mtc9000 resistor getting red hot

[jengineer]

I have a Hanty 9000, it came to me with a blown fuse, replaced the fuse, chassis is dead and the ceramic resistor got super hot so I turned it off

its the one with the yellow arrow pointing to it

it's even got James Cagney stumped

 

[RSwanson]

That's R81, it's wired in series with the LOPT primary. I'd guess the LOPT is shorted (or something putting a very large load on it).
Does the largest OCP resistor on the side get VERY hot too?

 

[jengineer]

That's R81, it's wired in series with the LOPT primary. I'd guess the LOPT is shorted (or something putting a very large load on it).
Does the largest OCP resistor on the side get VERY hot too?

from memory no, I don't think it did

also forgot to say there was a diode that had been totally de-soldered? so I re-soldered it,

jengineer2016-07-24 09:06:06

 

[big10p]

What diode number was it?

 

[jengineer]

I can check next week

 

[jengineer]

from memory it was ZD1

the picture below is one of my stock pictures

 

[big10p]

Not sure why that would have been de-soldered. I think ZD1 carries the voltage to power IC2, or something. If R81 is getting hot, it may be worth checking high voltage cap C36, which I think is in circuit with it. I've had a couple of C36 caps go. If it has gone, check the HOT too, as I think it can cause that to pop.

 

[RSwanson]

Strange, I can't see why that would be missing. It protects IC2 from overvoltage according to the schematics.
If it blows, then IC2 will likely die, so removing it (and not replacing it) serves no purpose.

 

[jengineer]

Strange, I can't see why that would be missing. It protects IC2 from overvoltage according to the schematics.
If it blows, then IC2 will likely die, so removing it (and not replacing it) serves no purpose.

it wasn't removed, it had been de-soldered, the diode was still in place, but definitely no solder on both legs, and no, not 'some bad soldering from an amateur' definitely no solder around both legs, clear as day it was,

weird because the chap I got the monitor off had it working on site, then one day it went faulty, and like I said above came to me with blown fuse and the rest etc

jengineer2016-07-24 18:09:56

 

[big10p]

That is odd. As far as I know, the chassis can't function without ZD1 in place. Then again, I'm just a mere amateur. Just bits and pieces I've picked up from fixing my own MTC9000 boards...

 

[tb2000]

R81 runs fairly hot anyway but not super hot. Just to rule things out i'd stick a new lopt in for testing for starters. That one looks like an original Eldor so there's every chance it's given up.tb20002016-07-24 19:54:02

 

[jengineer]

good point

 

[MKL]

Strange, I can't see why that would be missing. It protects IC2 from overvoltage according to the schematics.

The zener is used as a voltage regulator (note the grounded anode) together with R29 to get 12v from 24V. The chassis won't work without 12v so that zener is essential.

 

[RSwanson]

Strange, I can't see why that would be missing. It protects IC2 from overvoltage according to the schematics.

The zener is used as a voltage regulator (note the grounded anode) together with R29 to get 12v from 24V. The chassis won't work without 12v so that zener is essential.

It's the same thing. A 12v zener will conduct to GND at voltages over 12v, therefore it prevents more than 12v going to IC2, so whether you want to call it a regulator, 12v limiter or protection mechanism, it doesn't make any difference. R29 is simply acting as current limiting, it's not part of a voltage regulator.
Without the zener, voltage will still make it to IC2, and the chassis won't work you're right, but more specifically the 22ish volts will destroy IC2.