Ampeg SVT-VR making clicking noises when first turned on

[nevetslab]

One of my steady clients sent me an Ampeg SVT-VR which is(was) making clicking noises in the speaker when first turned on. They stated it may go away after 10 minutes or so. Yesterday, when I powered it up on the check-out bench, speaker connected, 120VAC mains, I powered up with a shorting plug in the power amp input jack. It came on quiet. I removed the dead-patch, and there it was...the clicking noise. Volume controls on the preamp both CCW. Turning either one up, and turning the treble up increased the clicking. Then it stopped. I haven't heard it since.

So, today, I pulled the amp apart, installed my clear lexan Test Panel so I could check the plate current on each of the power tubes, since I was seeing 260W being pulled instead of around 220-230W...though both GRN LED's were lit as was the Balance LED.

All of the power tubes were still well matched, so I then connected the speaker. So far today, I've yet to hear this clicking noise. I'm now cycling power up and down, waiting a good 10 minutes OFF time, after having powered up, with cold tubes, letting it run for just a couple minutes before turning it off, then turn on the cooling fans to bring the power tubes back to room temp. So far today, I've yet to hear anything.

As they stated the clicking may go away after ten minutes. They didn't say if this clicking remains for a long time, like thruout a set or thru rehearsal.

That's what made me think about cooling down the power tubes and preamp tubes between brief power on cycles. Finding initially it appeared to be in the preamp, since I had the power amp dead-patched, then unplugged that, and heard the clicking, then something in the preamp or power for the preamp. So far, it's elusive today. I hate problems like this.

[nevetslab]

Rear Fan Grill Panel floating

Having had no success in getting that clicking noise to reappear, I put it back together this morning. As I was installing the rear fan grille panel, I noticed there's no assurance it gets grounded. So, I checked with my Fluke to see if there was any static AC potential on the panel relative to the grounded Amp Chassis. Sure enough. Almost 27VAC. Turned the power amp off, and that potential went to zero. That's enough to warrant adding a small ground strap between the Fan Grille mtg and the chassis ground screw.



I haven't had any conformation that this 'floating' fan panel was responsible for the elusive clicking noises, but, perhaps it's smart to keep all metal surfaces at ground potential.

Now, while that interference noise hasn't been heard but once, what I AM hearing on this amp seems like too much AC hum on Ch 1 preamp. Pulling the preamp forward while still plugged in, didn't yield any improvement in that. I could get the slightest change in moving that AC Mains wiring from the Power Amp Chassis to the preamp (all low current wiring, as the AC Mains PCB handles all the high current). After plugging in the shop bass, having to back down the volume a bit, it seems ok. Not that I mind putting time in on where it's needed, but, not what it came in for. I'll wait to hear from my client if this clicking reappears. I had unplugged and re-connected all the internal connections, all of the tubes, so for all I know, I found the culprit in the process without ever knowing. Onward...........

[boroman]

Did you disassemble the preamp boards? I don't know if it's the same as in CL but in CL high voltages are fed from there and many problems start there... Had three CL's with weird noises (oscillation, clicking.popping) and the other one that would not go off standby. All of them were traced to the preamp board.

[nevetslab]

Did you disassemble the preamp boards? I don't know if it's the same as in CL but in CL high voltages are fed from there and many problems start there... Had three CL's with weird noises (oscillation, clicking.popping) and the other one that would not go off standby. All of them were traced to the preamp board.

The Preamp boards in the SVT-CL are vastly different than the -CL. First, there are 2 channels. The Ch 1 is similar to the SVT-CL, though the mechanics are very different. 5 tubes total in the preamp vs 2 in the -CL. I had the preamp open, but, had been waiting for re-occurrence of the stated problem (clicking). Hard to chase something that's not occurring.

Prior to installing the ground strap between rear panel and power amp chassis, I had only measured the static AC potential with a 10M input impedance DMM. I'll bet using my Fluke 893A Differential AC/DC Meter, which in diff mode, and dialed in to the true potential, has infinitely high input impedance, and wouldn't load down that static potential that the 10M Z meter read. Not that it's a shock hazard, though interesting that there appears to be some leakage from the AC fan thru it's plastic housing mounted to the grille. I'll look at that again after I finish with some other gear on the bench.