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

Post by chequered flag »

No, Spectrum is better than ZX81 video signal. I have better colors from my issue two both on CRT and digital TVs.

Maybe try to replace LM1889N Video Encoder IC, reasonable to try.
https://www.retroleum.co.uk/zx-spectrum-chips

Replace ULA, not so reasonable. It seem to do its job here.

Maybe bypass internal video all together and add external "Graphics Card" analog RGB SCART
http://www.fruitcake.plus.com/Sinclair/ ... erface.htm
There is a 3D printed case for SPECTRA now. I have it.

Or go digital HDMI
https://www.bytedelight.com/?product_cat=zx-hd
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

I watched the video now.
If the output turns blue, your TV now thinks the Spectrum is outputting a SECAM signal, so reverse the adjustment bit by bit.
Try to adjust to a more yellow kind of white. Like cream.

What if I can’t achieve a good video output?

Sometimes even given the best equipment available, the colour tinge cannot be removed from the signal. In this case, the following can be tried:

Recap the machine (this is recommended anyway, but faulty caps can cause noise on the video output)
If you have a frequency meter or a multimeter that can do basic frequency measurement, adjust TC2 to 4.433619MHz or as close as possible (probe pin 17 of the LM1889).
Installing a 4K7 resistor between pin 14 of the LM1889 and ground
Replacing the LM1889 chip.
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

Well, I fired it up today on the capture card with the intend of typing in my colourbars program and tuning it; and was met with black. There was signal there, but the capture card wasn't showing an image. The machine still works and still shows (glitchy) images on the other devices. So whatever's causing my other digital devices to be temperamental is now causing the capture card to be temperamental.

I don't have any way to directly measure frequency, but assuming my $16000 (2015 dollars) oscilloscope is accurate, a scanline is dead-on 64us, which should be right:

Image

I tried adjusting the pots with it connected to one of the glitchy monitors and it was a thoroughly frustrating experience. I think the monitor's automatic decoding is defeating my ability to make small changes and see the results. I also get the impression that the waveform varies over time.

My symptoms match those described here:

https://www.reddit.com ZZZ /r/zxspectrum/comments/5t1neb/heeelllppp_composite_mod_problems/ (ZZZ inserted to prevent automatic preview)

...which someone pins down to impedance mismatch issues, but there isn't a suggested solution. I can see the sync pulse on the scope and it looks nice and clean, with only a little bit of ringing. I've gone through the video diagnostics section of the service manual to the best of my ability and I can see all the various thingies twiddling as suggested, although I'm not sure about pin 13 --- the signal changes when I attach the scope.

I think I've found a source for a CRT, so I'll try that next, unless someone has a recommendation. If it is an impedance issue, maybe the transistor mod would help?
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Ast A. Moore
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Re: Brand new r2 48kB, not quite working

Post by Ast A. Moore »

This may not be related to your problem directly, but it’s about an Issue 2 machine, and the guy does show a few oscillograms and explains them. Might be of some use:

https://www.youtube.com/watch?v=KKFOxLqHl1s
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

With reference to the comments about the ZX81 video signal. The ZX80 was only designed with a monochrome (black and white) TV set in mind. The ZX80 does not generate the part of the video signal known as the 'back porch'. A monochrome TV will work without this part of the video signal being to the specification. The first and second versions of the ULAs used in ZX81s also don't generate the 'back porch' part of the signal. But the last (third) version of the ZX81 ULA does generate the correct 'back porch' part of the signal.

The ULA in the ZX Spectrum is far more complex than the ZX81 ULA, hence not only does it generate the 'back porch' part of the signal, but it actually generates the video signal on it's own (you will still get a picture with the Z80 CPU removed from the board). Unlike the ZX80 and the ZX81 where the Z80 CPU is fooled by the ULA in getting video data from the memory for it.

However, these computers don't produce a proper TV specification interlaced TV signal. Instead they repeat the same field every time (instead of an even field followed by an odd field making one interlaced frame).

