Amstrad GT65 monitor video width adjustment

[JonB]

Thanks Bryce, I'll do that - although twiddling the V-Hold doesn't restore sync (it looks like horizontal sync is out, as it is tearing). Apropos your concerns about altering that 400V capacitor.. this is highly unscientific, but the monitor has been on all afternoon and it hasn't popped yet...

Better not speak too soon 

[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.MaybeRay

.
Hi JonB
The weird thought was that as the scan coil is is Current driven the if you reduce the current ? Perhaps that may change the width ?
I had a look at the rest of the circuit and if you change anything else prior to that line then other parts will be affected.
PS:- Maybe NOT 500 Ohms but some thing less. Perhaps try 100 ohms first.
It would take about 2 Minutes to try my weird thoughts. ( Who knows maybe it would help).
These were only weird thoughts.
Thanks Ray

[Bryce]

If it's tearing then check that the contacts are clean on the Sync signal connectors.

@Audronic: A capacitor regulates the current on an AC signal just like a resistor. The only difference being that the current flowing is a function of the signals frequency.

Bryce.

[Audronic]

[quote
@Audronic: A capacitor regulates the current on an AC signal just like a resistor. The only difference being that the current flowing is a function of the signals frequency.Bryce.
Hi Bryce
Yes I am aware of that .
The 10uf cap introduces 1.009 ohms Reactance in the circuit.
So Maybe a 10 Ohm resistor would be a good starting point.
Just a weird thought.
Ray

[Bryce]

[quote
@Audronic: A capacitor regulates the current on an AC signal just like a resistor. The only difference being that the current flowing is a function of the signals frequency.Bryce.
Hi Bryce
Yes I am aware of that .
The 10uf cap introduces 1.009 ohms Reactance in the circuit.
So Maybe a 10 Ohm resistor would be a good starting point.
Just a weird thought.
Ray

And my statement is incorrect anyway . It's not the only difference. A capacitor shifts the phase of the current too, which a resistor doesn't.

Bryce.

[JonB]

Meanwhile, the GT65 is still not getting horizontal sync on power up without fiddling with the H-Pos (Hold?) inductor L703. Most annoying, and also the ferrite core is disintegrating. I now have a screw poked in the top of it to try and maintain sync, and this does work but not at power on. Now, I am replacing capacitors on the board, but to little effect.

It's frustrating that it will maintain a horizontal lock once tweaked, but not when power cycling the thing! So, I have set the H-POS to move the image more to the left (with experimentation, reaching a setting that will enable it to lock on with a power cycle, then used the toke magnets to shift it to the right.

Tentatively... voila!


(A picture of the "screw and blutac hack" follows... yes, believe it or not, this is part of the solution.)

[Bryce]

So you've been adjusting it with a metal screwdriver?? Tsk, tsk.

Bryce.

[JonB]

Alas, I fear "adjusting" is not the best word to describe what I did, Bryce. I tried to make a swizzle stick (as I think they are called) out of a plastic knitting needle, filing a hex shape on the end, but it just slipped (despite it being a reasonable fit). The core is stuck solid, so I had to try something else. In the end it did turn a tiny amount (or I lost some of the ferrite) when I used a hex screwdriver.

Bad, I know, and I rather wish I'd known about the high voltage capacitor trick sooner; but it is done now.

[LambdaMikel]

Tentatively... voila!


(A picture of the "screw and blutac hack" follows... yes, believe it or not, this is part of the solution.)

Hope it doesn't set your place on fire... MacGyver would have been proud of you!

Is this a permanent "solution" ? Reminds me a bit of that other post a while ago of Bryce where people were using plastic putty to fix a ribbon cable connector...but hey, if it works, why not! 

[JonB]

No, it cannot be permanent. The coil gets warm and this softens the Blu-tack, leading to movement of the screw. So I am considering gluing a plastic threaded PCB stand off to the top, then running a screw down it. Works, sort of, and gives more accurate adjustability.

[Bryce]

How about just replacing the entire component?

Bryce.

[LambdaMikel]

+1 for MacBryce
-1 for MacGyver

[Bryce]

+1 for MacBryce
-1 for MacGyver

I'm not suggesting he should swap the part with just a swiss army knife though!!

Bryce.

[JonB]

Hi Bryce

Yes, would be good to replace it, but what with? It doesn't look like a simple inductor - there are 8 pins for a start (although only three active connections) - and I've already searched for adjustable inductors and been disappointed. If you can suggest a suitable replacement I'd be very happy.. but the service manual doesn't even say what inductance value / range it is. L703 has a part number 305Y001 and searching for this returns some service manuals for other monitors / TVs, but no kinks to a discrete part that I could buy.

It looks like a variable inductor with a centre tap which makes finding a replacement even harder.

So, it'll be a "MacGyver special" or no picture!

Cheers
JonB

[Bryce]

The only place to get a replacement would be from a non-working GT65. I'm sure many here have one of those.

Bryce.

[JonB]

Might as well... http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/wanted-broken-gt65-monitor/

[LambdaMikel]

Would help if I could, but I only have one GT65... shipped from Germany to the US