[SOLVED] VCF not Working (Fake 2SC2291)

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pulsar256
 
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[SOLVED] VCF not Working (Fake 2SC2291)

Post by pulsar256 »

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:
Output on Pin 1 / VR4
Output on Pin 1 / VR4
VOLTCRAFT19_5.gif (17.84 KiB) Viewed 2517 times
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:
VCO DC Offset
VCO DC Offset
VOLTCRAFT19_3.gif (15.25 KiB) Viewed 2517 times
I can trace the signal down to R62, after that resistor I am picking up this:
Waveform after R62
Waveform after R62
VOLTCRAFT19_4.gif (19.33 KiB) Viewed 2517 times

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?
Last edited by pulsar256 on Tue Apr 02, 2013 4:59 pm, edited 1 time in total.

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pulsar256
 
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Re: VCF not Working?

Post by pulsar256 »

And some Closeups of the PCB and transistors:
Populated VCO and VCF
Populated VCO and VCF
2013-04-02 01.55.23.jpg (696.35 KiB) Viewed 2515 times
VCF / Transistors
VCF / Transistors
2013-04-02 03.00.13.jpg (636.95 KiB) Viewed 2515 times
2013-04-02 03.01.07.jpg
2013-04-02 03.01.07.jpg (388.27 KiB) Viewed 2515 times

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mmm
 
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Re: VCF not Working?

Post by mmm »

I must admit this looks weird.
Which is at the same frequency and somewhat resembles the original sawtooth, but is only 100mV pp.
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.
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?

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pulsar256
 
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Re: VCF not Working?

Post by pulsar256 »

Thanks for looking at my issue!
MMM wrote: I take it resonance is set to zero?
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: What's the code printed on Q22?
C2291 89C
MMM wrote: Can you measure (with the scope) both sides of C18?
Sure, here we go, I am triggering on channel 2 / sawtooth picked up directly at the waveform switch.
C18 Left
C18 Left
VOLTCRAFT20_3.gif (20.56 KiB) Viewed 2461 times
C18 Right
C18 Right
VOLTCRAFT20_4.gif (20.36 KiB) Viewed 2461 times
Unpopulated C21 Right
Unpopulated C21 Right
VOLTCRAFT20_5.gif (20.03 KiB) Viewed 2461 times
Thanks again!

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mmm
 
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Re: VCF not Working?

Post by mmm »

pulsar256 wrote:
MMM wrote: What's the code printed on Q22?
C2291 89C
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).
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

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pulsar256
 
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Re: VCF not Working?

Post by pulsar256 »

MMM wrote:
Please remove it and measure it with a transistor tester (if you have one).

2SC2291 is ECBCE
2SC1583 is BCECB
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:
C2291 89C - Fake?
C2291 89C - Fake?
2013-04-02 18.46.01.jpg (195.96 KiB) Viewed 2455 times

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mmm
 
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Re: VCF not Working?

Post by mmm »

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).

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pulsar256
 
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Re: VCF not Working?

Post by pulsar256 »

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).
I have a couple of 547's and 337's. Could these work as well? Sorry for being such a noob.

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mmm
 
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Re: VCF not Working?

Post by mmm »

pulsar256 wrote: I have a couple of 547's and 337's. Could these work as well?
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.

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pulsar256
 
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Re: VCF not Working?

Post by pulsar256 »

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.
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:
Provisory 2SC2291 Replacement: 2xBC337A
Provisory 2SC2291 Replacement: 2xBC337A
2013-04-02 22.44.29.jpg (243.2 KiB) Viewed 2412 times
Provisory 2SC2291 Replacement: 2xBC337A Waveforms
Provisory 2SC2291 Replacement: 2xBC337A Waveforms
VOLTCRAFT21_2.gif (21.61 KiB) Viewed 2412 times
Thanks again! I should have seen that one coming.

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mmm
 
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Re: [SOLVED] VCF not Working (Fake 2SC2291)

Post by mmm »

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.

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