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 » Wed Jul 18, 2018 11:31 am

(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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chequered flag
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Re: Brand new r2 48kB, not quite working

Post by chequered flag » Wed Jul 18, 2018 12:08 pm

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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chequered flag
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Re: Brand new r2 48kB, not quite working

Post by chequered flag » Wed Jul 18, 2018 12:28 pm

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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chequered flag
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Re: Brand new r2 48kB, not quite working

Post by chequered flag » Wed Jul 18, 2018 1:47 pm

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

Post by hjalfi » Wed Jul 18, 2018 2:31 pm

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 3:12 pm, edited 1 time in total.
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hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Wed Jul 18, 2018 2:42 pm

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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chequered flag
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Re: Brand new r2 48kB, not quite working

Post by chequered flag » Wed Jul 18, 2018 4:22 pm

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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1024MAK
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK » Wed Jul 18, 2018 4:28 pm

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 4:51 pm, edited 1 time in total.
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1024MAK
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Re: Brand new r2 48kB, not quite working

Post by 1024MAK » Wed Jul 18, 2018 4:34 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
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hjalfi
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Re: Brand new r2 48kB, not quite working

Post by hjalfi » Wed Jul 18, 2018 4:36 pm

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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