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- Halfbiass...Electron Herder and Backass Woof
I'm Buying a "Mule" PL400
- Thread starter mlucitt
- Start date
Sorry, I laced it all up thinking it would work perfectly, lesson learned. I followed the wiring chart and checked it against the overall PL400 schematic just to be sure.
Yes, I measure -15.27 and +15.10 at the respective test points. Also, with all the fuses in and all wires connected to the board, but the four driver transistors pulled out (the bottom row), the Dim Bulb stays dim. Could it be that MJ21196G transistors just won't work as drivers?
Yes, I measure -15.27 and +15.10 at the respective test points. Also, with all the fuses in and all wires connected to the board, but the four driver transistors pulled out (the bottom row), the Dim Bulb stays dim. Could it be that MJ21196G transistors just won't work as drivers?
The dim bulb is large and it measured 9 Ohms which seems low, but it does glow like a 150W bulb when I plug it into 120VAC.
I followed the guide and made sure the 2N5088 Bias Transistors were wired correctly. It is odd that I need to lift both 3L and 9R from the Control Board to keep the bulb dim. That would seem to indicate a bias problem in both Left and Right circuits. Unlikely that both new 2N5088 transistors are bad. I can test them easily enough.
The only difference between my amp and the guide is that I wired Star Ground to 5R instead of 5L because the wire was long enough. I can shorten it but I don't think it would make any difference with the jumper between 5L1 and 5R1.
I followed the guide and made sure the 2N5088 Bias Transistors were wired correctly. It is odd that I need to lift both 3L and 9R from the Control Board to keep the bulb dim. That would seem to indicate a bias problem in both Left and Right circuits. Unlikely that both new 2N5088 transistors are bad. I can test them easily enough.
The only difference between my amp and the guide is that I wired Star Ground to 5R instead of 5L because the wire was long enough. I can shorten it but I don't think it would make any difference with the jumper between 5L1 and 5R1.
BTW, the DC resistance on the speaker outputs (before the DCP board) is about 9.5K Ohms and immediately climbing on both channels. I'm not sure if that is normal but is certainly is not a short circuit.
I will try the DBT again with first Q11 and then Q12 in on the left channel only, then I will pull them and place them, one at a time again, in the right channel. I tested all four of the drivers and they are good. My assumption is that installing Q11 in the right channel will cause the DBT to fail and installing Q12 in the left channel will cause the DBT to fail. I'll see if I can prove my thesis.
Testing my knowledge here - when the dim bulb goes bright it indicates strong current flow into the power supply. The power supply is making the rail voltages, so current demand on the rails is high. Current flow in the rails is high because there is a low resistance where there should be a high/higher resistance such as a ground fault or a transistor that is conducting but should not be (with no input).
When I pull Q11 and Q12 I put a break in the high current flow and the bulb stays dim. I am going to assume that if I re-install Q11 and Q12 in both channels and pull the -80V rail fuse, and the bulb stays dim, that indicates the Control Board is working and the problem is on the chassis, past the -80V rail fuse.
I will try the DBT again with first Q11 and then Q12 in on the left channel only, then I will pull them and place them, one at a time again, in the right channel. I tested all four of the drivers and they are good. My assumption is that installing Q11 in the right channel will cause the DBT to fail and installing Q12 in the left channel will cause the DBT to fail. I'll see if I can prove my thesis.
Testing my knowledge here - when the dim bulb goes bright it indicates strong current flow into the power supply. The power supply is making the rail voltages, so current demand on the rails is high. Current flow in the rails is high because there is a low resistance where there should be a high/higher resistance such as a ground fault or a transistor that is conducting but should not be (with no input).
When I pull Q11 and Q12 I put a break in the high current flow and the bulb stays dim. I am going to assume that if I re-install Q11 and Q12 in both channels and pull the -80V rail fuse, and the bulb stays dim, that indicates the Control Board is working and the problem is on the chassis, past the -80V rail fuse.
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Latest round of testing complete and here are the results:
With both rail fuses in, all wires connected to the Control Board, and just driver Q11L installed - DBT is good.
With both rail fuses in, all wires connected to the Control Board, and just driver Q12L installed - DBT is bad.
With both rail fuses in, all wires connected to the Control Board, and just driver Q11R installed - DBT is good.
With both rail fuses in, all wires connected to the Control Board, and just driver Q12R installed - DBT is bad.
