[time-nuts] Re: Power supply buzzing in HP 58503A GPS receiver

[Chris Quayle]

Recently brought home 58503, cheap at industrial auction.

The power supply was faulty, seriously burnt  pcb area around
the line input circuit. Fixed that and put back together.

The psu runs very hot in the 58503, no fan cooling, so not
surprised it eventually failed. There is a design fault,
in that there is wirewound dropping resistor at the line input,
one assumes to lose volts to support the full range of input
line voltages. Perhaps originally a 115v psu, but hacked to
support a higher range of voltages. Anyway, that resistor
gets >100C and is placed right next to the input reservoir
capacitor. To improve reliability, bought in a TO220 film
resistor, mounted to the side of the case, which takes some of
the heat away from the input cap. The psu runs quite a  bit
cooler overall now. Long term, will probably find a more modern
equivalent psu, as the original is marginal, design wise...

Chris