Leave the bias as is unless it needs to be adjusted.
This will vary by tech but I set the bias fully down (generally CCW on the pot) and rotate until I just see a slight increase in the current draw from the 12v supply. Then drive about 1v of signal into a dummy load of 4 ohms (you can check at all acceptable loads for the amp) and look for crossover/notch distortion. It's more likely to be present at the 20kHz end of the spectrum but you will need to check the entire range. If it's clean and the bias holds for all temperatures, it's good enough. If you see any notch distortion, increase the bias just enough to get a clean signal.
You don't need a 200 watt load. a 10 watt dummy load would be plenty.
This will vary by tech but I set the bias fully down (generally CCW on the pot) and rotate until I just see a slight increase in the current draw from the 12v supply. Then drive about 1v of signal into a dummy load of 4 ohms (you can check at all acceptable loads for the amp) and look for crossover/notch distortion. It's more likely to be present at the 20kHz end of the spectrum but you will need to check the entire range. If it's clean and the bias holds for all temperatures, it's good enough. If you see any notch distortion, increase the bias just enough to get a clean signal.
You don't need a 200 watt load. a 10 watt dummy load would be plenty.
I have it back on the bench. When powering it on with no input, the current starts at 1A then jumps to about 6A for a second, then settles at 2A (using an analog ammeter, so I don't think it is super accurate).
Running for 30 min with no input, the current remained constant, and it stayed cool.
Also checked the DC voltage at the output at various points. One channel seemed to have .5v when I started, but after 30 min was (essentially) 0. .016 and .002 after 30 min. Interestingly, the .002 channel was the one that started at .5v
Photo of the board below. Note, VR101, VR201 and VR2. I am still wondering if they need to be adjusted based on all of the parts I changed out 2 yrs ago (at the start of this thread). I read your paragraph above, and not clear/confident on the steps. I am wondering if there is an service manual explination on how to set those 3 pots?
Thanks!
Running for 30 min with no input, the current remained constant, and it stayed cool.
Also checked the DC voltage at the output at various points. One channel seemed to have .5v when I started, but after 30 min was (essentially) 0. .016 and .002 after 30 min. Interestingly, the .002 channel was the one that started at .5v
Photo of the board below. Note, VR101, VR201 and VR2. I am still wondering if they need to be adjusted based on all of the parts I changed out 2 yrs ago (at the start of this thread). I read your paragraph above, and not clear/confident on the steps. I am wondering if there is an service manual explination on how to set those 3 pots?
Thanks!
What were you confused about?
VR2 appears to be the rail voltage target voltage. What's the rail voltage measured from the secondary center-tap to the positive rectifier output?
Did you measure the offset directly across the two speaker terminals/wires for the channel?
VR2 appears to be the rail voltage target voltage. What's the rail voltage measured from the secondary center-tap to the positive rectifier output?
Did you measure the offset directly across the two speaker terminals/wires for the channel?
Confusion I guess is more about looking for the crossover distortion- Maybe it is my OCD, but I would think there would be a less subjective and more objective way to do this. 🙂
+19.44V and -18.47V at the diodes. (that one volt difference is concerning?)
VR2 says "Set 4.7V" and the test point next to is measures 4.78V
Yes, the DC measurements I spoke of were at the speaker outputs.
Thanks for your continued help!
+19.44V and -18.47V at the diodes. (that one volt difference is concerning?)
VR2 says "Set 4.7V" and the test point next to is measures 4.78V
Yes, the DC measurements I spoke of were at the speaker outputs.
Thanks for your continued help!
4.7 or 47v?
Did you measure rail voltage using the secondary center-tap as the reference?
What's the voltage rating on the rail caps?
1v difference in rails isn't a big concern. The reference you used makes a difference.
Which diodes did you measure the voltage on?
Did you measure rail voltage using the secondary center-tap as the reference?
What's the voltage rating on the rail caps?
1v difference in rails isn't a big concern. The reference you used makes a difference.
Which diodes did you measure the voltage on?
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I measured the 19V and -18V at the large diodes near the rail capacitors (rated at 50V) to ground.
If I measure DC volts at the secondary of the transformer using the center as reference I get -6.28 and -3.82
It does say 47V. Not sure why I thought it said 4.7 Either way, the "DC test" (see photo) point next to it measures 4.78V Maybe a different test?
Adding some more photos for clarity.
If I measure DC volts at the secondary of the transformer using the center as reference I get -6.28 and -3.82
It does say 47V. Not sure why I thought it said 4.7 Either way, the "DC test" (see photo) point next to it measures 4.78V Maybe a different test?
Adding some more photos for clarity.
The DC test is to test the protection circuit.
Measure the rail at the center leg of the large D designated rectifiers clamped to the heatsink. Which one has the highest voltage. Place the black meter probe on the center-tap of the secondary on the transformer.
Measure the rail at the center leg of the large D designated rectifiers clamped to the heatsink. Which one has the highest voltage. Place the black meter probe on the center-tap of the secondary on the transformer.
18.86 and -18.88 after letting it stabilize. When I first turned the amp on to test, the difference was larger and fluctuating.
If you want a definitive answer, I think you'll have to contact Zed.
60v is more than I'd expect to see on 50v caps. Was this on the large rectifiers or on a smaller rectifier (TO-220)? I don't know if this amp uses driver supplies but this could be for drivers.
40v is just about enough (maybe a bit low) to produce rated power.
60v is more than I'd expect to see on 50v caps. Was this on the large rectifiers or on a smaller rectifier (TO-220)? I don't know if this amp uses driver supplies but this could be for drivers.
40v is just about enough (maybe a bit low) to produce rated power.
Are you measuring from a + 40v point to a -40v point?
Where are you making this measurement?
Don't make any adjustments yet. Zed may be able to give specifics.
Typically, the pot is used to set the highest rail voltage (this amp may have 3 rails like the attached photo). 47v seems a bit high for the rails for an amp rated for 100w into 4 ohms.
Look at the center leg of the output transistors and drive the amp to clipping (no load required). The rail voltage will likely step up as required when the amp goes to clipping. What's the highest rail voltage?
Ground the scope to the secondary CT.
Where are you making this measurement?
Don't make any adjustments yet. Zed may be able to give specifics.
Typically, the pot is used to set the highest rail voltage (this amp may have 3 rails like the attached photo). 47v seems a bit high for the rails for an amp rated for 100w into 4 ohms.
Look at the center leg of the output transistors and drive the amp to clipping (no load required). The rail voltage will likely step up as required when the amp goes to clipping. What's the highest rail voltage?
Ground the scope to the secondary CT.
Attachments
The center pin on all of the output transistors are common on each side of the board, so I don't think this is a 3 rail system.
Boltar VIII is rated at 200 W/ch in to 4 ohms and 350W/ch in to 2 ohms. If that matters??
"Set 47V" written directly on the board.. I guess there was not enough space to provide specific instructions.. LOL!
I sent Steve a PM, hopefully he responds sometime soon as I'd like to put the car back together.
Boltar VIII is rated at 200 W/ch in to 4 ohms and 350W/ch in to 2 ohms. If that matters??
"Set 47V" written directly on the board.. I guess there was not enough space to provide specific instructions.. LOL!
I sent Steve a PM, hopefully he responds sometime soon as I'd like to put the car back together.
It appears to have a rail voltage of about 50v (assuming that the scope is generally in calibration and the trace was aligned with the reference before the image was taken). You don't show where the signal clips so I can't know how much loss there is in the rail switching circuit and drive circuits.
Unless otherwise told, I'd leave the rail set point as is.
Unless otherwise told, I'd leave the rail set point as is.