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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Brand new r2 48kB, not quite working

Post by hjalfi »

(Cross-posted from r/zxspectrum, who suggested I ask here)

I've just bought my first Spectrum! And it almost works.

It's a working-when-it-went-into-the-attic purchase off ebay, and is a 48k series two with boxed Kempston-knock-off joystick interface, a boxed Amsoft joystick, and a decent collection of original cassette software, including Tasword Two and both Horace games...

It does mostly work: the power supply is producing about 4.5V, so I assume it's nadgered, and I'm ignoring it for now. My TV is unhappy about the RF signal so I've done the composite mod.

My questions:

(a) the composite signal is terrible. One of my monitors won't even lock onto it. Is that normal? Can I improve it? I see that the ZX81 composite mod requires adding a transistor to amplify the signal; would that work here?

(b) the bottom right row of keys on the keyboard don't work: B, N, M, SymShift, BreakSpace. (Which means I can't load games.) This means the membrane's dead, right? Is this repairable or am I getting a new membrane off eBay? Removing the connectors to do the composite mod was a bit hairy; the plastic doesn't feel like it's in good shape.

(c) there's a transistor patched between line A0 on the Z80 and somewhere in the RAM. (See pic: https://photos.app.goo.gl/GDDYvSrFkc4hpyK76) Is this original or an aftermarket addon?
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

Turn it off now!

You have the original capacitors. They need to be replaced to prevent further damage to memory chips.
https://spectrumforeveryone.com/technic ... -spectrum/
https://www.retroleum.co.uk/zx-spectrum-capacitors


Then you need this membrane.
https://www.sellmyretro.com/offer/detai ... brane-2502


Then check power supply. It should produce 12-14 Volts when not connected and at least 9 volts when connected. Be very careful about polarity of replacement power supply. Sinclair is non standard.
https://www.retroleum.co.uk/spectrum-cables
"Remember when soldering: The Spectrum PSU is unusual in that the inner pin is negative and the outer barrel is positive. (Getting these reversed will fry your Spectrum!)"

About now you are ready to power on.

Have fun!
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

Turn it off now!

You have the original capacitors. They need to be replaced to prevent further damage to memory chips.
https://spectrumforeveryone.com/technic ... -spectrum/
https://www.retroleum.co.uk/zx-spectrum-capacitors


Then you need this membrane.
https://www.sellmyretro.com/offer/detai ... brane-2502


Then check power supply. It should produce 12-14 Volts when not connected and at least 9 volts when connected. Be very careful about polarity of replacement power supply. Sinclair is non standard.
https://www.retroleum.co.uk/spectrum-cables
"Remember when soldering: The Spectrum PSU is unusual in that the inner pin is negative and the outer barrel is positive. (Getting these reversed will fry your Spectrum!)"

About now you are ready to power on.
(c) there's a transistor patched between line A0 on the Z80 and somewhere in the RAM. (See pic: https://photos.app.goo.gl/GDDYvSrFkc4hpyK76) Is this original or an aftermarket addon?
It is original. Compensating for errors in ULA.
https://spectrumforeveryone.com/2017/09 ... ariations/



Have fun!
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

(a) the composite signal is terrible. One of my monitors won't even lock onto it. Is that normal? Can I improve it? I see that the ZX81 composite mod requires adding a transistor to amplify the signal; would that work here?
http://blog.retroleum.co.uk/electronics ... ideo-mods/

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

Post by hjalfi »

Yeah, recapping it's a good idea. I think I may even have the right ones somewhere.

The power supply is buggered --- 4.5V disconnected, didn't bother to test further as it was missing the barrel connector anyway. I bodged up one from a new 9V power supply. Is the original worth repairing other than for completeness value? I assume that the problem's just capacitor failure.

I'll probably take the opportunity to replace the linear regulator while I'm working on the board; even in just a few minutes of testing it got scary hot.

Thanks for the links!
Last edited by hjalfi on Wed Jul 18, 2018 4:12 pm, edited 1 time in total.
hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

Incidentally: looking at the circuit, it seems that the unregulated 9V input is only used by the regulator (and a couple of smoothing capacitors). Is this correct?

As the recommended replacement regulator, the TRACO 1-2450, is rated for input voltages from 6.5V to 36V, then I could use pretty much any power supply of the right polarity... and if I can find a way to bodge in a rectifier next to the DC jack then either polarity would work too. (This would also make me much less likely to accidentally fry my valuable piece of retro hardware. I have a lot of power supplies which use that size of barrel connector.)
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

The unregulated 9V is only used to power peripherals that you may plug into the expansion port. Internally everything runs on regulated 5V. -5V +12V and 12V AC is created from regulated 5V. If you know that your expansion board do not use the 9V you may power the computer with 7V or so dictated by the needs of the internal regulator.

I use this modern internal regulator. MuRata Switch Mode 5v Regulator (replaces 7805), Got rid of the heat sink.
https://www.retroleum.co.uk/spectrum-repairs-mods
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

No, the on board DC/DC converter formed by TR4, TR5 and the coil (transformer) uses the nominal "9V" supply. This DC/DC converter produces the +12V and the -5V supplies. The -5V is critical, as the loss of this may damage the 4116 (or equivalent) 16k x 1 bit DRAM chips (that form the "lower" RAM).

