My Issue 2 continuing problems and Issue 6s fixed

redgatemodels
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Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

I've also finally found the cause of the keyboard issues, it's the 5 way ribbon socket, not sure if it is just not soldered in right, dirty, or I damaged it as a kid when I added the hardwired joystick socket but it's wobbly and prone to letting go of the ribbon periodically.

I've ordered a pair of new ones and will solder them in soon.
drdivago
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Re: My Series 2 Died (but now resurrected)

Post by drdivago »

Looking at the picture of the modulator it looks like you didn't disconnect the resistor from the RF connector before soldering the composite mod? :?:
redgatemodels
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Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

Yes it is disconnected :)

I've left the 5v and video feed connected though
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

redgatemodels wrote: Thu Sep 14, 2023 2:47 pm ULA pins 15 and 16: 1.8v, 1.68v

IC14 (LM1889)
Pins 2 and 4 of? 5v, 4v
DC voltages on pins 14, 15, 16, 12 and 3. 11v, 11v, 11v, 11v, 4.4v
On an issue one board that I have here (which has very similar analogue video circuitry to an issue two board), I get:

Code: Select all

Issue one board,  Voltages when on the copyright screen:

IC14 (LM1889) (use 20V range)
Pin Function                Fluke  DT-4000 XL830L
1   Chroma Lead              9.13V  9.19V   9.17V
2   R-Y (V) input            4.65V  4.67V   4.67V
3   Chroma Bias              4.64V  4.64V   4.66V
4   B-Y (U) input            4.56V  4.56V   4.58V
5   0V/GND
12  “Video input”           11.51V 11.49V  11.55V
13  Chroma Sub-carrier out  3.725V 3.719V   3.74V
14  RF Supply (+12V)        11.55V 11.53V  11.59V
15  Sound Tank (+12V)       11.55V 11.53V  11.59V
16  Chroma Supply (+12V)    11.55V 11.53V  11.59V
17  Chroma Osc output        8.63V  8.62V   8.36V
18  Chroma Lag               9.12V  9.13V   9.16V

ULA (use 2V range)
Pin Function                Fluke  DT-4000 XL830L
15  U                       1.913V 1.910V  1.920V
16  V                       1.675V 1.672V  1.682V
17  Y                       1.590V 1.589V  1.598V
I used three different multimeters to show that that there will be variations due to each meter responding slightly differently.

For a white screen (or copyright screen), the voltages on IC14 pins 2, 3 and 4 should all be the same or almost the same.
IC14 uses the +12VA supply, hence everything is referenced from this, so if this supply is a slightly different voltage, most of the other voltages on IC14 will also be slightly different.

The Service Manual is here. The set-up details are in section 3.1.3 (page 18). But you may find more detailed information elsewhere.

Mark
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redgatemodels
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Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

So pins 2 and 4 less than ideal?
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

Yes.

Mark
ZX81 Variations
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:!: 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 being good this year.
redgatemodels
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Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

If I can't balance them by adjusting vr1/2 then look at replacing ic14?
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

redgatemodels wrote: Sun Sep 17, 2023 4:44 pm If I can't balance them by adjusting vr1/2 then look at replacing ic14?
I’m not convinced that IC14 is faulty.

First, try VERY carefully, and gently adjusting VR1 and VR2. Adjust them slowly. Do them alternately.
If when adjusting one or both, the voltage jumps, or the colours jump rapidly from one hue to another, there may be dirt and dust on the carbon track. I normally spray some IPA (Isopropyl alcohol) on from an aerosol can to blast away any dirt and dust, then apply a very small amount of WD40 on to the track. Just a drop, enough to “wet” the wiper and to form a thin film on the track. This helps to reduce electrical noise caused by old carbon tracks.

If the above does not work, the next step would be to replace VR1 and VR2 with new preset resistors.

Obviously after each stage, you need to try setting up the colour again.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
redgatemodels
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Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

Do you mean real wd40 or their switch cleaner?
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Paul
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Re: My Series 2 Died (but now resurrected)

Post by Paul »

Better use special potentiometer cleaner like "Kontakt Chemiereiniger Spray 601" or their "tuner spray 600"
In theory, there is no difference between theory and practice. But, in practice, there is.
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