Did you replace the trim pots?

Yes, using the ones from the beoparts package. R59 adjusts offset just fine but R95 can't get the bias down to the 30 mv range. Setting R95 at anything but minimum activates protection circuit and orange light.

I'm certainly not an "expert" on these amps, but I'll try to help by asking some general questions. At the very least, there will be a bit more information here in case someone with in-depth knowledge of the common Penta amps failure modes chimes in. Are any of the main transistors or load resistors discolored? What are the actual voltages on the +68v and -68v power supplies? Did you replace the power supply capacitors? Have you reflowed the solder on the board connectors? Or at least carefully checked for cold solder joints? It also might be helpful if you could upload some pictures of the component and trace sides of the circuit boards. Glitch

Thanks, really appreciate it! Yes I replaced the psu capacitors. Problem is now amp is going into protect mode again, too fast to get a measurement on the +/-68V.   Transistors look fine to me but will check more closely. I have not reflowed the points of board connectors since I left them in, should that be done? I found a lifted trace at the ground wire that might have lifted more now, trying to fix it with wiring. So that's the weird thing in the middle of the picture of the trace side of the board.   Login to view image Login to view image

The transistors and load resistors "look" similar to my working Penta2's (i.e. no obvious overheating). I wouldn't mess with them right now. Probably not the problem, but are you sure you have the trim 100 and 220 trim pots in the correct location? While you are double checking... check the polarity of the power supply capacitors. I recall that the markings on the original capacitors could be misinterpreted. The soldering on some of the transistors doesn't look like I would expect coming from the factory. I would clean-up the flux residue and do a closer inspection. It might be just the camera angle, but the solder from TR32 seems close to the trace. I would carefully inspect the board connections even if you didn't touch the connectors. I use either a 7X loupe and a bright light or take a picture with a macro lens and zoom in on the pictures on the computer screen. Even if I don't find any obvious issues, I usually reflow solder on the board connectors as a precautionary measure. I had trouble setting the bias on one of my other B&O amps. It turned out to be a bad ground connection to the preamp. The symptoms were similar to what you described where the full range of adjustment on the trim pot wouldn't get the bias to a proper value. I am also interested in hearing from others the possible causes for the amp running hot. Glitch

For reference, the crack at pin 7 is the kind of thing to look for. The "stress rings" around the other pins look like they are getting close to failing. Login to view image Glitch

My first thought was also, if you had the trimmers mixed up when you fitted them. Martin

I also thought so but checked the ohm with dmm in circuit and it's correct. One thing that puzzled me was that while turned the 220 trimresistor would read above 220 ohm and after turning off would steadily decline down to the ~110ohm it was set to. Something to do with how the circuit design or indicative of a fault?

Thanks for the guidance, I'll go over the board once more and inspect the joints more carefully. Afraid I have a hard time making out the crack on pin 7 in the pic but gives me an idea of what to look for.

 

You can't do ohm readings on a powered circuit. Martin