400SII popped and died

Jess

Jess

New Around These Parts
Joined
Apr 28, 2019
Messages
27
Location
Placerville, CA
Well after my dumb mistake a few threads down with the caps I ordered some new ones from Joe and installed them correctly this time. Bring-up went great and was actually pretty fun (once I figured out a good method for setting in the output transistors lol). Once I finished the last bias check I turned the amp on it's side and plugged in my PL4000SI and my phone and listened to music for 30-40 minutes. It was glorious.

After a couple of days I moved it to it's permanent home and finally got to hook my record player up. Halfway through the second song, the system made a loud pop and the meters dropped. I didn't smell anything and I've since pulled the cover off and inspected everything. I don't see anything burned or fried looking but I did check the pico fuses on each board and the right channel ones are toast. I hooked the amp back up to the DBM+Variac and took some measurements at 60V with 10K ohm resistors on the ground/output test points. B+ at 40.70 and B- at -40.82 but the left output had 38.53v and the right had 0.24v. 12R/11R are also at 0.58v and 0.24v.

I'm not really sure where to go from here. I was thinking of pulling the output transistors and inspecting the back of the boards to see if something shorted there. I made sure none of my solder points were long and that I didn't over-torque the output transistors and checked for chassis shorts between each row but, maybe something got in there somehow, I dunno.

Sorry for the long thread, any help would be most appreciated.
 
With the mistakes that were made (bias transistor, backwards bulk caps etc...) Let's hope you have a cheap solution. I like to inspect my boards under the microscope to ensure no debris or bad solder joints. Although the new outputs are tough, I have had one fail in a fresh build a time or two.
 
Loud pops make me think of a high voltage short and associated arc. (don't ask how I know that :)) But that would presumably blow fuses. Any sign of an electrical arc inside the chassis? Generally looks like a small burn mark.
 
The only "pops" I've had were followed by smoke out of a cap....an arc has a totally different sound, more of a snap...
 
WOPL Sniffer said:
You can always remove the outputs and bring it back up in the DBT one row at a time
Click to expand...


Agree that would be a great first step on the affected channel. Only have the 2 bottom row devices installed after checking that those 2 devices are good.
 
Are both pico fuses on the right channel open? The power for the +/-15V regulators on the revE board comes from the right backplane board.

When you brought it up again on the DBT after replacing the pico fuses, What were your +/-15V regulators reading (be careful when probing around, hazardous voltages exist and an accidental slip can damage other components )
 
Jess it would be helpful to these experts on this thread to post some pictures of your control board, backplanes etc. Oftentimes pictures help with the debug.
 
On every forum I follow this is the #1 way for problems with builds to be solved and on most forums the good help will not even start until the pictures show up. The help you are getting right now is the best help possible form the best people on this forum but the frustration will start to show soon. Post pictures and your repair time will go down. :)
 
Thanks for all the replies. I'm at work right now but when I get home I'll post a bunch of pictures. I will say that yes, both pico fuses on the right channel are open.

Also I should clarify because after rereading my post I don't think I was very clear on this - the pop I heard came from the speakers, not inside the amp. It made a loud pop like if you yanked the wires out or something. I don't know if that matters but I just wanted to mention it.
 
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Sounds like the B+ & B- are okay. On mine, I accidentally shorted the -15vdc test point to the nearby heat sink. This took out the -15v supply and blew one of the pico fuses on the right board. This caused the left output to go B+ (or nearly so), so I suspect the cause of his left output going to B+ is that there is no -15vdc. And my right channel was dead, nearly 0v.
 
Pin 7 on the right backplane looks a little funky. Could be the photo. It looks like a solder joint to the insulation rather than wire.

Ditto for Ground_In for the left hand backplane. Don't see the solder connection.

Several more on the left backplane. Pin 5 and PNP and the Fused B- Out
 
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