[Test-Equipment] HP-140T/8555A/8552B/8445A repair (II)

[Dave Brown]

If the oven assy has been taken out of the 5245M counter, the associated
'always on' power supply has probably been taken as well, but maybe nothing
else.
Try selecting 'external timebase' with the rear panel switch, and feed 5 or
10 MHz at 1v rms or better into the timebase BNC -also on the back panel.
The input Z is high-(10K ohm?) so it's not too hard to get the level needed.

The oven assy power supply needs modifications anyway as it has no
overvoltage protection-I had to fix the mess in the oven that resulted when
that happened to mine. There's a big fat zener and added fuse sitting in
circuit now so it doesn't happen again. It's worth upping the voltage rating
on the series pass transistor as well-any half decent mains spike will take
it out.(I suspect that's what happend to mine in the first place.)
73
 Dave
ZL3FJ

----- Original Message ----- 
From: "Jose V. Gavila" <eb5agv at ctv.es>
To: <test-equipment at mailman.qth.net>
Sent: Monday, June 21, 2004 6:19 PM
Subject: [Test-Equipment] HP-140T/8555A/8552B/8445A repair (II)


> Hello!
>
> I have good and not so good news about my HP-140T system repair. I have
two
> good mainframes (another 140T and a 141A) but I wanted to get this 140T
> working as the other two host 8553B/8552B and 8554B/8552B pairs, so I
> looked at it yesterday to find that the usual LV supply voltages (-100V,
> +100V, +250V and -12.6V) were fine except for the last one, which was -22V
> on startup (without plug-ins) and then slowly drifted to close its nominal
> voltage. Circuit is not too complex but it took me some time to locate an
> open 432K resistor (without getting it out of the circuit I got a
> reasonable value). Once replaced, I was able to adjust all voltages to
> specifications and mainframe worked fine with plug-ins on it. Nice!!!
>
> So, once I had the mainframe working, I got back to the 8555A. I have not
> yet got a suitable counter to deal with the LOs (as it seems to me that
the
> YIG is not working) so all what I could check was DC voltages. All were on
> specs. My plan was to get an HP-5254C plug-in to be able to use my
HP-5245M
> up to 3GHz, which would be enough to check LOs (if not at full range).
> There was a unit at eBay which ended today at 3:10AM Spanish time (!) but
I
> have been brave enough to bid on it and have won the bidding war... BUT
now
> I have found that the 5245M I have is not working :-(!. It is missing
> timebase and, probably, some other items. Well, I had forgotten about
these
> 'small details'...
>
> So now I need some parts to get the 5245M working, to be able to use the
> 5254C once I get it here. Is there any available for parts?. Could I use
> the lower-spec 5245L timebase, if I locate one?
>
> Well, all in all I am happy as things are starting to work on this
project.
> And, for sure, I will learn a lot in the process :-)!
>
> Best regards,
>
> JOSE
> ----------------------------------------------------------------------
>   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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