My 8643A started doing a "recalibrate" every time I turned it on. Once the 8643A completes that "recalibrate" its RF output and frequency are OK, although I have not tested every aspect of modulation, sweep, etc. I assumed it was failure of the memory backup battery, but it was original and OK. But old and leakage would be a disaster, so replaced the battery with three AAA NiCad. Still "recalibrated", as expected.

The fault message at the end reported the 5.25V supply was not within spec. I checked a test point at the output of the power supply filter board and found less than 5.0V. Having experience with tired computer grade capacitors I assumed that would be the case here. Surprise: no transformer supply with big caps, just two switchers. I pulled both switching supplies and found that both the oven switcher and the main 5V/15V supplies voltages were OK. Particularly the 5.25 I tested with a 2A load current and it was perfect.

All capacitors in both supplies with Nichicon 105C and operating at a fraction of rated voltage. Nonetheless, considering the age and the disassembled state of the 8643A I have all new capacitors (again Nichicon 105C which were also my choice when I was working). I have replaced caps and retested the 24V oven switcher. The last capacitor for the main switcher is supposed to arrive today and I expect the main supply will test OK by tonight. Out of curiosity I tested the removed capacitors and all but one exceeded the rated capacitance and had tiny leakage current.

It appears I am losing about 300mV between a supply at 5.25 volts and the connector at the output of the power-filter board. Something drawing too much current? It would have to be a seriously large amount of current. That seems unlikely given that it seems to be working. All DC supplies pass through a passive filter board with series inductors, shunt capacitors.? With the supplies still out and the filter output connector disconnected, I will measure the shunt impedance of that filter board. Anyone with 8643A experience have any other ideas?

Bill

Hey everyone,

Having recently picked up an Hakko 474 I've decided to desolder every electrolytics in the PSU to test them before ordering some.
To my surprise they're all well within specs, close to new. I now understand why Sprague is regarded as such a good brand, I'm leaving them in. I'm only replacing the blown tantalum on the A7 board.

I'll post another update when some DC calibration will happen.

Arnaud.