Voltages too far out of spec?

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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?



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Re: Voltages too far out of spec?
On 2019/12/01 9:00 a.m., Cursitor Doom wrote:
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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 :-#)#

Re: Voltages too far out of spec?
On 12/1/19 12:38 PM, John Robertson wrote:
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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
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Re: Voltages too far out of spec?
On 2019/12/01 11:02 a.m., Fox's Mercantile wrote:
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the
f a

ts

ch
  
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tance,
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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 :-#)#


Re: Voltages too far out of spec?
On 12/1/19 1:29 PM, John Robertson wrote:
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Even so, a 100K load shouldn't load down a power supply.


--  
"I am a river to my people."
Jeff-1.0
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Re: Voltages too far out of spec?
On 2019/12/01 11:53 a.m., Fox's Mercantile wrote:
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e

istance,
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ter-impact-measured-circuit/  
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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 :-#)#


Re: Voltages too far out of spec?
On Sunday, December 1, 2019 at 8:15:26 PM UTC-5, John Robertson wrote:
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e
  
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istance,
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ter-impact-measured-circuit/  
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I dealt with troubleshooting and repairing a lot of consumer tube TV, stere
os, and radios during the 60's and 70's. (Still have a collection of servic
e 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 th
e B&K 177 VTVM.  On occasion, I would see a Sencore or Simpson 260 multimet
er. 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 wi
th 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 l
ab 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

Re: Voltages too far out of spec?
On 12/2/19 12:26 PM, three_jeeps wrote:
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Hewlett Packard isn't consumer grade.
If they say 80v on a power supply, they mean 80v.



--  
"I am a river to my people."
Jeff-1.0
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Re: Voltages too far out of spec?
On 2019/12/02 10:45 a.m., Fox's Mercantile wrote:
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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 :-#(#

Re: Voltages too far out of spec?
On Monday, December 2, 2019 at 1:45:38 PM UTC-5, Fox's Mercantile wrote:
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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

Re: Voltages too far out of spec?
snipped-for-privacy@flippers.com says...
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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.


Re: Voltages too far out of spec?
On 12/1/19 2:29 PM, Ralph Mowery wrote:
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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.

--  
"I am a river to my people."
Jeff-1.0
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Re: Voltages too far out of spec?
On Sunday, December 1, 2019 at 12:00:21 PM UTC-5, Cursitor Doom wrote:
The medium voltages are not so close to what  
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Over the years, I've seen voltages printed on PCs that were incorrect for t
he 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 u
se a different transformer to feed it.  Since they have may have boards alr
eady 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.

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