Help with dead Commodore 1802

[roadrash]

Re: Help with dead Commodore 1802

The bulb you are using has too high a resistance, and remember that 72.5Ω you measured is the DC resistance, it is going to increase even more when it lights.
The A/C voltage is being droped from 240vac to around 13vac to supply the diodes which then outputs 18vdc.
So you need to find a regular 60~100 watt Incandescent lamp

Ive connected 60w bulbs to both places now and when i power it up i hear a clicking noise and one of the bulbs flickers which then changes to a buzzing sound. Voltage at filter cap is still only 15v

[R_J]

Re: Help with dead Commodore 1802

The bulb you are using has too high a resistance, and remember that 72.5Ω you measured is the DC resistance, it is going to increase even more when it lights.
The A/C voltage is being droped from 240vac to around 13vac to supply the diodes which then outputs 18vdc.
So you need to find a regular 60~100 watt Incandescent lamp

[roadrash]

Re: Help with dead Commodore 1802

are you sure the rest of that cable is good?
you should see about 340v dc across the main filter cap

Yes that cable ok now because if t wasnt the bulb that is replacing that big resistor woulnt illuminate.
That is the resistor that kep popping if you remember and is what we were trying to diagnose..
What would cause a low voltage at that filter cap?
I just tested it again and I am getting 18v DC.

[stj]

Re: Help with dead Commodore 1802

are you sure the rest of that cable is good?
you should see about 340v dc across the main filter cap

[roadrash]

Re: Help with dead Commodore 1802

I just checked a couple incandescent lamps, these are 120vac 60 watt = 16Ω. (Christmas light) 120vac 7 watt = 189Ω, Appliance lamp 120vac 40watt = 27Ω

Sorry for delay getting back to you both but was in hospital yesterday getting steroid injections into my spine and boy was I was I very sore today.

Right well got all the bulbs now working as they should and when powered on the only bulb illuminated is the one across where the resistor R801 was. The bulb where L803 was does not light up.

The voltages now are now as follows:

At filter cap C803 it fluctuates between 6.5v DC and 16v. Although when first powered on it reads 6.2v DC and slowly rises the longer its turned on.

On T801 at pin 2 the voltage fluctuates between 9.5v and 13v DC but If I set the meter to AC I get a steady 30v which makes me think its AC and not DC voltage there.
Pin 4 it again fluctuates using DC scale between 8.5 and 13v but if tested using the AC scale I get 30v.

At the pins 4 & 5 of the LOPT again the DC voltage fluctuates and its impossible to get a steady reading. If I measure it using AC scales Pin 4 still fluctuates a little buts around 13v and Pin 5 its the same slight fluctuating but up to 15v.

According to your schematic RJ I should be using the DC scale to measure these voltages. I am taking the DC ground/negative from the C803 Cap on the side of Q803 & R803 & Q805 which according to your schematic RJ is the the DC ground.

So it seems I am not getting rectified voltage or am I doing something wrong still?

[roadrash]

Re: Help with dead Commodore 1802

I just checked a couple incandescent lamps, these are 120vac 60watt = 16Ω. (Christmas light) 120vac 7 watt = 189Ω, Appliance lamp 120vac 40watt = 27Ω

You are not going to believe this but that length of 3 core flex i was using had a break in it and after i cut 6 inches of each end it now works. What are the chances of that.
I didnt think to check the cable as it looked like it was good. Anyway i now have the bulbs iluminating as they should. I will have another go at it later now to see if i can make any sense now over what we checked.

[R_J]

Re: Help with dead Commodore 1802

I just checked a couple incandescent lamps, these are 120vac 60watt = 16Ω. (Christmas light) 120vac 7 watt = 189Ω, Appliance lamp 120vac 40watt = 27Ω

[stj]

Re: Help with dead Commodore 1802

it's really strange that you see a resistance but no life - be interesting to do an autopsy and cut the metal cap open!

[roadrash]

Re: Help with dead Commodore 1802

strange, you should see some voltage and some life from the lamp.

Well I just wired one of those ever ready bulbs to 240v mains and it doesnt light up. They all seem to be the same. They are nor open circuit and show a resistance but dont iluminate with 240v ac. I think these bulbs are what are fitted into ovens etc but surely they should light up. I will go andf buy some more of the bigger ceiling type bulbs tomorrow and have another go. No wonder we havent been getting anywhere.

