Seventh Circle Audio

Just completed the 4th of 4xC84s, first 3 working great, the fourth however...

Doing the first voltage test at pgnd and B+ = -9.1VDC, B- = -14.5VDC (both negative?!?),

this B- is typical so far for me with the trims all the way down.

Measuring pgnd and XLR pins 2 and 3 BOTH read +46.1 VDC.... yes both positive!!

I've scoured for shorts - none found, but I'm pretty much a noob, and have no idea what to check next.

Steve.

biasply \$m[1]:Just completed the 4th of 4xC84s, first 3 working great, the fourth however...

Doing the first voltage test at pgnd and B+ = -9.1VDC, B- = -14.5VDC (both negative?!?),

this B- is typical so far for me with the trims all the way down.

Measuring pgnd and XLR pins 2 and 3 BOTH read +46.1 VDC.... yes both positive!!

I've scoured for shorts - none found, but I'm pretty much a noob, and have no idea what to check next.

Steve.

Hi Steve,

I had a similar issue with one of my C84's as well and I had put the LM317 and LM337 in the wrong slots (swapped them around) and it had caused C34 (tantalum cap) to blow.

Might be way off, but it solved my issue!

James

biasply \$m[1]:
Measuring pgnd and XLR pins 2 and 3 BOTH read +46.1 VDC.... yes both positive!!

Steve.

This would be normal if you have the phantom power switch on (if the XLR is the mic input).

@jamesnorth - yes checking all my polarized caps, all Q's and U's was the first thing I did.

@DPLight - ah, ok, pardon me my noob is showing... could of swore pin 3 should be negative voltage.

I've continued with some of the other tests:

across R35-R38 = 135mV
across R50-R62 = 256mV
across pgnd-R30 = -12.78 V

biasply \$m[1]:@DPLight - ah, ok, pardon me my noob is showing... could of swore pin 3 should be negative voltage.

No pardon needed. With both XLR pins 2 and 3 at the same phantom power voltage there is little chance of toasting your dynamic microphones (the rare chance being when there is a current path to pin 1). Phantom powered mics use pin 1 as a power ground/return

With the phantom seeming stable, I decided to risk a mic, it appears to power up properly, but it's passing no audio signal. Also, when powering on the phantom, there is no familiar gentle thump.

I've rechecked for cold solders/solder bridges. Everything looks as clean as the other 7 pre's that work great.

Can anyone suggest tests to identify the possible weak link?
I know it must have something to do with the pgnd to B+ test being the wrong polarity and value, but I'm at a loss as to what parts would cause this...

Steve.

Pointless to connect a mic until you understand why your B+ is -9V. Re-check the orientation of all the polarized caps and diodes. Make absolutely sure the MJE172/182 transistors are all installed correctly. I can virtually guarantee you have a part installed wrong somewhere, and the longer you keep power applied the more likely you are to destroy it and associated parts.

Tim,
I just went over it again for the 4th time. Every polarized part is in the right place, all U and Q parts are also correctly oriented. I have 3 working C84s to compare to as well, and everything is in the right place, the right way around.

I was wondering if I fried the LM317 when I installed it, so I practiced desoldering one on a different dead project, it didn't go too well, so I think I might have to send it in for a rebuild because this is clearly out of my range.

Steve.

biasply \$m[1]:Tim,
I just went over it again for the 4th time. Every polarized part is in the right place, all U and Q parts are also correctly oriented. I have 3 working C84s to compare to as well, and everything is in the right place, the right way around.

I was wondering if I fried the LM317 when I installed it, so I practiced desoldering one on a different dead project, it didn't go too well, so I think I might have to send it in for a rebuild because this is clearly out of my range.

Steve.

In mine, because I put the LM337 where the LM317 was meant to be, it resulted in C34 frying - does C34 give you approx 22uF when testing in circuit? This caused my +B to give -B results around what you're getting.