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?
Composite mod done Zx81 great picture but somethkng is wrong
Re: Composite mod done Zx81 great picture but somethkng is wrong
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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- RetroTechie
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Re: Composite mod done Zx81 great picture but somethkng is wrong
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
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?
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.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.
- 1024MAK
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Re: Composite mod done Zx81 great picture but somethkng is wrong
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.RetroTechie wrote: ↑Mon Apr 22, 2019 3:05 pmSee 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.
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
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Re: Composite mod done Zx81 great picture but somethkng is wrong
[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]
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]
Many thanks all
quote=1024MAK post_id=33905 time=1555954985 user_id=1212]
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.RetroTechie wrote: ↑Mon Apr 22, 2019 3:05 pmSee 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.
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]