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.
Resonance very low: shorts on Q16 and Q17
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- mmm
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Re: Resonance very low: shorts on Q16 and Q17
Those are all fine; there's a trace beneath every one of those "shorts".
- 3SG
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- mmm
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Re: Resonance very low: shorts on Q16 and Q17
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?
Is the filter itself working fine otherwise with just the resonance being problematic?
- 3SG
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Re: Resonance very low: shorts on Q16 and Q17
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.
- mmm
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Re: Resonance very low: shorts on Q16 and Q17
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?
Is the resonance just weak or is it "strange" sounding, like warbling?
- 3SG
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Re: Resonance very low: shorts on Q16 and Q17
Resonance is weak. I'll inspect the board again.
- admin2
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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
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?
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?
- mmm
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Re: Resonance very low: shorts on Q16 and Q17
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.
- 3SG
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Re: Resonance very low: shorts on Q16 and Q17
I see. Would you recommend replacing them with a 2SC2291?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.
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?
- mmm
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Re: Resonance very low: shorts on Q16 and Q17
What type of transistor is used in the place of Q22 now?
- 3SG
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Re: Resonance very low: shorts on Q16 and Q17
There's smd transistors with adapter pcb's on Q12, Q21 and Q26. Q22 is a 2SC1845 pair.
- mmm
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Please be positive and constructive with your questions and comments.