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Mystery tube amp problem - can YOU solve it?

VTL IT-85 integrated amp. Manufacturer had it for three months, but couldn't reproduce the problem, so they didn't fix it. Local repair guy couldn't reproduce problem after 6 hours of listening to it. He's returning it unfixed, too.

Left channel cuts out. Happens more often when the unit is cold. It stays off until I do something to it. Turning the volume up to between 10:00 and 2:00 brings the left channel back in suddenly, and the volume can be turned down again for normal listening with both channels. If the unit has been warming up for a long time (overnight), but with no music playing, the volume trick will resolve the problem and the problem will not return until cooled down again. If the unit is cold (first one or two hours), the problem may or may not return (during that cold time), even after doing the volume trick.

Additional diagnostic info: when the unit is the sole source of amplification, the problem causes the left channel to drop out entirely. When I was biamping with the gain-matched ST-85 (IT-85 driving the higher frequencies, ST-85 driving the bass unit), the problem would result in dropping out the left channel high frequencies, but the left channel low frequencies would be boosted and distorted (tubbier, like the notes were made up of loud, low-frequency hum). If I was in a different room, that's how I would notice the problem (the bass got way loud and distorted). Connecting a separate preamp through the home theater bypass of the IT-85 allowed me to get the unit to a point where the problem was present (no left channel sound) when the IT-85's preamp section was part of the system, and not present (left channel operating fine) when the bypass switch was engaged (other preamp driving the IT-85's amp section). Sometimes engaging and disengaging the mute switch would get the left channel to come back, but that trick is not as reliable as the volume trick (volume trick always works). Also, I was able to get the left channel to return by hitting the unit with a rubber mallet, and to drop out by hitting it again. The repair person that suggested this diagnostic trick was unable to get the hammer to make it fail.

In case you have some simple suggestions to make, please note the following: it is indeed the amp, and not something else in the system. When auditioning my new speakers, I brought in the unit to the dealer's room, and the problem occurred (totally different system, upstream and downstream of the amp). Also, the amp is plugged in the wall socket, not a power conditioner or other filter. The unit was totally re-tubed by VTL when they had it, so that's probably not the problem. The tubes bias just fine, and the unit is not blowing fuses.

I'm having a hard time understanding why reproducing the problem is necessary for repair of the unit. Why can't my detailed description of the failure suffice to open up the amp and look for the problem? Reproducing the problem would only confirm that I'm not lying or delusional (remember: the problem occurred when the unit was dropped into a totally different system).

Thanks for any suggestions you might have. This has been a problem for 3.5 years now, and I feel like I'm out of options.

May the bridges we burn light our way....

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Topic - Mystery tube amp problem - can YOU solve it? - PranaBindu 15:17:15 01/19/15 (6)


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