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Old 17th May 2006, 02:01 AM   #1
Dylan is offline Dylan  United States
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Location: Oregon, USA
Default Trouble with my "new" PAS-3 Preamp

Hello, I've been lurking here for quite a while, but haven't posted much at all. So here's a long one!

First off, I'm fairly new to tube hi-fi, and I recently bought a Dynaco PAS-3 preamp along with the SDS Labs filter capacitor/heater supply upgrade board. I figured this would be a nice affordable preamp to start with.

I gave it a test shortly after it arrived, and found that the left channel played while the right channel was nearly nonexistent (No, I didn't turn the balance all the way left! :P). Checking voltages on the tube pins showed that they were all over the place, so today I carefully installed the SDS cap board hoping that it would fix this.

I fired it up and nothing exploded!

However, the left channel is now heavily distorted and the right still won't work! Back I went with my multimeter to the tube pins. All were very close to what they should be, except for two pins on the right channel 12AX7 of Board PC-6.

Pin 6 should be 200V but reads 311V
Pin 8 should be 1.25V but reads 0.21V

The other tube's pins on the same board check out fine.

Troubleshooting began, and this is where my knowledge isn't so strong.

First, I inspected the board for shorted traces, bad joints, or solder crumbs and clippings from installing the SDS board. No problems. I pulled up one side of each of the resistors connected to the tube and tested them. All were within tolerances. All caps are ceramics and new orange drops, and all tubes had tested good for emission and shorts.

My SDS board installation was ruled out because it is giving me the proper voltage, and all the other tubes' pins check out fine.

I've hit a wall. I don't know if these voltages are causing my initial problem, but they certainly are well out of spec. I'd appreciate any input on this, I really want to hear it play.

I have a schematic of the pre-amp that has the voltages at all tube pins which I've been using for troubleshooting, so I can refer to it if anyone has suggestions.

Thanks for any help!
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