varistor touble

[ballzdeep]

Hello all. I am up to my hips in a crate vc-6112. Replaced all caps and diodes in power supply circuit but am having trouble finding the value of the varistor. Not very sure what i can get away with as far as replacing it or bypassing it. I know it would be safer to replace it but with what? I have schematics but they are not helping with the value. I have attached them. The component in question is VR1 located between the fuse and the line connectors.

[Gtr_tech]

There will be a part # right on the varistor itself. Reference that number and get the ratings. Any MOV with those ratings will work. Looks like its there to create a soft start at pwr up since its in series with the primary.

[ballzdeep]

the component itself has a burned hole on the side with id numbers. only number i can make out is 10008. tried looking up lots of manufacturer's site but not coming up with anything. Parts list that came with schematics says part # 73-501-07 description 7A also tried looking up with these numbers with no luck.

[Enzo]

Varistor? MOV?

This is a thermistor - a series inrush limiting resistor. Its resistance drops after it warms up - a second or so after it turns on.

Varistors are voltage surge suppressors, usually parallel with the mains.


Says 7 amps next to it on the drawings.

Is yours bad?

Its value is not especially critical. One for any similar sized amp ought to work fine.

[ballzdeep]

yeah i am pretty sure its bad unless it is supposed to shoots sparks and smoke before the fuse blows several of the filter caps had popped open and were charred as well.

[Enzo]

Then they are what is causing IT to burn up. You can replace it with a piece of wire during testing.

Look up "light bulb limiter" - make one and use it.

If one cap fails, that is a bad cap. If a bunch of filter caps fail, it sounds like they are getting the wrong voltage - often as not it means AC comiong through a shorted rectifier. Check all the rectifiers in the power supply while you replace caps.

Then again I guess it matters WHICH caps. A shorted power tube can route high voltage to many other places it don;t belong.

[ballzdeep]

i replaced all that first but was not sure if i could bypass VR1 to test what else may be wrong. is thermistor the same symbol as varistor. will look that up. any other advice\tips for trouble shooting this area of the amp would be most welcome.

[Enzo]

The bulb will take its place.

This is exactly where a variac and current meter should be used.

The thermistor starts out at a few ohms, then drops to a low resistance as it warms. This added resistance limits the surge as the amp is furst powered. Helps save power switches and extends life of filters. But it won;t hurt the amp to live without it while under surgery.

[ballzdeep]

Word!!!

[olddawg]

The bulb will take its place.

This is exactly where a variac and current meter should be used.

The thermistor starts out at a few ohms, then drops to a low resistance as it warms. This added resistance limits the surge as the amp is furst powered. Helps save power switches and extends life of filters. But it won;t hurt the amp to live without it while under surgery.

As I am sure that Enzo is well aware of, many times thermistors are wound into power transformers. They are a major reason why primaries open up. I have unwound the first couple of turns on many transformers and simply jumpered the odd thermister when a replacement transformer is unavailable. I would say that you not only can jumper your thermistor while testing the amp, you could (although it isn't ideal) leave it that way as long as your mains are fused correctly. IMHO it's a bit of overengineering.

[Enzo]

NO! What is wound into tranformers is a thermal fuse. If it gets hot enough it opens - like a fuse- and then your tranny is dead. That is totally different from a resistor in series that varies its resistance. And in fact, the hotter you get the varistor, the lower its resistance gets.

The thermal fuse is meant to protect the amp from catching fire if the transformer is overloaded. the varistor is an inrush limiter, and it is designed to reduce stress on the empty filter caps and other circuit elements from the power-on current surge.

[Steve Conner]

yeah i am pretty sure its bad unless it is supposed to shoots sparks and smoke before the fuse blows several of the filter caps had popped open and were charred as well.

Did it get plugged into 240 with the voltage selector set to 120 by any chance?

[ballzdeep]

I do not know. This amp was given to me in this condition. Also i got a computer from the same guy that the caps in the power distribution circuits were also blown. I think this guy had a massive power surge or lightening strike at his house. Got the computer fixed but would rather have this amp working.
Enzo thanks for the correction on the component being a thermistor. Found part spec now that i know what it is. Look close to the same. Made light bulb limiter and replaced thermistor with jumper. was thinking of powering up with transformer disconnected to make sure power supply circuit was ok.

[Gtr_tech]

Varistor? MOV?

This is a thermistor - a series inrush limiting resistor. Its resistance drops after it warms up - a second or so after it turns on.

My bad. Brain fart......

[olddawg]

NO! What is wound into tranformers is a thermal fuse. If it gets hot enough it opens - like a fuse- and then your tranny is dead. That is totally different from a resistor in series that varies its resistance. And in fact, the hotter you get the varistor, the lower its resistance gets.

The thermal fuse is meant to protect the amp from catching fire if the transformer is overloaded. the varistor is an inrush limiter, and it is designed to reduce stress on the empty filter caps and other circuit elements from the power-on current surge.

I stand corrected. Nevertheless, I have jumpered these "thermal fuses" (they are usually marked in the schematic with a "T" also, if shown at all, and I was always told that they were a thermistor) dozens of times with no ill effects. You do have to fix the problem stressing the transformer. So much for my late night musings. I still wonder how necessary this thermistor really is in this case. Truthfully, I couldn't open the schematic last night.