Heat Sinks Used as Electrical Connections

Thread Starter

John Czerwinski

Joined Jun 19, 2017
48
This is the first time I've run into this situation, so I'm not sure exactly how to ask.

I'm working on an arcade monitor (Sanyo 18-Z2AB), which had vertical collapse. I believe I've located the issue as bad electrolytic capacitors and two of the vertical deflection transistors (TR402 and TR403).

After researching and asking around, a few of the more experience repair techs mentioned when you replace the transistors:
  1. Make sure you replace it with a metal package (the collector part) and not a full plastic package
  2. Don't use a mica or silicon heat insulator, just thermal paste because the heat sink electrically completes the circuit. This is a common comment and is proven when someone does put in something to electrically isolate the transformers from the heatsink, vertical deflection fails. When they have the transistor electrically connect to the heatsink via paste, it works.

My question is: Since I would have NEVER have thought of this when troubleshooting the monitor, how could I have found this out on either the schematic (see below...there's no indication of this...unless I completely missed it), or is there trick/best practice to find this out? Is this common practice? Is this a product of a "hot chassis?"

Appreciate any wisdom, insight, or reference material.


1654219069519.png
 

ElectricSpidey

Joined Dec 2, 2017
2,147
I doubt if the heatsink is being used to complete the circuit, that is normally done with the actual collector pin of the transistor. (but it is possible)

But you can check the continuity between the sink and the collector pin pad to discern whether you can place the transistor without any insulator, but you will always need some thermal compound.

If the tab is completing the circuit then you will need a metal screw to attach the transistor, as thermal paste can also be an insulator.
 
Last edited:

MrChips

Joined Oct 2, 2009
26,481
I am not sure that this is the correct answer but here goes.

If you have a bad transistor and need to replace it, one would assume that you will have to look up the datasheet.
Why? Because you want to know what you are dealing with, the type of transistor, specifications, package and pinout.
You would need to do this because maybe the exact replacement is hard to find and you might have to find a close match.

The datasheet for 2SA940 would indicate that this is a TO-220C package with a metal tab.
Very often the collector pin and the metal tab are connected. When you confirm this is true by measuring, you pay attention to how it is mounted on to the heat sink.

The collector pin should be soldered to a PCB pad. You would then confirm that the collector pin and chassis are COMMON.

Knowing this, you would double check the other transistor TR402, 2SC2073.
In this case, you want to make sure that the TO-220 metal tab is insulated from the heatsink.
 

crutschow

Joined Mar 14, 2008
30,102
that is normally done with the actual collector pin of the transistor.
Some packages, like the old metal TO-3 had no collector pin, only one for base and one for emitter.
You could use the heat sink or add a solder tab (below) under one of the mounting screws for the collector connection.
1654224977386.png
 

BobaMosfet

Joined Jul 1, 2009
1,996
This is the first time I've run into this situation, so I'm not sure exactly how to ask.

I'm working on an arcade monitor (Sanyo 18-Z2AB), which had vertical collapse. I believe I've located the issue as bad electrolytic capacitors and two of the vertical deflection transistors (TR402 and TR403).

After researching and asking around, a few of the more experience repair techs mentioned when you replace the transistors:
  1. Make sure you replace it with a metal package (the collector part) and not a full plastic package
  2. Don't use a mica or silicon heat insulator, just thermal paste because the heat sink electrically completes the circuit. This is a common comment and is proven when someone does put in something to electrically isolate the transformers from the heatsink, vertical deflection fails. When they have the transistor electrically connect to the heatsink via paste, it works.

My question is: Since I would have NEVER have thought of this when troubleshooting the monitor, how could I have found this out on either the schematic (see below...there's no indication of this...unless I completely missed it), or is there trick/best practice to find this out? Is this common practice? Is this a product of a "hot chassis?"

Appreciate any wisdom, insight, or reference material.


View attachment 268632
Sometimes heatsinks are attached to the metal fin on the transistor- and frequently that metal fin is tied to ground within the transistor. So, yes, it can complete a circuit. Get a Datasheet for that transistor, it should tell you.
 

sagor

Joined Mar 10, 2019
704
There are some switching supplies and UPS systems that bring a wire to the heatsink to distribute the power equally to 2 or more transistors. The heatsink is isolated from ground, just mounted on a PCB in some cases.
This is not a common practice, but is sometimes used that way. A schematic will seldom show this, it is up to the person looking at the heatsink to determine how the transistors are mounted. A tell tale sign the transistor is not insulated is a wire screwed onto the heatsink itself.
 

MisterBill2

Joined Jan 23, 2018
12,399
The correct approach to this question is to first verify which transistor element of the replacement transistors is tied to the metal tab. As stated, USUALLY it is the collector, almost always the middle lead of a power tab device. Then determine if the heat sink is insulated so that can be part of the circuit that is not common to the material that it is mounted to.
If the transistor and heat sink are mounted on a PC board it may be that the heat sink is isolated, in which case no insulator would be needed. That will require careful examination and probably a meter check to verify.
 

Irving

Joined Jan 30, 2016
2,769
Both the NPN and PNP output transistors are TO-220 cased and have their collectors (pin 2) also connected to the heatsink tab. But as shown in the schematic the collectors are NOT common, therefore one or both must be isolated. By inspection/continuity testing I think you'll find that TR403, whose collector is grounded has no isolation and relies on the heatsink for connection to ground whereas TR402 will probably be insulated from the heatsink.

1654350719586.png
 

MisterBill2

Joined Jan 23, 2018
12,399
Both the NPN and PNP output transistors are TO-220 cased and have their collectors (pin 2) also connected to the heatsink tab. But as shown in the schematic the collectors are NOT common, therefore one or both must be isolated. By inspection/continuity testing I think you'll find that TR403, whose collector is grounded has no isolation and relies on the heatsink for connection to ground whereas TR402 will probably be insulated from the heatsink.

View attachment 268705
As both heat sinks are mounted on a PC board probably neither is connected because that sort of connection is less uniform and not always reliable.
 

Irving

Joined Jan 30, 2016
2,769
As both heat sinks are mounted on a PC board probably neither is connected because that sort of connection is less uniform and not always reliable.
We don't know if its one heatsink or two - my comments assumed there was one and it was grounded so the NPN would need isolation, the PNP wouldn't. If its two seperate heatsinks then isolation may or may not be needed.

I know of several instances where the heatsink has been used as a route across the board for, typically, a ground rail, but, in at least one case, its been the +ve supply.
 

MisterBill2

Joined Jan 23, 2018
12,399
For the best production yield the heat sinks would be separate and the transistors not insulated. That is what I have seen in many monitors and televisions. It seems that adequate insulation is an extra production step that creates a point where failures are more common. In addition, separate transistor mountings allow more layout flexibility. Hence my presumption.
 

Thread Starter

John Czerwinski

Joined Jun 19, 2017
48
Thanks for everyone's help!

Closing this one out after investigating, disassembling, and tracing, here's what I found:

  • Both transistors are connected to separate heat sinks and their COLLECTORS ARE electrically connected to their respective heat sinks via the mounting screw.
  • Transistors use thermal paste, so no isolation thermal pad.
  • Each heat sink DOES electrically connect the collector to either the power rail, or the ground plane. This depends in its function. (removing the heatsink, there's no connection between the collector's solder pad to ground or power rail).
  • There's nothing in the schematic or data sheets to indicate this configuration.

I've updated the diagram image below.

Note: Replacing bad electrolytic capacitors fixed the overall vertical collapse. But now I cannot completely adjust the vertical image, as it's getting cut off at the top...back to the drawing board.

Verticle Circuit.PNG
 
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