High DC Offset On My PL400

[Forresthicks7]

Hi gents,

I’ve been working on a mid-70’s PL400 and am a little stumped on what could be wrong.
Initially, I was just going to recap the driver board and replace filter capacitors, but during the process, I made the mistake of shorting B+ to chassis. Whoops. I ended up replacing all 12 of the output transistors (but not pre-drivers), replacing filter caps and bridge rectifier with the snubber setup and other stuff from WO, and replaced the driver board with a matching PL14B board. I also replaced the bias transistors as they got fried when I shorted the B+ to chassis….

Now, when I go to check voltages, I have 40VDC on the outputs of both channels. I’m at a loss as to why that would happen. Any help is welcomed - if I missed any details, let me know.

 

[mlucitt]

You could lose sleep and pull your hair out trying to find the source of your problem. The easy and best solution is to replace your second obsolete driver board with a brand new dual mono Rev G1 Control Board from White Oak Audio, here:
https://www.whiteoakaudio.com/estore.aspx

Your amplifier will be grateful and your ears will be pleasantly surprised at the increase in sound quality.

 

[laatsch55]

That don't help ya get your chops. I believe trying to resurrect that old shit can teach some valuable lessons. Like how easy old traces lift, how long your iron can stay cuddled up to a lead to pull it and the trace comes with the lead. And old wire isn't vintage its a stiff pain in the ass. It did take a half a dozen 100.00 dollar broken 400's to be able to test in circuit and read a datasheet...And then one day it happened.....it worked when it left the bench....kinda cool....gotta earn your chops...

 

[Forresthicks7]

Rookie mistake. I had a floating ground at the inputs. Now voltages are looking much better, but need a replacement AGX-8 fuse. There’s no B+ going to the Watts Abundant relay setup I added. Will update when I am finished.
Also, I have every intention of picking up one of those control boards. I originally wanted to get one already assembled, but I think I value my money more than my time and I can solder just fine

 

[wattsabundant]

[QUOTE="Forresthicks7, post: 344206, member: 10589 "...There’s no B+ going to the Watts Abundant relay setup I added."[/QUOTE]

See step 23 & 24 on page 4 of the attached procedure. The first two photos on page 7 of the procedure

 

[Forresthicks7]

After some searching, I found the issues that solved the above problems. The fuse for B+ popped when I shorted to chassis a few weeks ago. Easy fix and we’re back to proper voltages. In addition to this, I had neglected to attach the new ground from the RCA inputs to chassis, therefore there was 35v trapped there with nowhere to go.
After solving these problems, a new one has arisen. On first power up, R23 (22ohm) popped and smoked. Ironically, your protection relay circuit is engaging and I am getting proper voltages everywhere except for the emitter of Q10 (supposed to be 1vdc, I am getting nearly zero). I’ll pull the transistor later today and check it. Thankfully I have some replacements on the original PL14B board that came with the amp.

 

[wattsabundant]

The likely reason R23 burned is Q10 has shorted. Did you use the light bulb test set in the relay board instructions? It's intent is to avoid this type of problem.

Start by checking every P/N junction on the board using the diode scale. Compare the two channels. See the troubleshooting section in the manual. Do not start changing out parts that haven't failed.

 

[Forresthicks7]

Yes, I did use the lightbulb test. It did not show any shorts. In addition to this, the relay engages properly. I will check with diode scale.

 

[mlucitt]

I had neglected to attach the new ground from the RCA inputs to chassis

The RCA Inputs should not be touching the chassis in any way. The RCA Inputs (signal and shield, or center and outer, or hot and common; regardless of what you call them) should only be connected directly to the control Board. The Right and Left Channel input common wires (usually Black) are connected together on the copper plate just inside the chassis, this prevents ground loops and interference from getting to other components.
Phase Linear used a solid bus wire to connect the RCA common wires to the Right Channel Speaker common terminal as a shortcut. This should be removed.

 

[wattsabundant]

The original input jacks used 3/8" x 1/2" phenolic shoulder washers to isolate the inputs from the chassis.