Sunday, September 16, 2007

When Good Amps Go Bad II

I previously explained that my Assemblage ST-40 power amplifier recently started blowing fuses. I have no idea how many ST-40s are still out there and in use, but I figured that I would post periodically on my progress, just in case others encounter the same problem and are looking for help.

The manufacturer or assembler of the old ST-40 kit was a Canadian company called The Parts Connection. TPC was, I believe, the original kit and parts company that later also made retail audio equipment under the name Sonic Frontiers. TPC fell on hard times, and the assets were sold to some of the old employees (and perhaps founders), who reopened the parts business under the name PartsConnexion. The website is at Although the new company is not formally the successor of the old one, there is I believe continuity of technical expertise.

For this reason, when I encountered my problem and was unable to figure it out, I emailed PartsConnexion. Within 24 hours, I received an email back that boiled down to the following: if the fuse still blows with all tubes removed, then the power transformer is shorted; if not, then one of the tubes is causing the problem.

Unfortunately, when I retested with all tubes removed, the fuse still blew. Looks like I need a new power transformer.

The power transformer in the amp is a Hammond H300338. Hammond is still in business, and PartsConnexion sells their transformers, but all the numbers seem to have changed. I've therefore emailed PartsConnexion for their advice on what I need. We'll see what they come up with.

The inside of the amp is pictured above. The underside of the power transformer is visible at the bottom center. You can click on the photo to enlarge. Note the tremendously bad wiring job, done by someone who at the time didn't know a resistor from a capacitor. Now I'll get to remove it and do it better.

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