Additionally, I checked the voltages on Q6R and Q6L from C-E and measured 2.06V and 2.07V, respectfully.
I rang out each wire disconnected from the Control Board for 0 Ohms between the end of the wire and its logical connection based on the PL400 Narrative Wiring Instructions - all good.
And I checked R38L/R38R and R39L/R39R and they measure 9.9-10 Ohms; R36L/R36R and R37L/R37R all measure 150 Ohms.
When I installed all the driver transistors, of course the DBT is bad, but when I pull the -80V rail fuse, the DBT is good. This tells me the Control Board is working fine, because it gets its driver power from the fused +80V rail on pins 7R and 7L.
So, the problem must be on the chassis, and associated with the -80V rail power.
First cold check was D14L/D14R and D15L/D15R (1N4004) and they are all good (new).
Second cold check was for shorted speaker outputs before the DCP Board and I measured 1.06 MOhms to ground on both channels.
Third cold check was to ensure no shorts to ground on any driver or output transistor connection point - all good.
The VU meters are not connected yet so those open ended speaker output wires are not a concern.
My next test is to pull Q14L/Q14R, Q16L/Q16R, and Q18L/Q18R one at a time, and see if I can get the DBT to stay dim with the -80V fuse installed.
Thanks for everyone's help and suggestions, very much appreciated.
With both rail fuses in, all wires connected to the Control Board, and just driver Q11L installed - DBT is good.
With both rail fuses in, all wires connected to the Control Board, and just driver Q12L installed - DBT is bad.
With both rail fuses in, all wires connected to the Control Board, and just driver Q11R installed - DBT is good.
With both rail fuses in, all wires connected to the Control Board, and just driver Q12R installed - DBT is bad.
Additionally, I checked the voltages on Q6R and Q6L from C-E and measured 2.06V and 2.07V, respectfully.
I rang out each wire disconnected from the Control Board for 0 Ohms between the end of the wire and its logical connection based on the PL400 Narrative Wiring Instructions - all good.
And I checked R38L/R38R and R39L/R39R and they measure 9.9-10 Ohms; R36L/R36R and R37L/R37R all measure 150 Ohms.
When I installed all the driver transistors, of course the DBT is bad, but when I pull the -80V rail fuse, the DBT is good. This tells me the Control Board is working fine, because it gets its driver power from the fused +80V rail on pins 7R and 7L.
So, the problem must be on the chassis, and associated with the -80V rail power.
First cold check was D14L/D14R and D15L/D15R (1N4004) and they are all good (new).
Second cold check was for shorted speaker outputs before the DCP Board and I measured 1.06 MOhms to ground on both channels.
Third cold check was to ensure no shorts to ground on any driver or output transistor connection point - all good.
The VU meters are not connected yet so those open ended speaker output wires are not a concern.
My next test is to pull Q14L/Q14R, Q16L/Q16R, and Q18L/Q18R one at a time, and see if I can get the DBT to stay dim with the -80V fuse installed.
Thanks for everyone's help and suggestions, very much appreciated.
To validate the test equipment, I can put an amp-clamp on the power cord to read the current as the DBT becomes brighter, but I'm not sure what level of current I should be looking for. I measured 20 mA with the Variac at 60VAC.
The bias pots were set to mid-scale, I have reset them to minimum (fully CCW).
Voltages across the 10 Ohm resistors with DBT bright (60V indicated on Variac):
R38L - 512mV
R39L - 305mV
R38R - 513mV
R39R - 306mV
Voltages across the 150 Ohm resistors with DBT bright (60V indicated on Variac):
R36L - 512mV
R37L - 389mV
R36R - 570mV
R37R - 393mV
Results of pulling minus side output transistors:
With Q14L/R out, DBT is Bad
With Q16L/R out, DBT is Bad
With Q18L/R out, DBT is Bad, so no effect on the problem.
A new wrinkle, if I disconnect 3L and 3R from the Control Board, the amp comes up just fine on 120VAC with 0mV on the speaker outputs.
The bias pots were set to mid-scale, I have reset them to minimum (fully CCW).
Voltages across the 10 Ohm resistors with DBT bright (60V indicated on Variac):
R38L - 512mV
R39L - 305mV
R38R - 513mV
R39R - 306mV
Voltages across the 150 Ohm resistors with DBT bright (60V indicated on Variac):
R36L - 512mV
R37L - 389mV
R36R - 570mV
R37R - 393mV
Results of pulling minus side output transistors:
With Q14L/R out, DBT is Bad
With Q16L/R out, DBT is Bad
With Q18L/R out, DBT is Bad, so no effect on the problem.
