Help with dead Commodore 1802

[R_J]

Re: Help with dead Commodore 1802

I would replace the caps, They are about the only parts that will deteriorate overtime, This is a crude switching circuit and if it doesn't operate properly it won't work.

I just repaired a monitor power supply that would turn on and off about 10 times before it would stay on. The power supply startup cap was a 22µf/50v. It's capacity checked 21.5µf. However the scope showed about 3~5v of high frequency ripple on it. replaced it and the power supply was working fine.

C806: c=223nf = 0.22µf. On the schematic it looks like 2.2µf/160v

[roadrash]

Re: Help with dead Commodore 1802

I would replace the caps, They are about the only parts that will deteriorate overtime, This is a crude switching circuit and if it doesn't operate properly it won't work.

I just repaired a monitor power supply that would turn on and off about 10 times before it would stay on. The power supply startup cap was a 22µf/50v. It's capacity checked 21.5µf. However the scope showed about 3~5v of high frequency ripple on it. replaced it and the power supply was working fine.

C806: c=223nf = 0.22µf. On the schematic it looks like 2.2µf/160v

I ordered the two caps as suggested and they arrived today. However I was shocked at how much smaller they are than the originals. WIll this be OK or should I find some others a similar or nearer size.

[R_J]

Re: Help with dead Commodore 1802

Hold them up to the monitor so I can see them I'm sure they will be fine, With newer technology they can make them smaller as long as the capacity and voltage is correct they will work.
Check the new 2.2/160v capacitor [C806] on your meter just to see what it displays.

[Agent24]

Re: Help with dead Commodore 1802

I would replace the caps, They are about the only parts that will deteriorate overtime, This is a crude switching circuit and if it doesn't operate properly it won't work.

Optoisolators can degrade over time too, with the LED inside becoming dimmer like any LED does.

[R_J]

Re: Help with dead Commodore 1802

Optoisolators can degrade over time too, with the LED inside becoming dimmer like any LED does.

I was talking about THIS ciruit ONLY it was not a ment to be a general comment, This monitors power supply does'nt us an optoisolator. Since the monitor worked befor it was stored, components like transistors, ic's, resistors, normally won't deterorate, but electrolytic capacitors can dry out.
The crt will degrade over time also.

[Agent24]

Re: Help with dead Commodore 1802

Can you point out the Optoisolator in THIS circuit?

I'm guessing there isn't one, but that wasn't my point. Just figured I'd add the information as it may help someone in the future.

[R_J]

Re: Help with dead Commodore 1802

I'm guessing there isn't one, but that wasn't my point. Just figured I'd add the information as it may help someone in the future.

I made an edit after you quoted me.

[roadrash]

Re: Help with dead Commodore 1802

Hold them up to the monitor so I can see them I'm sure they will be fine, With newer technology they can make them smaller as long as the capacity and voltage is correct they will work.
Check the new 2.2/160v capacitor [C806] on your meter just to see what it displays.

Ok here is comparison picture. They are about half the original size.

[R_J]

Re: Help with dead Commodore 1802

They will be fine

[roadrash]

Re: Help with dead Commodore 1802

They will be fine

Ok soldered in and still the monitor is tickimg and lilght bulb is flashing. So no change

[roadrash]

Re: Help with dead Commodore 1802

Just a thoght and i think STJ mentioned it before, it couldn't be a transformer could it? Running out of things to test now. We could just replace all caps in psu.

[R_J]

Re: Help with dead Commodore 1802

Ok soldered in and still the monitor is tickimg and lilght bulb is flashing. So no change

What is the voltage across C803, If this voltage is too low the circuit won't start
Did you check that C806 did measure different with your meter ?

[roadrash]

Re: Help with dead Commodore 1802

What is the voltage across C803, If this voltage is too low the circuit won't start
Did you check that C806 did measure different with your meter ?

Its fluctuating between 50 & 69v DC.

[R_J]

Re: Help with dead Commodore 1802

Its fluctuating between 50 & 69v DC.

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

[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?

[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

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

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

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

Check if ic801 is shorted, pins 3 to 4