View Full Version : Commodore 1901 colour monitor repair...

10th June 2014, 10:06
ive just finished repairing a commodore 1901 monitor i got here off macc.

it had loads of problems due to old age,but is now fully working with some extra features.:)

when i got this it had a bad mains switch that had fused contacts but that wasnt the end of this so i had to repair it.after the repair.

this had no colour on luma/chroma and was displaying black and white and needed a real overhaul as it had alot of problems switching between pal/rgb modes,so i thought i would service it and add a few nice features.

such as:
colour on composite:)(i will document this later) its black and white normally...very interesting this one.
scart for rgb analogue input its digital rgb i normally as well on a 9 pin d sub as standard for the commodore 128 rgb output...
chroma luma repair with colour fix

interesting really, this monitor as its actually a thomson internally.

i will post a few pictures with various inputs being used to show the display quality and pictures of the front and rear,i will still have to fix the front flap as it has broken hinges...i'll do that at a later date as ive been very ill.
and a parts list of what i replaced and added to make this monitor really useful...

more later chaps...

here is a list of parts:

parts added:
1 toggle switch(was going to use a push to make latch switch but i havent got one )
1 scart socket

parts replaced:
1 mains switch(fused contacts monitor could not be turned on)
1 AN 5620 X (PAL)(on pal/rgb module) replaced to extend the life of the monitor)
2 BU508D's(tired components)
1 7812 regulator(as above)
1 TEA 2017(partly failed)
1 TDA 3506(replaced to extend life)
1 2K trimpot(i broke the original during the repair,doh)
removed one resistor grounding the chroma signal hence why it was black and white on luma chroma,cant remember the value.its on the back of the chroma port..didnt replace its not needed...
1 1.6 amp fuse due to corrosion.

i may also make a nice wiring loom for the tube neck assembly at a later date,original looks tired..

i'll upload pictures and descriptions later for anyone interested.no pictures of the actual repair that was a long night so didnt bother..

10th June 2014, 16:17
Cannot wait for the pictures! :)

10th June 2014, 16:40
here is a picture of the back of the monitor,the switch is connected to the pal/rgb board on pins 4 and 5 you cant miss them...(they are on the right of the board as standoffs with nothing connected to them,the numbers are printed on the board)
found this out by looking at the service manual...monitor only uses the composite for a sync only normally.
the switch is for colour in composite to be fed into the pal/rgb board.without it there is either no colour on composite if the the 4 and 5 pins are bridged permantly chroma wont work hense the switch...monitor is originally setup for chroma luma only.

this is a c64 via chroma luma...

this is a couple of pictures of my ps1 via composite in colour:)



my spectrum 128 via analogue rgb via scart:)


and last but not least an amiga 600 via analogue rgb



pictures are a bit naff because i was shaking(recovering form illness),but you get the point they are actually sharp and i really like the fact the blacks really are black,the only down side is the monitor is mono...

im pleased with the result,my wife already has dibs on this for here machines...

im a bit tired now...so if anyone has any questions regarding this please let me know and i'll do my best to answer.

im off now for some rest,i'll log in in the morning.

10th June 2014, 20:40
Lovely job Roy, some nice contrast on that last picture

10th June 2014, 21:06
Nice work! They are a great monitor alright.

Did you replace any of the caps? Might help to give a brighter, crisper picture. I'm due to do an overhaul on mine too, where did you get the IC's?

10th June 2014, 22:39
Great Work as always Roy :-)

10th June 2014, 23:51
hi,i didnt replace any of the caps there fine.checked them with a esr meter.the caps where the least of this monitors problems trust me.

you can get the ic's anywhere even ebay,they are really cheap for this monitor.and very common.

i really suggest getting hold of the service manual,it helps somewhat...but be careful reading it,its not totally accurate .so common sense is advised so only replace what needs to be replaced especially in the switchmode supply you have to be careful there.
some parts have to be insulated on the heatsinks,such as one of the BU508D's care is needed because some come in a package that has a metal plate where a plastic one is on some and it will short the supply blowing fuses like a good un...
and check the 12volt regulator this monitor needs it for switching modes on the pal/rgb module.

carefully inspecting all the solder joints is a good place to start on old monitors,they do go dry.
and the cables can corrode over time,hence the reason im thinking about making a new cable loom for this one.

the picture is crisp and sharp its just my camera work thats shoddy:)

11th June 2014, 00:18
Great info, thanks!

11th June 2014, 00:47
Great work Roy! Nice to see it back in action and better than ever!!

12th June 2014, 10:38
thanks everyone,this one is a keeper!

my wife definitely has dibs on this already,lol... debbie wants to know when i'll set this up for her machines!
it is a nice monitor though and really reliable now,glad i did it.

12th June 2014, 17:22
You turned it into the monitor it should have been from the start!

15th June 2014, 13:51
just a small update.

i have now finished making new hinges for this and glued them on...

i think this is completely done now:)

here are the pics:


and thats all folks.

