Blazing Hot U5 & U6 and incorrect U1 voltage... Help!

[nohjoh]

I successfully put together a J99 & A12 yesterday - they sound great. Kudos to you, Tim and your great service... now for the C84, though. I have problems...

Upon power up for testing, The U5 & U6 heatsinks are blazing hot. (too hot to touch).
Voltage readings at U1 are at times (pins 1-8): 12, 14.4, 14, 12.3, 14, 14.4, 12, 14.4 (yep, all positive)
AND THEN SUDDENLY CHANGE TO: -12.8, -12.4, -12.5, -14.3, -12.5, -12.1, -12.8, -12.1 (yep, all negative)

similar results in U2 & U3 .

I Triple checked and as far as I can tell, I have all caps & diodes installed the correct way.
I also checked for shorts at the voltage regultors.

I put a fresh battery in the DMM just to make sure and, again, got the positive readings as above on all 8 pins at U1. (+14.4 at pin 8, +12.3 at pin 4)

what other readings should I take to pinpoint the issue?

[nohjoh]

I've been working on this all day - I had a a friend come over who knows his way around this stuff a lot more than I do... We checked and re-checked the diodes in question, we swapped out the negative regulator and the neg 1500uF cap. Still blazing hot U5 & U6 (though U5 -the negative- gets the hottest).
Here's an image - maybe one of you can spot the issue??? maybe a diode is backwards contrary to the silkscreen or something???

[tpryan]

What voltage do you measure at the cathode (banded end) of D13?
What voltage do you measure at the anode of D16?

[nohjoh]

cathode (banded end) of D13:
28.9

anode of D16:
-29.6

[tpryan]

What voltages do you measure directly across C34 and C38?

[nohjoh]

C34 is +14.4
C38 is -12.2

[tpryan]

Can you adjust R64 and R68 to get 18V across those capacitors?

[nohjoh]

well... I just tried...
R64 & R68 were already fully CCW, so I gradually turned R68 up while measuring across C38 - it made the voltage waiver a bit but didn't change much from -12.2.
While turning R64 & measuring across C34, though, Q7 sparked/popped and now I get a 1.7v reading across C34 - both fully CW & fully CCW.

I now get just a 0v reading across C38.

Oh well... I had a 4 out of 5 success rate... the A12, J99 & 2 N72's came out perfectly!

Should I get another BC560 and try something else... or should I email you for an RMA# and send it in for you to take a look at?

[tpryan]

You almost certainly have some components installed incorrectly, probably the MJE172s/MJE182s but quite possibly others as well. Get a new complement of BC550s and BC560s and replace all of them. Recheck your work and find the errors before you power up again. You can find the mistakes yourself as easily as we can, it just takes some patience.

[nohjoh]

you're right - crap! I installed an MJE182 at Q6 - and the MJE172 at Q9 - I mis-read Q6 & Q9 because the 6 & 9s are printed upside-down next to the Q5 & Q10!
Do you think i'll need to replace other components other than the BC560 & BC550s? Should I replace the MJE172s/MJE182s as well? Do you think I may have dameged some other stuff as well?
Thanks for working with me on this.

[tpryan]

The MJE172s and MJE182s are probably OK, but it's much easier to snip them out and replace them with new parts than to desolder them. You can order all of the replacement transistors directly from Mouser, they have no minimum order and will ship next day if you care to pay for it. Be sure to double check the orientation of all the protection diodes as well, since incorrectly installed diodes can cause similar symptoms.

[nohjoh]

I thought I'd check my local electronics supplier before I ordered and found that they didn't have some of the components - but the guy said here were some acceptable subs. What do you think?
Can I substitute the MJE172/182s for MJE 171/181s? The guy at the electronics store said they are exactly the same except rated up to 60volts instead of 80volts... is he right?
What about using BC549c/BC559c instead of BC550/BC560?

[nohjoh]

ok - I was patient and waited for the right parts to come from Mouser. I put them in and smoked another transistor!!! I can't figure it out and so I admit defeat. I double checked everything again thinking I was absolutely right this time... but noooo...
I guess I'll have you guys re-build this one... but I want to order another C84 and make another attempt at it. Frustrating, but still fun considering the A12, J99 & N72 modules I made came out splendidly.
Tim, I sent you an email - wondering if I need authorization before I send it to you... ?

[nohjoh]

Just a follow up for anybody interested...
I had a short in a component and Tim found it and fixed it without having to do a complete rebuild. The price was reasonable and the work was done great... People should be prepared for a long wait, though, if you ever need work done. Be patient!
The module sounds EXCELLENT. I have 2 N72s, an A12 and a J99 - the C84 is only cheaper because of the components involved. It sounds awesome, just like the other ones.
Thanks, Tim.

I just ordered another Chassis, PS, harness and C84 to make for a friend. Hopefully I won't screw it up this time!