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Old 28th Nov 2022, 4:20 pm   #1
Sanxion
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Default Commodore 1901 switching power supply

Good afternoon all,

I am new to the site so I wanted first to say "hello" to you all and secondly, I hope I have posted my question in the correct forum!

Issue: I am in the process of diagnosing a fault with a 35 year old monitor that I acquired recently...to give some background, I replaced the mains filter capacitor and the monitor did indeed work. However, a few days later, I switched it on and there was no activity at all. I have already been advised to replace the capacitors which I have started doing although according to my testing, the original ones seem fine. Nevertheless, I have also observed some corrosion on the switching transformer, see attached image. I am assuming it would be a good idea to replace it although I have not actually tested to see if it has failed.

According to the schematics, the transformer outputs 24DC and 114DC, see attachment. I have not been able to find a replacement.

What would you guys suggest I do? I must add, I am no expert. I know how to use a soldering iron and multimeter and understand basic fault finding.

Thanks!
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Old 29th Nov 2022, 9:08 pm   #2
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Default Re: Commodore 1901 switching power supply

I recently repaired a TV that also had a PSU using the TDA4601. Definitely replace all the capacitors around that IC because on mine they tested ok but replacements cured the fault. Also check the resistance of any high value resistors above 100K as sometimes they drift high. Also test the transistor that's on the heatsink next to the transformer. The corrosion on the transformer might have been caused by the glue, sometimes it goes conductive and can cause malfunction where the glue is spread across several components. Try cleaning off the corrosion and inspecting the wires.
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Old 29th Nov 2022, 9:26 pm   #3
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Default Re: Commodore 1901 switching power supply

Hi Sanxion,

That is a block diagram....not much help. Do you have a proper full circuit? I agree with the above by Andrew. There should be a 'start-up' resistor somwhere around 100K so worth checking any around the TDA4601. Probably some low-value electrolytics as well that will be worth changing. They used these chips in quite a lot of TV's and in most cases of no power, it was either the start-up resistor or one of the low-value (typically 1uF - 10uF) capacitors that was faulty. Very rarely the chip itself.

The transformer doesn't actually output those voltages....it feeds the circuits that output those voltages. You won't be able to measure much from the transformer itself on a standard digital meter....at least it won't make much sense even on a working power supply. The transformer runs at a high frequency....at least several tens of kilohertz and a standard meter won't indicate anything meaningful at those frequencies. Concentrate on the suggestions by Andrew first and come back if still no luck....oh and try and locate a proper circuit.
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Old 30th Nov 2022, 10:33 am   #4
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Default Re: Commodore 1901 switching power supply

Thanks Andrew and SIdeband. I will follow your suggestions and then report back.
The service manual is available online for this monitor. I have attached an image of the schematics. I would have uploaded it but it is 26mb:

Regarding the transformer...Do you know of a modern equivalent that I could use in its place?
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Old 30th Nov 2022, 12:41 pm   #5
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Default Re: Commodore 1901 switching power supply

Don't worry about the transformer yet. I doubt you will find one anyway except from a scrap monitor. They are usually dedicated devices so there won't be a modern replacement. Chances are the transformer is OK unless it's developed shorted turns but that is less likely with a switching transformer.

The resolution of that circuit is not good enough to make out any detail. I can see the start-up resistors but can't identify value or reference.
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Old 30th Nov 2022, 1:00 pm   #6
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Default Re: Commodore 1901 switching power supply

OK had a quick look on line and found the circuit. Items to check: RP12 10k 3watt, CP21 22uF 25V, CP10 100uF 16V, CP11 100uF 25V.

Of those, RP12 (start-up resistor but 3 Watt so less likely) and CP10 (high probability) would be my first checks (check CP10 by replacement.....had plenty of those fail). You need to fit low ESR type electrolytics here (105 degree types).

If still no luck it might be something outside the power supply causing a shut-down condition. Come back if necessary.

Slightly better image attached (I hope)
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Old 30th Nov 2022, 1:00 pm   #7
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Default Re: Commodore 1901 switching power supply

Sanxion (Good game, great Hubbard soundtrack!) we should maybe explain that the forum engine downsizes images to 800 x 600 pixels which is usually death to any detail on a large diagram like that.

