Change in bias test point readings upon adding 2nd row of power transistors - FullComp PL400 build

[ChuckD]

Greetings all, typical newb here ... I've enjoyed reading all y'alls posts over the past few years as I've dreamed of giving my beloved PL400 the FULL WOPL treatment. I pulled the trigger a few months ago and have been working as time permits to get it done ...

So, I'm merrily rounding what I hope is the home stretch in my first Full Complementary build of a PL400, with the WhiteOak RevG1 Dual Mono Control Board Kit , full conp backplane kit, LED light board, (back-ordered) 15,000 uF capacitors, WattsAbundant DCP board, the works. It has been a fun project so far.

I'm working my way through the back-plane bring up recommendations procedure and got the first row of power transistors installed and bias adjusted to 350mV, no problem. Things are looking good!

I install the second row and find that the right channel holds right around 350mV +/- a few, but the left channel's bias reading (on the control board) jumps to around 705mV, beyond what I can adjust back down to 350mV with the trimmer pot. I can only get back down to 580mV. With that setting on the pot, on the left chan backplane board Bias+ reads 580mV, Bias- reads -413mV. Swap the pair of power transistors that I just installed in the 2nd row on that channel with another pair and ... nope ... nearly identical readings.

So I suspect something is wrong with the backplane build for the left channel, but I'm unsure where to look. Intensive visual inspection under magnification prior to installing both of them into the chassis checked out and I test resistance, capacitance and diode forward voltage drop (as applicable) at the pads after each component's solder step to ensure I didn't fry anything. My guess would be to start at R5/R8 and R35/R36 as components directly contributing to voltages at TP1 & TP5 (bias+, bias-), but this circuit is complex enough for me to want to ask for any advice or tips folks might have. Should I start there or look elsewhere or is 580mV within the range of acceptability? I dunno.

 

[Gepetto]

Greetings all, typical newb here ... I've enjoyed reading all y'alls posts over the past few years as I've dreamed of giving my beloved PL400 the FULL WOPL treatment. I pulled the trigger a few months ago and have been working as time permits to get it done ...

So, I'm merrily rounding what I hope is the home stretch in my first Full Complementary build of a PL400, with the WhiteOak RevG1 Dual Mono Control Board Kit , full conp backplane kit, LED light board, (back-ordered) 15,000 uF capacitors, WattsAbundant DCP board, the works. It has been a fun project so far.

I'm working my way through the back-plane bring up recommendations procedure and got the first row of power transistors installed and bias adjusted to 350mV, no problem. Things are looking good!

I install the second row and find that the right channel holds right around 350mV +/- a few, but the left channel's bias reading (on the control board) jumps to around 705mV, beyond what I can adjust back down to 350mV with the trimmer pot. I can only get back down to 580mV. With that setting on the pot, on the left chan backplane board Bias+ reads 580mV, Bias- reads -413mV. Swap the pair of power transistors that I just installed in the 2nd row on that channel with another pair and ... nope ... nearly identical readings.

So I suspect something is wrong with the backplane build for the left channel, but I'm unsure where to look. Intensive visual inspection under magnification prior to installing both of them into the chassis checked out and I test resistance, capacitance and diode forward voltage drop (as applicable) at the pads after each component's solder step to ensure I didn't fry anything. My guess would be to start at R5/R8 and R35/R36 as components directly contributing to voltages at TP1 & TP5 (bias+, bias-), but this circuit is complex enough for me to want to ask for any advice or tips folks might have. Should I start there or look elsewhere or is 580mV within the range of acceptability? I dunno.

Not acceptabele at 580mV Charles...

 

[George S.]

Chuck, start with the basics with that bottom row installed that biases correctly. Check your fuses first, then +and- voltages at the storage caps, then your pico fuses that are mounted on the backplane. Move on to your control board. You have one channel that biases correctly, so ground your multimeter at the star ground and compare test points voltages between both channels on the control board. If everything is equal up until you insert a output in the problem channel, then I think your issue is directly there at that second row. Have you overtightened a output causing the output case to ground through the silpad or is there a burr puncturing the pad? You do have the backplane installation assembly "help" bolts and insulators installed so a second row output bolt isn't grounding? Those bolts and insulators hold that backplane in position so the outputs can be installed without grounding.

 

[ChuckD]

Thanks, George and Joe.

I found it. R21 (1/2 W 10 Ohm) seemed to test ok at the pads, but testing the resistance between XQ7 emitter and TP4 I found a couple megaohms resistance. Reworked it with a bit more solder on the top side of the board and all is bueno now.

Onward.

Cheers and thanks for the guiance.

 

[George S.]

Cool! Let us know how you like it once you get some time with it up and running.

 

[MarkWComer]

Thanks, George and Joe.

I found it. R21 (1/2 W 10 Ohm) seemed to test ok at the pads, but testing the resistance between XQ7 emitter and TP4 I found a couple megaohms resistance. Reworked it with a bit more solder on the top side of the board and all is bueno now.

Onward.

Cheers and thanks for the guiance.

Good! An easy fix is the best fix!
I never adjusted bias until all of the transistors were in- was there a change in the procedure or is that typical of the latest backplane revision?

 

[George S.]

Good! An easy fix is the best fix!
I never adjusted bias until all of the transistors were in- was there a change in the procedure or is that typical of the latest backplane revision?

I put in a row with bias turned all the way down. Checked each output case for no continuity to ground. Powered it up(Variac and DBT). Checked and set bias for each channel. Turned the bias pots all the way down. Powered it down. Discharged caps with my fancy resistor with leads and insulated clips. Put in the next row. Repeat.
Doing it this way I think saved my ass when I had a red bias transistor wire burn through due to unseen broken strands.
Can't recall if doing it this way was in Joe's fine print or not. I do think it's probably the safest way to limit potential damage if somethings not correct.

 

[Gepetto]

The fine print...

 

[George S.]

Forgot about the resistors and volt meter across the output jacks, been awhile.

 

[mlucitt]

I found it. R21 (1/2 W 10 Ohm) seemed to test ok at the pads, but testing the resistance between XQ7 emitter and TP4 I found a couple megaohms resistance. Reworked it with a bit more solder on the top side of the board and all is bueno now.

It is a good feeling when you correct the problem. Over the years, I have found most of the problems to be mechanical in nature (bad solder connection) and not faulty components, especially new or previously good components. Good work!

 

[ChuckD]

Follow up: completed re-assembly and final testing last night., And ... first tune was Funk #49. Oh. My. Gosh. So good. Out of the gate. It's time to let her run for a while and see where we go with this. BIG thanks to Joe for providing a truly superior product at a very reasonable price. His support with the occasional question has also been prompt and helpful. He does way better than the old saying "Price, Service, Quality: Pick two." suggests is reasonable to expect. He definitely exceeds my expectations!

 

[laatsch55]

Funk 49....yeah...good tune!!