Monthly Archives: August 2013

Bad diodes and bad caps

No schematics, but found a few leaky caps. Pulled them off the board, the rails went to 4.7, 11, 38 and the odd one out, the -12 was at about +4.7… see a pattern emerging?

Going to trace the -12 and see if it’s decoupled anywhere.

Looks like this has been a problem for a while because the -12 smoothing capacitor has bulged out. Yuk.

HP 700/44 Terminal

Rescued an HP 700/44 terminal from work today. Going to be discarded so I figured what the hey and snagged it.

Got it home, tried the power on it to check for bangs and smoke. It liberated neither, but it didn’t come on either. No raster, no beeps.

Pulled it apart, checked the voltages.
12 was about 7.6, 5 was dead in the millivolt range and 38 was a healthy 38 on the nose.

Lifted the board and tube out. Sometime in the past it’s had a repair, the warranty on it was out in 2001. Probed around the power supply section of the board, two of the three 33D diodes tested funny and looked a bit toasty so I de-soldered then and tested them out of circuit. They tested good so went back in.

I’m at a bit of a loss with it. I’ve found the user manual but that’s not much help right now.

I don’t think I can power it up out of the case either because the HT is integral to the board. The cathode heater glows, and that’s about it. Somehow I’ve managed to stop the latch mechanism on the power switch from working also, the power switch is now just momentary…