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Post by burgerbassist on Feb 21, 2018 18:42:40 GMT
Greetings,
I recently built a G1217 Project Sunrise. I had a few issues at first (due to some bad solder joints) that caused bias voltage on both channels to swing wildly all over the place from ~7v to ~16v. Turning the bias pots didn't do anything to help the issue. I re-flowed the joints and everything was working fine (bias voltages normal and way easier to dial in), but now the right channel bias shoots up to ~20v and I can't dial it back down with the right channel bias pot. Does anyone have an idea of where to start poking around on the board to address this?
Thanks, Dan
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solderdude
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Post by solderdude on Feb 21, 2018 19:10:32 GMT
Hi Dan,
It is possible the tube is not making proper contact in the tube socket. Before looking at solderjoints or broken parts I would remove the tube. Have a close look at the tube socket and maybe bend the individual contacts a bit more inwards.
If that doesn't help we will zero in on the issue and see if anything needs fixing or replacement.
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Post by burgerbassist on Feb 26, 2018 15:30:09 GMT
I went ahead and did what you suggested with the socket pins. I didn't really bend them at all, but gave them all a solid push inward. It did seem to help, but only for about 5 minutes, so I'm not sure if it was what I did that helped the issue, or if it's something to do with the amp warming up that causes the problem to come on. It didn't seem like anything was out of whack with the socket to begin with, though - all of the tube pins seem like they should be making contact.
Anything else you'd suggest I look at to narrow things down? I also wanted to mention that I had headphones on when the bias started to drift and I adjusted it a little before I remembered to take them off and unplug them, but adjusting the bias produces a very scratchy sound. I'm not sure if that's normal or not.
Thanks!
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solderdude
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Post by solderdude on Feb 26, 2018 20:24:41 GMT
It can be the tube itself (pins being corroded) but of course something may be wrong somewhere.
The scrathing sound when adjusting the bias is normal when the amp is on an a headphone is connected.
Time to measure some voltages.
As 1 channel works fine a few parts can be considered O.K. already.
In the faulty channel the BC560 (Q2) could be suspect. Measure the DC voltages between B and E of both Q2 to compare (don't short the pins)
Measure the voltage across R15 (should be almost immeasurable small)
measure the voltages over R16 (both channels) these should be around 1.25V
Measure the DC voltage between R15 (doesn't matter which pin) and + of C3 (output cap) The voltage should be between 2V and 3V.
measure the voltage from R15 to ground. Measure the voltages from +C3 to ground.
See if the voltages vary when adjusting the bias pot P3.
Measure the voltage across R12 (should be a low voltage)
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Post by burgerbassist on Feb 26, 2018 20:40:46 GMT
Just to note, this is happening with multiple tubes, so I don't think it is an issue with a specific tube.
I'll take the measurements in a few hours and post back here with the results.
Thanks!
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Post by burgerbassist on Feb 27, 2018 2:19:39 GMT
Q2 - both Q2's (near the tube socket) measure ~+0.55v R15 - right channel is bouncing around between ~+24mv to ~+39mv, left channel is hovering around -5mv R16 - both channels right around -1.25v R15 and +C3 - -2.18v both sides R15 to ground - right channel is bouncing between +10.25v and +10.5v, left channel is steady at +5.5v +C3 to ground - right channel bouncing between +9.15v and +10.15v, left channel is steady at +3.5v R12 - right channel bouncing between -.365v and -.385v, left channel is steady at -.149v
R12 is steady when adjusting the pot on the right side, R15 and R16 move with pot adjustment, as does +C3 and ground. R15 and ground seemed to move with pot adjustment, but then kept jumping around after I stopped adjusting the pot, so I'm not sure it wasn't just jumping around in the first place. R15 to +C3 moved with pot adjustment, as well.
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solderdude
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Post by solderdude on Feb 27, 2018 6:16:07 GMT
It looks like there might be leaking a current.
How high and constant is the voltage across led D4 ?
How high can you turn up the bias voltage ?
In the channel that always has the lowest voltage can you lift 1 pin of R15 and see how the bias voltage can be adjusted now ? Measure between the bias test point (anode of the tube) and ground.
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Post by burgerbassist on Feb 28, 2018 1:04:43 GMT
D4 will sit constant at -1.8v
The bias voltage can be turned up to +23.5v
With 1 pin of R15 lifted, bias voltage adjusts as normal.
Interestingly, when I turned the unit on this evening everything was running perfectly. I then adjusted bias to where I needed it (+12v on each side, then fine tuned using the LEDs), and it ran correctly for about 10-15 min, at which point the LEFT channel voltage started bouncing around, exhibiting the symptoms the right channel had yesterday.
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solderdude
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Post by solderdude on Feb 28, 2018 5:45:47 GMT
If I had to make a wild guess either the MOSFET's have some latent damage or solderflux is causing a leakage current somehow.
Clean the PCB (the solder residue) from the pins of the MOSFETs (IRL510).
If that does not help:
Perhaps remove the MOSFET's and solder in 2 new ones, observing ESD countermeasures. So not touching the pins and unpacking them only just before soldering them in. Also ensure the parts and PCB are not on a plastic surface and the soldering iron is grounded.
Could be caused by 'latent' damage due to ESD discharge (especially with freezing temperatures outside) when mounting those parts or current leakage from not removed solderflux.
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