Troubleshooting a variac

[Larryhere40]

I have a elenco xp-800 variac! Doesn’t adjust with the DC voltage. Works on all the others stays at the top DC voltage and doesn’t go down when you turn the knob replaced a couple transistors and a voltage regulator, not a lot of other stuff in there. Any ideas on what would cause that? Capacitor? Any advice would be great for what to look for. Thanks.

 

[rickl]

if this is the schematic, not a lot of parts.

I'd check the power supply but my variac is an old school.

 

[Larryhere40]

if this is the schematic, not a lot of parts.

I'd check the power supply but my variac is an old school.

This 40 to 50 years old! No transformer or anything, just a big giant poinometer kind of thing used to change the voltage on both AC and DC from the wall plug of 115

 

[Larryhere40]

This 40 to 50 years old! No transformer or anything, just a big giant poinometer kind of thing used to change the voltage on both AC and DC from the wall plug of 115

Seems to be in the small printed circuit board, because all the rest work fine

 

[petehall347]

first off it's not a variac .its an adjustable power supply . post high res full size pics of the insides .

 

[Larryhere40]

 

[Larryhere40]

View attachment 3314175View attachment 3314178View attachment 3314179View attachment 3314180

first off it's not a variac .its an adjustable power supply . post high res full size pics of the insides .

just wondering, as I am a novice, what is the difference other than, ac and dc, is the difference? Thanks Larry

 

[petehall347]

variac = variable ac . well that's how i see it .

 

[Robisme]

Is that 8 pin dip supposed to be the regulator? Is it a variable regulator?

perhaps that transistor in the heat sink is shorted. Unless that’s one you replaced.



Rob

 

[whoaru99]

There is definitely a variable autotransformer (aka variac) in there. That's the coppery winding gizmo hooked to the bigger knob.

That variac (T1) provides the variability for the straight AC 0-120V output and through the step down (T2) the variable 0-40AC and varies the AC input to the DC power supply for that adjustability too.

 

[Larryhere40]

Is that 8 pin dip supposed to be the regulator? Is it a variable regulator?

perhaps that transistor in the heat sink is shorted. Unless that’s one you replaced.



Rob

Replace that one. It has a voltage regulator of 7805 I replaced that also.

 

[Larryhere40]

Replace that one. It has a voltage regulator of 7805 I replaced that also.

Thanks

 

[whoaru99]

Have you put a load on the DC output to see what happens? That's a pretty big capacitor and maybe holding the high charged voltage without a load.

 

[Larryhere40]

Thanks

Everything works on the AC side. The DC side stays on the high end doesn’t reduce.

 

[Larryhere40]

Have you put a load on the DC output to see what happens?

It works, just that it doesn’t adjust

 

[whoaru99]

It works, just that it doesn’t adjust

Confirm if you actually loaded it down rather than just reading or measuring the voltage.

 

[Larryhere40]

Confirm if you actually loaded it down rather than just reading or measuring the voltage.

Not a full load, just load of an amp

 

[whoaru99]

Sorry.. 1A current on the DC output?

 

[Larryhere40]

Not a full load, just load of an amp

Actually the last time I just did a reading

 

[Larryhere40]

Actually the last time I just did a reading

After changing a couple transistors and what not