Help with dead Commodore 1802

[roadrash]

Re: Help with dead Commodore 1802

i'll go over the schematic again on saturday

When i turn it on a can hear a faint bong sound which might be the audio amp. I checked and there is 240v at the filter cap..

[stj]

Re: Help with dead Commodore 1802

you should have 340v DC at the cap more or less.
the sound is probably the degausing coil around the edge of the crt.

[roadrash]

Re: Help with dead Commodore 1802

you should have 340v DC at the cap more or less.
the sound is probably the degausing coil around the edge of the crt.

Well I actually get 245v volts measuring between the filter cap we replaced and L802, but I measure between the filter cap and chassis ground I get 11v.

Its encouraging that it makes that bong when its powered up as it just used to blow that resistor before but I still have the light bulb in place of the large resistor at present or should I remove that as well now so its back to what was original.

[stj]

Re: Help with dead Commodore 1802

worth a try, it;s obviously not drawing enough current to damage a resistor

[roadrash]

Re: Help with dead Commodore 1802

worth a try, it;s obviously not drawing enough current to damage a resistor

Well it blew the resistor again but not the fuse and once I solder the light bulb back in place of the resistor its still powering up in that I get that bong sound, but the tube heaters not glowing or anything. This is frustraiting. It the bong sound means its powering up then why doesnt anything ele. where do I measure the PSU output from, I want to see if when its going bong is it actually powering up and giving any power out and if so how much.

[R_J]

Re: Help with dead Commodore 1802

The "BONG" sound is just the deqausing coil. Unplug it. It has nothing to do with the power supply. look at the schematic, The deqausing coil is in series with the ptc PR801 and it is connected across the a/c line.

[roadrash]

Re: Help with dead Commodore 1802

The "BONG" sound is just the deqausing coil. Unplug it. It has nothing to do with the power supply. look at the schematic, The deqausing coil is in series with the ptc PR801 and it is connected across the a/c line.

Thanks for that info RJ.
I decided that the only way I will find this hopefully is to check every single component one by one and any that dont give me the result I expect I remove from circuit to test again. So far ive tested quite a few caps, resistors and a few transistorsand so far all are testing ok except a resistor that is showing as a short. Its red,red,red & gold which is 2.2k I think. Should that be detected as a short?

[R_J]

Re: Help with dead Commodore 1802

Maybe, it depends on what is across it in the circuit. I could guess where it is. The only 2.2k in the power supply circuit is R806 (they mark the locations) If it checks shorted Q806, C824 could be shorted.
How are you checking it? What resistance does it check?

[roadrash]

Re: Help with dead Commodore 1802

Your spot on its r806 and i removed it to check i was seeing those colours as reds and not an orange or something. Once i removed it and hooked up to my component tester it shows as 2.2k or within tolerence. Since then ive contined and checked every component within the psu area and nothing else is reading as abnormal. That surely leaves only things like inductors and transformers doesnt it?

[R_J]

Re: Help with dead Commodore 1802

So did you figure out why R806 was checking shorted in circuit?

[stj]

Re: Help with dead Commodore 1802

you cant really test the psu chip with a meter.

[roadrash]

Re: Help with dead Commodore 1802

So did you figure out why R806 was checking shorted in circuit?

Not entiely sure but might be because I had my meter on diode setting so it beeps, but when isolated, it gives a reading of 71.59k so its within tolereance isn't it. Its a a big grey cap and marked as 3w 68k. Another resistor I was suspicious about as it was showing as a short or beeping on my meter and I lifted one end to isolate was R808. But again when isolated measures ok at 2243 ohms so this I beleive is also ok. Could some of these readings be beacause these all go through the big transformer T801.

Since then I removed the L805 again and reconnected the bulbs like we had before and powered it up again and looked back through this thread and took some voltage reading again now I have a better idea wheer everything is and here is what I got:

At filter cap C803 =165V AC
On T801 at pin 2 and pin 4 there is = 165 V DC on both
At the pins 4 & 5 of the LOPT = 10V DC

I hope these make sense.
I get confused whether we are dealing with AC or DC voltages because although with AC there is no positive (+ ) & negative (- ) and I can use either of the 2 240v inputs from the mains. With DC voltage there is polarity and I dont know where I should be putting the black probe to. I usually connect the black probe to a heatsik or the outer connection on a RCA jack as the negative and these are usualy Negative. Am I doing this right? I am not sure at which point in the circuit (diodes) the voltage is recified and we get a Positive and negative voltage. Once I can get my head round this I shouldnt be as confused checking voltages whether AC or DC. Where is the best place to use as a reference negative side when taking a DC voltage.

[stj]

Re: Help with dead Commodore 1802

after the mains input diodes - it's all DC

[R_J]

Re: Help with dead Commodore 1802

I don't know how many times I have said it but you can NOT use the BEEP function for testing components. Most meters will beep between 0Ω and 150Ω
As for the a/c or DC. You can check for A/C only to the two input (~) of the bridge rectifier, after that it is DC with reference to HOT ground (- of input capacitor C803) for any measurements in the primary side of T801, All DC measurements on the secondary side use the chassis for ground.

If you are measuring 165v AC on C803 you are using the wrong ground!

[stj]

Re: Help with dead Commodore 1802

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

[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!

[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

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.

[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

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..