Beta22 Help?

Well, I just built my first active ground Beta22 and it looks as though I need some help troubleshooting what I blew out. After populating the three Beta22 boards and the one Sigma22 board, I did the initial checks. Everything checked out OK... I adjusted VR1 to 4.5 V across R9, put 60mV across R34 (about 128mA of quiescent current), and adjusted VR3 so the DC offset was exactly 0.0 mV. When I got all 3 Beta22 boards up to spec, I mounted them in my chassis. Before I played some tunes through them, I decided to double check all my measurements. After verifying 4.5V across R9 on the ground channel board, the mini-grabber adapter on my positive probe got stuck on the resistor lead. When pulling it off, it cut through to the ground plane and shorted something out. Looking at the schematic diagram, it shorted the 28.85V from the top of R9 to the ground plane. I immediately turned off the power and dislodged my probe. I powered everything back on and checked the sigma22. Everything checked out ok for the PSU, so I went to check the damage done to the Beta boards. The left and right boards are fine, but something is blown on the ground board. I'm only getting about 0.8V across R9 and R10, while I'm getting 0V across R11 and 4.5V across R12. Very puzzling. I am also getting an enormous DC offset of 4.5V. Any insight into what could have blown? I'm almost positive the diode D11 must have blown, but the weird voltage readings I'm getting are leading me to believe something else went awry. Could I have blown some of the transistors too? Any help would be much appreciated! I have attached some pics to show the damage.

Also, I have set up my Beta22 as a 2-channel build for the time being, and it sounds absolutely phenomenal. Best amp I've ever heard, by a long shot. I can only imagine how good it will sound when I get my ground channel operating again!

Quote:

Originally Posted by johnwmclean 

Check voltages relative to ground at the outer leads of R9 and R11 should be roughly (+/-) 28V, either Q25 or Q26 could be blown.

 

Tip: I always raise R9 about a millimeter and half off the board, shorting with probes/grabbers is a very common mistake.

I was afraid I'd blown one of the transistors. Is there any easy way to check them with a DMM? I just checked some of the voltages around the diodes, and I definitely blew D11. It reads the PSU voltage one one side, and -1.5V on the other side (which is what the top of R9 measures). D12, on the other hand, seems to be reading OK, and the voltage on one side of R11 and R12 is correct. R11, for some reason, has no voltage drop across it, while R12 has the correct voltage drop. Could my problem be as simple as D11 being blown? I'm going to head to the electronics store tomorrow to see if I can find a suitable replacement to test if anything else is busted. I didn't short anything other than one lead of R9 to the ground plane, so I'm crossing my fingers hoping that all I need to replace is a diode!

Also, thought I'd throw in that I inspected the board multiple times and couldn't find anything that looked physically burnt. I could have missed something though.

Thanks for the tip on raising the resistor, I'll have to keep that in mind for next time!

Quote:

Originally Posted by amb 

Most likely Q25 is blown.  This is a very common accident.

Q25 was blown. Thanks for the tip on testing, Holland, and thanks for the tip on which transistor might be blown AMB! I checked the diode, and it seems to have been fine. I suppose that doesn't really surprise me because it's a rather beefy diode. I replaced it anyways though, as they're only 50 cents a piece. Anyways, I replaced Q25 with a transistor I could find locally, a NTE199. It's a low noise preamp transistor, with a Vces of 50V, an Ic of 100ma and a ft of 90Hz min. The specs resemble a BC550C fairly closely, so I figure it's ok for now. Do you anticipate any problem using the amp with this transistor for a while? I turned it on for a test, and boy does it sound good! Major kudos to everyone involved in the design.