Replacing a Varistor

[clk1981]

I'm working on a MC2105 for a friend and had it working great. He brought it back a week later with a blown main fuse. After examining, the varistor near the main filter caps started smoking, cracked, and the main 5A fuse blew. It looks like he may have had a power surge of some sort. I replaced the varistor with one that I had in stock temporarily but the replacement I have is much higher rated. I briefly turned the unit on just to see if there were other issues and the unit is working fine now.

I am trying to find an equivalent to the original. I don't see it listed in the service manual but it seems to be a 150V MOV, but I don't know any of the other specs. I'm guessing the +80 - 20 is the operating temperature range?

Attached is a picture. Any help would be greatly appreciated. Thank you!

 

[c_dk]

I believe if you google RMC capacitor you will find that this is a ceramic cap.

Best to replace them both (should be one on each AC leg) with a modern safety cap, I believe. Modern safety caps fail open while the old ones have a nasty habbit of failing shorted.

 

[clk1981]

I believe if you google RMC capacitor you will find that this is a ceramic cap.

Best to replace them both (should be one on each AC leg) with a modern safety cap, I believe. Modern safety caps fail open while the old ones have a nasty habbit of failing shorted.

Thank you. This one did fail shorted! What voltage and capacitance do you suggest? There seems to be a lot of different ones.

Does this look like a suitable replacement?

http://www.mouser.com/ProductDetail/Murata/DE1B3KX471KJ4BP01F/?qs=/ha2pyFadujx42u/QhnRwgAcVkiuYaZ5CwBeE%2b8ELTuv5q8sJJ6/MZTQ6l6ETFlO

 

[clinic-audio]

I'm working on a MC2105 for a friend and had it working great. He brought it back a week later with a blown main fuse. After examining, the varistor near the main filter caps started smoking, cracked, and the main 5A fuse blew. It looks like he may have had a power surge of some sort. I replaced the varistor with one that I had in stock temporarily but the replacement I have is much higher rated. I briefly turned the unit on just to see if there were other issues and the unit is working fine now.

I am trying to find an equivalent to the original. I don't see it listed in the service manual but it seems to be a 150V MOV, but I don't know any of the other specs. I'm guessing the +80 - 20 is the operating temperature range?

Attached is a picture. Any help would be greatly appreciated. Thank you!

This is not a varistor , this is a simple ceramic capacitor .(C_DK is right)
In MC2105 you have thermistors but they are in serie with the primary of the power transformer and are not in the same color

 

[c_dk]

X1/y1, x2/y2 capacitors are specially designed to be placed in the AC circuit and are rated by Underwriters labs as well as other agencies specifically for this function.

Google the above terms to acquaint yourself for your liability in repairing this for your friend and choose wisely what you replace them with.

 

[MACKIE1975]

I'm guessing the +80 - 20 is the operating temperature range?

It is tolerance.

 

[MACKIE1975]

Check your schematic for the specification. Base on your picture, I think it is 0.1uF at 150V. You can replace with MLCC capacitor.

 

[AbSoundLabsMN]

I'm working on a MC2105 for a friend and had it working great. He brought it back a week later with a blown main fuse. After examining, the varistor near the main filter caps started smoking, cracked, and the main 5A fuse blew. It looks like he may have had a power surge of some sort. I replaced the varistor with one that I had in stock temporarily but the replacement I have is much higher rated. I briefly turned the unit on just to see if there were other issues and the unit is working fine now.

I am trying to find an equivalent to the original. I don't see it listed in the service manual but it seems to be a 150V MOV, but I don't know any of the other specs. I'm guessing the +80 - 20 is the operating temperature range?

Attached is a picture. Any help would be greatly appreciated. Thank you!

The part that your picture shows is not a capacitor. The capacitors across the AC mains are rated at 1500 v breakdown. There are two of the varistors in the MC-2105. The original part number for that device was 144012. It has been updated to part number 144151. The schematic ID is R301 and R302. This part is available from the Parts department at (800) 538-6576. Ask for Brian. We strongly recommend that you replace both of the devices with the updated part. Mark - McIntosh Service Department.

