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Classic analog board voltage problem

genie_mac

genie_mac

6502
I got this Classic of a friend's friend that's in really poor condition technically and cosmetically. Sure enough it showed the usual garbled screen. I recapped the logic board which is now working OK (tested in another Classic). But still the garbled screen. So a voltage check showed that they were way below spec. So time to recap the analog board. Still no luck after that. So time to investigate further. Here's what's going on:

- voltage on start up is about 11.4V and slowly ramps up to 12V and machine chimes and boots no problem

- screen wobbles but as voltages ramp up screen gets more stable

- pre-heating the board with a hot air gun reduces time to boot considerable (however I haven't tried applying localised heat)

- some components are running rather hot though (transistors, voltage regs, about 25degrees C hotter than in my good Classic)

- the main switching transistor (I think it is anyways, a IRFBC40 600V 6A MosFET) is running very hot (doesn't have a heat sink)

Just a few thought:

- there still is a bad cap / caps in there somewhere (replaced all of them except a high voltage one, but may be of dubious quality)

- rectifier problem

- switching MosFET doesn't turn on fully and therefor heats up or transistor is faulty

What do you think? Any advice would be much appreciated. I really need a "daily driver" to mess around with / mod and an old tatty one would be ideal.

Cheers!

 
AH!

yeah you have the 220 version of the A/b too.

can you take some pictures? i would like to see what the differences are.

the classic was not a auto switching psu… it was either 120 or 220...

 
Well, mine shows on the protector cardboard that it only has or does not have a jumper in place. That switches the voltage.

 
Just a quick update on this.

I think I got it working again. Voltages are stable from turn on and machine boots straight away. Need to check temperatures and compare them to my other Classic but so far, so good.

All I did was resolder the QP2 MosFet and replaced a dodgy looking 1uF cap (which I had already replaced).

Regarding 120-240V conversion, here's what Larry Pina says (also as Ike says):

For 110VAC, jumper JP1 must be installed.

For 220VAC, jumper JP1 must be removed.

[attachment=0]IMG_0931.JPG[/attachment]

[attachment=1]IMG_0928.JPG[/attachment]

 
Last edited by a moderator:
On my both 220 boards, there is a big filter cap there, nice to know there can also be a jumper there.

BTW, note that jumper JP1 is between the two filter cap spots, but might be completely covered in a dab of hot glue, as in my case!

 
Spoke too soon! I still have problems with this. Tried a cold start (room temperature of about 10 Celsius / 50 Fahrenheit) and voltages were about 10% below spec. A quick blast with the heat gun and voltages were in spec within about 15 seconds and stayed there. Rock solid after this for an hour until I turned it off.

So I'm wondering if there are any other components apart from the caps that could cause this. I desoldered most caps again and tested them for ESR. All good so far. Could it be the feedback that needs warming up?

It's a nice little machine, would really like to get this one going.

 
check the optoisolator in the feedback circuit, it may need replaced.
Click to expand...
Sure enough that was the problem. Thanks man :)

Funny thing is the diode in the optoisolator tested good at room temperature, but once I hit it with cold spray it would stop working. And even stranger testing with a multimeter it checked out OK, no matter what the temperature.

I replaced it with a another IC from an old power supply, same chip but lower gain in the transistor and it gave me some really mad screen problems. Found another similar IC with similar specs and works OK now. Touch wood :)

 
That was probably what was wrong with mine!

Oh, well, it's all moot now; I stripped it for MinerAl.

Glad you got yours working!

c

 
That was probably what was wrong with mine!
Click to expand...
Ah no! This one was driving me mad for a few weeks as well. I was close to giving up on it too.

Well here's what I had to replace to get it working again.

Symptoms: low voltages on start-up. Voltages stabilise after a while and computer boots and works OK after that.

Solution: replace optocoupler CNY75GB or replace with CNY75B. I found other CNY75's don't work. Tried CNY75A and no good. Needs a high CTR (coupling transfer ratio) of about 100-200% it seems.

[attachment=0]IMG_0928.JPG[/attachment]

 
Optoisolators have an LED in them, and they have a finite life. Eventually they will wear out and it will slowly lose regulation.

 
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