As I was dropping off some surplus household items at the local recycling center where people can leave or take useful items, I observed an older man come in with a perfect-looking Heath SB-1000 linear amplifier and place it on the shelf.
I immediately pounced on it and the owner said it had a weak 3-500Z and a broken meter switch. I took it home and will do the following with it:
- Replace the broken meter switch with a quality ceramic and silver switch. The old Heath switch is a cheap plastic thing.
- Test and reform the Aerovox filter caps and ensure the HF supply is working properly. Replace any items as necessary.
- Add a HV fuse of 750 or 1000 ma, immediately after the rectifiers, to blow should any downstream short occur, including the filter caps. This will be encased in a protective housing, taken from a junked microwave oven, to prevent the fuse from sending glass shards around the amplifier’s chassis, in case it blows.
- Add a 50 ohm “glitch” resistor immediately after the HV fuse, to absorb brief overloads. Wattage TBD after I explore the junkbox.
- Investigate a way to gently activate the HV when the HV switch is thrown. The power transformer contains both the filament and HV windings so this will require some creativity. Perhaps only a 1000 ohm wirewound resistor in the HV secondary winding’s output. Old school, but might work.
- Replace the two small DC blocking capacitors in the pi network with a more robust “doorknob” type.
- Check out the meter lamps to see if they are working.
- Replace the HV indicator lamp, which is physically broken.
- “Rejuvenate” the original 3-500Z that was said to be weak, using a slightly higher filament voltage for about 30 minutes.
- Once it’s reassembled, bring it up to power slowly using the Heath assembly manual to properly test it using a driver (Yaesu 840 xcvr), SWR meter and dummy load, with a field strength meter to watch the leaking RF signal if present.
- Then conduct some on-the-air testing, using the original 3-500Z tube that the owner said was “weak.” Should this tube prove unusable, I have a new Eimac 3-500Z that I bought for $95.