The LM1889 is a colour encoder, it only generates the colour signal. Colour being added to the monochrome signal. Without the LM1889, you would get a monochrome black/grey/white picture.

There are FOUR ways to composite mod a 16k/48k/+ ZX Spectrum. These are listed from least complex (and cheapest) to most complex. as you go further down the list, so the resulting signal 'should' be more compatible with more TVs and monitors.
  1. Just use a piece of wire.
  2. Use an electrolytic capacitor, typical values are 100uF 16V
  3. Use a NPN transistor in emitter follower configuration, followed by an electrolytic capacitor (100uF, with a 100nF paralleled)
  4. Use the configuration used in the 128k machines.
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

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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Re: Brand new r2 48kB, not quite working

Post by Ast A. Moore »

1024MAK wrote: Fri Jul 27, 2018 10:42 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:
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

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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Re: Brand new r2 48kB, not quite working

Post by Seven.FFF »

Good stuff!

For the ZX81, you could try doing the front porch mod: https://youtu.be/1irH3KuGyl0
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

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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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

hjalfi wrote: Sun Jul 29, 2018 6:19 pmThe 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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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

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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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

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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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

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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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

hjalfi wrote: Thu Aug 23, 2018 7:15 am ...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?
Well... maybe...

In theory, a transistor rated for HF (high frequency) signals should be used rather than a general purpose one. A composite video signal has frequencies between a few tens of Hz through to 6MHz.

The larger the value of capacitor, the greater the frequency range it will pass. However, electrolytic capacitors become slightly inductive at higher frequencies. So 100nF ceramic types are sometimes wired in parallel with them. Also larger values of capacitor allow more current to flow.

The value of the resistor should really be determined in order for the circuit to have a 75 ohm output impedance. 100 ohms is not too bad, 120 ohms is a bit further away...

But I'm not absolutely convinced making any changes would make much difference. As most of these 'buffers' designs were put together by experimentation.

It could be that your monitor does not like the non-standard video signal from the ZX Spectrum, or it could be that parts of the signal it is receiving are marginal, hence dropping from colour to monochrome.

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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

I really like it when I find a problem that turns out to be caused by me doing something really dumb --- because it's usually very easy to fix.

Turns out the Spectrum was just poorly calibrated, and five seconds' work with a screwdriver fixed it.

It looks really nice on my Trinitron PVM monitor, and a quick colourbar program shows that they're all the right colour, as well. Now I just need to find some good games...

Thank you very much for the assist!
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

Excellent news :D

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Re: Brand new r2 48kB, not quite working

Post by Contact Sam Cruise »

1024MAK wrote: Wed Jul 18, 2018 5:34 pm Oh, and it's normal for the heatsink attached to the 7805 voltage regulator to run hot. As long as the nut and the bolt holding the heatsink to the 7805 is tight, the 7805 will be okay. If the 7805 gets too hot, it will reduce it's output, and then you will know about it. It is however worthwhile putting a very thin smear of heatsink compound on the 7805 metal tab where the heatsink makes contact to aid heat transfer. Also resolder it's legs.

The ULA also runs hot. Some 4116 DRAM chips run warm. Everything else (with the lid off) should run cool or barely warm.

Mark

I've been going through this forum trying to find a solution to my issue - this thread seemed to come the closest to it.

I have gotten a refurbished issue 4B 48K Spectrum that starts up fine but then loses the display image after a couple of minutes. The case above the power supply feels very hot when it shuts down. I opened it up and it still has the original 7805 voltage regulator. I'm using a modern 9V power supply that works well with a Spectrum+. I'm also using the ZX-HD add-on for HDMI output - this also works fine with the Spectrum+ but the display is lost after a few minutes with the 48K Spectrum.

I also notice that there's no heatsink in the case - is this normal? I had thought that the heatsink could be eliminated if using an upgraded voltage regulator, but my Spectrum has the original 7805.

