Notes from the bench

[Doc B.]

I'm the repair guy for the next couple of weeks while Kelvin studies for finals.

Yesterday he had a Seductor come back that he had repaired a few weeks ago. The issue was hum in one channel. Turned out to be a 6AU6 tube, which was determined by swapping tubes from side to side. We sent it back with a fresh, tested tube. Then we cooked the offending tube in overnight in our house Seductor and today it's nice and quiet.

This afternoon I had a Crack on the bench. The complaint was hum in both channels. As always, the first thing I did was wiggle the tubes in the socket. This created a lot of noise with the hum cutting in and out. Reflowing all the joints fixed the problem, which appears to have been a cold solder joint at pin B2.

A third repair, one channel out on a Stereomour II, took some sleuthing with a scope and required the replacement of a transistor that had blown due to intermittent connections.

Just want to point out that fairly often (two out of three cases here) the repair we do is something we explain how to DIY in the troubleshooting section of the assembly manual. It may be worth trying to reflow all the joints and to let the tubes run in for 50 or 100 hours before giving up on a hum problem.

[Reap]

I highly second this. I'm pretty ignorant about circuitry but have a natural incline to build things and do pretty well. I also took a couple of college classes on electronic fab. it seems they could not stress enough the importance of making sure you do not have cold solder joints. They also went in depth to show the causes and cures. I have had components I purchased that worked great, for a while. And then opened them up to find a cold solder joint that just needed to be reflowed and worked fine forever after. Even the factories suffer from this.

[jjvornov]

I have a BeePree that after being put back in the system had one channel that after warming up for a few minutes put out a loud rhythmic thump, not affected by volume or input. I thought there was no way I was going to figure out this intermittent problem, but dutifully put the preamp on my desk and started reflowing every solder joint I had access to. I figured I worked on the boards and joints under the boards on a second wave.

Sure enough, I found a 215 ohm resistor at C2 that had a broken lead at the body of the resistor. I remember wrestling all of wires at that tube socket into place and the short resistor leads I had ended up with. I ordered a replacement from Mouser hoping that there was nothing else going on.

Sure enough, after the repair the BeePre sounds fantastic once again. And I can now say there is satisfaction not only in building the gear but repairing it as well.

James