A new wrinkle, if I disconnect 3L and 3R from the Control Board, the amp comes up just fine on 120VAC with 0mV on the speaker outputs.
Joe, your PL14_PL20 Rev E4 schematic shows one voltage TP on the Base of Q3L (0.928V). Are there any other voltage TPs that I can use to troubleshoot? Specifically, the Base of Q8L and the Base of Q7L. It seems that R36 and R38 are carrying too much current.
I measured these voltages, referenced to ground, with all wires connected and all fuses in. The DBT was glowing with 60V on the Variac:
12L 11L 10L 9L 8L 7L 6L 4L 3L 10R 9R 8R 7R 6R 5R 4R 3R
-9 9 .5 1 .03 9 .009 -.02 -8.2 .52 1 .03 9 .009 0 -.02 -8.2
Do these levels look good? I know they may be low due to the DBT.
Also, a question because I have been studying the Rev E4 schematic intently. Is the signal on 6L a Negative Feedback signal going back into the Control Board from the Output of the amplifier?
I measured these voltages, referenced to ground, with all wires connected and all fuses in. The DBT was glowing with 60V on the Variac:
12L 11L 10L 9L 8L 7L 6L 4L 3L 10R 9R 8R 7R 6R 5R 4R 3R
-9 9 .5 1 .03 9 .009 -.02 -8.2 .52 1 .03 9 .009 0 -.02 -8.2
Do these levels look good? I know they may be low due to the DBT.
Also, a question because I have been studying the Rev E4 schematic intently. Is the signal on 6L a Negative Feedback signal going back into the Control Board from the Output of the amplifier?
Hi Mark
I reformatted your measurements since it was difficult to be sure if I had it right when they all ran together like they are. Also added plus symbols to what I assumed to be positive readings. I am assuming that you have all wires connected, please confirm. Also do you have all output devices in?
Check them to ensure that I transcribed them correctly.
I also do not know why you are getting only 9V rail voltages with a 150W light bulb in series and 60VAC in. Seems low for that wattage.
I measured these voltages, referenced to ground, with all wires connected and all fuses in. The DBT was glowing with 60V on the Variac:
12L=-9 (OK but low)
11L=+9 (OK but low)
10L=+0.5 (s/b +0.35 when bias is properly set)
9L=+1 (correct when bias is set to +0.35V)
8L=+0.03 (should be closer to zero when bias is set properly)
7L=+9 (OK but low)
6L=+0.009 (s/b close to +0.001)
4L=-0.02 (should be closer to zero when bias is set properly)
3L=-8.2 (Close to right)
10R=+0.52 (s/b +0.35 when bias is properly set)
9R=+1 (correct when bias is set to +0.35V)
8R=+0.03 (should be closer to zero when bias is set properly)
7R=+9 (OK but low)
6R=+0.009 (s/b close to +0.001)
5R=0.0 (Good)
4R=-0.02 (should be closer to zero when bias is set properly)
3R=-8.2 (Close to right)
I reformatted your measurements since it was difficult to be sure if I had it right when they all ran together like they are. Also added plus symbols to what I assumed to be positive readings. I am assuming that you have all wires connected, please confirm. Also do you have all output devices in?
Check them to ensure that I transcribed them correctly.
I also do not know why you are getting only 9V rail voltages with a 150W light bulb in series and 60VAC in. Seems low for that wattage.
I measured these voltages, referenced to ground, with all wires connected and all fuses in. The DBT was glowing with 60V on the Variac:
12L=-9 (OK but low)
11L=+9 (OK but low)
10L=+0.5 (s/b +0.35 when bias is properly set)
9L=+1 (correct when bias is set to +0.35V)
8L=+0.03 (should be closer to zero when bias is set properly)
7L=+9 (OK but low)
6L=+0.009 (s/b close to +0.001)
4L=-0.02 (should be closer to zero when bias is set properly)
3L=-8.2 (Close to right)
10R=+0.52 (s/b +0.35 when bias is properly set)
9R=+1 (correct when bias is set to +0.35V)
8R=+0.03 (should be closer to zero when bias is set properly)
7R=+9 (OK but low)
6R=+0.009 (s/b close to +0.001)
5R=0.0 (Good)
4R=-0.02 (should be closer to zero when bias is set properly)
3R=-8.2 (Close to right)
Joe, thanks for your time. Sorry about the formatting, I didn't see that all my spaces came out...
