spectrum 128 with only 12 volt

jojo
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Joined: Mon Mar 16, 2020 5:25 pm

spectrum 128 with only 12 volt

Post by jojo »

This 128 is also sitting on my workbench for about half a year now .......

came a long way , but right now things are getting more clear .
going by the service manual voltage check ;

i have 12 volt on ic33 pin 14
no voltage on ic34 pin 1
and no voltage on d19 anode

so , the 7805 is working , also the dc/dc is working .

but why cant i find the other voltages on the ic's ??

i have replaced about anything in the dc/dc section .

i can keep the unit on for hours , nothing overheats and nothing changes .

anyone with a cool idea to go futher


jojo
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: spectrum 128 with only 12 volt

Post by 1024MAK »

jojo wrote: Fri Jun 10, 2022 5:49 pm i have 12 volt on ic33 pin 14
Good :D
jojo wrote: Fri Jun 10, 2022 5:49 pm no voltage on ic34 pin 1
That’s normal, I think this is an error in the service manual. Pin 1 goes to the keypad socket…
jojo wrote: Fri Jun 10, 2022 5:49 pm and no voltage on d19 anode
Not good :-(

I presume you have a healthy +5V on the 7805 voltage regulator output?

What do you get on these points:
  1. Capacitor C111 negative lead or diode D28 anode?
  2. IC33 pin 1?
Mark
jojo
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Re: spectrum 128 with only 12 volt

Post by jojo »

Hi Mark ,

my workbench is too small for all projects , so have to finish one to get room for the 128 again .


Indeed 7805 is fully happy at 5 volt .

so i would expect -12v present ( as + 12 volt is there ) .
strange there is no 5 volt on the ic's

anode d28 is -10,62
ic33 pin 1 - 10.65
i would expect -12v at c80 + , but it is + 6,93 - side is + 8,75

jojo
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: spectrum 128 with only 12 volt

Post by 1024MAK »

jojo wrote: Sat Jun 11, 2022 11:31 am anode d28 is -10,62
ic33 pin 1 - 10.65
So the negative -12V is being generated (it will always be a bit low).
jojo wrote: Sat Jun 11, 2022 11:31 am i would expect -12v at c80 + , but it is + 6,93 - side is + 8,75
Keep in mind that in a lot of places, including “official” documentation, the use of the “~” (tilde) symbol gets mixed up, misprinted or replaced with a (-) dash. Hence the edge-connector contact (23A or 23B again depending on what documentation you look at) shown as -12V is supposed to be ~12V, meaning 12V AC. Either side of capacitor C80 is a pulsed AC signal, not a simple DC level.

As you have -10.62V at the anode of D28, then the fault looks like it’s either a broken track between D28/C111/IC33 pin 1 and resistor R79. R79 open circuit or gone high in value (should be 2.2k). An open circuit fault between R79 and zener diode D19. D19 short circuit, or fitted with the incorrect polarity or a short circuit fault on the -5V rail.

Mark
jojo
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Re: spectrum 128 with only 12 volt

Post by jojo »

Thanks Mark ,



r79 is 2.187 k
d19 replaced by new 5v1 zener
connection to pin 1 ic33 is ok

right now there is no video-output , so quite hard to test that.


jojo
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
jojo
Posts: 109
Joined: Mon Mar 16, 2020 5:25 pm

Re: spectrum 128 with only 12 volt

Post by jojo »

is there a chance i have a faulty ula here ??



jojo
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: spectrum 128 with only 12 volt

Post by 1024MAK »

If you have no video, yes, that’s a possibility.

Do you have a 3.5MHz clock on pin 6 of the Z80?

Mark
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1024MAK
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Re: spectrum 128 with only 12 volt

Post by 1024MAK »

I’m assuming this is a U.K. toast rack rather than a Spanish one…

How are you connecting it to your display?
RGB / SCART, Composite video or RF/UHF?

Do you get any key click sounds?

What do you get on ULA (IC1) pins 19, 20, 21, 22, and 23?

Do you have +12V across C131 (47µF)?

Mark
jojo
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Re: spectrum 128 with only 12 volt

Post by jojo »

output is rgb , no sound.

23 = 3,77
22 = 0,33
21 = 3.42
20 = 3.38
19 = 3.38

c131 = 11.21


mmm , brightness 0,33 looks strange to me
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: spectrum 128 with only 12 volt

Post by 1024MAK »

Brightness is likely correct. The screen is normally non-bright unless a program sets the brightness attribute.

I don’t have a similar 128K model to hand so that I compare your results, but they look like they are in the range that I would expect.

The +12V is a little low at 11.21V. But I would not have thought that on it’s own it would cause this problem.

Has your RGB connection been tested with another machine? Are you using a SCART connection on a TV? Or more directly, does your display need the correct function switching control voltages on pin 8 and pin 16 of its SCART socket?

Do you have a logic probe or an oscilloscope (I keep asking because I can remember if you have said)?

Mark
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