Re: TDS794D, alleged shorted power supply?


These power supplies have a power factor correction booster right up front.
This will start at around 90V DC, at which point the rest of the supply
will run full tilt. A variac is not advisable on these, as they will draw
the same power regardless of input voltage.
On the bench I ran the PSU under test with a lab supply, unloaded. These,
as most switchers will run fine on DC, so a current-limited lab supply is a
reasonable way to test for a short.

Þann þri., 28. júl. 2020 kl. 21:27 skrifaði Richard Knoppow <

Do you have or have access to a metered Variac? If you do
connect it and start with low voltage and watch the current
meter. Be ready on the Variac power switch. Current can come up
pretty fast even if there is no short but a short will generally
draw a lot of current right away. I would not be too concerned
with the seals since this is a "reject". How do you know the
line cord is good under load if you haven't powered it up?
Either put it on a metered Variac or dim bulb tester or stick
it in a display case.
Of course, I would open it up first and look for any signs of
something shorting. Have someone else follow you up, its easy to
miss even fairly obvious things.

On 7/28/2020 2:30 PM, Roy Thistle wrote:
I've got this TDS794D in skookum cosmetic shape... that is just daring
me to plug it in, and boot it up. Only... the... assumed... "dumpster
diver" I got it from claims... "It has a shorted power supply." Further
query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO,
anti-tamper/cal sticker on it... and none of the usual clues a thing has
been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to
work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted
power supply...that have crossed my mind... is that that is what SIMCO
reported... or documented... if the 794D was indeed sent to them for
cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least
to error checking, would it report and error indicating a shorted (low?)
power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to
watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service
flow chart... so if I dare to plug it in... and it starts to boot... I can
work from there. (I don't see anything about error reporting... or
explanations/table of error codes.)

Richard Knoppow

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