- Voltages too far out of spec? - 10 Updates
| Cursitor Doom <curd@notformail.com>: Dec 01 05:00PM 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 <spam@flippers.com>: Dec 01 10:38AM -0800 On 2019/12/01 9:00 a.m., Cursitor Doom wrote: > 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 <jdangus@att.net>: Dec 01 01:02PM -0600 On 12/1/19 12:38 PM, John Robertson wrote: > "over-voltages" may simply be the result of under-loading the circuit > with your modern DVM. > John :-#)# 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 <spam@flippers.com>: Dec 01 11:29AM -0800 On 2019/12/01 11:02 a.m., Fox's Mercantile wrote: >> John :-#)# > If a 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/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/ Spoiled by DMMs one gets rusty on one's assumptions. Thanks! John :-#)# |
| Trevor Wilson <trevor@rageaudio.com.au>: Dec 02 06:36AM +1100 On 2/12/2019 4:00 am, Cursitor Doom wrote: > 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 <ohger1s@gmail.com>: Dec 01 11:37AM -0800 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 <jdangus@att.net>: Dec 01 01:53PM -0600 On 12/1/19 1:29 PM, John Robertson wrote: > 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 am a river to my people." Jeff-1.0 WA6FWi http:foxsmercantile.com |
| Ralph Mowery <rmowery28146@earthlink.net>: Dec 01 03:29PM -0500 In article <mYWdnU_ED71flXnAnZ2dnUU7-VfNnZ2d@giganews.com>, spam@flippers.com says... > 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 <jdangus@att.net>: Dec 01 03:24PM -0600 On 12/1/19 2:29 PM, Ralph Mowery wrote: > 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. 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 WA6FWi http:foxsmercantile.com |
| John Robertson <spam@flippers.com>: Dec 01 05:15PM -0800 On 2019/12/01 11:53 a.m., Fox's Mercantile wrote: >> 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 :-#)# |
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