June 16, 2010

I didn't know Vactrols could do that!

I recently repaired a 227. It has several problems: A dead channel, reverb wasn't working, I forget. It seemed like a quick turn around repair, until I sent it home and got the dreaded, "this thing's broken again!" phone call. Only now, it was completely shutting off the main output whenever it got hot. So, as long as I kept it on my bench, with air flowing around it, no problem showed up. After 5 minutes inside the boat, the main output went dead. This problem drove me nuts for a while, and in the end it was a Vactrol! Since the main output buss is quad, the panel volume control is actually a 4 channel Vactrol VCA. I guess diodes sometimes go open with heat if they're old enough. In this case the diode was a vintage Vactrol.