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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 10th Sep 2020, 6:02 pm   #1
coopzone
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Default Amstrad PCW 8256 Screen Brightness

Does anyone know if it's possible to increase the brightness (or what would be faulty more like!) As the brightness on the front still leaves the screen looking dim.

Derek
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Old 10th Sep 2020, 6:52 pm   #2
AC/HL
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Default Re: Amstrad PCW 8256 Screen Brightness

Do you have the user manual: https://www.manualslib.com/manual/10...d-Pcw8256.html
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Old 10th Sep 2020, 6:59 pm   #3
GeoffB17
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Default Re: Amstrad PCW 8256 Screen Brightness

Hello,

I bought my PCW when it first came out in 1986, I still have it, I still use it. I don't think I've ever had the brightness setting at max, it's always been bright enough, although if there's a lot of sunlight on the screen it isn't ideal but that's a problem for most monitors, and it's better to solve the sunlight (curtain etc).

Was your brightness OK, and now isn't, and getting worse. Maybe something wrong with the power supply? I've got the service manual and tried to find where the control is circuit-wise, but not found it yet. I'll keep looking, wonder if it's on the disk circuit board?

What range of brightness do you get between max and min. On mine max is slightly too bright (in normal light) and min is almost nothing visible.

Geoff
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Old 10th Sep 2020, 7:06 pm   #4
KeithsTV
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Default Re: Amstrad PCW 8256 Screen Brightness

In case you need it I have copy of the PCW service manual but it's just too big for the forum. PM me with your email address if you want a copy.

Keith
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Old 10th Sep 2020, 7:42 pm   #5
GeoffB17
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Default Re: Amstrad PCW 8256 Screen Brightness

You can view the manual online at:

http://www.progettosnaps.net/manuals/pdf/pcw8256.pdf

The brightness control and associated components are upper left of the monitor pcb. Not much to go wrong there. There's a VR, 2 R and a C. Capacitors can go wrong, but I don't think this is the type that fail?

Geoff
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Old 10th Sep 2020, 7:49 pm   #6
coopzone
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Default Re: Amstrad PCW 8256 Screen Brightness

Thanks for the comments.

The machine. is positioned well away from windows and in a relatively dim area. The brightness goes from black to just about usable / ok. But not as bright as another PCW i have.

I do have the service book for it.

I'm thinking along the lines of it will be a cap gone leaky some place. I have not yet checked voltages. I was hoping someone would say - yes that will be ...... to give me a starting point.

As far as i can tell the PSU is ok, after fixing the disk drive, it boots fine and even the printer works (needs re-inking, but works)

Derek
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Old 10th Sep 2020, 8:16 pm   #7
GeoffB17
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Default Re: Amstrad PCW 8256 Screen Brightness

Page 17 of the PDF I linked to refers to 'Cut Off Adjustment' - this shows an adjustment to VR4004 so that VR2001 (the Brightness Control) has the correct range.

I wonder if that would help?

But then, if it WAS OK once, why has it gone wrong? You said it's less bright than another PCW you have, but didn't say if it was better, but has got worse?

Geoff
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Old 10th Sep 2020, 8:49 pm   #8
coopzone
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Default Re: Amstrad PCW 8256 Screen Brightness

Hi Geoff, i saw that to - but had not had time to follow it though yet, i'll have a tweak and update the thread.

I don't know if it got worse, i only got the machine today - and spent the first 2 hour completely stripping the disk drive and rebuilding it after replacing the belt and a faulty transistor.

Thanks i'll update this thread tomorrow.

Derek
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Old 11th Sep 2020, 7:24 am   #9
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Default Re: Amstrad PCW 8256 Screen Brightness

Morning,

I've just had a closer look at VR4004, it looks like it controls the main grid on the CRT. and adjusts it's voltage from around -50v to 0v. It's primary use is to set the blackness level (greeness in this case) for the CRT. But as Geoff said this also effects the range of brightness available.

So by adjusting it a little i'm able to give the "high" end of the brightness a bit more range, and am pleased to report it's now perfectly acceptable. However as a consequence you do loose the ability to dim the screen to no-output. But this is not really an issue - as when do you not want to see the screen anyway? And it still goes low enough to be ok.

I can only assume that the CRT has lost some of it's emission over the years and that was the primary cause of my issue.

I'm not big on TV tech. But it was curious to see that the video drive is to the cathode, a sort of common control grid setup. I always assumed the video would drive the control grid not the cathode. Interesting - i must try to remember to take a look at a few TV driver stages and see if this was "the way it was done" in the day.

Derek
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