Brand new r2 48kB, not quite working

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Fri Jul 27, 2018 11:59 am

Thanks for the links; all useful. The transistor mod is simple enough I may just try it, and see what happens --- it can't make things worse.

It's still really weird that there's no sign in the scope of anything wrong. The timing looks fine (64us per scanline). The voltages look fine (1V from the bottom of the sync pulse to the white level), which suggests the monitor's producing the right loading. Twiddling the pots causes the monitor (actually, the composite-to-HDMI upscaler in this setup) does, in fact, have the right impedance. The waveform shape looks about right, too. (Turns out that what I thought was interference noise in the video is, in fact, perfectly correct signal modulation.)

One really interesting thing is that my big TV shows the image in black-and-white, with mosaic patterns instead of colours. That suggests it's not seeing the colour burst. So it's probably worth me checking that the colour modulation is actually happening at 4.43MHz. I see now that the colour clock is generated from a different crystal; is there a chance that has failed?
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Ast A. Moore
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Re: Brand new r2 48kB, not quite working

Post by Ast A. Moore » Fri Jul 27, 2018 1:35 pm

1024MAK wrote:
Fri Jul 27, 2018 9:42 am
  • Use the configuration used in the 128k machines.
Um, that would be quite a complicated piece of circuitry. The TEA2000 needs digital RGB signals, and the 48K ULA only provides the YUV output. Converting that to RGB isn’t particularly easy. I’ve seen a schematic for converting YPbPr to RGB using four LT6552 video difference amps—I think it could be adapted for YUV signals. You’ll still need a sync, though.

EDIT: Oh, found another schematic that generates the sync signal:

Image

You could take the existing Y, R-Y, and B-Y signals from the Speccy and thus greatly simplify the circuit above.

Then again, you might as well use these signals to drive a SCART input directly. :lol:
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hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Sun Jul 29, 2018 5:19 pm

Or maybe, and bear with me, I know this is a long shot... maybe I could just connect the composite decoupling capacitor the right way round?

Okay, so I also added the transistor mod, so maybe that made a difference too, but all the monitors I've tried this with are quite happy now. (Although the instructions I found, at http://www.projectavr.com/spectrum-comp ... nd-refurb/, said to use a PNP transistor. I picked a 'universal PNP' from the parts drawer. Sadly I couldn't find a 100R resistor or another 100uF capacitor (certainly wasn't going to reuse the old one), so I picked the closest I had at 120R and 47uF, reasoning that this is fuzzy analogue stuff; it seems fine.)

Image

That image is after calibrating the colours. The white's not very bright and the yellow's kinda sickly.

Anyway, thanks for all the help. I've learnt a tonne and I have a nice apparently-functioning Spectrum. Cassette recorder's on order, and now I think of it I should recap the joystick interface.

I also have a ZX81 on order. That'll need recapping too; maybe I should do that one in ceramics.
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Seven.FFF
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Re: Brand new r2 48kB, not quite working

Post by Seven.FFF » Sun Jul 29, 2018 5:22 pm

Good stuff!

For the ZX81, you could try doing the front porch mod:
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chequered flag
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Re: Brand new r2 48kB, not quite working

Post by chequered flag » Sun Jul 29, 2018 7:24 pm

That link is now collected. Really nice.
http://www.projectavr.com/spectrum-comp ... nd-refurb/

Did you see the "Sinclair Spectrum Composite Mod PCB" that fits inside the RF box.
http://www.projectavr.com/esp-01-esp-03 ... ing-board/
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1024MAK
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK » Sun Jul 29, 2018 9:19 pm

hjalfi wrote:
Sun Jul 29, 2018 5:19 pm
The white's not very bright and the yellow's kinda sickly.

Anyway, thanks for all the help. I've learnt a tonne and I have a nice apparently-functioning Spectrum. Cassette recorder's on order, and now I think of it I should recap the joystick interface.

I also have a ZX81 on order. That'll need recapping too; maybe I should do that one in ceramics.
The 'white' is not supposed to be a bright white (unless the BRIGHT attribute is set). The normal white should be more like a light grey.

I'd be supprised if there are any electrolytic capacitors in a joystick interface. Unless it gives problems, there is no need to change the capacitors in a joystick interface.

The main reason for replacing the capacitors in a ZX Spectrum is to improve the reliability, as a failure of an electrolytic capacitor in the DC-DC converter section (or a capacitor supplied by, or supplying it) can cause it to fail. The loss of the -5V rail then may cause damage to the 4116 (or equivalent) DRAM chips. Also the inside of the ZX Spectrum 16k / 48k machines gets rather hot, and increased temperature decreases the life of electrolytic capacitors.

With the ZX81, there are only two electrolytic capacitors, one to smooth the "+9V" rail and one in the /RESET circuit. Both are non-critical in that a failure of the capacitor will not cause any semiconductors to become damaged. So no need to routinely replace them. Just replace on failure. Not that these cause many problems anyway.

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hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Wed Aug 22, 2018 9:46 pm

My nice new Sony PVM has arrived!

The Spectrum displays in mono on it. It displays in colour on two out of my three composite-capable LCDs (the third is a big-screen TV and just shows black).

I was kinda expecting a real CRT to do better than a modern LCD. Is this behaviour symptomatic of anything?

(And yes, I have verified that the Sony does actually work with a colour signal!)
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1024MAK
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK » Wed Aug 22, 2018 11:02 pm

Odd. Is the Sony a multistandard monitor? If yes, does it have manual or automatic switching? And if automatic, is there a way to force it to PAL I standard? (that's I as in aye!). Or maybe just the PAL 50Hz setting.

Some multistandard monitors and TVs sometimes get confused by the Spectrums video output.

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hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Thu Aug 23, 2018 6:15 am

It does PAL and SECAM, with a switch. It also has lights to let you know whether it detects colour. Interestingly, if I route the video through an CV-to-HDMI-to-CV cover I get nice vibrant colour and the PAL light lights up. Connected directly I get mono and no light.

This suggests that the monitor's not detecting the colour burst, right? That implies that my dubious transistor mod is dubious. That would explain the skewed colour on one of my LCDs, actually.

I used the circuit from http://www.projectavr.com/spectrum-comp ... nd-refurb/:

Image

...although I had to make some substitutions: the resistor's a 120R, the transistor is an off-the-shelf 'general purpose' PNP (which the article said should be fine). Do any of these values actually matter?
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