PL 400 II--DC at both speaker outputs

[AntMast]

[FONT=&quot] Working on a PL 400 II. upgraded with: new resistors on driver board--new op-amps--recapped all boards--replaced the 2 PS zeners-- [/FONT][FONT=&quot][FONT=&quot]new main 10,000ufd filters--checked output & driver transistors(reinstalled with new mica insulators & thermal grease).
I now have the amp hooked up to a scope & fluke meter. Some times I can power up & have 0 DC volts at speaker outputs. Next time, it's same result or I get 76V DC(on both channels)! It's a random thing--never know what will happen. I pulled the drivers & outputs for a second time & retested--nothing shows up--no shorts---leakage. Using a Atlas transistor tester.[/FONT][/FONT][FONT=&quot] Being random is this a output or driver transistor failing or is there something else that could cause DC at the speaker outputs?? In troubleshooting, pulled all the small transistors off the driver board & tested--new resistors[/FONT][FONT=&quot] & caps checked [/FONT][FONT=&quot][FONT=&quot]--[FONT=&quot][FONT=&quot]all good[/FONT][/FONT]. Hope one of the experts have seen this problem before[/FONT][/FONT] & can point me in the rite direction. Thanks in advance.

 

[Gepetto]

Working on a PL 400 II. upgraded with: new resistors on driver board--new op-amps--recapped all boards--replaced the 2 PS zeners-- new main 10,000ufd filters--checked output & driver transistors(reinstalled with new mica insulators & thermal grease).
I now have the amp hooked up to a scope & fluke meter. Some times I can power up & have 0 DC volts at speaker outputs. Next time, it's same result or I get 76V DC(on both channels)! It's a random thing--never know what will happen. I pulled the drivers & outputs for a second time & retested--nothing shows up--no shorts---leakage. Using a Atlas transistor tester. Being random is this a output or driver transistor failing or is there something else that could cause DC at the speaker outputs?? In troubleshooting, pulled all the small transistors off the driver board & tested--new resistors & caps checked --all good. Hope one of the experts have seen this problem before & can point me in the rite direction. Thanks in advance.

What op amps did you put in? Did you change any values on the control board? Since you are not blowing any fuses (that you have mentioned), the amp is happy being in that state (which usually is related to...)

 

[AntMast]

What op amps did you put in? Did you change any values on the control board? Since you are not blowing any fuses (that you have mentioned), the amp is happy being in that state (which usually is related to...)

Hi Gepetto, op-amps are TI OPA227(I've used these in 2 other PL 400 II with great results). No values were changed on driver board. No blown fuses on 400. Current spikes to about 1 amp on power up & settles down to about 1/4 amp.

 

[laatsch55]

The intermittent nature of the DC offset and not blowing fuses has thinkin small wire cutoff, solder whisker or cold solder joint..

 

[Gepetto]

Hi Gepetto, op-amps are TI OPA227(I've used these in 2 other PL 400 II with great results). No values were changed on driver board. No blown fuses on 400. Current spikes to about 1 amp on power up & settles down to about 1/4 amp.

since it is happy in this latched up state and not blowing fuses, you should be able to use you oscilloscope to debug it while latched up in this condition. I would start by figuring out whether the op amp is behaving. All the control loop on the PL36 board starts and ends with this op amp.

 

[AntMast]

since it is happy in this latched up state and not blowing fuses, you should be able to use you oscilloscope to debug it while latched up in this condition. I would start by figuring out whether the op amp is behaving. All the control loop on the PL36 board starts and ends with this op amp.

Ok..looked at the 400 for awhile tonite & here's what I saw. I can cause the problem every time if the unit is ramped up on the variac. Took some volt readings & found when 75VDC is at the speaker terminals the -15V supply reads .7V! I'll look into it further tomorrow nite. Thanks for the tips & suggestions so far.