Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old September 24th 03, 07:46 PM
Michael Caplan
 
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Default RV power converter Todd PC40 -- insulator problem

Does anyone have information on or experience with the PC40, 40 amp
converter made by Todd Engineering Sales of Tucson (now out of business)?
I'd sure like to get hold of a schematic, but my main interest right now is
in identifying the special insulator used under the mosfets.

The power supply is used to provide the 12 DC used in recreational travel
trailers.

It's basically three paralleled SMPS's mounted inside a large aluminum heat
sink (which is also the case), one of which, as far as I can tell, is the
master.

The unit came into my shop with no output. After getting the case opened
(it's all pop-riveted) and pulling out the master board, I found the IRF740
mosfet on this board shorted and the resistor next to it open (looks like
0.24 Ohms, 2 Watt -- red, yellow, silver, then gold on a gray body). I
also found signs of arcing from the mosfet to the heat sink. The arcing
appears to have been through the mosfet's insulator, and that is where I am
puzzled.

The mosfet, an IRF740 in a TO-220 type case, is insulated from the heat sink
by a piece of what at first looks like a standard silicon rubber heat
conductive insulator. However, I was attracted to the fact that there is a
grounding wire soldered to the insulator. On much closer examination I
confirmed that the insulator has a metallic core ("middle of the sandwich").
The point where the heat sink was marked by the arcing corresponds to the
narrow edge of the insulator where the metallic core is now exposed.

I suspect that the insulator is compromised and should not be reused. But I
haven't seen this type of insulation material before and was wondering if
anyone has come across it. Is there a description and trade name for it?
What about sources?

Michael Caplan CET



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Old October 6th 03, 04:07 AM
Tony Marsillo
 
Posts: n/a
Default RV power converter Todd PC40 -- insulator problem

Mike,
I am working on the same unit. I am trying to find out the part number of U2
on the main PCB. It is a 3 terminal TO-92 plastic device, it burned up and I
can not read the markings.. My unit has a standard insulator, no wire
soldered to it. The main board near the large chassis terminal has a spade
terminal soldered to the PCB and held in contact with the heatsink with a
clip. Also do you know if the telephone type connector is required to turn
the unit on.

Thanks

--
Tony Marsillo
Nutmeg Repair
-----------------------------------------------------------------
"Michael Caplan" wrote in message
...
Does anyone have information on or experience with the PC40, 40 amp
converter made by Todd Engineering Sales of Tucson (now out of business)?
I'd sure like to get hold of a schematic, but my main interest right now

is
in identifying the special insulator used under the mosfets.

The power supply is used to provide the 12 DC used in recreational travel
trailers.

It's basically three paralleled SMPS's mounted inside a large aluminum

heat
sink (which is also the case), one of which, as far as I can tell, is the
master.

The unit came into my shop with no output. After getting the case opened
(it's all pop-riveted) and pulling out the master board, I found the

IRF740
mosfet on this board shorted and the resistor next to it open (looks like
0.24 Ohms, 2 Watt -- red, yellow, silver, then gold on a gray body). I
also found signs of arcing from the mosfet to the heat sink. The arcing
appears to have been through the mosfet's insulator, and that is where I

am
puzzled.

The mosfet, an IRF740 in a TO-220 type case, is insulated from the heat

sink
by a piece of what at first looks like a standard silicon rubber heat
conductive insulator. However, I was attracted to the fact that there is

a
grounding wire soldered to the insulator. On much closer examination I
confirmed that the insulator has a metallic core ("middle of the

sandwich").
The point where the heat sink was marked by the arcing corresponds to the
narrow edge of the insulator where the metallic core is now exposed.

I suspect that the insulator is compromised and should not be reused. But

I
haven't seen this type of insulation material before and was wondering if
anyone has come across it. Is there a description and trade name for it?
What about sources?

Michael Caplan CET




  #3   Report Post  
Old October 6th 03, 04:07 AM
Tony Marsillo
 
Posts: n/a
Default RV power converter Todd PC40 -- insulator problem

Mike,
I am working on the same unit. I am trying to find out the part number of U2
on the main PCB. It is a 3 terminal TO-92 plastic device, it burned up and I
can not read the markings.. My unit has a standard insulator, no wire
soldered to it. The main board near the large chassis terminal has a spade
terminal soldered to the PCB and held in contact with the heatsink with a
clip. Also do you know if the telephone type connector is required to turn
the unit on.

Thanks

--
Tony Marsillo
Nutmeg Repair
-----------------------------------------------------------------
"Michael Caplan" wrote in message
...
Does anyone have information on or experience with the PC40, 40 amp
converter made by Todd Engineering Sales of Tucson (now out of business)?
I'd sure like to get hold of a schematic, but my main interest right now

is
in identifying the special insulator used under the mosfets.

The power supply is used to provide the 12 DC used in recreational travel
trailers.

It's basically three paralleled SMPS's mounted inside a large aluminum

heat
sink (which is also the case), one of which, as far as I can tell, is the
master.

The unit came into my shop with no output. After getting the case opened
(it's all pop-riveted) and pulling out the master board, I found the

IRF740
mosfet on this board shorted and the resistor next to it open (looks like
0.24 Ohms, 2 Watt -- red, yellow, silver, then gold on a gray body). I
also found signs of arcing from the mosfet to the heat sink. The arcing
appears to have been through the mosfet's insulator, and that is where I

am
puzzled.

The mosfet, an IRF740 in a TO-220 type case, is insulated from the heat

sink
by a piece of what at first looks like a standard silicon rubber heat
conductive insulator. However, I was attracted to the fact that there is

a
grounding wire soldered to the insulator. On much closer examination I
confirmed that the insulator has a metallic core ("middle of the

sandwich").
The point where the heat sink was marked by the arcing corresponds to the
narrow edge of the insulator where the metallic core is now exposed.

I suspect that the insulator is compromised and should not be reused. But

I
haven't seen this type of insulation material before and was wondering if
anyone has come across it. Is there a description and trade name for it?
What about sources?

Michael Caplan CET






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