Could it be that the 7805 is overheating and reducing the output (as above), and then the ZX-HD no longer has enough power to drive the HDMI display? As mentioned before, the ZX-HD, TV, and power supply all work fine with the Spectrum+ I have.

Thanks for any help.
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Re: Brand new r2 48kB, not quite working

Post by Ast A. Moore »

Contact Sam Cruise wrote: Sun Nov 24, 2019 10:45 pm I have gotten a refurbished issue 4B 48K Spectrum that starts up fine but then loses the display image after a couple of minutes. The case above the power supply feels very hot when it shuts down. . . . it still has the original 7805 voltage regulator. . . . I also notice that there's no heatsink in the case - is this normal?
The 7805 definitely needs a heatsink, especially if you’re running additional hardware (although yours probably uses the 9V directly from the PSU). But, yeah, you don’t want to run it without a heatsink. Even a small slab of aluminum will be better than nothing. As a test, remove the heatsink from your working Spectrum and put it into the 4B. See if it operates as normal. They rarely burn out when properly heat sunk, but in your case, I’d also be looking into replacing the 7805.
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

The drop-in switch mode regulator has worked like a dream for me --- no heat sink, no heat, no problems. Strongly recommended!
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Re: Brand new r2 48kB, not quite working

Post by Contact Sam Cruise »

Ast A. Moore wrote: Sun Nov 24, 2019 11:42 pm
The 7805 definitely needs a heatsink, especially if you’re running additional hardware (although yours probably uses the 9V directly from the PSU). But, yeah, you don’t want to run it without a heatsink. Even a small slab of aluminum will be better than nothing. As a test, remove the heatsink from your working Spectrum and put it into the 4B. See if it operates as normal. They rarely burn out when properly heat sunk, but in your case, I’d also be looking into replacing the 7805.
Thank you. I will try the other heatsink. I don't own a soldering iron, is there anything else I'd need on top of that to replace the 7805?
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Re: Brand new r2 48kB, not quite working

Post by Ast A. Moore »

Contact Sam Cruise wrote: Mon Nov 25, 2019 12:30 am I don't own a soldering iron, is there anything else I'd need on top of that to replace the 7805?
Solder, flux, and basic desoldering/soldering skills. The latter is of particular importance, given that you’re going to be working on an old PCB, which poses a higher risk of damaging/lifting the traces.
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

When removing or fitting a heatsink, don’t try to use the 7805 voltage regulator chip as a mechanical fastening. Always use a suitable spanner to hold the nut and a suitable screwdriver to turn the machine screw. Otherwise you risk damaging the regulator chip, it’s legs or the PCB tracks/pads.

No flat insulators or insulated bushes are needed on the 7805 and heatsink in the ZX Spectrum. But do make sure that the metal heatsink does not touch (and short out) any other conductors or components (other than the large area of copper track under the 7805 voltage regulator chip).

The machine screw, washer and nut should be done up tightly so that the heatsink makes good firm contact with the metal tab of the 7805.

The idea is that heat can freely flow from the tab of the 7805 to the heatsink with no air gaps.

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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

If you're new to soldering, I'd definitely recommend practicing on something other than the Spectrum first. It's pretty easy to work on, with huge joints and tracks the size of your thumb, but Sinclair definitely skimped on the materials and the board does have a bit of tendency to fall apart while working on it. I got lucky.

You will need: a simple 25W soldering iron, a solder sucker (https://www.banggood.com/Manual-Solder- ... 29699.html), some leaded solder (don't get the lead-free stuff, it's noticeably harder to work with and the total amount of lead you'll be using is negligible), a set of small needle-nosed pliers and wire cutters, an old chopping board or cutting mat to work on because you will be dribbling molten metal around the place, and Big Clive's soldering tutorial video: https://www.youtube.com/watch?v=VtJ5gngTm-A If you've got three hands you will be at a significant advantage.

For desoldering just find some random old junk like a radio or TV (new stuff has much smaller joints, get something old) and try removing components. After all, you can't break it if it's already broken.
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