Composite mod done Zx81 great picture but somethkng is wrong

Discussions about Sinclair ZX80 and ZX81 Hardware
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RS232
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Re: Composite mod done Zx81 great picture but somethkng is wrong

Post by RS232 » Mon Apr 22, 2019 11:58 am

Replaced cap, soldered negative side to video out.
I used a 10v 100uf for this.
Got rid of the extra wire
Insulated both cap and resister together soldered to transistor.

I have noticed the screen still does the screen issue not as often. Now every 5 to 15 minutes. So problem has still not completly gone. This is with the case off. Interesting the longer it is on the less frequent it becomes now!

I notice it's true the ULA does still gets a bit hot, but only in the center of the chip which surprised me given i have replaced the regulator.

Pictures attached.. what are your thoughts?
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20190422_103718.jpg
20190422_103631.jpg
20190422_105305.jpg

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RS232
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Re: Composite mod done Zx81 great picture but somethkng is wrong

Post by RS232 » Mon Apr 22, 2019 2:53 pm

Attached is the mod applied.. i'm dipping my toe in the water of doing electronics didn't realize it could get so troublesome.. i wonder if it is the BC108 i am using and not the BC108B i should have in place.. sigh
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ZX81-MOD.jpg
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RetroTechie
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Re: Composite mod done Zx81 great picture but somethkng is wrong

Post by RetroTechie » Mon Apr 22, 2019 3:05 pm

RS232 wrote:
Mon Apr 22, 2019 11:58 am
I notice it's true the ULA does still gets a bit hot, but only in the center of the chip which surprised me given i have replaced the regulator.
Hmm yeah, center of the chip is where the heat gets released... ;) The ULA's dissipation is not changed by a regulator replacement, only the temperature of its surroundings.

Maybe capacitor was damaged indeed. But
1024MAK wrote:
Mon Apr 22, 2019 1:10 am
The transistor you used. In one of your other threads, I can see that it is marked BC108B.
See no -B in photo just before this post, but yes I noticed that too. Chinese ripoff bought from eBay, AliExpress or so? I really don't understand why people save pennies on parts like this, as opposed to buying from trustworthy distributors like Mouser, Farnell etc (or brick & mortar electronics store, if there still is one in your area). Those pennies saved just aren't worth the headaches of fakes / out-of-spec issues.

More to the point: RS232, how did you determine pinout for this transistor? :?:
I still strongly recommend trying a different TV / monitor/ display. This is because the video output from the ZX81 is not fully compliant to broadcast TV standards and it is possible that the TV / monitor/ display you are using just does not like the video signal from your ZX81.
Good tip! I have a small 'camping' TV myself (flatscreen) that has great trouble to produce an image from my ZX81's. The one with CPLD-based ULA works slightly better than original Sinclair ULA's (read: still unstable), when set to white text on black background. With the normal black-on-white text, TV tends to loose sync or otherwise f** up the image. I suspect it doesn't like a composite video signal without the color burst, sync signal timing or whatever. With other LCD screens or CRT's, no problems.

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1024MAK
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Re: Composite mod done Zx81 great picture but somethkng is wrong

Post by 1024MAK » Mon Apr 22, 2019 7:43 pm

RetroTechie wrote:
Mon Apr 22, 2019 3:05 pm
1024MAK wrote:
Mon Apr 22, 2019 1:10 am
The transistor you used. In one of your other threads, I can see that it is marked BC108B.
See no -B in photo just before this post, but yes I noticed that too. Chinese ripoff bought from eBay, AliExpress or so? I really don't understand why people save pennies on parts like this, as opposed to buying from trustworthy distributors like Mouser, Farnell etc (or brick & mortar electronics store, if there still is one in your area). Those pennies saved just aren't worth the headaches of fakes / out-of-spec issues.
Sorry, the photos I was looking at are those in one of RS232’s other threads, but they were a bit blurry. Hence when I trawled through the results of a search engine, the closest match I could find with the sideways printing was a so called ‘BC108B’. Hence my comments... So sorry if this caused any confusion.

If it is a ‘BC108’ then that part number has a wide range for the actual gain (hFE) figure. A BC108B has a more defined range for the gain figure. But in this circuit, as long as the gain is 60 or greater, it should work (although we prefer a higher gain figure).

A datasheet for a real BC108 is here
The minimum gain (hFE) for a BC108 is given as 110 when Ic=2mA.

Mark

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RS232
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Re: Composite mod done Zx81 great picture but somethkng is wrong

Post by RS232 » Mon Apr 22, 2019 10:14 pm

[Re your message thanks all working, it looks like a minor fault with my tv

Many thanks all


quote=1024MAK post_id=33905 time=1555954985 user_id=1212]
RetroTechie wrote:
Mon Apr 22, 2019 3:05 pm
1024MAK wrote:
Mon Apr 22, 2019 1:10 am
The transistor you used. In one of your other threads, I can see that it is marked BC108B.
See no -B in photo just before this post, but yes I noticed that too. Chinese ripoff bought from eBay, AliExpress or so? I really don't understand why people save pennies on parts like this, as opposed to buying from trustworthy distributors like Mouser, Farnell etc (or brick & mortar electronics store, if there still is one in your area). Those pennies saved just aren't worth the headaches of fakes / out-of-spec issues.
Sorry, the photos I was looking at are those in one of RS232’s other threads, but they were a bit blurry. Hence when I trawled through the results of a search engine, the closest match I could find with the sideways printing was a so called ‘BC108B’. Hence my comments... So sorry if this caused any confusion.

If it is a ‘BC108’ then that part number has a wide range for the actual gain (hFE) figure. A BC108B has a more defined range for the gain figure. But in this circuit, as long as the gain is 60 or greater, it should work (although we prefer a higher gain figure).

A datasheet for a real BC108 is here
The minimum gain (hFE) for a BC108 is given as 110 when Ic=2mA.

Mark
[/quote]

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