Help with dead Commodore 1802

[roadrash]

Re: Help with dead Commodore 1802

Thank you RJ, Luckinly now I am back to full health again (well apart from having MS) woopee!

So what is the voltage across C803?

No nothing

Check if there is a short between pin 3 and 4 of ic801

No short

Check if Q806 or Q805 are ok and not shorted

I removed these two yesterday and checked them with my component tester which recognised them and said no problems.

I was afraid this might happen with a thick jumper and that's why I used a small thin wire the first time but I guess that thin it probably would have blown just under current draw through it.
Thanks again for all this RJ.

[R_J]

Re: Help with dead Commodore 1802

So what is the voltage across C803?
Check if there is a short between pin 3 and 4 of ic801
Check if Q806 or Q805 are ok and not shorted
Something was not working properly or the wire across would not have been an issue.
Check that IC801 is not shorted to the heatsink again.

[roadrash]

Re: Help with dead Commodore 1802

Just been waiting for some 4A fuses to come as i only had 2A fuses left.

I would get it back to where this is working. I don't know how you have things connected so I will assume you have the bulb in place of R801, correct?

Yes its back to the same as when it was working but its not now of course

You were getting 121 volts on C812, Were you getting 121v all the way to the deflection circuit and Q402 collector?

Not sure and anyway I cant find Q402 in the PSU area where its it connected too?
or just to the lamp across C812?

Bulb across R801 is lit up but the monitor seems dead again and no hissing from audio like we had. Putting that bit of wire across R801 blew something else other than the fuse I bet.

[R_J]

Re: Help with dead Commodore 1802

I fitted a different 60w halogen bulb i got the other day and it almost starts the PSU without another bulb. But If I add another with this one it wont start up. its certainly starting the PSU in as much as its powering up the audio section definitely.
C812=121VDC and C814=9.9VDC So its not far off.

I would get it back to where this is working. I don't know how you have things connected so I will assume you have the bulb in place of R801, correct?
You were getting 121 volts on C812, Were you getting 121v all the way to the deflection circuit and Q402 collector? or just to the lamp across C812?

FROM post52: Ive removed L805 and connected the bulb across C812.

[roadrash]

Re: Help with dead Commodore 1802

If the wire is too thin it won't work, If the monitor still had a power supply problem you would not be getting the 121 volts for the monitor.
I suspect the original problem was a bad C806, then when you changed ic801 you caused the short that blew the resistor. If you install a wire in place of the resistor and there is a problem, the 4 amp fuse should blow.

Sorry in delay ive been ill. I plugged it in with resistor bridged as you said and it popped a 5a fuse straight away.

[R_J]

Re: Help with dead Commodore 1802

If the wire is too thin it won't work, If the monitor still had a power supply problem you would not be getting the 121 volts for the monitor.
I suspect the original problem was a bad C806, then when you changed ic801 you caused the short that blew the resistor. If you install a wire in place of the resistor and there is a problem, the 4 amp fuse should blow.

[roadrash]

Re: Help with dead Commodore 1802

L802 is the inductor before the resistor. It will have AC NOT DC voltage. If you have 240vdc on C803. that is close to what it should be
Do you have one bulb or two in place of the resistor? if only one try a second one in parallel. I suspect everything is working, so make sure everything in the secondary is connected as it should be, Then Check the voltages (DC VOLTAGES) on C812 (121VDC) and C814 (12VDC)
If those voltages are close, then remove the lamps and replace it with a jumper wire for now. If the monitor works, then replace the jumper wire with the proper resistor.

Did you recheck these caps, and compare the values to the new ones? I will bet that C806 was the problem.

Thanks for the info, I fitted a different 60w halogen bulb i got the other day and it almost starts the PSU without another bulb. But If I add another with this one it wont start up. its certainly starting the PSU in as much as its powering up the audio section definitely.
C812=121VDC and C814=9.9VDC So its not far off.
I did try bridging across the R801 resistor but I was a little cautious about what gauge of wire to use so as not to do any other damage somewhere else if there is still a short.
So I used a single strand of copper from a small twisted wire and when I plugged the monitor in it went crack! and blew it up. So either its still shorting or it might be the wire I used couldn't handle the current. What thickness of wire should I use?
Iv'e tested it since then and it is still working ok so no harm done by that.

[R_J]

Re: Help with dead Commodore 1802

L802 is the inductor before the resistor. It will have AC NOT DC voltage. If you have 240vdc on C803. that is close to what it should be
Do you have one bulb or two in place of the resistor? if only one try a second one in parallel. I suspect everything is working, so make sure everything in the secondary is connected as it should be, Then Check the voltages (DC VOLTAGES) on C812 (121VDC) and C814 (12VDC)
If those voltages are close, then remove the lamps and replace it with a jumper wire for now. If the monitor works, then replace the jumper wire with the proper resistor.

C806 & C825 when tested give the following results:

C806: c=233nf vloss=1.5% ESR=1.6 ohms
C825 c=12.25uf vloss=0.7% ESR= 3.0 ohms.

Did you recheck these caps, and compare the values to the new ones? I will bet that C806 was the problem.

[roadrash]

Re: Help with dead Commodore 1802

Looks like good news l as when i powered it on after putting the IC back in carefully and making sure of no short or too much heatsink paste squeezing out and getting over other things, it powered up properly I believe because it its making a sound like its powered up and I can hear a hissing from the audio stage which i think was the only thing not isolated after removing that inductor and replacing it with a bulb.
there is 240v at filter cap and output of L802 is flickering between 9 & 12v DC but psu is not completely stable which is why its fluctuating using the bulb instead of the resistor. Even though the bulb is flickering its outputting voltage because I can definitely hear a sound coming from the speakers.
What should I try now?