What you can try is to put your original image into a .zip file and attach that to your post - it should work as long as the size of the .zip does not exceed the maximum allowed size for a .zip.
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Old 30th Nov 2022, 1:06 pm   #8
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Default Re: Commodore 1901 switching power supply

Quote:
Originally Posted by Sideband View Post
OK had a quick look on line and found the circuit. Items to check: RP12 10k 3watt, CP21 22uF 25V, CP10 100uF 16V, CP11 100uF 25V.

Of those, RP12 (less likely) and CP10 (high probability) would be my first checks (check CP10 by replacement.....had plenty of those fail). You need to fit low ESR type electrolytics here (105 degree types).

If still no luck it might be something outside the power supply causing a shut-down condition. Come back if necessary.

Slightly better image attached (I hope)
I will do. Thank you for having a look!
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Old 30th Nov 2022, 1:08 pm   #9
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Default Re: Commodore 1901 switching power supply

Quote:
Originally Posted by SiriusHardware View Post
Sanxion (Good game, great Hubbard soundtrack!) we should maybe explain that the forum engine downsizes images to 800 x 600 pixels which is usually death to any detail on a large diagram like that.

What you can try is to put your original image into a .zip file and attach that to your post - it should work as long as the size of the .zip does not exceed the maximum allowed size for a .zip.
That is useful to know, thanks.
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Old 12th Dec 2022, 8:04 pm   #10
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Default Re: Commodore 1901 switching power supply

Quote:
Originally Posted by Sanxion View Post
Quote:
Originally Posted by Sideband View Post
OK had a quick look on line and found the circuit. Items to check: RP12 10k 3watt, CP21 22uF 25V, CP10 100uF 16V, CP11 100uF 25V.

Of those, RP12 (less likely) and CP10 (high probability) would be my first checks (check CP10 by replacement.....had plenty of those fail). You need to fit low ESR type electrolytics here (105 degree types).

If still no luck it might be something outside the power supply causing a shut-down condition. Come back if necessary.

Slightly better image attached (I hope)
I will do. Thank you for having a look!
OK...I have replaced all the capacitors mentioned - including the "big ones" : CP04, 14 & 15 - and CP10. I then decided to give it a try...suffice to say that I know my smoke alarm works! There was a loud bang and then plumes of smoke emanated from CP04 (100uF 385V)..I also noticed the 5A fuse wire I was using as a temporary fuse had been obliterated!

So I am either making progress, or going backwards.

What next?
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Old 13th Dec 2022, 10:52 am   #11
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Default Re: Commodore 1901 switching power supply

Correction: CP04 is actually 100uF 160V.
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Old 13th Dec 2022, 11:31 am   #12
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Default Re: Commodore 1901 switching power supply

Capacitors only go bang like that if (a) they are old and worn out, (b) if they are severely overvolted, (c) if they are replaced the wrong way round (reversed polarity), (d) if the rectifier was shorted and they were subject to AC voltage. Using an inappropriate fuse is NOT a good idea. CHECK: that the capacitor was fitted the correct way round, CHECK that the rectifier is not short circuit (it may well be now if the cap was fitted incorrectly....huge current flow!!!
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Old 13th Dec 2022, 1:57 pm   #13
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Default Re: Commodore 1901 switching power supply

Quote:
Originally Posted by Sideband View Post
Capacitors only go bang like that if (a) they are old and worn out, (b) if they are severely overvolted, (c) if they are replaced the wrong way round (reversed polarity), (d) if the rectifier was shorted and they were subject to AC voltage. Using an inappropriate fuse is NOT a good idea. CHECK: that the capacitor was fitted the correct way round, CHECK that the rectifier is not short circuit (it may well be now if the cap was fitted incorrectly....huge current flow!!!
Thank you!
I realized I made a terrible mistake...I soldered a 100uF 160V capacitor in place of the 100uF 385V.
The monitor is working...but it is now green monochrome only.
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