Help with dead Commodore 1802

[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

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

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

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

[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

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

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

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

[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

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

[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

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

[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.

[roadrash]

Re: Help with dead Commodore 1802

Just powered it on and the light bulb flashed for anout 8-10 secs and then it stopped and now monitor doesnt turn on any more.

[stj]

Re: Help with dead Commodore 1802

that's good, the faulty part may have burned out.
check the fuse though

[roadrash]

Re: Help with dead Commodore 1802

I checked and fuses are ok and so far I have checked all the diodes they test with the multimeter as ok. So must be something else like maybe the STR54041 regulator i already replaced earlier or another transistor thats now failed. What do your best guest to go for 1st?

[stj]

Re: Help with dead Commodore 1802

no idea, i would probably be using a meter on the board.
but testing all transistors is simple - there arent many.

[roadrash]

Re: Help with dead Commodore 1802

I had a few of these nice little oven lights turn up today and ive solderd one in to replace the halogen one so it might be better. I will start removing all semiconductors now one by one (except the caps & diodes that ive checked), until I find whats failed. At least there are not too many.

[stj]

Re: Help with dead Commodore 1802

just a thought, check the mains smoothing cap - the big electrolytic.