Amstrad GT65 monitor video width adjustment

[JonB]

Hi

Hoping some electronics expert out there can help.

First off, there's no need to reply with the usual warnings about high voltages around the tube. I know about them already and am taking the proper precautions.

I'm trying to use a GT65 monitor with another computer (a Superbrain). The image is clear and solid, but it's too wide to fit on the screen, so I'm looking for some way to adjust it on the GT65 chassis. Referring to the service manual I can see an adjustable inductor in the horizontal sync circuit labelled L703, but this is for horizontal hold. Adjusting it has the effect of shifting the image left and right.

I read on this useful FAQ http://repairfaq.cis.upenn.edu/sam/tvfaq.htm#tvhpssla that width is sometimes set by having another inductor in series with the horizontal deflection coil in the yoke. On the GT65 schematic there is such an inductor, L702, and I've experimented with different inductances. I bought an adjustable coil having the same nominal inductance - 100uH - but it made no difference. Connecting it in series or parallel with L702 didn't change things, neither did a straight short across L702's pads.

The FAQ also suggests the B+ voltage adjuster can alter the image size. I think on the GT65 this is VR502 and it does have this effect but the image doesn't get small enough to correct the problem. It also becomes very dim when turned down, so it's necessary to up the brightness / contrast.

I've tried to research this, in the hope of finding a way to add width adjustment to the GT65. Unfortunately, I've come up blank, and found very little in the way of explanation as to how a typical width adjustment circuit might work. It seems logical to me that the speed of the horizontal scan, relative to horizontal sync is the key, but I can't see how it is regulated on the GT65 (or rather, I don't understand the horizontal circuit).

I also tried to adjust the magnets on the yoke but again, they are more about moving the image than altering its width.

I've attached the schematic and a picture of where I am at (the last 10 characters are missing on the right hand side of the screen).

Any ideas?

[Bryce]

Hi JonB,

The width isn't dependent on the speed of the scan, rather the peak voltage to the Horizontal coil. In the case of the GT65, the horizontal scan voltage is being determined by quite a lot of components and isn't just limited to a single component that you could adjust or swap out. You could try increasing the value of C712 which would narrow the width but would also pull the picture slightly towards the left. This would then have to be componsated by adjusting the H.Pos.

Bryce.

[TotO]

Is-it the same with an other CPC connected? 


The width should depend of the CRTC clock... So, what is the main clock on this computer?
Had been great to have a 320x240 screen with overscan! 

[JonB]

@TotO : It's OK with a 6128 connected, but that is in a 40 column mode with a generous border. The Superbrain is 80 columns and there is no border I can see. Indeed, it doesn't even show a back porch.

@Bryce : C712 is a bit unusual as it appears to be a non polarised electrolytic. This is the first time I've encountered such a thing. The PCW chassis has one, too, which I swapped thinking there might be a problem with the one in the GT65. Not so, unfortunately. I came to the conclusion that the Superbrian's video output is the root cause because I've tried it on a couple of other monitors and got the same problem.

The Superbrain is using an 8350 CRTC to generate the signal, but its clock input comes from the main system clock. It is 10.92 Mhz derived from a 16Mhz crystal.. There's a bunch of very complex logic that controls the attributes (underline, blinking, half intensity) which makes it trickier to see what is going on...

[Bryce]

Unfortunately I don't know the superbrain very well. If I get a chance I'll take a look at the schematics and see if anything could be changed on the computer side.How well do you know the system? Are there CRTC registers you could change to improve the situation?

Bryce.

Edit: Looking at the 8350 Datasheet. I looks like there are quite a few registers you could experiment with to narrow the screen / enlarge the border.

[JonB]

I did have a skim of the datasheet but I must have skipped that.

A monostable is used to tweak the horizontal sync. There is a small ceramaic capacitor and resistor that affects the timing. Altering this had no effect..

