ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello, hoping to get some advice/help on diagnosing a penta 3 amplifier issue. Model number 6633. I bought a used pair of towers. When I received them, one was fully functional, the other was tripping circuit protection and not playing music at all. I refoamed all the mid ranges. Then I rebuilt the crossovers and amplifiers with the kits from beo parts. The original good tower is now great. The original bad tower will play now but it only plays at a very low volume compared to the good tower. After the rebuild, I set the potentiometers in both amps in the same manner, following the instructions that came with the rebuild kit. I’ve tested both towers with my one good amp to confirm there is no issue in the crossovers or drivers. Both towers function well when connected to the good amp. However, I've now noticed the bad amp will still slowly shift into protection (orange light) when no input signal is present. So I took both amps off the towers for some bench testing.
Hey,
Sounds like you've done some brilliant troubleshooting. I haven't specifically seen such an issue. But in my YouTube video I do show about replacing the transistors and some tests you can do on the transistors - this might help?
https://www.youtube.com/watch?v=psY_RV1NjHM
Good luck!
Bristol based B&O fan with a collection including the following: BC9500, Beogram 9500, Beosystem 3, BeoVision Avant DVD 32, AV9000, Beolab 3500, Beolab 3000s, Beolab 4s, Beolab 6000s, Beovox Pentas, Beovox 3000s.
Hi david656,
I watched your video (particularly the section on transistor testing). Today I removed TR30, 31, and 32 from the circuit board and tested them per your guide. They all appear to be working correctly. The only thing I noted is that I got voltages closer to .4 VDC vs the .5 VDC you listed. While doing this work I noticed I could wiggle TR23, suggesting loose or broken solder joints. Indeed it looked like the connections for TR23 were loose for pins B and C. I reflowed these joints and reinstalled TR30/31/32 on a hunch that TR23 may have been the issue all along. Put everything back together, set the potentiometers, fired it up and its still preforming exactly like it was before.
Is it worth popping out TR 25/26/27 and checking them off the circuit board as well?
Thank you.
Closing out this topic. The main stage was totally fine. A bad transistor on the input board was pulling the input signal to ground at the quad bilateral switch on the main amplifier board. I had to use a function generator and oscilloscope to trace the input signal through and find the problem. I think the issue was the transistor responsible for the “mute circuit”.