Which is at the same frequency and somewhat resembles the original sawtooth, but is only 100mV pp.
I am also posting the populated PCB images showing the populated VCO/VCF areas and a closeup of the transistors I've got in my kit used. I have read a couple of VCF related troubleshooting threads and did not find any similar issues to mine. Can anyone point me in the right direction?
I don't know if that can be an issue, but my Offset-Voltage of the VCO ist around 8.2v instaed of 8.5v:
I can trace the signal down to R62, after that resistor I am picking up this:
[SOLVED] VCF not Working (Fake 2SC2291)
Moderators: altitude, adafruit_support_bill, adafruit, phono, hamburgers
Please be positive and constructive with your questions and comments.
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
[SOLVED] VCF not Working (Fake 2SC2291)
Last edited by pulsar256 on Tue Apr 02, 2013 4:59 pm, edited 1 time in total.
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
Re: VCF not Working?
And some Closeups of the PCB and transistors:
- mmm
- Posts: 517
- Joined: Tue Apr 24, 2012 1:51 am
Re: VCF not Working?
I must admit this looks weird.
I take it resonance is set to zero?
What's the code printed on Q22?
Can you measure (with the scope) both sides of C18?
The 100mV pp are perfectly normal (within the filter ladder it drops even more) however I find the waveform strange. With a working filter it would still look like an (increasingly fuzzy) sawtooth.Which is at the same frequency and somewhat resembles the original sawtooth, but is only 100mV pp.
I take it resonance is set to zero?
What's the code printed on Q22?
Can you measure (with the scope) both sides of C18?
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
Re: VCF not Working?
Thanks for looking at my issue!
Thanks again!
Does not make any difference at all, resonance, cuttoff or env-mod do not have any effect on the output. I tried all pins on VR4 and figured the right side of C21/C20 (easier to probe). See the last / third image capture which shows that once again with the second channel triggered on the sawtooth.MMM wrote: I take it resonance is set to zero?
C2291 89CMMM wrote: What's the code printed on Q22?
Sure, here we go, I am triggering on channel 2 / sawtooth picked up directly at the waveform switch.MMM wrote: Can you measure (with the scope) both sides of C18?
Thanks again!
- mmm
- Posts: 517
- Joined: Tue Apr 24, 2012 1:51 am
Re: VCF not Working?
Very likely that's the problem. This particular charge (and some others) are known to be fakes with a different internal layout (essentially those are re-labeled 1583 or something similar).pulsar256 wrote:C2291 89CMMM wrote: What's the code printed on Q22?
There may be legit "89C" parts but since this has been a problem in the past my guess is this could be a fake, too.
Please remove it and measure it with a transistor tester (if you have one).
2SC2291 is ECBCE
2SC1583 is BCECB
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
Re: VCF not Working?
I don't have one at hand, but after desoldering I am measuring a voltage drop of 0.69V on pin1 - pin2 (+) and pin4 - pin5 (+) in diode mode. I guess, we have the smoking gun then? For further reference, here a closeup of that transistor:MMM wrote:
Please remove it and measure it with a transistor tester (if you have one).
2SC2291 is ECBCE
2SC1583 is BCECB
- mmm
- Posts: 517
- Joined: Tue Apr 24, 2012 1:51 am
Re: VCF not Working?
Well, this 2291 looks a bit like it has been tampered with but this may have been the result of soldering and desoldering. But the traces could also be the results of someone removing the old markings.
There have been several instances (also on the forums) of the "89C" charge being fake so my guess is this could be the reason.
If you have two spare 536 you could use them as a makeshift dual transistor to see if the filter puts out something then (the results will not be the same, of course).
There have been several instances (also on the forums) of the "89C" charge being fake so my guess is this could be the reason.
If you have two spare 536 you could use them as a makeshift dual transistor to see if the filter puts out something then (the results will not be the same, of course).
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
Re: VCF not Working?
I have a couple of 547's and 337's. Could these work as well? Sorry for being such a noob.MMM wrote:If you have two spare 536 you could use them as a makeshift dual transistor to see if the filter puts out something then (the results will not be the same, of course).
- mmm
- Posts: 517
- Joined: Tue Apr 24, 2012 1:51 am
Re: VCF not Working?
It probably doesn't hurt to try. (After all, this is just to see if something happens within the filter.) Just make sure you connect the proper terminals; maybe it's useful to use some (isolated) wires instead of soldering the transistors directly to the PCB.pulsar256 wrote: I have a couple of 547's and 337's. Could these work as well?
- pulsar256
- Posts: 42
- Joined: Mon Apr 01, 2013 8:22 pm
Re: VCF not Working?
You right on the money, it was a Fake. The other 2SC1583 checked exactly the same as my 2SC2291. I added a socket to the board and hacked a small adapter together based on two BC337As. They actually sound not *that* bad, I am getting some nice resonance and can continue on with the VCA. The obligatory pictures: Thanks again! I should have seen that one coming.MMM wrote:It probably doesn't hurt to try. (After all, this is just to see if something happens within the filter.) Just make sure you connect the proper terminals; maybe it's useful to use some (isolated) wires instead of soldering the transistors directly to the PCB.
- mmm
- Posts: 517
- Joined: Tue Apr 24, 2012 1:51 am
Re: [SOLVED] VCF not Working (Fake 2SC2291)
Nice to hear you're making progress (of course, the 2291 being a fake is not so nice...). It looks like the number if fake 2291's has increased lately; there's another thread where the builder got three different fakes.
Please be positive and constructive with your questions and comments.