Help with dead Commodore 1802

[roadrash]

Re: Help with dead Commodore 1802

it will work for the task, although a not-halogen bulb has slower inrush.

So would have been better with a standard incandescent bulb like in a bedside lamp cos i thought about them but they are quite big still.

[stj]

Re: Help with dead Commodore 1802

it will work for the task, although a not-halogen bulb has slower inrush.

[roadrash]

Re: Help with dead Commodore 1802

Yes its got U shape pins (see attached) and I had to solder them to a couple bits of wire I soldered into the holes the resisitor goes. I hope this will do.

[stj]

Re: Help with dead Commodore 1802

is that really a 240v lamp? i thought the ones with straight pins where 12v?

[roadrash]

Re: Help with dead Commodore 1802

Really good idea that STJ. I managed to get a nice little 240v 20w halogen bulb and removed the resistor soldering the bulb it its place.
Before I power up the monitor I want to check this will do the job.

[stj]

Re: Help with dead Commodore 1802

yes, it will act like a resistor but instead of blowing it will just get brighter.

[roadrash]

Re: Help with dead Commodore 1802

yes,
but it's a pain in the ass.
replace the 10w resistor with a 15-25w lamp to save resistors,
then remove the transformer T801 and see what happens.
if the lamp goes full-on then we start to lift things 1 by one to find the cause.

my suspicion is a diode is failing at full voltage but looking o.k. on a meter that only tests with about 3v
i have seen this a number of times and it's a pain to track down.

Thanks a lot STJ for that reply. That 15-25w lamp do you mean AC 240v bulb? like we use here.

[stj]

Re: Help with dead Commodore 1802

yes,
but it's a pain in the ass.
replace the 10w resistor with a 15-25w lamp to save resistors,
then remove the transformer T801 and see what happens.
if the lamp goes full-on then we start to lift things 1 by one to find the cause.

my suspicion is a diode is failing at full voltage but looking o.k. on a meter that only tests with about 3v
i have seen this a number of times and it's a pain to track down.

[roadrash]

Re: Help with dead Commodore 1802

desolder coil L808
then you only have the psu running.
the str device could be bad btw, you cant test those with a meter properly.

I know this thread is a bit old now but I have to make a choice now what to do with this monitor, I did what you said STJ and the problem keeps being there is a short somewhere in the PSU which is still blowing the Wire wound resistor. I checked every component in the PSU which currenty has L808 removed and a light bulb soldered across C812 as you suggested.
If I cant resolve this I will have no choice but to dump it which I absolutely hate doing to such rare items. Is there one last thing I can try before I have to give up?

[stj]

Re: Help with dead Commodore 1802

desolder coil L808
then you only have the psu running.
the str device could be bad btw, you cant test those with a meter properly.

[roadrash]

Re: Help with dead Commodore 1802

Please anyone how to I isolate everything except the PSU to trace this fault..

[stj]

Re: Help with dead Commodore 1802

you can lift L808 to isolate the audio stage.

im starting to wonder if the transformer could be breaking down, but few people have the equipment to check one,
(although i do)

[roadrash]

Re: Help with dead Commodore 1802

so a refresh of the facts,
with the coil out and just the psu & audio sections running your still burning resistors??

Yes thats right. Ive done as was said and checked all componnets in the PSU area for shorts incuding ceramics caps ansd cant find anything shorted. I wonder is there a way we can also isolate the audio section so we can just make sure its in the audio section.

[stj]

Re: Help with dead Commodore 1802

so a refresh of the facts,
with the coil out and just the psu & audio sections running your still burning resistors??

[roadrash]

Re: Help with dead Commodore 1802

yes, the coil is the link from the psu stage to the tube handling.
it does not isolate the audio amplifier though.

I am more than a bit stuck as to what I should be looking for now as there doesnt seem to be a component in the PSU area of the board thats short circuit.
Do you think its in the PSU area that could be causing this or ius it in the Audio amp area? Can you point me where I should look for theis short next?

[roadrash]

Re: Help with dead Commodore 1802

coils should look short - they are just a bit of wire,
ceramic caps are a bastard, they can go short without external signs, and withstand a LOT of current.
i have seen shorted ceramics blow tracks off the board!

Sorry for delay in continuing with this but now we are in summer i've been swamped with work rebuilding motorcycles and their engines. I have 5 bikes still to do yet but they are awaiting parts so must get on with something else.

Ok i've checked all components in the PSU area again including ceramics and all I could find causing a short was resistor R805 (0.27ohms) This was all I could find that could be responsible for that wire wound resistor to keep popping.

[stj]

Re: Help with dead Commodore 1802

coils should look short - they are just a bit of wire,
ceramic caps are a bastard, they can go short without external signs, and withstand a LOT of current.
i have seen shorted ceramics blow tracks off the board!

[roadrash]

Re: Help with dead Commodore 1802

I only checked inside the marked area of the psu and didnt check the ceramic caps. I checked all diodes and electrolytic caps. I didn't know that ceramic caps went short circuit. It is ok when the coils (Lxxx) show as a short I think isn't it?

[stj]

Re: Help with dead Commodore 1802

so no shorted diodes or ceramic caps?

[roadrash]

Re: Help with dead Commodore 1802

I am not sure where to look now after not finding any shorted components in psu area of the pcb. Can you point where else I should check?