I took measurements with one channel unloaded and the other at 85% bias as that is where I left it when I started troubleshooting.
Unloaded channel is V+ at 40.4v and V- is -38.8v. The biased channel is 34.2 V+ and -34.8 V-. AC is 11mV biased channel and 5mV unloaded channel.
Measuring DC after the BA-3FE output caps:
-2.8v on the biased channel (between the FE output cap and the output stage) and +4 to +6 volts with the output stage unpowered (but still attached to the FE output cap).
At this point, I have no DC on the speaker outs as I zeroed that out and both channels still make sound in addition to the tapping/pulsing noise.
Could something be wrong with the output stage and it's sending the excess DC back towards the FE board?
Btw, Mighty has a mono block chassis project going on with Modushop. Could not find the thread right now, but I’d check it out
I think not. And I also think it is a bit strange that you are able to zero out several volts of offset, I would not think the pots on the OS have range for that. Something seems very fishy.
You asked «pics of what?». The answer is pics of the amplifier, several angles, overview an closeups, so that we are enabled to help you.
If measured at the correct test points, your post-cap offset on the front end indicates something severely wrong with both your front end circuit, but also that the output caps are not working.
I recommend you post pics, then come back with front end measurements:
1: Voltage drop across R11 and R12 on both channels. Adjust to equalize @ 800mV drop or so.
2: FE DC offset before the cap. Mount probes at input side of output cap, the other @ audio gnd, either @ FE board or @ PSU 0v / gnd takeoff.
Leave post-cap measurements alone for now, and let’s see what you get.
Once this is done, we can look at post-cap measurements and the output stage. Many here will be able to help you along, starting with good pics
Regards,
Andy
Assuming you are measuring the voltage across R10 and R11 in the ~800+mV pics for both left and right channels
AND
that you're measuring the offset in the correct place... and getting 29 and 16mV for R and L
THEN
That looks good to me. No crazy voltages like before. Did you do anything differently?
I can't see if your FE board has ground connections. Does it? I can pretty clearly seen V+ and V- along with the Input signal wiring and the signal to the output stage boards, but I can't see the GND. Not a great angle in any of the pics. Have you posted a wiring diagram previously?
AND
that you're measuring the offset in the correct place... and getting 29 and 16mV for R and L
THEN
That looks good to me. No crazy voltages like before. Did you do anything differently?
I can't see if your FE board has ground connections. Does it? I can pretty clearly seen V+ and V- along with the Input signal wiring and the signal to the output stage boards, but I can't see the GND. Not a great angle in any of the pics. Have you posted a wiring diagram previously?
Pics are enlightening. Looks like you are handy with the soldering iron.
In conclusion: measurements indicate FE is OK. Do however check the following:
1: Those wires going through the riser panel holes: make sure they are fully intact. Plan to reroute or protect them better, for the long term. Those holes are quite sharp.
2: I think you allready did this, but if not make sure the front end MOSFET heatsinks are not touching anything, especially the nuts under them.
3: Let’s move on to the output stage. Starting with closeups of the boards. We need closeups of:
When all this is confirmed, I suggest moving along to powering up with bias levels completely turned down.
Regards,
Andy
In conclusion: measurements indicate FE is OK. Do however check the following:
1: Those wires going through the riser panel holes: make sure they are fully intact. Plan to reroute or protect them better, for the long term. Those holes are quite sharp.
2: I think you allready did this, but if not make sure the front end MOSFET heatsinks are not touching anything, especially the nuts under them.
3: Let’s move on to the output stage. Starting with closeups of the boards. We need closeups of:
- bias boards
- both output stages.
- wiring to and from
- Also, please remove the big washers on your transistors, and confirm all the P channel and N channels are in the right place.
- also, confirm with DMM that there is no direct conductivity between transistor and heatsink due to metal fragments or crooked mounting. Put one probe at center pin (middle of the three solder joints on output stage), the other probe at the sink (use another CLEAN hole at the sink, so that the anodization doesn’t hinder conductivity. You want to se «OL» (over limit), or very high resistance.
- provide closeups of your zener diodes, and try to confirm correct orientation and good solder joints.
When all this is confirmed, I suggest moving along to powering up with bias levels completely turned down.
Regards,
Andy
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Well some of the measurements are funky still.....I am still getting voltages too high from power supply, which is a different issue. Not sure if that damaged anything...I am getting 36-40v on V+ and supposedly the JFETS don't like that long term...though I have only ran this amp for 5-10min at the most any given time. The DC at the input side of the cap does fluctuate but my meter cannot register it fast enough to match the tapping noise. I wish I still worked at the local college so I could take it there to the electronics classrooms and get help using an oscilloscope on it.
Just thinking outside of the box here....could I take my currently unused Korg Nutube B1 kit and wire the output directly to the BA3 output boards to isolate the FE from the OPS to narrow down the source of the noise? I also have Dynaco PAS2 I could use as a FE for testing purposes.
I will get to work on the other steps suggested over the next few days and report back. Thanks Andy!
Just thinking outside of the box here....could I take my currently unused Korg Nutube B1 kit and wire the output directly to the BA3 output boards to isolate the FE from the OPS to narrow down the source of the noise? I also have Dynaco PAS2 I could use as a FE for testing purposes.
I will get to work on the other steps suggested over the next few days and report back. Thanks Andy!
That sounds very relevant.
Don’t have time to check the B1K schem right now, but isolating is generally a very good idea. I’d go with that, and wait for 6L6 or the likes on how to connect it (gnd and so on).
But, seeing as you have a PSU issue, I’d basically recommend you sort that first. The other steps I mentioned are done without PSU connected, and are mostly just control checks that are good to do anyways.
Are you saying the V+ rail fluctuates 4 volts? If so, that is rather alarming. I recommend you disconnect both FE and OS and troubleshoot the PSU first. The rest of your work looks very good, perhaps it all lies in the PSU.
To check the JFETs, just probe the two outer legs of them (drain-source) at DMM set to DC volts, and compare the measurements. That’s for N channels, hope it is the same for P?
Fluctuation of offset at the FE is normal. But, not with extreme speeds. They should move up or down, mostly, then stabilize. Temp dependant.
Regards,
Andy
You’ll prolly wanna connect it to PSU 0V/audio gnd, and I guess that is what you suggest. Really no other place to put it.
But that puts it @ high current return from speaker. So try it and post back results for us all to learn from, is my suggestion.
Spec is 1R x 4, but most builders seem to run lower than that, for example 0R47. Depends on how much ripple you have whether you need it all or not.
In fact, I would wait for, or PM, 6L6 or ZM regarding wiring scheme with FE outside the box, before deciding.
Wrt PSU R I personally ended up with 1R x 4, but ran insane Iq, so in 4U400 maybe you are good with 4x0R47.
Regards,
Andy