[stj]

Re: Help with dead Commodore 1802

strange, you should see some voltage and some life from the lamp.

[roadrash]

Re: Help with dead Commodore 1802

What do you have in place of the resistor? if there is nothing there, You will not get any voltage across the capacitor because you are not getting any AC to the diodes.

I put this bulb ain place of the resistor maybe i need a bigger bulb? It measures 72.5 ohms.

[R_J]

Re: Help with dead Commodore 1802

What do you have in place of the resistor? if there is nothing there, You will not get any voltage across the capacitor because you are not getting any AC to the diodes.

[roadrash]

Re: Help with dead Commodore 1802

At filter cap C803 there is no AC voltage. This is correct! IT IS DC VOLTAGE!!!!

Yes thats right. No voltage at filter cap. I am right measuring accros the 2 cap contacs (+and -).

[R_J]

Re: Help with dead Commodore 1802

At filter cap C803 there is no AC voltage. This is correct! IT IS DC VOLTAGE!!!!

[roadrash]

Re: Help with dead Commodore 1802

This is the a/c path and where the 4 diodes (in a bridge configuration) convert to DC. After that it gets more complicated as the STR54041 IC takes that DC and switches it, at HIGH FREQUENCY (around 20KHz) into A/C to drive the transformer.

Hey RJ thats really helpful of you. Sorry been a delay in getting back to you as ive been unwell all week with my MS. Anyway what yiou did there makes it a lot more plain where the AC and DC changen and where I take the measurements from. Going by your Shematic I now find I am getting no readings where I had some before.

At filter cap C803 there is no AC voltage. (before I was using one off the AC inputs as a pole but now if I measure across the capacitors 2 contacts -+ there is no voltage reading and I checked fuse again and its ok. Should be a voltage across the 2 poles of C803?

I will have to look in to this further because there is something amiss here. Now I use your schematic RJ, I am getting no readings where I was getting something before.
I will sleep on it and start again tomorrow with a fresh start when I am not so fatigued.
thanks again..

[R_J]

Re: Help with dead Commodore 1802

This is the a/c path and where the 4 diodes (in a bridge configuration) convert to DC. After that it gets more complicated as the STR54041 IC takes that DC and switches it, at HIGH FREQUENCY (around 20KHz) into A/C to drive the transformer.

[roadrash]

Re: Help with dead Commodore 1802

after the mains input diodes - it's all DC

Ok I will have to use my other automotive meter for voltage checking. I am a little confused here. I know you said that everything after the mains input diodes is DC. The mains input diodes are these the diodes marked 4700 500V x4 on the schematic? It might be easier for me for the time being until I learn a bit more if you could give me the positions as to where to take the reading from. Sorry its a pain I am still getting to grips with the stage where AC is coverted to DC and the place where DC & polarity +- takes effect.

[roadrash]

Re: Help with dead Commodore 1802

I have just realised there is a fault with my multimeter as its only showing 160v and not 240v. This would mess the things up a bit. Ive had a lot of problems with meters going wrong like incorrect reading of resistance and now on this meter its voltages that are wrongly measured. Can you reccomend me a good reliable digital meter on Ebay thats not going to coast a arm and leg.
For examle I have a really nice USpro EM129 automotive meter here that for some reason gives incorrect resistance readings but there isnt any information about how I can calibrate it, if thats possible. The meter I have been using to test this monitor is a XL830L I bought from Ebay only about 6 months ago and is my 2nd one to go wrong now.

[roadrash]

Re: Help with dead Commodore 1802

Thank you both for that help and its spot on as usual. RJ I know exactly what you mean about the meter beep. It's not a accurate way of checking for continuity as how many times has my meter beeped a short when its just a low resistance. You got that right too STJ I forgot we isolated the PSU so using any part our of that area to take a measurement is not going to be valid. Why did I not think of that. I am learning loads here and a lot more confident working on things like monitors now than I was when I would be reluctant to delve around in one if it was playing up. And it would get dumped eventually. I am going away for a week so wont get anything done now until I get back but I am more and more confident that with your help we will get this thing sorted now like we did before.
At least you will get a break from me for a week lol.
Speak again when I get back. Cheers!

[stj]

Re: Help with dead Commodore 1802

the input is isolated, for testing the psu the ground must be on the same component.