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Coco 1 F Motherboard Power Issue.

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dabone

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I've got a Coco 1 F board that keeps popping q2 (MPSW01A).

I'm trying to kinda follow the original board schematics, in the color computer technical reference manual, but it's a little different.


jFCbmDi.png


I've got a MPSW01 instead of the tip29 at q3, (q2 on my board.)
Now, this is getting hot enough to burn me, and fails quickly.
I tossed in a TIP29 to see how bad it got, (swapping pins around to match) and it too gets very hot.

I'm wondering it my 2n6594 has failed and the first transistor is trying to supply all the +5 voltage to the machine.
Looking at the 723c data sheet it says..
By itself, it Transistor will supply output currents up to 150 mA; but external transistors can be added to provide any desired load.
Click to expand...

The 2n6594 doesn't have any shorts to itself, but I'm unsure how to test further, and I don't have any spares.
 
The next think is to check the caps for shorts and the zener CR17. You might get lucky with C10.
 
No shorted caps, and please give me your theory as to why cr17? It's a overvoltage protection that shorts to gnd when the voltage exceeds the junction voltage.
(It tests correctly for a diode out of circuit with a meter.)

Screenshot 2022-07-10 004007.png


I'm struggling to get my head around how q3 and q2 work together to form the +5 rail.
Any explanation by one of the resident geniuses would be much appreciated.
 
dabone said:
No shorted caps, and please give me your theory as to why cr17? It's a overvoltage protection that shorts to gnd when the voltage exceeds the junction voltage.
(It tests correctly for a diode out of circuit with a meter.)

View attachment 1243419


I'm struggling to get my head around how q3 and q2 work together to form the +5 rail.
Any explanation by one of the resident geniuses would be much appreciated.
Click to expand...
If the zener shorts it's tied to the collector of Q2. You could lift one end of the zener and see what happens. There's not much in the overall circuit and its just a process of elimination. I know it can be tedious. Also, verify the input voltages.
 
Last edited:
I removed the zener from circuit again and double checked it, and it's testing like a good diode.

For as long as q3 (in the schematic) survives, all voltages look good. Including those at the 723 for reference voltages.
Machine works properly, plays games off my coco sdc fine, etc.

All the socketed chips from my coco2 were removed, and I tested the ones from this machine, and everything worked fine.
I have felt all the large components, and can't feel anything getting toasty except the one transistor. The large 2n6594 isn't getting warm, which is weird to me, being that it's equipped with a large heat sink.

Here's the voltages present on the 723c. The circuit is a little different on the f model around pin 6, it has a 980ohm at r61 and a 2.2k at r60 that goes to ground without a r62 being in the circuit.

1 - 0
2 - 5.23
3 - 4.96
4 - 4.96
5 - 4.96
6 - 7.10
7 - 0
8 - 0
9 - .125
10 -5.81
11 -12.23
12 -12.23
13 -7.16
14 -0

I also just tried removing the 2n6594 and the output voltage dropped to under 2v, so it's doing something.
 
May be possible that Q3 is breaking down. Can you change it out? I'm running out of ideas.
 
It would be interesting to know what the voltage drop across R66 is - i.e., how much current is actually being drawn? But given that the device works OK until the transistor fails, it's presumably (approximately) correct.

I'm thinking that the 2N6594 is the problem - either it has very low current gain or the base-emitter junction is shorted. Probably the latter. Then all the current flows through the MPSW01 or TIP29, and they are rated at 1W & 2W respectively. If the circuit draws 1.35A as indicated in the schematic and the +9V input is approximately correct, that means that the MPSW01 or TIP29 is trying to dissipate 5.4W, which would cause it to get very hot and fail. Which is what you're seeing. The reason the output voltage drops so low when you remove the 23N6594 is that the current now has to flow through R59, and this will limit it.
 
These notes are referencing the schematics shown above.

I replaced the q3 with another MPSW01A, and Q2 with the MJ2955.
Now the heatsink for q2 is HOT... not to the point of not being able to touch it, but toasty like it should be.
The MPSW01A is room temp as far as I can tell, so it was Q2 the entire time. Partial failure..
Nothing showed open or direct short when I removed the old 2N6594 and tested it, but it wasn't actually doing its job.

That was an annoying repair. But it's been up for a hour now. :)
 
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