Resonance very low: shorts on Q16 and Q17

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3SG
 
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Resonance very low: shorts on Q16 and Q17

Post by 3SG »

Hi,

I'm troubleshooting a x0xb0x (pcb 1.02) for a friend. It works fine but has very little resonance. I've adjusted the trimmer which helped a bit, but the resonance is still very low.

After a first cleaning of the PCB - there's flux leftovers everywhere ... - I've noticed shorts on Q16 en Q17.
Pics of Q39 (short on Q39 seems to be required) and the region around Q16, Q17.

https://imgur.com/a/wvtmL2l

I believe these shouldn't be shorted, right?

Thanks for your help.

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mmm
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

Those are all fine; there's a trace beneath every one of those "shorts".

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3SG
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

mmm wrote: Mon Sep 12, 2022 12:47 pm Those are all fine; there's a trace beneath every one of those "shorts".
Ok thanks. Got another answer stating: Q13,14,15,16,17,23,39 should all have 2 of their pins shorted. I'll have to recheck, but that's dfntly not the case.

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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

Q13 to Q17 and Q23 are forming the filter ladder and will each have a direct connection between base and collector. Q39 is not in the filter circuit.
Is the filter itself working fine otherwise with just the resonance being problematic?

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3SG
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

mmm wrote: Mon Sep 12, 2022 1:27 pm Q13 to Q17 and Q23 are forming the filter ladder and will each have a direct connection between base and collector. Q39 is not in the filter circuit.
Is the filter itself working fine otherwise with just the resonance being problematic?
Filter is working fine. Resonance is problematic... which is the whole point of a typical 303-bassline.
I'd expect lows to drop when reso cranked like on a typical ladder filter, but that's not happening.

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mmm
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

I'd check the area and components around Q18. Resonance is essentially a feedback loop; this signal passes through VR4 and then the whole circuit around Q18.
Is the resonance just weak or is it "strange" sounding, like warbling?

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3SG
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

mmm wrote: Mon Sep 12, 2022 1:54 pm I'd check the area and components around Q18. Resonance is essentially a feedback loop; this signal passes through VR4 and then the whole circuit around Q18.
Is the resonance just weak or is it "strange" sounding, like warbling?
Resonance is weak. I'll inspect the board again.

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Re: Resonance very low: shorts on Q16 and Q17

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3SG
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

After further investigating the board, I've found this on Q22. Following the build guide, this should be a 2SC2291.

https://imgur.com/a/h2M6Uni

Looking at the schematic, this is at the end of the ladder filter (if I'm reading the schematic correct).

Could this be the problem with the low resonance?

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mmm
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

When 2SC2291's are unavailable people have substituted two single transistors instead. If those two transistors were selected for identical values it should work. If they are slightly mismatched it would probably also affect the filter sound in general, not just Resonance.

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3SG
 
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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

mmm wrote: Thu Sep 15, 2022 7:48 am When 2SC2291's are unavailable people have substituted two single transistors instead. If those two transistors were selected for identical values it should work. If they are slightly mismatched it would probably also affect the filter sound in general, not just Resonance.
I see. Would you recommend replacing them with a 2SC2291?

Looking at the pair of transistors, I believe the emitters are connected. In another thread - viewtopic.php?p=218098#p218098 - there's a picture with base connected. What's the correct way to pair transistors?

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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

What type of transistor is used in the place of Q22 now?

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Re: Resonance very low: shorts on Q16 and Q17

Post by 3SG »

There's smd transistors with adapter pcb's on Q12, Q21 and Q26. Q22 is a 2SC1845 pair.

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Re: Resonance very low: shorts on Q16 and Q17

Post by mmm »

3SG wrote: Fri Sep 16, 2022 1:59 am Q22 is a 2SC1845 pair.
As far as I can see this has indeed the bases connected. So this is at least in principle fine.

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