PL400 II Resurrection

The transistors on the amp are all still measuring OK. Would it be acceptable to replace the 66546 with MJ15024 while I'm trying to figure out what's going on ? Those 66546's are hard to xome buy, and If I toast them I'll be set back for months. I'll put them back once I have it figured out...
 
Yes you can use the MJ15024's temporarily during debug. Don't use them long term though in the 2nd and 4th column starting with column 1 being next to the transformer.

You can use those MJ15024's all day long in column 1 and 3.
 
OK, so if I finally get it working to I install 15024/66456/15024/66456 or do I dispense with the 15024 altogether and leave the spaces unpopulated ?

Pete
 
Hi Pete
You need something in that lower row location, you cannot leave the spaces unpopulated.

So it either needs to be:
Transformer/15024/66456/15024/66456/Binding Posts

OR

Transformer/66456/66456/66456/66456/Binding Posts

Either configuration works.

Above the bottom most row, you can use MJ15024 or MJ21196 (better)
 
OK ,after finding yet another dodgy wire I now feel I have made some progress.
I used all 15024 on the bottom so as to keep the 66456s of which I have only three left out of harm’s way. I can now once more bias left to 375 mV without issues. The right only gos up to 260, but since it also starts lower, the back end looks ok. How do I proceed ? Thanks, Pete
 
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Carefully, very carefully!

Measure Q6 from C-E on the left channel (where you can bias correctly) with the pot set at the minimum. Then move the pot to the 0.375V set point and measure Q6 C-E again. Make these measurements where the Q6 wires attach to the control board.

Left Q6 C-E (pot min) = ??
Left Q6 C-E (pot adjusted to 0.375) = ??

Rinse and repeat for Q6 on the right channel except substitute the max pot setting this time for the 0.375V setting that you did in the first case.

Right Q6 C-E (pot min) = ??
Right Q6 C-E (pot max) = ??

If you are seeing the lower voltages on these readings, you can deduce that your outputs are not the culprit and can focus on what is wrong in the circuit surrounding Q6, or Q6 itself.

If, however, the performance of your Q6 circuit is comparable from the left to right channel, then you can deduce the problem is somewhere in the output driver section.

Make sense?

Joe
 
I made following measurements :

Turning both bias pots fully counterclockwise, I measure 240 mV on the left channel, and 120 mV on the right. Setting the left bias at 375 mV, and the right pot in approximately the same physical position, I measure 168 mV, which I kind of expected. Q6 L and R are around 78V at all times.
 
TapeHeadOne said:
I made following measurements :

Turning both bias pots fully counterclockwise, I measure 240 mV on the left channel, and 120 mV on the right. Setting the left bias at 375 mV, and the right pot in approximately the same physical position, I measure 168 mV, which I kind of expected. Q6 L and R are around 78V at all times.
Click to expand...
Hi Pete
Q6 C and E should be hovering around about a volt or two around zero start by trying to figure out why you are seeing 78v
 
Gepetto said:
Hi Pete
Q6 C and E should be hovering around about a volt or two around zero start by trying to figure out why you are seeing 78v
Click to expand...

That's what I'm not seeing. 75V comes in through L7 but R35 reduces that. So it must be base voltage. Which points to Q9. But on both channels?
 
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Hi Pete
I think you are confusing Q6L and Q6R on the control board with the bias transistor that was classically named by PL as Q6 and mounted in the Pstrap on the back wall of the amp. There are three leads that emanate from there and terminate at the control board. The red wire is C, the black wire is B and the brown wire is E.

Those colors are traditional colors used but may vary if someone has been in there before you and may have replaced or substituted.

Thinking this may get you more consistent results on that Q6.
 
:scratch::banghead::cussing:

OK...

Starting with the left and right bias pots fully counterclockwise :

L Bias : 246 mV R Bias : 115 mV
L Bias 2N5088 C-E : 2V R Bias 2N5088 C-E: 1.96V

L Bias : 375 mV R Bias : 169 mV at approx the same position
L Bias 2N5088 C-E : 2.11V R Bias C-E : 2.11 V

So it's biasing. I'm missing half of the amplification somewhere.

Pete
 
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Now with both channels set at 2.11v C-E on the 2N5088 start moving outward to Q6R and Q9R measuring differences you see between the right and left channel. Recommend that you take all the outputs out of the right channel except the drivers in the bottom most row. You may already be at that configuration
 
I'm seeing a markedly higher voltage on the E of Q9R. Where Q9L is at -0.1V approx, Q9R is at -0.35V. Drivers in or out, no difference.

Pete
 
That would be a markedly lower voltage since the polarity is minus in your readings?

What is the output voltage on each channel? Zero V?
 
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