Help with dead Commodore 1802

[stj]

Re: Help with dead Commodore 1802

i have never seen as resistor fail without help.
a 40w lamp bulb in series with the monitor is a good idea to limit the current.
and leave the degausing coil unplugged - it's the thing that causes the bong sound - and causes a big surge.
it's around the outer edge of the front of the tube, and runs to a 2pin connector near the mains input.

[roadrash]

Re: Help with dead Commodore 1802

It blew the 3.3Ω 10 watt resistor! but not the fuse? Ok check the 4 power supply diodes, then check the resistance across the main capacitor. You must have a dead short somewhere in the primary circuit
Post a picture of that new resistor that blew open

R505 shows as a dead short. Would this be the culprit?
All others like 4 diodes are ok

[stj]

Re: Help with dead Commodore 1802

no, it's supposed to be 0.27ohms

desolder coil L805, connect a 40w mains bulb to the pins of C812 and try it again.
then your only running the psu and the audio amp - not the rest of the circuit.
you need the light because the psu expects a load.

[R_J]

Re: Help with dead Commodore 1802

So no short across the main capacitor?, You can also check the output transistor in the str54041 (E=pin4,C=pin3,B=pin2) however I suspect that if the str was bad it should have blown R805 (0.27Ω)

[roadrash]

Re: Help with dead Commodore 1802

8

no, it's supposed to be 0.27ohms

desolder coil L805, connect a 40w mains bulb to the pins of C812 and try it again.
then your only running the psu and the audio amp - not the rest of the circuit.
you need the light because the psu expects a load.

I will give this a go as i went and got some old 40w bulbs.
On my last 3.3uf resistor here so dont reallywant to blow it up too. Ive had some mox type 3.3uf resistors turn up but can i use them?.

[roadrash]

Re: Help with dead Commodore 1802

So no short across the main capacitor?, You can also check the output transistor in the str54041 (E=pin4,C=pin3,B=pin2) however I suspect that if the str was bad it should have blown R805 (0.27Ω)

Just to make sure what is the number of the main capacitor?

[R_J]

Re: Help with dead Commodore 1802

It's the large one in the power supply C803

[roadrash]

Re: Help with dead Commodore 1802

Can i use these metal oxide resistors if i blow my last wire wound one?

[R_J]

Re: Help with dead Commodore 1802

How many resistors do you have?, Use a metal oxide one and save the last wire wound. Use a incandescent lamp in series with the ac line untill you locate the problem.

[roadrash]

Re: Help with dead Commodore 1802

So no short across the main capacitor?, You can also check the output transistor in the str54041 (E=pin4,C=pin3,B=pin2) however I suspect that if the str was bad it should have blown R805 (0.27Ω)

No short across main cap. Took out str54041 so how do i test this thing. My component tester only has 3 test leads. Can it be done with a multimeter? Here isna picture of what i removed there is not any room to get into it much.

[R_J]

Re: Help with dead Commodore 1802

You can test the output transistor with a meter, see post #44

[roadrash]

Re: Help with dead Commodore 1802

You can test the output transistor with a meter, see post #44

with a meter, see post #44[/QUOTE]

Thanks. It tests ok. Ive been testing everything including the STR54041 for a short and nothing tests as a short in the power supply area of the pcb except a odd iduction coils and that new metal oxcide resistor. Should i refit the STR transistor and try power it up using the light bulb as was mentioned earlier. Ive removed L805 and connected the bulb across C812. Is there anything to check before i try this and most likely blow another resistor probably.

[roadrash]

Re: Help with dead Commodore 1802

Ok i gave it one final check after solderig in the STR transistor we checked and as i was afraid of it blew the now mox resistor. Those resistors give a much bigger flash than the wire wound one's. Well that is still a bit strange because i checked all component for the psu and none were shorting. Am i right in assuming that that coil i removed is the output voltage from the psu to the monitor itself. So this rules out a monitor fault and the short is isolated to the psu circuit somewhere?

[stj]

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.

[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?

[stj]

Re: Help with dead Commodore 1802

so no shorted diodes or ceramic caps?

[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

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

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.

[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?