Our Sharp R-4H88 microwave popped a fuse. Door switches, all OK. High voltage rectifier shows "infinite" R. both directions. H.V. xformer hums when plugged in with door shut. I'm stumped!

Main fuse may have blown due to a possible failure in the high voltage doubler circuit. Check high voltage rectifier to see if it is shorted. Discharge high voltage capacitor first so you don't electrocute yourself.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/david_29ad5d1dd86564b0

The service manual is here:
http://www.microwavedisplay.com/r820bc_bk_bw_sm.pdf

You can get Sharp parts from Encompass here.

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.
See http://www.microtechfactoryservice.com/cap_test.html

If it went dead several seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
See: http://www.microtechfactoryservice.com/xformer.html

If it goes dead or blows the breaker (or GFI) when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.
See this: http://031d26d.namesecurehost.com/mwd/doorsw.txt

You have a failed high voltage transformer, magnetron, or high rectifier diode, or possibly both of the last two.

A loud buzz, hum, or groan is usually a shorted high voltage rectifier diode, but a sound like gurgling into an empty coffee mug is one symptom of a failed magnetron, and yet the mag can be bad without that sound.

The service manuals you need are here:

http://031d26d.namesecurehost.com/mwd/r1490_r1491_r1492_sm.pdf

http://031d26d.namesecurehost.com/mwd/r1480_r1481_r1482_sm.pdf

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/mag_test.html
http://www.gallawa.com/microtech/xformer.html

You can order parts from Encompass here or you can call them toll-free at 866-779-5835.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.

If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.

If it goes dead or blows the breaker (or GFI) when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.

If you or someone you know decide to look into it, we have critical safety information and disassembly information at our site, and our link is at our listing here on FixYa.

Here are some links you or someone you know can use (in order given) for test help, but read the safety warnings first:
http://www.gallawa.com/microtech/cap_test.html
http://www.gallawa.com/microtech/diode.html (if it is NOT shorted, it's probably fine)
http://www.gallawa.com/microtech/xformer.html

You can usually find exploded views Sears parts site.

BUT Once you have the part numbers, the best place to order Sharp parts is Tritronics.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.

If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.

If it goes dead or blows the breaker (or GFI) when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.

If you or someone you know decide to look into it, we have critical safety information and disassembly information at our site, and our link is at our listing here on FixYa.

Here are some links you or someone you know can use (in order given) for test help, but read the safety warnings first:
http://www.gallawa.com/microtech/cap_test.html
http://www.gallawa.com/microtech/diode.html (if it is NOT shorted, it's probably fine)
http://www.gallawa.com/microtech/xformer.html

You can usually find exploded views Sears parts site.

BUT Once you have the part numbers, the best place to order Sharp parts is Tritronics.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Apparently you're in the UK where 220V is used, so a 10A fuse would be correct.

If it goes dead or blows the breaker (or GFI) when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.

If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.microwavedisplay.com/safety.txt
http://www.microwavedisplay.com/disassembly.txt
http://www.microwavedisplay.com/doorsw.txt
http://www.gallawa.com/microtech/cap_test.html
http://www.gallawa.com/microtech/xformer.html

Info on ordering Sharp UK parts / spares is here.

We're happy to help you with free advice and we'd appreciate your thoughtful rating of our answer.

Then the most likely suspects are:

- loose / corroded connection at the magnetron terminal
- bad magnetron (may buzz or hum, may not)
- bad high voltage transformer (may smell / smoke, may not)
- shorted high voltage rectifier diode (may hum, may not)

If you or someone you know decide to look into it, we have critical safety information, info on door switch diagnosis and replacement, and disassembly information at our site, and our link is at our listing here on FixYa.

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/mag_test.html
http://www.gallawa.com/microtech/xformer.html
If it's within the magnetron portion of the warranty (usually 5-9 years) but out of the original full warranty (usually 1 year), then your manufacturer will cover the part cost, but not the cost to visit you or the labor charge to do the work.

If you want to find a local Sharp-authorized servicer, visit http://www.sharp.co.uk

We're happy to help you and we'd appreciate your thoughtful rating of our answer.

If the turntable runs when you try to cook with the microwave portion, then the problem is either a bad relay on the control board (RY2, primary interlock relay) or something wrong in the high voltage section.

If a high voltage problem, it may be as simple as a loose connection on the magnetron terminal or a bad high voltage rectifier diode or as costly as a high voltage transformer or magnetron.

Most of the time the magnetron will have a 5-9 year warranty, and you would only have to pay labor charges to have it fixed. If you take it to the shop yourself, you can save quite a bit.

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/mag_test.html
http://www.gallawa.com/microtech/xformer.html

We're happy to help and we appreciate your thoughtful rating of our answer.

If it went dead almost immediately after pressing the Start pad, that's usually a shorted high-voltage capacitor.

If it went dead a few seconds after pressing the Start pad, that's usually a failing high-voltage transformer.
If it goes dead or blows the breaker when you plug it in or open or close the door, then there's a problem with a door switch or door switch mount.

But you might replace the fuse and do another quick heating test to see if it blows again. If so, you'll need to do some checks.

Here are some links you or someone you know can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/cap_test.html
http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/xformer.html
We're happy to help and we appreciate your thoughtful rating of our answer.

William E. Miller, AS-EET
[email protected]
http://www.microwavedisplay.com

This is tough to call remotely, but it will almost certainly be the magnetron or high voltage capacitor or (if you're lucky) the high voltage rectifier diode.

If you or a friend decide to work on it, we have *critical* safety, disassembly, and troubleshooting info at our site, and our link is at our listing here on FixYa: http://tinyurl.com/yzjozk

Here are some links you or a friend can use for test help, but read their safety warnings first:

http://www.gallawa.com/microtech/diode.html
http://www.gallawa.com/microtech/mag_test.html
http://www.gallawa.com/microtech/xformer.html

You can order Sharp parts from Tritronics at http://www.wehaveparts.com

If yours is not a Sharp, you can usually find helpful exploded view diagrams and order parts by entering your full model number here: http://tinyurl.com/gv383

We're happy to help and we appreciate your thoughtful rating of our answer.