Moderators isaac42 Posted December 15, 2013 Moderators Share Posted December 15, 2013 Finally got around to looking at my 200S. The fuse blows a few minutes after switching from standby to operate. Fuse doesn't blow with no tubes. For reference, I have replaced the power supply filter caps with the SDS Labs circuit board (http://www.triodeel.com/dynaco.html) and the rectifier tube with diodes, also on the circuit board. I put one power tube back in. No problem. I removed that tube and put the other in. Fuse blew. Okay! Bad tube, right? Wrong. I put the first tube in the second socket, and the fuse blew again. So, something about that socket or the circuitry associated with that tube, but not the other one. Tricky. Looking at the schematic, I can't see anything obvious that might cause this. Time for some voltage measurements. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted December 16, 2013 Author Moderators Share Posted December 16, 2013 With no tubes, there were voltage discrepancies. On top of that, I made a dumb technician mistake. Not a bad one. My probe went through a hole in one of the socket terminals. No damage done, but when I did I noticed that the resistor connected to that terminal shorted out to the next terminal. Aha! That explained the voltage discrepancies. That's when I noticed the burnt resistor. Burnt bad. Couldn't tell the value. Back to the schematic.Result was that the resistor wire was shorting from pin 5, the grid, to pin 4, the screen. That'll draw a bit of current, even through a 100K resistor. Which is what burned out. So, moved the resistor away from the neighboring terminal and replaced the resistor. Replaced the tubes, fired it up. No problem.Until I started playing through it. Volume was low, and faded in and out. I remembered that I had encountered something similar in the past and that it had been the 6AN8 splitter/driver tube. Poked around until I found another. Replaced that, and voila! Amp is up and working fine.I'll leave it on for a while to make sure, but everything looks good. I assume that I pushed the resistor over toward the other terminal when I put in the SDS circuit board. Another dumb technician mistake. But one that looks to be fixed.Woohoo! Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted December 16, 2013 Author Moderators Share Posted December 16, 2013 So let that be a lesson to you. If you work on a piece of gear, and there's something wrong with it afterward, it's probably something you did. Link to comment Share on other sites More sharing options...
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