I'm Buying a "Mule" PL400

Gepetto · Mar 31, 2020

Does it go up and down on each channel with adjustment?

And when you measure the screw on Q6 you have +80V and the screw on Q9 you have -80V? It feels like your bias is getting dragged down by having no gain in Q6/9 and the drivers that you have installed. That is usually a result of missing collector voltage on one or both transistor sets. You normally expect that bias voltage to be ~3 - 3.1V in a full comp setup.

mlucitt · Mar 31, 2020

I did not think to adjust the bias pot during the measurement to see if the 2.16 VDC would change. I need at least one more arm to do that.

I do have the rail voltages on the collectors of the predrivers.

If Q7 and Q8 were conducting for some reason, would that affect the bias voltage? What should I expect at the base of Q7 and Q8?

I will take some voltage measurements referenced to ground tomorrow.

I'm looking for +0.9V at Q6 Base and +0.42V at Q6 Emitter, I think? I'm looking for -0.8V at Q9 Base and -0.2V at Q9 Emitter, right?

Any other measurements I need to make?

Gepetto · Apr 1, 2020

mlucitt said:

Hi Mark
At idle, in a PL400 with +/-80V rails, with the output offset at 0V, you should measure ~193mV at the base of Q7 and Q8 with your DMM negative probe referenced to the output.

In the full comp configuration, the base of Q6 and Q9 should be close to symmetrical with respect to the output with approx +1.55V on the base of Q6 and approx -1.55V on the base of Q9 when the bias is set to 0.35V. Emitter of Q6 and Q9 should be approximately +0.95V and -0.95V respectively (a diode drop lower than the base). PNP transistors generally speaking have a slightly lower Vbe than NPN transistors so you may see Q6 slightly higher than Q9 but not by very much.

mlucitt · Apr 1, 2020

Checking...
Here are the results of the Predriver voltage checks, all on Left Channel with Dim Bulb in place:
Connection ............................ To Ground ..................... To Output Post
Q6 B .......................................... +0.653V ............................ +0.710V
Q6 C .......................................... +84.7V .............................. +84.6V
Q6 E .......................................... +0.145V ............................ +0.221V

Q9 B ......................................... -1.197V ............................. -1.104V
Q9 C ......................................... -84.7V ............................... -84.6V
Q9 E ......................................... -0.665V ............................. -0.592V

For fun, I tested the SOA Protection transistors:
Connection ............................ To Ground ..................... To Output Post
Q7 E .......................................... -0.055V .............................. +0.015
Q7 B .......................................... +0.606V ............................ +0.674V
Q7 C .......................................... -0.048V ............................. +0.22V

Q8 E ......................................... -0.062V ............................. +0.008V
Q8 B ......................................... -0.269V ............................. -0.197V
Q8 C ......................................... -1.115V ............................. -1.025V

While looking at the PL14_PL20 Rev E BOM and Schematic together, I saw where I might have made some errors. Could you help me verify the correct values for the Control Board Left Channel for Fully Complimentary units:
On the Schematic .................. Currently on the Control Board and listed in the BOM
R20L, R26L 13K ....................... 56K
R21L 22.1 ohm ........................ 56 ohm
R36 56 ohm ............................. 1 ohm

I think these values, as built on the board, might change the bias voltage felt at the Q6 backplane transistor? I have verified that the parts to be left off the fully comp Control Board are omitted.

Finally, I traced the source of the 3-3.1V Emitter to Collector voltage across the bias transistor. I have the full rail voltage at R31L (10K) and on the other side it drops to 22.11V. This voltage is sensed on one side of R20L (3.32K) and then drops to .986V on the other side to feed the Collector of Bias transistor Q6.
This seems low, is it possible that the Q6 Bias Collector voltage is being pulled down by something common to both channels such as a shorted capacitor or a missing ground on the backplanes?
On the Emitter side of Bias transistor Q6, the voltage is negative (-1.168) is this normal?

Still searching...
Mark

I'm Buying a "Mule" PL400

Gepetto

Administrator

mlucitt

Veteran and General Yakker

Gepetto

Administrator

mlucitt

Veteran and General Yakker