Sytek preamp issues: distortion and modulating output levels

Sytek preamp with intermittent distortion issues.  Momentarily sounds like a dirty pot, thin papery light fuzz with a fading quality, then signal back again.  Seems to occur on all channels.  Infrequent enough that you might not hear it for several hours, and it might not show up at all on the bench.  PSU output looks solid on a meter.  So, I multed a 100Hz tone into all 4 channels, and recorded 25 minutes of the outputs.

Channels 3 and 4 both, at differing times, dropped about 3 dB in gain over the course of several minutes.  I then raised the gain again to rematch the channels.  1-2 held (mostly) consistent output levels, while 3 and 4 always show a rapid level modulation, 4 the most, about 0.1 dB variation.  Channel 4 also had the obvious distortion notch, which slowly disappeared as the gain dropped, then back again when raised to re-match.  I note a few spots where 1-2 both modulated a bit, together. 

Ideas? 

I haven't had a Sytek apart. How are the boards connected? Ribbon cables? Motherboard and daughter boards? One big PCB? Are the opamps socketed?

Have you sprayed a little CaiLube into the pots?

Peace,
Paul

One big board for everything including power supply. Mic ins and outs are hard wired to the board. The op-amps in mine have sockets.

I had a similar problem, but only on CH1 of my Sytek. Drifting gain, intermittent light fuzz. I let it go and eventually the peak indicator light came on and stayed on, and the channel became totally un-useable. PS is fine. So, I'd love to hear what everyone thinks about this as well.

As described by Adam, in all regards.  Mine is rev. A from about 1996-97. 

You should ask Greg Norman over at Electrical Audio if they've encountered this.  

http://www.electrical.com/staff.php

I talked with a tech friend of mine and he mentioned that sometimes syteks come into his shop with similar problems because the "zener diodes go bad" I'm not at my bench right now, but I remember there being these weird little diodes kind of daisy chained together up off of the pcb on mine. Near the IC's. Are there schematics for this these floating around? I looked on the sytek-audio-systems website and had no luck.

I'm not in front of mine either, but do recall a diode/R/R intersection that's one of the very few things built up off the board in mid-air. 

No schematics have surfaced, may have to start drawing.  My experience points to Sytek wanting to do all tech in house, with no reference materials ever provided.  I've sent support an email a week ago describing the problem, and have heard nothing so far. 

hey there,
channel 1 on my sytek just started distorting in the output. tried swapping the ICs with channel 2 with no success. did you determine if the problem was the diodes? i am thinking about ordering up some caps...

BD

Haven't done a thing yet. I'll take this as a reminder to track down management and inquire further.

FWIW, i tried lifting one leg of the diodes that are connected in series off of the PCB, still getting white noise from the output of channel 1 even with no input connected.

I started a new thread, but thought I might follow up here too to see if you guys had any luck. seems like we have similar symptoms. i was able to probe around for voltages and noticed that when i hit a point where 48v is being passed the white noise in my broken channel disappears and then comes back after a few seconds. I also found some posts about using caps to keep 48v out of the circuit on another thread. i replaced almost all electrolytics including the first two 47uf caps after the input jack. also swapped the first ceramic after the input and the the ceramic in close to the inputs coming from 48v with no change. any ideas?

BD

I'll yank my sytek out of the rack this week and crack it open, I'll check and see if I'm getting the same 48v problem. I don't think I've turned the thing on in 3 months.

I sent mine in to Sytek for assessment, and ended up with a good trade-in deal.  Mine was about 15 years old, so general wear combined with discontinuation of original parts made repair of mine potentially more expensive than worth, versus new unit price. 

So, no real answers from a tech end. 

Mike's comments to me suggested that he thought it better to leave them turned on, rather than have long periods w/o power.