The Superbrain II schematics are not generally available (yet), but I have a printed schematic pack that includes the diagram. See attached...

[Bryce]

I meant firmware tweaks, not hardware. On the second last page of the Datasheet there's an "options Table" which seems to be register values. If you can find the setup routine in the ROM you may be able to tweak these.

Bryce.

[JonB]

Yeah, I had a look in the ROM.

The chip appears to be not set up as programmable as far as I can see. SB uses a PPIA to control some of its pins (such as 50/60Hz setting) but I can't see where it sets the internal registers. Yet.

Horizontal tweak at Z78 on the schematic by the way.

[LambdaMikel]

I remember I had the very same problem when I connected my Amiga 500 in the early 90s to a GT65.
The father of a friend of mine happened to be an electronics engineer and he actually looked at this problem.I remember he somehow managed to pull the CRT magnets / deflection coils somewhat into a different position and that gavea slight change, but the solution was not satisfactory. I must say I don't recall the details of what he did, butI remember he opened it up in my presence and wiggeled and changed something with the tube and that changed
the width of the image. I was impressed, because he did all this with the GT64 being on and showing the image ofthe Amiga. Apparently, he knew what he was doing - he happened to be a TV repair person at some point also.

DON'T TRY THIS AT HOME, RISK OF ELECTRIC SHOCK 

[JonB]

I remember I had the very same problem when I connected my Amiga 500 in the early 90s to a GT65.
The father of a friend of mine happened to be an electronics engineer and he actually looked at this problem.I remember he somehow managed to pull the CRT magnets / deflection coils somewhat into a different position and that gavea slight change, but the solution was not satisfactory. I must say I don't recall the details of what he did, butI remember he opened it up in my presence and wiggeled and changed something with the tube and that changed
the width of the image. I was impressed, because he did all this with the GT64 being on and showing the image ofthe Amiga. Apparently, he knew what he was doing - he happened to be a TV repair person at some point also.

DON'T TRY THIS AT HOME, RISK OF ELECTRIC SHOCK 

To repeat my original post: First off, there's no need to reply with the usual warnings about high voltages around the tube. I know about them already and am taking the proper precautions.

The deflection coils and magnets can be moved about and this moves the image slightly, but doesn't affect the width (much).

I connected the Superbrain to a PCW monitor (composite sync + video) and got the same image. Have also connected it to a Tatung monitor (colour, for an Einstein) via a composite adapter and got a rock steady image, again showing the same problem (very very straight and square though). Finally, the cute little 9" CCTV monitor that I recently acquired and repaired, also shows the problem but has a V-Width adjustment (a variable inductor) which when wound all the way out can display the full image, nearly.

As shown here: https://stardot.org.uk/forums/download/file.php?id=43363&mode=view

Hence, I really do think the Superbrain has an out-of-specification video signal, rather than any of the monitors I tried it with. I have posted various 'scope traces on StarDot; take a look at this one:

https://stardot.org.uk/forums/download/file.php?id=43361&mode=view

..you can see it is generating no back porch, for example (Yellow=Composite Video, Cyan=HSYNC, Blue=VSYNC ).

[JonB]

I've swapped out the CRTC for a known good one and that is not the problem..

@Bryce : Maybe I could try adding the other non polarised capacitor (the one fitted to the PCW chassis) in parallel to the one in the GT65. It would double the capacitance (I think) and I could see if this has any effect on the width?

[LambdaMikel]

To repeat my original post: First off, there's no need to reply with the usual warnings about high voltages around the tube. I know about them already and am taking the proper precautions.

The deflection coils and magnets can be moved about and this moves the image slightly, but doesn't affect the width (much).

Exactly, that was the problem.


I also remember the guy tried to find a pot or similar to change the width, but to no avail.
So you might have to change the circuitry along the lines Bryce suggested or modify the
deflection voltages.

[Bryce]

I've swapped out the CRTC for a known good one and that is not the problem..

