GE Microwave,no heat?

[RCA2000]

Re: GE Microwave,no heat?

But Kaboom...did you ever see THIS::

An early 80's IIRC Kenmore MWV. Came in with a blwo n fuse. Replaced. closed door...no shorts on HV area.. Lit off...worked FINE.,..cooked well... UNTIL...I shut it OFF..or opened the door. THEN..there would be a loud "rumbling noise" from the HV tranny...and the fuse blew again.

at first...figured a weird door sw. problem or such.. SO I unhooked the primary of the HV tranny...and replaced the fuse...and opened and closed the door MANY times..NO problem.

Unlike most--if not ALL..modern MWV"s...this one used a TRIIAC..instead of a relay to break the primary line the the HV tranny. At FIRST...I thought "maybe it has a bad triac..which is SOMEHOW not "conducting right"..and somehow "chopping" the power to the tranny...which it would NOT like. BUt this did N OT help...powered up...worked great..until stopped or door opened AGAIN..SAME rumble from tranny and blown fuse.

I was not sure where to look next. THEN..I recalled a tv set which used a triac for control...and it ALSO did something similar. That turned out to be a bad OPTOCOUPLER.. which drove the gate of the triac. Somehow it was NOT conducting fully or such...and chopped the power to the VRT tranny (this was a Zenith CC-II set)...which then tripped the breaker after some seconds.

Turned out it was the SAME case HERE. The control board DID have an optocoupler--and evidently it WAS acting that way. After I located another opto (this was NOT a "modern opto" it was a LOT larger--as was the one in that tv set..) and installed it on the board ALL was GOOD..

I think I saw that..TWICE On an MWV and mabye 3 times on a tranny-powered tv set...

[goontron]

Re: GE Microwave,no heat?

But Kaboom...did you ever see THIS::

An early 80's IIRC Kenmore MWV. Came in with a blwo n fuse. Replaced. closed door...no shorts on HV area.. Lit off...worked FINE.,..cooked well... UNTIL...I shut it OFF..or opened the door. THEN..there would be a loud "rumbling noise" from the HV tranny...and the fuse blew again.

at first...figured a weird door sw. problem or such.. SO I unhooked the primary of the HV tranny...and replaced the fuse...and opened and closed the door MANY times..NO problem.

Unlike most--if not ALL..modern MWV"s...this one used a TRIIAC..instead of a relay to break the primary line the the HV tranny. At FIRST...I thought "maybe it has a bad triac..which is SOMEHOW not "conducting right"..and somehow "chopping" the power to the tranny...which it would NOT like. BUt this did N OT help...powered up...worked great..until stopped or door opened AGAIN..SAME rumble from tranny and blown fuse.

I was not sure where to look next. THEN..I recalled a tv set which used a triac for control...and it ALSO did something similar. That turned out to be a bad OPTOCOUPLER.. which drove the gate of the triac. Somehow it was NOT conducting fully or such...and chopped the power to the VRT tranny (this was a Zenith CC-II set)...which then tripped the breaker after some seconds.

Turned out it was the SAME case HERE. The control board DID have an optocoupler--and evidently it WAS acting that way. After I located another opto (this was NOT a "modern opto" it was a LOT larger--as was the one in that tv set..) and installed it on the board ALL was GOOD..

I think I saw that..TWICE On an MWV and mabye 3 times on a tranny-powered tv set...

Rather common in SMPSs actually.... Im fixing a server PSU right now and the optoiso wont cut out. It blows switchers.

[kaboom]

Re: GE Microwave,no heat?

But Kaboom...did you ever see THIS::

An early 80's IIRC Kenmore MWV. Came in with a blwo n fuse. Replaced. closed door...no shorts on HV area.. Lit off...worked FINE.,..cooked well... UNTIL...I shut it OFF..or opened the door. THEN..there would be a loud "rumbling noise" from the HV tranny...and the fuse blew again.

Weren't those the Sanyos? Sears didn't switch to Goldstar for their MWOs til the late 80's AFAIK... Even when working properly, those triacs still needed their little heatsink. But they often half-shorted- light dimmers did this after those spectacular lamp failures/arcs. Since the arc in a blown/blowing lamp is assymetrical, one half-wave of current can greatly exceed the other; that peak which is beyond the triac's capabilities destroys that side of the device. The result was the bulb(s) would be stuck on, at about half nominal wattage, even with the control all the way down!

