Composite video for ZX81

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Composite video for ZX81

Postby tezza on Sun Jan 03, 2016 1:47 pm

What to do on a rainy day. Why modify your collection pieces of course...
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Re: Composite video for ZX81

Postby LilJoe on Sun Jan 03, 2016 4:34 pm

I always have trouble getting a readable picture on mine, something like this could work well...
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Re: Composite video for ZX81

Postby kevman3d on Sun Jan 03, 2016 6:56 pm

That is pretty cool! I'd tossed about the option to purchase an AV mod unit from RWAP - but I figure that this may be the cheaper option given how expensive those kits are... Thanks for posting.

I will need to get a new membrane though - thinking it was well gone, I decided to lift it out. Its pretty well glued (taped) down, and it becomes a 'once you take it off, you will likely need to replace it anyway' type of affair. However it still works, just looks kinda messy now. Doh! :roll:

Tuning my ZX81 into the RF worked, but the picture was almost black on black. Some of the kits claim to resolve this, but then I also saw someone mention the emitter-follower approach may also help. Any ideas on this, or perhaps I just try it and find out. :)

As for transistors, what would you recommend. I have a handful of modern transistors laying about that I used to replace the power transistors in the ZX spectrum, but would they create a problem (ie. overheat, or damage the machine) ot should I go strip some old-school ones from a circuit board?
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Re: Composite video for ZX81

Postby tezza on Sun Jan 03, 2016 9:57 pm

Well the info I used for the mod just said "a simple NPN transistor". I used a 2n3904 as I had some lying around here.
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Re: Composite video for ZX81

Postby kevman3d on Mon Jan 04, 2016 7:12 pm

Ah, OK. I've got a few new TIP31C transistor. The 31C seems to handle much higher voltages and temperatures, but whether that makes the ZX81 work harder I have no idea.

I'm just not that au fait when it comes to this area (ie. electronics in general :roll: ) apart from being able to follow and understand things with instructions. :lol:
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Re: Composite video for ZX81

Postby arjoll on Thu Jan 07, 2016 10:48 pm

kevman3d wrote:Tuning my ZX81 into the RF worked, but the picture was almost black on black.

Most ZX81s don't output the back porch - a reference level after the sync at the start of each line which sets black level. Older TVs don't mind, but anything new-ish (probably within the last 20 years!) will have problems. It's the output of the ULA, so a simple composite mod probably won't help, although it's such as well known problem there's a ton of info online.

My ZX81 is still sitting in the massive (and very old) to-do pile (sorry Tezza), but the plan is to leave it as-is and use a 35 year old NEC 13" TV I've hung on to for home computer use, which should cope just fine - and if not, dad still has the little 12" Thorn black and white I used with my original ZX81 in the 80s.
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Re: Composite video for ZX81

Postby kevman3d on Fri Jan 08, 2016 1:06 pm

arjoll wrote:Most ZX81s don't output the back porch - a reference level after the sync at the start of each line which sets black level. Older TVs don't mind, but anything new-ish (probably within the last 20 years!) will have problems. It's the output of the ULA, so a simple composite mod probably won't help, although it's such as well known problem there's a ton of info online.


Just bought a new ZX81 membrane from RWAP, and obviously he suggested his ZXVid (also suggested the much cheaper ZX8CCB (not his product) though its been out of stock for a while) as they both correct the signal issues caused by the missing back porch. That said, A friend at work has an old 20" CRT that he says he'll (eventually) drop off so I'll test that when he gets around to it. :)

But thanks for that clarification - I was 'umming' over whether that mod would have helped, but obviously not. In a way I'm glad at this stage I haven't ripped the poor machine apart to try it just yet...
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Re: Composite video for ZX81

Postby tezza on Fri Jan 08, 2016 1:19 pm

kevman3d wrote:
arjoll wrote:Most ZX81s don't output the back porch - a reference level after the sync at the start of each line which sets black level. Older TVs don't mind, but anything new-ish (probably within the last 20 years!) will have problems. It's the output of the ULA, so a simple composite mod probably won't help, although it's such as well known problem there's a ton of info online.


Just bought a new ZX81 membrane from RWAP, and obviously he suggested his ZXVid (also suggested the much cheaper ZX8CCB (not his product) though its been out of stock for a while) as they both correct the signal issues caused by the missing back porch. That said, A friend at work has an old 20" CRT that he says he'll (eventually) drop off so I'll test that when he gets around to it. :)

But thanks for that clarification - I was 'umming' over whether that mod would have helped, but obviously not. In a way I'm glad at this stage I haven't ripped the poor machine apart to try it just yet...

Results must vary. With the mod I did, there was no problem with lack of contrast on the composite signal. It was perfect. Perhaps it depends on the monitor?
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Re: Composite video for ZX81

Postby kevman3d on Fri Jan 08, 2016 1:54 pm

tezza wrote:Results must vary. With the mod I did, there was no problem with lack of contrast on the composite signal. It was perfect. Perhaps it depends on the monitor?


Its also depends on what ULA is in the ZX81. Updated ULA's later on fixed the back porch issue - specifically the one I see from your photo's being a 210E, so that's why yours looks great. 8)

Here's an interesting explanation (translated by google).
Last edited by kevman3d on Fri Jan 08, 2016 2:08 pm, edited 1 time in total.
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Re: Composite video for ZX81

Postby acsi on Fri Jan 08, 2016 1:56 pm

The ZX-Vid is a great little unit. Costs a bit but is well worth it.
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Re: Composite video for ZX81

Postby tezza on Fri Jan 08, 2016 2:44 pm

kevman3d wrote:
tezza wrote:Results must vary. With the mod I did, there was no problem with lack of contrast on the composite signal. It was perfect. Perhaps it depends on the monitor?


Its also depends on what ULA is in the ZX81. Updated ULA's later on fixed the back porch issue - specifically the one I see from your photo's being a 210E, so that's why yours looks great. 8)

Here's an interesting explanation (translated by google).

Ah right. That explains it. Thanks.
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Re: Composite video for ZX81

Postby kevman3d on Sun Jan 10, 2016 9:03 pm

acsi wrote:The ZX-Vid is a great little unit. Costs a bit but is well worth it.


Yup, I've heard nothing but positive things about that unit.

I just got a message from Karl (Pokemon) today that his ZX8-CCB units are back in stock, so I've ordered one of those (it only cost NZ$18 inc. postage). It was more in my budget range (ZXVid would cost an additional $60) so which one was based completely on a monetary decision.

I'll let you know how it works out.
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Re: Composite video for ZX81

Postby acsi on Mon Jan 11, 2016 10:13 am

I have also used the ZX8-CCB's in both ZX81's and a ZX80 and they give a real nice picture too.
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Re: Composite video for ZX81

Postby kevman3d on Tue Feb 09, 2016 12:52 pm

acsi wrote:I have also used the ZX8-CCB's in both ZX81's and a ZX80 and they give a real nice picture too.


I finally got around to fitting the ZX8-CCB on the long weekend. What a great image too. And the machine works perfectly - which has made my inner 11yr old extremely happy. :lol:

I wrote it all up in this blog post if anybodies curious. It wasn't as quick or easy as I'd imagined - not the ZX8 per'se, but the process of removing the damn RF modulator components. :roll:
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Re: Composite video for ZX81

Postby tezza on Sat Feb 27, 2016 11:19 am

That's a great blog post. It really captured the adventure. Congratulations.

Yes, 1k. It's amazing what some programmers fitted into that. A toy really, but it was affordable and got people into computing. Many went on to work in IT.
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