This Hewlett-Packard 606A HF Signal Generator is another piece I’ve recently completed repairing and refurbishing in my quest to use the recommended (now vintage) test equipment to align my Yaesu FT-101ZD Transceiver. This unit looks great but it didn’t operate well when I got it on the bench. It is loaded with tubes for the various functions – the power supply alone has 7 or 8 – and quite a few tubes measured bad. Once I screened the tubes I was able to get good ones installed and get the -200V and +300V supplies checked out. Moving through each function in turn, I replaced a number of faulty components scattered throughout the unit, and I went ahead and replaced all the old multi-section electrolytic capacitors as well.
There were a lot of loose wires so I spent a good amount of time with the schematic, following the nets, and getting those wires put in the right places. One of the tuned tanks on the oscillator turret was broken free and hanging in place so I reattached – it worked surprisingly well after all of that. The final issue was that the copper brushes for the oscillator turret switches had broken loose, making electrical contact with the turret intermittent. Once sorted, this unit operates very nicely across all bands.
The feature I love the most is the crystal calibrator, which generates harmonics at 100kHz or 1MHz intervals and mixes them with the oscillator signal. As the oscillator is tuned toward one of these calibration points, the mixing product is audible using a front-panel connected crystal earpiece. Continue to tune the oscillator so the mixing product moves to lower and lower frequencies until it’s too low to hear, at which point you are tuned exactly on the 100kHz or 1MHz mark. The technique is called “zero beating,” and is actually quite popular in vintage HF radios as well. Once through the procedure, you can tune confidently using the front panel dial and it is quite accurate. I’ve verified with a modern frequency counter that the technique is surprisingly good and this oscillator is amazingly accurate.