Now imagine a large transformer with low DCR- that MOT for example. If the triac's half shorted, the fuse blows upon plug-in; if improperly/assymetrically driven, the "net" result is the same- net DC thru that transformer- saturation and blown fuse.

at first...figured a weird door sw. problem or such.. SO I unhooked the primary of the HV tranny...and replaced the fuse...and opened and closed the door MANY times..NO problem.

Unlike most--if not ALL..modern MWV"s...this one used a TRIIAC..instead of a relay to break the primary line the the HV tranny. At FIRST...I thought "maybe it has a bad triac..which is SOMEHOW not "conducting right"..and somehow "chopping" the power to the tranny...which it would NOT like. BUt this did N OT help...powered up...worked great..until stopped or door opened AGAIN..SAME rumble from tranny and blown fuse.

If they used an odd (at the time) photo-triac optoisolator, the little triac inside the opto could also have one side go bad. Or leakage. Or both. Hence the "randomness" of the noise; you mentioned rumbling.

An internal diagram of that optocoupler would've been interesting...

I was not sure where to look next. THEN..I recalled a tv set which used a triac for control...and it ALSO did something similar. That turned out to be a bad OPTOCOUPLER.. which drove the gate of the triac. Somehow it was NOT conducting fully or such...and chopped the power to the VRT tranny (this was a Zenith CC-II set)...which then tripped the breaker after some seconds.

Turned out it was the SAME case HERE. The control board DID have an optocoupler--and evidently it WAS acting that way. After I located another opto (this was NOT a "modern opto" it was a LOT larger--as was the one in that tv set..) and installed it on the board ALL was GOOD..

I think I saw that..TWICE On an MWV and mabye 3 times on a tranny-powered tv set...

Again, almost sounds like a hybrid module- like a "mini STK."

[RCA2000]

Re: GE Microwave,no heat?

But NOT so common..on a 60HZ line powered supply. I agree optos TODAY..are VERY often at fault--when an SMPS will not run, runs away or latches and blows the switching device. And it is NOT easy to check them other than by subbing...

I STILL WONDER...just how much FUN it was..back in '72,'73..for TV techs...when they encountered a Motorola Quasar with the JA SMPS power supply. These techs were used to 60HZ power trannies or HOT chassis sets. The JA was a combinatin of BOTH.. (HOT primary side and COLD output side) .and a WHOLE LOT like a modern SMPS ckty-wise. . But for the "old timers"...I can just IMAGINE...them finding a shorted switching transistor, putting a new one in and as SOON as power is applied BAM !! OUT it goes INSTANTLY--and THEN..not having ANY idea..where to look next !!

My guess is they usually just SUBBED the WHOLE JA module" and "let the factory sweat it out"..

[RCA2000]

Re: GE Microwave,no heat?

That was over 20 years ago...when I saw that Kenmore with that problem. There was NO internal diagram of the Opto..or for THAT matter control board. Nealry ALL techs would just "replace the board"...but I have LONG tried my BEST to REPAIR boards..then and NOW...when it is possible.

Actually..on the MWV...IIRC..it was a black, epoxy-encased device like today...BUT a LOT bigger and NOT with a standard DIP layout. ON the TV.(this was the late 1980's IIRC) I THINK..it was a "blob" of some sort--all wrapped in fiber tape--with a pair of wires on each end. Pretty sure THAT one showed a LAMP and a PHOTOCELL..inside on the schematic.

[michael dranfield]

Re: GE Microwave,no heat?

Two microwaves I have fixed both just had a loose connection at the magnetron. I suspect the magnetron vibrates and weakens the connections. If either one breaks that way again, I'm soldering the spade plugs on.

The other two I fixed were worn door switches.

I don't think the magnetron, diode and caps go out as often as people online make it out to be.

Its a very common fault having a poor connection at the magnatrons filament terminals , the magnatrons filament can draw upwards of 10 amps so any resistance here becomes critical .