 

[c_dk]

Who knows what Mark is posting....RMC was a capacitor brand.....

Since this could involve causing the chassis to become HOT WITH AC if the wrong part is installed I will ask the OP what serial number his unit is and waste 20 minutes of my life shorting out what Mark might be trying to post......the schematic I grabbed starts with serial #26M08.

OP your first picture shows a ceramic capacitor which on units after #28M65 is located after the transformer across the 4 Delco main diodes forming the bridge rectifier. This according to my schematic is C304 part number 061-113 a dual disc .1 100 volt cap.

According to the schematic there are two varistors located before the transformer in series with the power switch. The temp sensors mounted on the heat sinks are in series on the other leg of the AC.

You could have a blown varistor but they look different than your picture.

A shorted varistor would not necessarily blow the fuse, by design as it heats up it's resistance lowers to a near short, a ceramic cap shorting across the bride rectifier would create a issue however....I have replaced varistors hit by lightning and they usually just blow up and open and the pieces do not conduct anything.....

 

[c_dk]

To the OP and our Moderator Mike......

Please, if I am correct that the part you pictured is located across the bridge rectifier, correct the name of the part and location in the original post.

A quick search through a number of Mac amp schematics did not show them using any of the so called "death caps" from AC to the chassis except for their use in preamps, receivers and tuners.

My fear is that some day a lurker will skim through this thread and say " Oh I have one of those" and install the incorrect part not after the transformer in the right place but before it to chassis ground and cause for themselves a serious problem.

Posting about high voltage issue when we know that often people do not read through the whole thread and the cautions that need to be exercised concerns me.

 

[MACKIE1975]

I think the OP should have the following details:

1. A broad view picture of the affected PCB
2. A closeup of the component is still on the PCB
3. What happened before and after

All the gurus (not me) in this forum answered in a good spirit so do your part. Help us, Help you.

 

[AbSoundLabsMN]

Who knows what Mark is posting....RMC was a capacitor brand.....

Since this could involve causing the chassis to become HOT WITH AC if the wrong part is installed I will ask the OP what serial number his unit is and waste 20 minutes of my life shorting out what Mark might be trying to post......the schematic I grabbed starts with serial #26M08.

OP your first picture shows a ceramic capacitor which on units after #28M65 is located after the transformer across the 4 Delco main diodes forming the bridge rectifier. This according to my schematic is C304 part number 061-113 a dual disc .1 100 volt cap.

According to the schematic there are two varistors located before the transformer in series with the power switch. The temp sensors mounted on the heat sinks are in series on the other leg of the AC.

You could have a blown varistor but they look different than your picture.

A shorted varistor would not necessarily blow the fuse, by design as it heats up it's resistance lowers to a near short, a ceramic cap shorting across the bride rectifier would create a issue however....I have replaced varistors hit by lightning and they usually just blow up and open and the pieces do not conduct anything.....

Let me see if I can try to clarify my previous post. It was early and I apparently ran two seperate thoughts together, making it confusing, even to me. Sorry Folks, must be my old age. What I was trying to indicate was that if it is a Varistor, it should replaced with the updated part from our Factory Parts Department. Typically speaking, when a varistor of the old type fails, it separates and then becomes an open circuit. The original Varistor has been updated. The replacement Varistor is encapsulated in a different process, to be more secure against breakage, due to heat. As far as the fuse blowing, a larger than normal current flow, in the primary circuit could ultimately blow the fuse and perhaps open the Varistor. Since the value of the Varistor is very low, shorting it isn't really a significant concern its failure alone, would not, in all likely hood create a significant problem. My mention of the caps on the AC side of the Power Supply are special and should not be eliminated, but replaced, in pairs, if two and as a single if it contains two (ie. 3 lead). Failure or leakage of these parts can create a shock hazard as previously pointed out. Regarding the Delco Style rectifiers a Factory Charge Bulletin removed those devices and replaced them with a Bridge Pack rectifier device. When a unit using the Delco type rectifiers is received it is a Factory procedure to install the newer design part. Sorry for all the confusion. Hope this clarifies what I was attempting to say. Mark - McIntosh Service Center