Due to a limitation of the design, voltages above about 11.9V on the "9V" input will cause the DC/DC converter to stop working, this causes a loss of the important-5V supply, while the +12V is supplied by current "leakage" via the coil.

Sinclair corrected the problem in later machines (designed in to the PCB layout) or by service personal altering the components on existing boards. But many machines did not receive the upgrade, as they were not returned or serviced.

Some expansions (ZX PrInter, ZX Interface 1, Microdrives and various others) also use the +9V.

I recommend using an external regulated supply of between 7.5V to 11.5V. Or an unregulated supply of 8V to 9V.

Mark
Last edited by 1024MAK on Wed Jul 18, 2018 5:51 pm, edited 1 time in total.
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

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
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

Good to know; I'll stick with the regulated 9V supply. I think I'll attempt to repair the original power supply brick, but I suspect it's going to spend its time on a shelf somewhere.
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

Keep the original power supply as a novelty item for display. When the ZX Spectrum was introduced in Sweden back in 1983 the Sinclair power supply had to be replaced to pass electrical noise regulations.
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

The external Sinclair "PSU" is a normal transformer - bridge rectifier - smoothing capacitor arrangement.
The 1.4A type should be used for 48k ZX Spectrum and ZX Spectrum + machines. Although some 0.7A and 1.2A (designed for the ZX81) were apparently supplied with early machines.

Typical failures are (in no particular order):
  • Output cable failed (copper conductor broken, but insulation appears fine).
  • Broken PCB tracks or broken transformer legs/terminals (some designs)
  • Blown thermal fuse (mounted under the transformer or inside it)
  • Damaged/blown rectifier diode(s)
  • Life expired electrolytic capacitor
Sinclair used a number of companies to produce these, so there are a number of variations. From the outside, they look the same, or very similar (PSU for the 16k and 48k models). A new case was used for the ZX Spectrum + model, but internally the arrangements are the same.

Most use four individual diodes. Most use a single 4700uF 16V electrolytic capacitor. But some use two x 2200uF.

Any with broken tracks, broken transformer legs/terminals, or blown thermal fuse should be scrapped. If a diode has failed and badly burned the board, again, scrap it. Otherwise repair is straightforward. Renew any damaged or suspect diodes, renew the capacitor(s) with a new 4700uF 25V or 35V part. 105 degree type being preferred.

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

Post by hjalfi »

Nothing looks particularly explodey in the power supply, but it generates about half the voltage it should; it's got two 1000uF capacitors and a single 2200uF one, all in parallel across the output lines. Wow. It's been a while since I've seen a PSU quite this primitive.

Naturally, my parts drawers have precisely one fewer than the required number of capacitors to recap the whole lot today: it's that 2200uF one in the PSU. Everything else I've got (including the one for the composite mod).

New membrane and TRACO switching regulator on order. Thanks!

Additional questions; see https://photos.app.goo.gl/n1CDAmenxYHrSU338:

- there's an extra 4.7uF capacitor (of a different make) spider patched between the positive end of C43 to R58. Original or aftermarket?

- on the east side of the board next to KB2 there's a couple of what look suspiciously like configuration links, labelled N / H / I. They're connected underneath according to the white lines. Any idea what these are?

- so far I've removed the keyboard twice and plugged it in once; the connectors feel really grim and don't grip the membrane very well. I'm hoping the replacement will be a bit stiffer, or at least less likely to fall apart if I bend it too much. Is it worth trying to clean or service the connectors to make removing the keyboard easier?
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

- there's an extra 4.7uF capacitor (of a different make) spider patched between the positive end of C43 to R58. Original or aftermarket?
The radial capacitor soldered to C34 is a recommended service modification. Maybe original from factory.

http://blog.retroleum.co.uk/electronics ... -spectrum/

- on the east side of the board next to KB2 there's a couple of what look suspiciously like configuration links, labelled N / H / I. They're connected underneath according to the white lines. Any idea what these are?
ROM select links. N-N for NEC ROM. Matches your NEC ROM in the picture. H-H for Hitachi ROM

http://blog.retroleum.co.uk/electronics ... -an-eprom/
- so far I've removed the keyboard twice and plugged it in once; the connectors feel really grim and don't grip the membrane very well. I'm hoping the replacement will be a bit stiffer, or at least less likely to fall apart if I bend it too much. Is it worth trying to clean or service the connectors to make removing the keyboard easier?
Gentle clean. If the new membrane doesn't grip you can make it thicker on the non conducting side. The old membrane may have a sticky transparent piece of plastic for that purpose that you can reuse.

https://www.youtube.com/watch?v=76gyKWRI7dY

https://www.youtube.com/watch?v=-PJLPEI8RW8

https://www.youtube.com/watch?v=O61Yy00GHEc

https://www.youtube.com/watch?v=Q9cgCAZti98
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1024MAK
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

It's well worth reading the service manual, here is a link to a HTTP version

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

Post by hjalfi »

Status update: caps replaced, though I cocked up and I might need to replace some again --- I'll find out when I try to put reassemble later; replacement membrane is here and I've disassembled (and cleaned, ugh) the old keyboard; TRACO regulator should arrive tomorrow. Kudos to RWAP for delivering to Switzerland amazingly quickly.

