开云体育

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