15th June 2014, 17:51
ok to make more sense of why there is no colour or why i fitted a switch so composite in colour actually works,i have a couple of pictures of the schematic of the pal/rgbi module and marked where the swicth goes to help people understand why it dont work normally.



as you can see the composite signal dont go into the an 5620 normally,but there is prevision for it to on the board i have marked where the switch is fitted to feed the signal into the an 5620 for colour output.
hopefully you can see why the switch needs to be fitted apposed to permanently bridging the points 4 and 5 on the board.

but...for the people that think hey cant i just plug the composite cable into the chroma port and it'll work?...no it wont.
because the that signal also has to go into the tea 2017 and tda 3506 for sync.
so,normally you would either get black and white or nothing at all,this fixes this and you will agree its very simple to do if you understand how this works.
hope this makes sense...

15th June 2014, 19:56
That would explain why every speccy i tried on it was always b+w!

15th June 2014, 20:23
That would explain why every speccy i tried on it was always b+w!

yes,and any other pal composite device.

i thought i would share this info for anyone else that has one of these monitors as i havent seen any info on a fix for this on the net anywhere,only people saying composite is in black and white but nothing more.and the odd reference to scart!
it just made sense to add it,and make the monitor more useful!

it makes sense really that it was only used for the sync signal because of the scart option on the main board.
but,i have never had one of these monitors before so it was an interesting find.
maybe they did a version with composite instead of chroma luma or even scart with/without rgbi on the thomson one or all the options at the factory,i'll never know.
still dont know what the extra settings are for on the swicth on the front,there isent even a mention of that in the thomson manual(its the same monitor)

the best part about this is,if anyone adds the switch and you happen to have a early c64 thats luma only or composite its colour through luma as well:lol:(luma on its own without chroma,early c64's dont have chroma as far as i know)

15th June 2014, 20:47
Pretty sure i heard that the 1902 had the scart as standard but that was the 1901's american brother

15th June 2014, 20:51
Pretty sure i heard that the 1902 had the scart as standard but that was the 1901's american brother

i'll have to have a look at that one!:thumbsup:

maybe get a hold of the service manual.

EDIT: got hold of the schematics and the 1902 is a completely different monitor internally adding a scart socket is simple enough though from the look of it.

16th June 2014, 16:28
just a small update.

i have now finished making new hinges for this and glued them on...

i think this is completely done now:)

here are the pics:

and thats all folks.

Brilliant stuff here! Very neat way of fixing an all too common problem on these things :)

4th August 2017, 15:38
Thread from the dead :)

I'm just about to add scart to my 1901, but wanted to check pinouts... It looks like I simply need to fit a scart socket, upside down, with the bottom row going farthest away from the back and the top row closest... Will PM roy and share if he replies :D

4th August 2017, 16:06
get one of these the item is wrongly listed as 90 degrees,its actually 180


4th August 2017, 16:23
Thanks for that! So with the 180 (straight out the back, I guess) connector, I need to install with the wider bit to the bottom, then connect the top row of pins on the connector to the row nearest the back, and the bottom row of pins furthest away?

This is what I'm going on - http://elgensrepairs.blogspot.co.uk/2012/12/finding-repairing-and-enhancing.html . If you've got any more pics of the scart and connection that would be great!

4th August 2017, 16:44
no check the link i gave here,180 degree is opposite to 90 not straight out the back

i have seen that link and that was a straight out pin socket he had already i suggest you get a socket in the link i provided,no guesswork required and just simply solder it in,simple

why have all the extra work of wiring a socket with 21 pins? when you can get a socket made for the job?

4th August 2017, 17:55
Perfect, if that fits without messing around so much the better :D

31st August 2017, 14:35
Finally getting around to fitting the scart socket... Found that CP2 has disintegrated! It`s a paper capacitor next to the power switch that has exploded, so that needs replacing.

Other than that it looks straightforward to fit - everything`s there already to go and the connectors are in the right place, thanks Roy! I`ve had to cut the side mounts off to get it to fit and I think I may need to shave a bit off the back to rotate it into the holes whilst the metal surround is still fitted.

31st August 2017, 21:54
good good,the x2 cap is a common failure

11th September 2019, 05:07
I'm very late to this thread, but I also just installed a SCART port into my 1901 after replacing all of the capacitors (the image was shaky after 15mins of use, this appears to be fixed now). However there's another problem that's ticking me off a bit:


This is my USA SNES connected via RGB SCART. Notice the gap to the right? Well that's with the horizontal position POT turned all the way in that direction... The 1901 has no POT for horizontal amplitude adjustment.

It does have vertical amplitude (and v-sync) but no vertical position - who the hell designed this thing?!

Super late edit: I ended up just adjusting the positioning rings on the yoke and got it to where I wanted it. It's interesting to note that despite being near identical internally to this Commodore 1901, my Thomson 4165 (similar to the Thomson 4120) has a much thinner horizontal amplitude when displaying 60Hz analog RGB...