Display bars

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roganjosh
Posts: 10
Joined: Fri Oct 01, 2021 3:40 pm
Location: Leeds

Display bars

Post by roganjosh »

All my Dragons, before a couple were modified to accept an external RGBTOHDMI, suffered from vertical jail bars and something resembling horizontal hum bars on the display.

My original display setup was:
Original Dragon PSU->Dragon->SCART to HDMI converter->LCD display.

The vertical jail bars were no worse than those seen on other retro computers of the era (Spectrum, C64 etc). When upgrading the RAM to 64K I took the opportunity to replace the 100nF RAM decoupling with a full complement of 1uF MLC decouplers. I can just about convince myself that they made a slight improvement. The jail bars were by no means off-putting though.

More of a distraction were the horizontal bars. On the default green display they occupied approximately one third of the screen height, were a slightly lighter shade of green and moved slowly down the screen. The bar moved over both the main screen and border. As the bar moved over screen text it distorted the text such that it moved a few pixels to the right. The display setup had no earth connections so a ground loop wasn't possible. I did, however, notice that disconnecting the colour burst from the video output stage (in preparation for RGBTOHDMI which does not, of course, produce bars) gave no horizontal bars on the resulting monochrome display. So the bars appear to be associated with the stock chroma circuit.

All the above is a long prelude to asking whether there are any existing recommended mods to help reduce or banish the above artefacts?

Alan
Last edited by roganjosh on Tue Jan 04, 2022 10:22 am, edited 1 time in total.
sorchard
Posts: 530
Joined: Sat Jun 07, 2014 9:43 pm
Location: Norwich UK

Re: Display bars

Post by sorchard »

Hmm, strange one.

In my experience, jail bars on Dragons are usually caused by missing bypass caps on the DRAM array, and fitting all eight 100n caps virtually eliminates them.

I don't think I've ever seen hum bars on a Dragon. That tends to indicate a fault, particularly if you say 1u caps didn't make a lot of difference to the jail bars. It's unlikely that multiple Dragons have the same problem so I'm tempted to suggest there is an issue with the converter or wiring. One thing worth bearing in mind is the Dragon doesn't produce proper interlaced PAL which confuses the hell out of some converters (no difference between odd and even frames). I have one which randomly decides the signal is SECAM and refuses to show colour.

How is the converter powered? That could be a source of noise.

Have you tried going direct to the composite input of a TV?

Do you have more than one Dragon transformer to try?
Stew
roganjosh
Posts: 10
Joined: Fri Oct 01, 2021 3:40 pm
Location: Leeds

Re: Display bars

Post by roganjosh »

sorchard wrote: Mon Jan 03, 2022 9:45 pm
How is the converter powered? That could be a source of noise.

Have you tried going direct to the composite input of a TV?

Do you have more than one Dragon transformer to try?
The converter is powered by a 5V DC wall wart. The original wart failed mechanically rather than electronically but its replacement didn't improve matters. The converter behaves flawlessly with other retro computers. I've tried the set up with two original transformers and also a Ramoth PSU board. No difference there.

Stupidly on my part the direct-to-TV test slipped my mind, probably as it's a faff to connect. No horizontal bar was displayed, though the jail bars seemed more pronounced and the overall picture quality was, of course, vastly inferior to RGB.

So it looks like a good argument can be made for the converter getting a little confused by the odd PAL output. Regarding the jail bars, I tried adding more decoupling at various points in addition to the DRAM but nothing much seemed to improve matters. Part of the reason for my post was to see whether others had found any good strategic point(s) to add extra decoupling. I also take it from your reply that there are no recommendations internal mods to improve the standard display circuit.

Anyway, many thanks for helping to sort that out. I will, of course, be using the problem-free RGBTOHDMI as a matter of course on two Dragons but I like to keep one Dragon pretty bog standard, as a reference, and my post exclusively referred to that.

Alan
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