[bside123]

Just thought I'd do a little shout out for some troubleshooting help on my VTL 5.5 Linestage Preamp as a last ditch effort before I send it off to California for repair. Here's the situation:

1) I finally found a tube compliment that I was really enjoying... dontcha' know it? Whatever... the preamp uses 4 X 12AT7 and 2 X 12AX7 tubes.
2) Up until the day before yesterday, the preamp seemed to be working flawlessly and sounded just great.
3) Two days ago a family friend was visiting. I was not in the house at the time. She asked to listen to the stereo, as she has done dozens of time before without any problems. She simply turned on the power switch to the preamp. The preamp has a warm-up/safety mode for approximately one minute. After the warm-up period, she noticed a "bad" smell... she said like something burning. She immediately powered off the system.
4) The result has been the following: a DRAMATIC drop in the left channel's gain along with period bad noises... crackling and popping.

I've run the following tests:

* tested all the tubes - check - all good
* reversed the position of the output tubes - problem remains unchanged
* tested ALL connections including speaker cables, source and preamp>amp ICs - check - all good
* reversed the leads at the power amp - problem moves to the other channel - power amp checks good
* fuses - check - all good
* changed the input channel on the preamp to another selection, i.e., CD to Aux as well as to the Tape Input and listened via the monitor. Problem unchanged.

The preamp has a balance pot. Here's the thing, when rotating the balance switch back and forth, at anytime the switch passes through the middle position (12 o'clock) a loud crackle pop is emitted from the left channel. If the balance pot is turned completely to the left channel, there is enough signal to hear the music but only very softly. If the balance switch is turned to the right channel, again as it passes through the middle position, a big crackle pop is emitted from the left channel. The left channel will also make the same crackle pop as the balance knob reaches the full right position, with only the right channel playing. The pops are also emitted through the left channel whether music (a signal) is playing from the source or not.

I do not see a way to test the preamp by DEFEATING the balance circuit. Can anyone think of anything else I have not tried? Otherwise, off to VTL in CA. Thanks!

[richidoo]

Sounds like a high voltage resistor in music signal gone bad in the left amplifier circuit. The smell, the noise, attenuation, and one soft channel.  If it has dual mono power supplies or distributed electrolytic storage caps, look at those first as they are more likely to have leaked and burned, but not likely to cause the noises you describe because they are not in the signal path. If it is a single supply, it would be effecting both channels. If there are no obvious burnt caps or wire, etc, then it was probably the ink or paint on a high power resistor, it might not look burnt as they are designed to resist high heat. But it may have cracked inside, or somehow broken. If it is on a PCB maybe there is a burned or darkened spot under it. If a part in amp circuit is replaced then it would be good for a tech to start it up according to VTL startup procedure to make sure there is not another defect and to make any adjustments necessary to accommodate the new part's tolerance. I know it's expensive but VTL would do the best job to preserve the performance and your investment. Super sweet preamp, good luck with the repair.

[bside123]

Thanks for your response Richidoo. The preamp is dual mono with two large toroidal power supplies. I can see no sign or visible evidence of burning, etc. from the top of the board. I'll call VTL today.