Query: the tape holding the old faceplate on appears to have metastasized into a foul, yellow, chunky mucous-like substance, except mucous looks more appetising. The big problem is that it's lumpy, so when I stick the faceplate on it probably won't lie flush. I'd like to clean it off and use new tape. What's safe to use on the case without risk of melting it? Will isopropyl alcohol work? I'd be very hesitant about using acetone of white spirit.
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

I bathe my plastic cases in warm water and mild washing-up liquid. Not too hot, don't want to melt the plastic. The heat makes the glue soft and then I scrape it off with a small screwdriver. Any scratches I make will be under the new double sided tape.
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

I did actually use a water bath to get the faceplate off (it wasn't like the old membrane could get any more broken!); I'll have another go. I have a soft plastic spudger that'd be ideal for scraping. Apparently IPA isn't a good idea because it can discolour plastics.

I can't find out what the plastic case is made of. I assume it's something very, very cheap. Anyone know?
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Re: Brand new r2 48kB, not quite working

Post by Seven.FFF »

I've also used a steam cleaner to soften that kind of gunk.
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK »

hjalfi wrote: Tue Jul 24, 2018 1:56 pmI can't find out what the plastic case is made of. I assume it's something very, very cheap. Anyone know?
Note: this applies to both ZX81 membranes (which are stuck on the case) and the glue (or double sided sticky tape) used on rubber keyed ZX Spectrums. No other machines.

The black plastic case is ABS. Normal IPA solvent does not damage ABS plastic as far as I know (been using IPA on ABS plastic for many years). But IPA alone may not help a great deal with the glue.

When I do it, I use a hair dryer and a wide flat bladed screwdriver at a shallow angle. I then wipe the screwdriver in paper towel. Then scrape again... Only once I have got as much as possible off with this method, do I use IPA. In my experience, the IPA softens the glue, but does not fully dissolve it. So you have to wipe it off. As soon as the IPA evaporates, the glue goes back to being glue!

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

Post by hjalfi »

Hot water and a iFixit-clone stainless steel spudging tool worked reasonably well --- I managed to get most of the lumps off. It'll lie flat now, so I think I'll leave it at that, as it's good enough.

Man, that glue is repulsive. I actually tried a nylon pot scourer thing, and the nylon came off on the glue.

Thanks!

(Still waiting for the regulator to arrive.)
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

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

Post by hjalfi »

Woohoo!

Image

I own a really very nice Tektronix 7603 oscilloscope weighing 13kg, so calibrating the video was a cinch --- thanks for the links; I'd never have thought to do it otherwise.

However...

I have a mini composite monitor, which won't even admit there's a signal there; a big TV with composite in, which will show a picture, but very reluctantly, with fadeouts, glitches, and it can't see the colour burst so everything's a mosaic of black-and-white; a composite-to-HDMI converter, which will show colour, but has a lot of trouble locking onto the image and it rolls and glitches; and a ultracheap composite capture device, which shows a rock solid image and nice, rich, and wrong colours. The image above came from that. You can see that the white is grey, and that the bright0/bright1 bands are... weird.

Is this just digital TVs not liking composite, or is there something I can do? When working on the board, I added a 100uF capacitor between the composite video signal and output pin (as recommended).

(I have a lot of this on video; I'm editing.)
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Re: Brand new r2 48kB, not quite working

Post by chequered flag »

Is this just digital TVs not liking composite, or is there something I can do?
I have three different digital TVs. Newer and higher resolution gives better picture.

There are some adjustments to do on the TV.

All auto detect is bad. Lock it to PAL video signal if possible.

Disable all filters that are supposed to improve the picture. These filters get confused by the dot crawl that is always present. These filters will also introduce a delay that is sometimes noticeable when you play games. Select "game mode" if possible.

Increase brightness until bright white becomes white. Normal white should be grayish.

Increase contrast until you get sharp edges between the color bars.
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Re: Brand new r2 48kB, not quite working

Post by hjalfi »

Way ahead of you own the TV settings, sorry! Image 'enhancement' filters drive me nuts and I always try to turn them off...

But there's something more than that going on. I'm trying to source a real composite CRT and see if that's any better. I hear that the ZX81 omitted various important parts of the video signal for simplicity; did the Spectrum?

In other news, I have produced a massive 2h30m video of me working on the board. I'm not expecting anyone to actually watch this, but:

https://www.youtube.com/watch?v=24WUYppOExA

Useful bits are at about 2:20:20 where I have the output video hooked up to an oscilloscope, and 2:28:00 where I have it connected up to a monitor (via a HDMI upconverter) and you can see sync problems.
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