• Please review our updated Terms and Rules here

Commodore 1084ST monitor (horizontal width problem) - looking for inductor

S

Steven Marshall

Member
Joined
Feb 17, 2024
Messages
17
The existing Horizontal Width has little to no effect. I assume the inductor is busted. I used my multimeter and actually did measure something like .028mH - .080mH (min vs max position when turning the inductor) but I am skeptical does it actually work as it is supposed to.

Unfortunately it is a rare 1084ST monitor and I have no idea which inductor it is as finding as service manual for this is impossible. This is the only clue I have, the photo:

https://imgur.com/7QgK1IE

The monitor is freshly recapped and works excellent.

Would it be of any help if I unsolder it and look for some sort of part item code on the bottom side of the inductor?
 
If the inductor was "busted" I assume you mean open circuit, the H scan would have likely collapsed. If it had shorted turns the width would be too great. Is the raster currently over-scanned or under scanned ie, why do you want to adjust the width ?

Other circuit factors such as the B+ voltage have significant effect on the width too (have you checked that), so if it is currently over scanned, there may be an issue elsewhere. Also Have you changed any capacitor that you perhaps should not have such as the S correction cap in series with the yoke ? More details will help solve it, but I would not blame the width coil just yet.

A number of people have been turning up with VDU problems after re-caps because they did not check the function of the VDU after each part was changed, the big question therefore, was this fault there before the recap, or appeared afterwards?

Some of these sets had a dynamic width manipulation system, it could be another part responsible. Perhaps post the schematic and we can have a look at the likely cause of the problem. And if you did replace that cap that is in series with the width coil and yoke, please post a photo of it and the original part if you still have it.
 
Thank you for the response!

This issue was present before recapping. I checked again with the multimeter and you were correct, the coil is not short, so it actually seems to be ok. The problem is - the picture is literally from edge to the edge, which for Amiga system should not be, there should be overscan area on each side.

Here a few images to illustrate this. Here are the edges of the screen at -minimal- setting for the horizontal width:
https://imgur.com/p6G1T0Q
https://imgur.com/7bTPfHs

And after turning the horizontal size knob for around 20 turns and putting it at -max- size I get barely a 2 mm in width increase. This is a bit strange.

For comparison, here is how it looks like on my 8833 mkII when i set horizontal size to -minimal-. Not only I get the Amiga overscan area shown but a black border outside:
https://imgur.com/Pm7mkEt

The coil is followed by another coil which seems to be ok, I am measuring 250 uH, so no short. There's a poly capacitor connected to the coil (I assume going to the ground) it is measuring 270 nF so it's exactly as shown on the label (27k4). So yeah I'm out of ideas. I was thinking about brute force method of decreasing the poly capacitor for 10%, but again what's the use, when the width control has so little control. Unfortunately I am not the original owner. I could see the newer caps in the PSU area, but the rest of the unit had those old 90's grey caps which are all now replaced with Nichicons.

The problem is there's no schematic for 1084ST.
 
Here are the values that I have measured. I temporary desoldered the resistor and the poly cap to measure them:

large_pic.jpg

Just realised there's a small trimmer in there in the circuit. I wonder if it relates to the overall screen width.

EDIT: the image is flipped, sorry about that. I flipped it to make it easier to track bottom traces from below and draw them on top. But now I realised I forgot to flip the picture back.
 
Last edited:
With no schematic, it may be required to trace it out at some point. Perhaps somebody on the forum will know where to find a similar set's schematic.

Have you checked the B+ voltage to the H output stage, There maybe a voltage adjustment preset in the the power supply section. If that was running a little high the width would be greater.

The width is relatively easy to manipulate by adding additional inductance in series with the width control or yoke to reduce it. But the fact that the H size control is having so little effect suggests there is a fault in that part of the circuit. So ultimately we would need to see that or trace it out.
 
Hugo Holden said:
The width is relatively easy to manipulate by adding additional inductance in series with the width control or yoke to reduce it. But the fact that the H size control is having so little effect suggests there is a fault in that part of the circuit. So ultimately we would need to see that or trace it out.
Click to expand...
Ok I think I will go with the brute force method. :)

What voltage / current rating should the series inductance be?
 
Measure the inductance of the width coil that is there with the slug fully screwed into it. You will probably need at least that again. Ideally the inductor is a high current rated type at least greater than 3 amps with a low DCR, less than 0.5 to 1 Ohm, if the wire that it is constructed from is close to 1mm thick or more, normally that would be ok.
 
Hugo Holden said:
Measure the inductance of the width coil that is there with the slug fully screwed into it. You will probably need at least that again. Ideally the inductor is a high current rated type at least greater than 3 amps with a low DCR, less than 0.5 to 1 Ohm, if the wire that it is constructed from is close to 1mm thick or more, normally that would be ok.
Click to expand...
Excellent. Thank you! I think that will do the trick.

Will also inspect all of the transistors and diodes around the board something I should have already. There's really not that many of them.
 
Hugo Holden said:
Have you checked the B+ voltage to the H output stage, There maybe a voltage adjustment preset in the the power supply section. If that was running a little high the width would be greater.
Click to expand...
Ok I found one trimmer next to TDA4605 I think this is what needs adjustment.

And now comes the bad news I never worked with crt, so I am totally scared. :)

If someone could tell me some typical location on where to check B+ voltage that would help me a lot. Still I will work in ski gloves. ;) Don't want to touch the DC.
 
Steven Marshall said:
Unfortunately I don't have schematic.

Cheers!
Click to expand...
The 1084 is normally a repackaged Phillips 8833.

Search for these drawings and compare to what you see on the board. I know there are at least two variations of one board with all the same components but a different flyback pinout.

I can't see pictures at work, but if you have one of the circuit board I will try to match it up with the service manuals

EDIT

But I see that the ST does look like its not a 8833 :(
 
Last edited:
UPDATE: Big thanks to Hugo Holden !!!!!

It was exactly as he described it.

The B+ voltage was too high. It was 113V DC. Now I have set it down to 104V DC and the border looks so much better. Of course now it's trial and error to find which voltage is correct since there is no manual. 1084S uses 110V DC, so I guess 100-105 is optimal for 1084ST. I will simply try to mimic what 8833 MkII is giving me at minimal H-Size setting which should put me in the ballpark.

Once again, thank you Hugo!

I occasionally provide technical help on one (music instruments related) forum, so I guess this is good karma coming back at me.

Still wearing ski glove. As of voltage measurement, I took the second cap in the PSU and tapped in there (having +B and GND), then soldered a long twin lead cable out of the monitor and placed two open ends into the desk area into multimeter. Yes I am that scared of CRT monitors. Not that I am afraid of electricity. I did held 30kV with my left hand and a fluorescent bulb in the other which was glowing, but that was a AC Tesla transformer, so it was all fun! DC is not fun.

@bburley: the design of the PSU does indeed look similar, I actually used 1084S-D1 version as a research material. It helped a lot. There's a difference on 1084ST is that there is no choke in between the two caps on the secondary of the main transformer, but a resistor, the choke seems to be near the Flyback. On a positive side I have learned how a CRT works and now I can repair them.
 
Last edited:
Hugo is the main monitor man!

That is exactly what I do. Solder insulated wires onto the points I want to measure, and clamping the free ends into an insulated 'chocolate block' connector. I can then probe in complete safety...

Dave
 
You must log in or register to reply here.
Back
Top