ZX80 issue 2 kit does not start
ZX80 issue 2 kit does not start
Hi, I am pretty new around here so sorry in advance for the noob question.
I have soldered siiue2 replica kit, this one https://www.ebay.co.uk/itm/PCB-ZX80-iss ... SwLgVcjfAU
I also read some early replicas had some problems with MREQ pin on CPU, but this PCB looks okay.
So after soldering done there is literally nothing on the screen, out of sync
I am using a simple composite mod for my telly with a transistor to amplify the video signal, taking it directly from the modulator input. All chips are getting slightly warm after some time, so I presume they are working, include CPU, but there is no output.
I have no oscilloscope, but simply measuring the voltage on composite gives me 0V. Picking the vid signal gives me something in-between 0.11 and 2.3V
Any tips where I should look would be helpful.
Cheers ps Don't worry about R12 it has been soldered later on
I have soldered siiue2 replica kit, this one https://www.ebay.co.uk/itm/PCB-ZX80-iss ... SwLgVcjfAU
I also read some early replicas had some problems with MREQ pin on CPU, but this PCB looks okay.
So after soldering done there is literally nothing on the screen, out of sync
I am using a simple composite mod for my telly with a transistor to amplify the video signal, taking it directly from the modulator input. All chips are getting slightly warm after some time, so I presume they are working, include CPU, but there is no output.
I have no oscilloscope, but simply measuring the voltage on composite gives me 0V. Picking the vid signal gives me something in-between 0.11 and 2.3V
Any tips where I should look would be helpful.
Cheers ps Don't worry about R12 it has been soldered later on
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Re: ZX80 issue 2 kit does not start
We need to know more about how your composite transistor circuit is wired up. Is it a NPN transistor, or a PNP. What other components did you use? Maybe you can draw a schematic diagram.
Mark
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: ZX80 issue 2 kit does not start
Hi Mark, thanks for your answer.
I am using the simple mod with NPN mod ripped from here
https://www.bytedelight.com/?page_id=3560
(cheers to Ben, btw)
On my photo below that 2N3904 is flipped, so the wiring is mirrored
I am using the simple mod with NPN mod ripped from here
https://www.bytedelight.com/?page_id=3560
(cheers to Ben, btw)
On my photo below that 2N3904 is flipped, so the wiring is mirrored
- 1024MAK
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Re: ZX80 issue 2 kit does not start
Right, except that with the ZX80, you have two different TTL chips providing the sync and video data. The resistors R30 (1kΩ) and R32 (330Ω) then combine these signals, the resulting signal is then fed to the input of the modulator.
But with a 2N3904 NPN transistor wired as an emitter follower configuration, different voltage levels may be needed.
You say that the composite output is 0V. That indicates to me that the NPN transistor is not being turned on, which seems strange to me.
What voltages do you have at either end of R30 and at either end of R32?
Mark
But with a 2N3904 NPN transistor wired as an emitter follower configuration, different voltage levels may be needed.
You say that the composite output is 0V. That indicates to me that the NPN transistor is not being turned on, which seems strange to me.
What voltages do you have at either end of R30 and at either end of R32?
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: ZX80 issue 2 kit does not start
I've cheked both R30 and R32, it is 1kOhm and 280 Ohm. But all sides of both resistors (ie all 4 points) give me only 0.16-0.17 V
Re: ZX80 issue 2 kit does not start
So there is progress, had a closer look I have discovered R24 and R20 are missing (I must have didn't notice that before). Since they are somewhat critical for vid signal, installing them give some interesting consequence.
R32 and R30 give me 3.65 Volts at the output. I think it is okay now. My telly synchs with SYNCH. But the picture is now black with some white random noise, a very weak one. It only occurs if I only use video modulator's input, raw, no amplification.
However, using the amplifier lead only to a black picture as if the amplifier does not work at all. I replaced that transistor but with the same result. I am somewhere quite close but still have some trivial problem, I am sure.
Here is a video of what I got with modulator input (not the npn-emitter)
https://youtu.be/O5RB_gvwCGc?t=74
Is there a quick way to use somewhat different as an amplifier?
R32 and R30 give me 3.65 Volts at the output. I think it is okay now. My telly synchs with SYNCH. But the picture is now black with some white random noise, a very weak one. It only occurs if I only use video modulator's input, raw, no amplification.
However, using the amplifier lead only to a black picture as if the amplifier does not work at all. I replaced that transistor but with the same result. I am somewhere quite close but still have some trivial problem, I am sure.
Here is a video of what I got with modulator input (not the npn-emitter)
https://youtu.be/O5RB_gvwCGc?t=74
Is there a quick way to use somewhat different as an amplifier?
Re: ZX80 issue 2 kit does not start
After soldering some vital RR, I came to 3.6V for R30 and 32, same as I do have now after on NPN transistor amplifier.
So I moved to old CRT telly, and I have somewhat out os sync picture
https://www.youtube.com/watch?v=p84BBV2FhsQ
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Re: ZX80 issue 2 kit does not start
I just built one of these:
http://www.myprius.co.za/ZX80.htm
No transistor buffer was necessary, only a back-porch generator. The kit I bought had the WRONG frequency resonator. I had to put in a 6.5536MHz crystal myself.
http://www.myprius.co.za/ZX80.htm
No transistor buffer was necessary, only a back-porch generator. The kit I bought had the WRONG frequency resonator. I had to put in a 6.5536MHz crystal myself.
ZX80/ZX81/ZX Spectrum
Re: ZX80 issue 2 kit does not start
I got 3 pin decriminator as 6.5Mhz supplied with this kit, is that okay?albertusj wrote: ↑Mon Aug 05, 2019 3:06 pm I just built one of these:
http://www.myprius.co.za/ZX80.htm
No transistor buffer was necessary, only a back-porch generator. The kit I bought had the WRONG frequency resonator. I had to put in a 6.5536MHz crystal myself.
https://gd3.alicdn.com/imgextra/i1/0/TB ... em_pic.jpg