Thermistor in a MC240 blew, violently

It looked right. The green wire on the ground connection on the left cap is part of the winding with the 12µF cap. Both 270µF are tied at negative, going to red winding. Positive of left should be at ground and positive of right should be at cathode of SR2 and black wire side of the main choke.
I was consulting an online schematic, which appears correct, the power supply is incorrect on yours re that elyticap.
Check this one;
https://goo.gl/images/ez8KdT
 
Here's the setup with one can isolated from the chassis, if it helps you. It is as the original setup was. Tying the two negative leads directly seems to be your issue.


L1120384.JPG
 
I can see, now that we are off the road, that that is a solder joint, not a diode. Turn the elyticap around, replace the inrush limiter and fuse(s), check that it is set, wired for the higher line voltage.
 
How are you doing on this?

Letting it sit.... :(

Can I use a 100Ω power resistor and then short it to replace the thermistor before I power the unit on again? All else I need to do is test and rotate the e-cap. The 2/10 amp fuse blew so I'm also gonna use two .1A fuses in parallel.
 
Fuse(s) 3.2 Amp slo-blow.
The resistor will likely overheat almost immediately, so, no. A 100 Watt 120 Volt incandescent lamp bulb would be fine, but do be sure the amp power is set for the higher 125 Volt option.
 
I believe OP is referring to the fuse on the small board with SR1, SR2, SR3. It will blow if there is a B+ problem.L1120380.JPG
 
So, to the OP, you can use another 3.2 amp fuse and a variac to do a slow start on the amp if you don't have a thermistor. The 100ohm resistor in place of the thermistor will do nothing for you except burn up.
 
Fuse(s) 3.2 Amp slo-blow.
The resistor will likely overheat almost immediately, so, no. A 100 Watt 120 Volt incandescent lamp bulb would be fine, but do be sure the amp power is set for the higher 125 Volt option.
Tested the backwards ecap out of circuit. 281µF, esr .25Ω, Vloss iss .4%. Any chance it could still be bad? Drew at least 3.2 A when connected backwards.
 
Schematic is wrong, C23 is shown backwards. Turn it around. What I took for a diode between the elyticaps may be a solder joint. Confirm please.


yep, agree. replace the thermistor, double check the diode orientation. then bring her up on a variac and watch the current draw as you bring the AC voltage up from a few volts. If current draw stays low, then move up a bit more. by the time you get to 80-90VAC, current draw should be minimal.

dont feel bad, mistakes happen. A few months ago, I was rebuilding an MC60. was late, was tired, should have gone to bed but decided to solder in the bias cap first. confirmed the orientation and still soldered the cap in backwards. did not notice it til I was bringing up the AC lines on the variac. sure enough at around 50VAC input that darn magic smoke started to appear. Shut it down. realized what a bone head move I had done, tossed the newly installed diode, cap and resistor in the bias section with all new parts, and brought it back up on line slowly. The second time no issues.
 
You can try it with the proper fuses in place, it may be ok.

Lastly, to (for now) replace the thermistor,
You can try it with the proper fuses in place, it may be ok.

I wired up two 60W bulbs in parallel where the thermistor was. They glow dimly but decently. Where 435 should be, there is 260.25VDC. All ecaps get proper voltage direction. I can list the other voltages and compare them to the proper rail level. Can I short the bulb or get a proper thermistor for closer voltage? The line setting is at 125, close to the 122.2VAC where I am in Missouri.
 
So, to the OP, you can use another 3.2 amp fuse and a variac to do a slow start on the amp if you don't have a thermistor. The 100ohm resistor in place of the thermistor will do nothing for you except burn up.


Using 200W worth of bulbs in place of the thermistor, still get 325.7VDC where I should get 435VDC.
 
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