Last month I was at the UK “National” hamfest and in the flea market area a
seller had a HP 58503B for sale. It was missing the cover and power supply (AC
mains version)including mounting. However it was only £3 (<5$) so it was a
no-brainer to buy it. Research shows that PSU failure is fairly common on these
units. There does not even seem to be an assembly level service manual readily
available for the 58503B never mind a CLIP. I could not identify the power
supply used but assumed it was a standard 3rd party SMSU. Connecting up to a
triple output bench PSU (+5V, +15V -15V) and antenna and the unit powered up
and locked. Checking he fault log confirmed that the PSU had failed with
voltage out of range. I measured the current on the supplies and surprisingly
the current on the -15 supply was the highest during warm-up at around 750mA.
Once warmed up the -15V current fell and +15V current went up. Unfortunately
all the 30- 40W SMPSUs (the 58503B is rated t 35W max input) I had handy had
significantly lower -15V supply current ratings. I decided to build a mostly
linear supply. The basis was a dual secondary 15V toroidal transformer, two
full wave bridge rectifiers, 7815 and 7915 regulators and a switching “7805”
replacement for +5V. Calculations for a 30VA transformer show that the winding
for the negative regulator would be slightly overloaded during oven warm-up
however the “positive” winding will be well under loaded at this point because
the +15V current is low. So saturation is not an issue and the total I2R
heating is less than for full rated load on both windings. So I knocked this up
with a 30VA toroid, an a 194mm x 90mm alloy plate and a couple of bits of
extruded angle to pick- up on the existing locations. A 20mm fuse holder was
added to an existing hole in the back panel. A cover for a 53131A was bought on
ebay for $20 (I was in the USA for 10 days so got it delivered to my hotel so
postage was cheap). So it all worked fine but the GPS week roller-over bug was
there. Lady Heater corrects this but I had some other Oncore timing receivers
about so I tried a VP and it gives the correct date and operation. I have
another GPSDO for under $50.
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