On 2019/12/01 11:53 a.m., Fox's Mercantile wrote:
On 12/1/19 1:29 PM, John Robertson wrote:
On 2019/12/01 11:02 a.m., Fox's Mercantile wrote:
On 12/1/19 12:38 PM, John Robertson wrote:
On 2019/12/01 9:00 a.m., Cursitor Doom wrote:
Gentlemen,

On the HP audio spectrum analyser I'm now working on, I've found
all the
low voltage regs seem fine (sub 30V I mean) within a few hundredths
of a
volt to what they should be. The medium voltages are not so close
to what
those printed on the PCB state they should be, however. I've got
points
on the board that are supposed to be 150V reading 170V and (less
far out)
an 80V rail reading 84V. 150 reading 170 seems to me to indicate a
regulator gone short, possibly. Anyway, question No.1 is: has anyone
encountered readings that were this far out from those stated but
which
turned out to be nothing to worry about in the end?
Question No.2 is: should I investigate the 80V -- 84V anomaly?




On the HP schematic notes what was the VOM loading factor? Back in
the day 100K Ohms/volt (analog meter) was not unusual so your
"over-voltages" may simply be the result of under-loading the
circuit with your modern DVM.

John :-#)#

If aÂ* power supply voltage can be loaded down by a 10-20 Meg resistance,
there's something seriously wrong with the supply.



Sorry, I believe I was thinking of 1000 Ohms/V meter when I was
speaking of loading.

https://www.allaboutcircuits.com/tex...sured-circuit/


Spoiled by DMMs one gets rusty on one's assumptions.

Thanks!

John :-#)#


Even so, a 100K load shouldn't load down a power supply.



I do recall schematics of old (mostly tube equipment) warning people
that the readings were done with a specific Ohms/Volt meter and that
readings would vary accordingly.

Mostly it is something to keep in the back of your mind when reading
schematics and voltage test points...

Nice chatting with you!

John :-#)#