|
Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
|
Thread Tools |
28th Nov 2022, 4:20 pm | #1 |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
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! |
29th Nov 2022, 9:08 pm | #2 |
Pentode
Join Date: Aug 2008
Location: Taunton, Somerset, UK.
Posts: 135
|
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.
|
29th Nov 2022, 9:26 pm | #3 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
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.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 29th Nov 2022 at 9:39 pm. |
30th Nov 2022, 10:33 am | #4 |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
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? |
30th Nov 2022, 12:41 pm | #5 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
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.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
30th Nov 2022, 1:00 pm | #6 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
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)
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... Last edited by Sideband; 30th Nov 2022 at 1:07 pm. |
30th Nov 2022, 1:00 pm | #7 |
Dekatron
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 11,484
|
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. |
30th Nov 2022, 1:06 pm | #8 | |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
Re: Commodore 1901 switching power supply
Quote:
|
|
30th Nov 2022, 1:08 pm | #9 | |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
Re: Commodore 1901 switching power supply
Quote:
Good, I am glad I am not the only one who appreciates Mr Fasoulas' endeavors. |
|
12th Dec 2022, 8:04 pm | #10 | ||
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
Re: Commodore 1901 switching power supply
Quote:
So I am either making progress, or going backwards. What next? |
||
13th Dec 2022, 10:52 am | #11 |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
Re: Commodore 1901 switching power supply
Correction: CP04 is actually 100uF 160V.
|
13th Dec 2022, 11:31 am | #12 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
|
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!!!
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
13th Dec 2022, 1:57 pm | #13 | |
Diode
Join Date: Nov 2022
Location: Burnley, Lancashire, UK.
Posts: 7
|
Re: Commodore 1901 switching power supply
Quote:
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. |
|