Voltages too far out of spec?

Cursitor Doom

unread,

Dec 1, 2019, 12:00:21 PM12/1/19

to

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?

--
This message may be freely reproduced without limit or charge only via
the Usenet protocol. Reproduction in whole or part through other
protocols, whether for profit or not, is conditional upon a charge of
GBP10.00 per reproduction. Publication in this manner via non-Usenet
protocols constitutes acceptance of this condition.

John Robertson

unread,

Dec 1, 2019, 1:39:06 PM12/1/19

to

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 :-#)#

Fox's Mercantile

unread,

Dec 1, 2019, 2:02:41 PM12/1/19

to

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

--
"I am a river to my people."
Jeff-1.0
WA6FWi
http:foxsmercantile.com

John Robertson

unread,

Dec 1, 2019, 2:29:57 PM12/1/19

to

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

https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/

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

Thanks!

John :-#)#

Trevor Wilson

unread,

Dec 1, 2019, 2:36:46 PM12/1/19

to

On 2/12/2019 4:00 am, 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?
>
>
>

**Are the Voltages that are out of whack at the end of a Voltage
regulator, or are they unregulated Voltages? Is yours mains supply bang
on spec?

--
Trevor Wilson
www.rageaudio.com.au

John-Del

unread,

Dec 1, 2019, 2:37:04 PM12/1/19

to

On Sunday, December 1, 2019 at 12:00:21 PM UTC-5, Cursitor Doom wrote:
The medium voltages are not so close to what
> those printed on the PCB state they should be, however.

Over the years, I've seen voltages printed on PCs that were incorrect for the version of board as loaded. IOW, manufacturers will often make running changes to a product design and revise parts loaded on the board and even use a different transformer to feed it. Since they have may have boards already screened, they're not going to scrap them out.

If HP has archived schematics, they may have a revised schematic that more closely or even exactly matches the voltages you're encountering.

Fox's Mercantile

unread,

Dec 1, 2019, 2:53:18 PM12/1/19

to

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

Ralph Mowery

unread,

Dec 1, 2019, 3:30:03 PM12/1/19

to

In article <mYWdnU_ED71flXnA...@giganews.com>,
sp...@flippers.com says...

>
> 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.
>
>
>

On some of the old vacuum tube circuits I have seen schematics with
several voltages at test points and a note as to a 20,000 ohm/volt or a
VTVM is being used for the test.

Probably won't see much of that any more as with the solid state most
resistances are low enough an analog meter will not load it enough and
most are using digital meters to do the testing.

Fox's Mercantile

unread,

Dec 1, 2019, 4:25:04 PM12/1/19

to

Those were almost always grid voltages.
Those DID require a high resistance volt meter to not load them down.
The other voltages in the set, plate, screen filamant were not that
critical about loading.

John Robertson

unread,

Dec 1, 2019, 8:15:26 PM12/1/19

to

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 :-#)#

three_jeeps

unread,

Dec 2, 2019, 1:26:44 PM12/2/19

to

I dealt with troubleshooting and repairing a lot of consumer tube TV, stereos, and radios during the 60's and 70's. (Still have a collection of service manuals filling 4 large bookcases from my fathers shop). One of the most cited volt meters in the service manuals and schematics in that era was the B&K 177 VTVM. On occasion, I would see a Sencore or Simpson 260 multimeter. As consumer electronics moved to all solid state (80s), I saw a B&K 3-1/2 digit DMM specified (cant remember the model). At some point we went with HP gear.
I think they speced equipment that was what the TV technicians were buying, as lab bench gear was quite a bit more expensive and did not require the lab quality accuracy & repeatability numbers.

Occasionally I would run across a voltage tolerance spec on a schematic or SM, something like 5%, but dont recall exactly...
J

Fox's Mercantile

unread,

Dec 2, 2019, 1:45:38 PM12/2/19

to

On 12/2/19 12:26 PM, three_jeeps wrote:
> Occasionally I would run across a voltage tolerance spec

> on a schematic or SM, something like 5%, but don't recall
> exactly...
> J

Hewlett Packard isn't consumer grade.
If they say 80v on a power supply, they mean 80v.

John Robertson

unread,

Dec 2, 2019, 2:14:32 PM12/2/19

to

On 2019/12/02 10:45 a.m., Fox's Mercantile wrote:
> On 12/2/19 12:26 PM, three_jeeps wrote:
>> Occasionally I would run across a voltage tolerance spec
>> on a schematic or SM, something like 5%, but don't recall
>> exactly...
>> J
>
> Hewlett Packard isn't consumer grade.
> If they say 80v on a power supply, they mean 80v.
>
>
>

Yes, I would have agree, His readings indicate power supply regulation
issues not VOM errors.

My mistake for going off on this tangent!

Sorry about that wild goose chase,

John :-#(#

three_jeeps

unread,

Dec 3, 2019, 12:08:10 PM12/3/19

to

I know that about HP. Been specing lab equipment for my uses in our/my labs for 35+ years. Apologies if it came across that way