1176 Rev F noise

[mitsos]

Having a problem with a Urei 1176 Rev F.  It was recapped and calibrated about a month or so ago, worked fine for a couple of weeks, then, literally overnight, it developed a hiss.  Detail, one of the lamps blew out soon after the recap, but I don't think that had much to do with it. then again..

Voltages seem fine, at least within reasonable limits, though I got a weird reading on Q15 (output of GR amp) where the B was more positive than the emitter by a couple of volts. But that went back to normal by itself, so I'm not sure if it was my DMM freaking out.

Hiss is audible with I/O knobs at max, with either at minimum it disappears. Also, with the input XLR disconnected it also disappears. I tested each amp by itself by disconnecting at the output pot, output seemed fine, input amp had some similar noise (frying pan noise?).  Here is a clip of the noise with everything connected.  First half is with XLR disconnected, noise starts as soon as it's connected.

https://soundcloud.com/mitsoball/1176-noise

Already swapped transistors in input amp, a couple in the output amp as well, just for kicks, but I'm stumped.
Also, thanks to niels for the suggestion, I disconnected the GR amp from the input (wire coming off the attack/release pots). 

disconnected the meter circuit as well, noise continues...

thanks for any help.

 

[12afael]

it sound like normal hiss to me. provably noise from a resistor. what happen if you short the input? you still have noise? I have repaired some guitar amps with noise from dirty connectors. it's not a bad idea to clean the xlr connector just to discard.
if the unit was recaped provably a resistor was overheated while removing the caps. I would try to find the guilty stage and change some resistors. 

 

[mitsos]

I wish it were normal.  It sound to me like a bad transistor, sort of that frying sound. I am at the point where I'm considering rebuilding this because I've tried everything else.  I didn't short the input exactly, I had a 150R XLR "dummy plug" that I use to check preamp noise, and plugged it in at some point. The noise stopped. But it also stops if the input is left open, nothing plugged in.  I've tried removing the Tpad, new FETs, transistors, even a new XLR pigtail, but nothing helped. I would like to think it's normal hiss, but it shows up on the meter. The other 1176s don't show anything on the meter with both controls at max. At least, IIRC, I don't have any others right here to check, but I've been through a few of them recently.

 

[radardoug]

When you say the noise disappears when you disconnect the input XLR, do you mean something plugged into the input XLR, or are you disconnecting at the board? If it is something plugged into the input XLR, then the noise is coming from that source.
If you mean disconnecting at the board, is your input grounding correct? Are you picking up r.f. on the input?

 

[mitsos]

I have this on the bench for a while now.  Signal is coming/going through a digi003. It passes signal but with noise, bunches of that frying pan sound.  Voltages are mostly OK, except for the one occasion that Q15 base was more positive than the emitter. 

If I plug the 1176 I/O to the digi003, with no signal, I get the noise in the post above (second half of the clip). If I unplug the input, I get normal hiss, like the beginning of the audio clip.  If I plug the digi I/O direct to itself, I get perfect silence. 

I've used this same digi (same I/O in fact) with other equipment, both 600 Ohm and modern line impedance, with no issue.  I don't have any other 1176s here atm, but I plugged it into a 600 Ohm input pultec and it was fine.

Could the input trafo have fried somehow? I would think it would go all or nothing, not cause this kind of noise.

thanks!