[R_J]

Re: Help with dead Commodore 1802

Between ic801, pin4 and ground there is a 0.27Ω resistor. so it will check close to a short.
Well if the mica was'nt damaged what was causing the short. It would be a good idea to identify what caused the short. Check if the ic itself is shorted between pins 3 & 4. If you just replace it you won't know what is causing the problem.
The pin 3 is connected to the metal on the back of the ic, that is why you need the mica insulator.

If you don't apply heatsink paste to the mica, the ic will run hot, The paste is to help conduct the heat from the ic to the heatsink.

[roadrash]

Re: Help with dead Commodore 1802

Continuity could mean anything, What is the resistance?, Look at the schematic... Between ic801 pin4 and ground there is a 2watt "continuity". If IC801 pin 3 is shorted to ground what do you think that is going to do with 330vdc on C802? The mica insulator is likely damaged and the ic could be damaged by now.

Thanks for your patience with this RJ. I meant in saying continuity that there was no resistance. Sorry if I miss desrcribed that. I thought that continuity meant a perfect conection with no resistance. The Mica piece looks fine. It was coated each side with heat sink paste. Other than that I couldnt see anything. Tomorrow I will put that new IC in and check it for a short before testing it. Should I apply heat sink paste to both sides of the mica again?.

[R_J]

Re: Help with dead Commodore 1802

Continuity could mean anything, What is the resistance?, Look at the schematic... Between ic801 pin4 and ground there is a 2watt "continuity". If IC801 pin 3 is shorted to ground what do you think that is going to do with 330vdc on C802? The mica insulator is likely damaged and the ic could be damaged by now.

[roadrash]

Re: Help with dead Commodore 1802

made a few more checks and it turns out that pin 3 on the IC has continuity with the metal backing of the IC. Also pin 4 on the PCB has continuity with the heatsink the IC is bolted to.
So if there was contact between the back of the IC and the heatsink it would cause a short between pins 3 & 4. I cant see how that can happen as there is a bit Mica between the IC & heatsink. But still its a thought.

[roadrash]

Re: Help with dead Commodore 1802

Check if ic801 is shorted, pins 3 to 4

I think your right RJ, there is a dead short for sure. I also have a new STR54041 here as well we can use.

Edit: Just removed it and when removed it doesn't show a short

[R_J]

Re: Help with dead Commodore 1802

Check if ic801 is shorted, pins 3 to 4

[roadrash]

Re: Help with dead Commodore 1802

Did you have any voltage on either of the secondary capacitors, C812 or C814?
You could try disconnecting L806 and see if you get any voltage on C814. This would lighten the load the power supply has to supply and if it is going to work. Or leave L806 connected, and disconnect L805 and place one of your lamps across C812 and see if it lights.
This power supply needs to generate a square wave to drive the regulator ic 801. If there is not enough voltage on C803, the cpapacitors, C807 and C825 do not charge enough to make the circuit start to switch (like a flip flop circuit)
Q805 and Q806 generate the square wave that is applied to ic801 pin2. Then ic801 pin3 switches on and off at about 20khz and produces the 350 volt peek to peek waveform (number 25). at pin 2 of the transformer.

Something weird has happened, because the 83v DC I was getting at the filter cap has now dropped to just 0.5v DC but the two bulbs connected in parallel to where the resistor was are still lit up. I tried removing the extra bulb we added and the other bulb just stays on and not flashing anymore like it used to and the ticking noise has gone as well. Nothing has changed so I don't understand. Last night I really thought we were getting somewhere.

[R_J]

Re: Help with dead Commodore 1802

Did you have any voltage on either of the secondary capacitors, C812 or C814?
You could try disconnecting L806 and see if you get any voltage on C814. This would lighten the load the power supply has to supply and if it is going to work. Or leave L806 connected, and disconnect L805 and place one of your lamps across C812 and see if it lights.
This power supply needs to generate a square wave to drive the regulator ic 801. If there is not enough voltage on C803, the cpapacitors, C807 and C825 do not charge enough to make the circuit start to switch (like a flip flop circuit)
Q805 and Q806 generate the square wave that is applied to ic801 pin2. Then ic801 pin3 switches on and off at about 20khz and produces the 350 volt peek to peek waveform (number 25). at pin 2 of the transformer.

[roadrash]

Re: Help with dead Commodore 1802

most of the voltage is being dropped by the lamp as soon as the power supply starts the voltage drops even lower so it will never start. It likely needs around 150vdc to operate.
Since you don't have a variac, try adding a second 60watt bulb in parallel with the 60watt bulb you have in place of the resistor you have now. This will provide more dc voltage and it might start but still provide protection

Ok done that and after about 15 secs the bulbs stopped flashing and just iluminated. Voltage at c803 filter cap is now 83v DC. So I guess thats the psu started and running. What was the point behind this? Whas it to establish the psu will run when fault is found?

[R_J]

Re: Help with dead Commodore 1802

most of the voltage is being dropped by the lamp as soon as the power supply starts the voltage drops even lower so it will never start. It likely needs around 150vdc to operate.
Since you don't have a variac, try adding a second 60watt bulb in parallel with the 60watt bulb you have in place of the resistor you have now. This will provide more dc voltage and it might start but still provide protection

[roadrash]

Re: Help with dead Commodore 1802

That is likely too low for the power supply to start.

How does this happen or is it a tranformer fault or maybe a regulator?