@Bryce : Maybe I could try adding the other non polarised capacitor (the one fitted to the PCW chassis) in parallel to the one in the GT65. It would double the capacitance (I think) and I could see if this has any effect on the width?

Just make sure the PCW capacitor is rated for the correct voltage. As for doubling the capacitance, that will depend on the values. Remember, we are talking about capacitive reactance here, so Xc= 1/2pi.f.C so parallel will be Xt=Xc1 + Xc2 but series would be Xt = Xc1 x Xc2 / Xc1 + Xc2

Bryce.

[JonB]

Thanks Bryce - the PCW cap is exactly the same (capacitance, type, voltage - 10uF 25v non polarised) as the one in the GT65. I'm not hoping that connecting them together will solve the problem; I am testing to see what effect the capacitance has on the problem.

So, parallel gives 20uF and series gives 5uf? That's good, it will give me a range on either side of standard.

[Bryce]

I hope it's nF and not µF !

Bryce.

[JonB]

Nope, they are both 25v 10uF non polarised electrolytics. Attached are the results of connecting two of them in series / parallel as well as "standard". As you can see, the variation in capacitance has little effect, if any.

[JonB]

Schematic of C712 : "25V10 (NP)" and a picture of the actual components (the one on the right is the GT65 one, the left hand one is from the PCW).

[Bryce]

Ok. I would have expected a much lower value. As for further suggestions, there's not much else you could try on the monitor side of things.


Bryce.

[Fessor]

Some guy over here https://www.edaboard.com/showthread.php?374112-GT65-AMSTRAD-to-18-432KHz-upscale&p=1602703 reported about changes on C729

[Bryce]

Ooo, I'm not sure how safe that would be. He's messing with the LOPT outputs which is a tuned circuit. That could have other unintentional effects (especially on the life of the LOPT).

Further suggestion: Have you tried other monitors with a real composite input or feeding the signal through a modulator?

Bryce.

[JonB]

I can experiment. The PCW chassis has a similar circuit but the equivalent capacitor is 33nf 400V. In other words, I have an appropriate part to hand!

@Bryce yeah, I tried a small 9" CCTV monitor with the H-width coil wound all the way out: https://stardot.org.uk/forums/download/file.php?id=43363&mode=view

It sort of works, but as it's only 9" I find the characters a bit small.

[Audronic]

@JonB


I had a weird thought about the WIDTH
Put a 500 Ohm to 1K 5 Watt resistor in series with the Non polarised capacitor.
And see if that helps.
Maybe


Ray

[JonB]

Well, replacing that high voltage polyester cap with a lower value seems to have helped. That was a good find, Fessor!

The test pattern (grid) is a small program written in MS BASIC-80 that uses the SB graphics characters. You can see there's some pin cushioning but it's otherwise bearable. The screen is on a sort of soak test for the time being (as I am mindful of Bryce's warning!).

This is great, as it means I can now build a case for it (the SB II is a board only, I don't have the cabinet), safe in the knowledge that the GT65 bits can be used. Did I say that this monitor cost a pittance, and was only bought to scavenge the tube for a PET I was repairing? The guy who sold it to me said it was actually broken, but it looks like that's not the case. I shall be replacing some of the electrolytic capacitors, though, because it doesn't sync on startup - you have to twiddle the B+ voltage pot to get it to lock onto the video signal (not practical once it is enclosed).

[JonB]

@JonB


I had a weird thought about the WIDTH
Put a 500 Ohm to 1K 5 Watt resistor in series with the Non polarised capacitor.
And see if that helps.
Maybe


Ray

Hello Ray. Care to explain the reasoning behind this? I'm interested...

[Bryce]

Using the resistor just adds resistance instead of reactance (AC resistance), but if the capacitor didn't help, I doubt a resistor would give much different results.

If the monitor has problems syncing on startup, try cleaning the V-Hold pot.

Bryce.