HP 651B Oscillator restoration and an OOPS!!
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651B 1975 edition.
Replaced all of the electrolytics and tantalums, quite a chore to get the exactly correct values Several caps were replaced with two or three in parallel. Same or better tolerance and voltage. BEWARE. I bought what were advertised as Spragues from Mouser, they came with 30-50 YEARS of oxidation on the leads. Severe oxidation which indicates either very old stock or storage in an aggressive atmosphere. The caps in the unit had much less oxidation. Anyway, went most of the way through alignment, the unit is close enough in 2020 with easy availability of an external freq counter, that adjusting resistor values wasnt necessary. Didnt want to risk damaging the switch wafers. Back in the day, counters etc werent as cheaply available. Now they are and its not necessary to tweak this unit right on frequency, especially that its a mechanical VFO. Points:
* replace ALL electrolytics. No old stock, get new fresh stock from Digi Key only. They religiously track stock and age. This is (or was) high end test equipment, not an old stereo.
* Ranges dead- cleaned switch segments with Q Tip and isopropyl alcohol. DELICATE WORK.
This fixed the 'dead range' problem. There was a LOT of black oxide residue. Spraying cleaner is NOT enough, because that doesnt clean the inside surfaces of the switch fingers that ride on the wafer segments.
* Used a Beckman DMM to monitor TP2, no 1K resistor needed. 10 Mohm input.
* used an AWG and good analog scope in place of distortion meter (its just a sine wave...)
Fed reference signal at 1Kc into one scope channel and the HP gen output into the other, then synchronized the frequencies and matched amplitude, then did an ADD and SUBTRACT on the scope channels. The trace was dead flat- no distortion
* when checking levels and tracking, FIRST check for spurious oscillation. If there is any, REDUCE the value of A2C21, 12 pF. Its very touchy, I used 10 pF leaving a very small amount of oscillation. Avoid changing the resistor value, it reduces output voltage.
Even after 30 years experience on the bench, I did something very bad..,.
Id shorted A1C11 with a pencil lead attempting to mark the stator plate with power on and destroyed A1Q9 and A1CR9 in the Monitor circuit. Yes, its a very sensitive circuit. Well, it **was** a very sensitive circuit... Anyone have parts? (yes I just put an ad up...) Am considering replacing the Monitor circuit and meter with a voltmeter unit. Thoughts?
73 Dave
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