Yes, for the above measurements, all wires were connected and all devices were installed.
I should be clear, these test voltages were with the dial on the Variac pointing to "60". The actual voltage coming out of the Variac was 65VAC. The DBT is glowing quite bright at that Variac setting. I verified the voltage coming into the amplifier is 65VAC.
In reference to the low rail voltages, when I remove the rail fuses the following voltages are observed:
12L= -42.9
11L= +43.0
Closer to normal with 60VAC applied, no? Also, the DBT stays dim, naturally.
If I put the positive rail fuse back in, these voltages stay good. But when I put the negative rail fuse back in the voltages fall to:
12L= -8.8
11L= +8.8
I will try with just the drivers installed and see what happens. I tried the amp without Q14L/Q14R, Q16L/Q16R, and Q18L/Q18R and the DBT still failed. I'll pull out the positive side as well this time and report back.
Yes, for the above measurements, all wires were connected and all devices were installed.
I should be clear, these test voltages were with the dial on the Variac pointing to "60". The actual voltage coming out of the Variac was 65VAC. The DBT is glowing quite bright at that Variac setting. I verified the voltage coming into the amplifier is 65VAC.
In reference to the low rail voltages, when I remove the rail fuses the following voltages are observed:
12L= -42.9
11L= +43.0
Closer to normal with 60VAC applied, no? Also, the DBT stays dim, naturally.
If I put the positive rail fuse back in, these voltages stay good. But when I put the negative rail fuse back in the voltages fall to:
12L= -8.8
11L= +8.8
I will try with just the drivers installed and see what happens. I tried the amp without Q14L/Q14R, Q16L/Q16R, and Q18L/Q18R and the DBT still failed. I'll pull out the positive side as well this time and report back.
Joe, all outputs pulled and the drivers still installed, the DBT fails.
With all other outputs still out except the drivers, when I pull Q11L and Q11R, the DBT passes. If I put Q11L and Q11R back in and pull Q12L and Q12R, the DBT passes.
If there is both Q11 and Q12 installed on either side the DBT fails. The last test I did, I left the DBT on for a bit and R39L started to smoke (maybe it was the insulation on the wire. All the outputs on that side were out except driver Q12L. The only other transistors in were Q11L and Q11R. When I pulled Q12L, R39L went back to normal and the DBT passed.
So it looks like any pair of Q11 and Q12 on the same side (L or R) are conducting when they should not be. I can confirm this because disconnecting pin 3L and 3R on the Control Board prevents them from conducting and the DBT passes. Why would it be common to both sides?
With all other outputs still out except the drivers, when I pull Q11L and Q11R, the DBT passes. If I put Q11L and Q11R back in and pull Q12L and Q12R, the DBT passes.
If there is both Q11 and Q12 installed on either side the DBT fails. The last test I did, I left the DBT on for a bit and R39L started to smoke (maybe it was the insulation on the wire. All the outputs on that side were out except driver Q12L. The only other transistors in were Q11L and Q11R. When I pulled Q12L, R39L went back to normal and the DBT passed.
So it looks like any pair of Q11 and Q12 on the same side (L or R) are conducting when they should not be. I can confirm this because disconnecting pin 3L and 3R on the Control Board prevents them from conducting and the DBT passes. Why would it be common to both sides?
Q11 and Q12 should be conducting at the same time Mark. That is what makes bias in the amp. Q11 and Q12 work in Class A mode.
When you thought that R39 was smoking, were you measuring the voltage across it? Prior test results were showing that you had about 0.5V across it. If you still have ~0.5V across it then the wattage you are dissipating is 25 milliwatts. The 1/2W resistor you have in the R38 and R39 positions are 500mW resistors so you should never see any heat in them with only 25 mW being dissipated in them.
When you thought that R39 was smoking, were you measuring the voltage across it? Prior test results were showing that you had about 0.5V across it. If you still have ~0.5V across it then the wattage you are dissipating is 25 milliwatts. The 1/2W resistor you have in the R38 and R39 positions are 500mW resistors so you should never see any heat in them with only 25 mW being dissipated in them.