Hello my friends!
It is time <pun intended> for some update...
Last friday I picked-up locally the eBay-special 57 Euro HP-5370A :-). It
was complete but unworking. Only thing which lighted in the front panel was
the 'oscillator clock loss' LED at the left side. Internally, fan run and
all the different power supplies LEDs were also on, as was the 10MHz CLOCK
LED. I checked all voltages and found them were all on specs.
What seemed to happen, IMO, is that the CPU didn't work. So I began to
study the Service Manual. Wow!. In order to work on the thing you need a
full array of extenders and an special card. Google told me it was a US$250
service kit!. Bad thing :-(!
OK, I said, go back to the basics: is the CPU clock working?. Some
oscilloscope probing later showed that it was not. Good point to start!. So
I went to A8 (REFERENCE FREQUENCY BUFFER) assembly troubleshooting, as it
is the source of the 10MHz clock to the CPU clock circuit. Manual shows
some waveforms and yes!, one of them was bad... obviously, the one which
has to do with the 10MHz CPU oscillator. Some more probing later I decided
one of the transistors in the circuit could be bad. But I replaced them
(both 2N2904) to not avail... Then I saw a couple tantalums and a
suspicious small resistor which seemed overheated. Looking at the manual it
was no less than a fuse resistor. And it was open... and one tantalum was
shorted. Yes, this supply goes only to the 10MHz CPU clock generator circuit!
After replacing the burnt resistor and the tantalum (with a 105C 10uF/50V
electrolytic as I hate tants!), I powered up the unit... and IT WORKED!!!
I think I made a good buy :-)!
Unit has some cosmetic damage and one button does not work properly (it
sticks) but with a careful cleaning and some front panel work it should end
as an 8.5+ looking instrument.
Now I have to learn how to properly use it. I will sure ask you ;-)
Best regards and sorry for the ramblings but I am a very happy camper today!
JOSE
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73 EB5AGV / EC5AAU - JOSE V. GAVILA
La Canyada - Valencia (SPAIN) - Loc: IM99SM
Vintage Radio: http://jvgavila.com
Vintage Test Equipment: http://jvgavila.com/testeq.htm
European Boatanchors List: http://groups.yahoo.com/group/euro_ba_swap
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