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Troubleshooting a variac

whoaru99 said:
Confirm if you actually loaded it down rather than just reading or measuring the voltage.
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Looking at the schematic I see there is a 470 ohm 2 watt resistor (R260 that is connected (shunted) between the rectifiers output and ground and that should drain, if not open, and keep the the big main filter cap`s DC from floating to any higher voltage level than the desired dial selected level.
 
Bill Ferris said:
Looking at the schematic I see there is a 470 ohm 2 watt resistor (R260 that is connected (shunted) between the rectifiers output and ground and that should drain, if not open, and keep the the big main filter cap`s DC from floating to any higher voltage level than the desired dial selected level.
Click to expand...

That's the unstated, sly aspect behind my question. There has to be something to discharge the cap. The resistor should, but so would a load. Point being, does an actual load on the DC output change the symptom.

Bear in mind this appears not to be a regulated power supply. All the ICs and such are for the meter display scaling and function.
 
Even with the 470 ohm resistor across the cap, it could take an unexpected long amount of time for the voltage to drop.

If you dial it up to 28V with no external load, then dial back the adjustment to say 14V, if I did the calc properly it could take something like 30-40 seconds for the voltage to get down to display 14V just on the internal resistor alone.

If the OP is expecting the voltage display to respond quickly/immediately that might be part of the issue.
 
whoaru99 said:
That's the unstated, sly aspect behind my question. There has to be something to discharge the cap. The resistor should, but so would a load. Point being, does an actual load on the DC output change the symptom.

Bear in mind this appears not to be a regulated power supply. All the ICs and such are for the meter display scaling and function.
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Yes, I understand your reasoning, as I observed by looking at the schematic about the metering`s circuitry`s requirements, a set of alligator jumpers connected to a table lamp, etc. with a medium wattage incandescent bulb fitted & it`s power plug connected to the DC output would easily be household readily be available and used as a quick test to pull the main cap`s voltage down if R26 is in fact open with the feeding variac turned all the way down.
 
whoaru99 said:
Even with the 470 ohm resistor across the cap, it could take an unexpected long amount of time for the voltage to drop.

If you dial it up to 28V with no external load, then dial back the adjustment to say 14V, if I did the calc. properly it could take something like 30-40 seconds for the voltage to get down to display 14V just on the internal resistor alone.

If the OP is expecting the voltage display to respond quickly/immediately that might be part of the issue.
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This ^^ calculated 30~40 sec. time constant would be for a fresh up to snuff cap.
How old is this PS, I wonder ?
 
Bill Ferris said:
This ^^ calculated 30~40 sec. time constant would be for a fresh up to snuff cap.
How old is this PS, I wonder ?
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Is a point, yes, but being what looks to be one of those big "computer grade" type caps, I'd guess it's not going to be super huge difference. I have some pretty old ones albeit not quite that old, but still have respectable leakage numbers.

Would be nice if the OP reported back on the results of actually applying a load to the DC output.
 
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whoaru99 said:
I'd be surprised if it's actually 40-50 years old. Ain't no spring chicken for sure, but 40-50 seems like could be a stretch.
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Eighty four is 40 yrs. ago this yr.
And 1974 is just 10 yrs. older.
I might inclined to speculate the unit is probably a eighties ~early nineties manufactured device.

The date code on the big main DC computer grade cap would be telling, at least it`s date would not be after the PS was manufactured and possibly short months before, in any case, as those are a rather expensive component to stock pile in large amounts while waiting to build a device that wasn`t going to be in great demand and shipping out in great quantities.

I have no recollection of ever seeing that particular combo AC/DC PS.
 
I was guesstimating 30-ish at the outside based on 1991 copyright date and 2002 revision date showing on the manual. But, who know if there were older iterations.

The other thing is would some of those ICs even been around 40-50 years ago? I dunno...

Anyway, that point seems largely academic. The capacitor seems to be staying charged and, afaik, advanced age of the capacitor would only make it drop faster, not hold out longer.
 
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whoaru99 said:
The capacitor seems to be staying charged and, afaik, advanced age of the capacitor would only make it drop faster, not hold out longer.
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I absolutely agree !
Could the DC voltage controlling variac malfunctioning with shorted windings, some how and not be able to reduce it`s output all the way down to zero ?
 
Possible, but I thought it was said the AC outputs work as expected. The DC runs off that variac just the same, albeit through step down T2, rectified, and filtered.

Probably not a whole lot more to say/discuss without some feedback from Larryhere40.
 
whoaru99 said:
Is a point, yes, but being what looks to be one of those big "computer grade" type caps, I'd guess it's not going to be super huge difference. I have some pretty old ones albeit not quite that old, but still have respectable leakage numbers.

Would be nice if the OP reported back on the results of actually applying a load to the DC output.
Click to expand...
Bill Ferris said:
whoaru99 said:
Possible, but I thought it was said the AC outputs work as expected. The DC runs off that variac just the same, albeit through step down T2, rectified, and filtered.

Probably not a whole lot more to say/discuss without some feedback from Larryhere40.
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After
I absolutely agree !
Could the DC voltage controlling variac malfunctioning with shorted windings, some how and not be able to reduce it`s output all the way down to zero ?
Click to expand...
 
whoaru99 said:
Possible, but I thought it was said the AC outputs work as expected. The DC runs off that variac just the same, albeit through step down T2, rectified, and filtered.

Probably not a whole lot more to say/discuss without some feedback from Larryhere40.
Click to expand...
Got back, looking at it, after changing the regulated voltage transistor, I never applied a large test bulb, just use the voltmeter. So after applying a larger draw, it seems to work fine now. Seems like it needed more than just the meter. Thanks for all the help.
 
Cool.

You may want to check that 470 ohm resistor though. It should eventually come down on its own albeit sorta slowly from that.

If that resistor is open or not connected as Bill mentioned, it would take a VERY long time to come down without external load.
 
Larryhere40 said:
Got back, looking at it, after changing the regulated voltage transistor, I never applied a large test bulb, just use the voltmeter. So after applying a larger draw, it seems to work fine now. Seems like it needed more than just the meter. Thanks for all the help.
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Ah, more info leaks out with updates on this not common and unique bench variable PS !

Thanks for finally letting us know !

10 meg per volt sensitivity DVMs don`t load down squat (rightly so), the PS was all but measuring air equivalent (all but no loading)

Member whoaru99 was the individual who first exclaimed the no/low risk of misinterpreting your no load output voltage DVM readings, I just followed up reinforced his reasonably suggestive trouble shooting theory !
 
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