ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 8:41 am

1024MAK wrote:
Wed Oct 04, 2017 8:53 am

Professional and consistent joints, but with signs of flux, any unused pads free of solder - hand soldered by someone who is proficient

Mark
Thanks Mark. What are the signs of flux?

Also I have been assembling a parts list to order all the bits I need and have a question (probably first of many) -

What tolerance do the resisters need? I can see looking at them there are carbon based and film based with the latter being more expensive but a better tolerance. Do I need to spend the extra or will the cheaper ones work well enough?

Lardo
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
Posts: 1822
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Oct 06, 2017 1:53 pm

Lardo Boffin wrote:
Fri Oct 06, 2017 8:41 am
Thanks Mark. What are the signs of flux?
Typical flux residue from this period is brown/dark brown in colour and may be slightly sticky to the touch if the surface is disturbed.

5% tolerance, 1/4W (250mW) carbon film is the minimum requirement in most circuits (including the ZX81) unless otherwise specified. But 1% or 2% or 5% 250mW metal film are fine as well. In the ZX81, carbon film is good enough.

Mark

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Oct 06, 2017 1:55 pm

You often find flux left on the pads of the mic, ear and power jack socket solder connections.

Mark

Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 2:35 pm

1024MAK wrote:
Fri Oct 06, 2017 1:53 pm
Lardo Boffin wrote:
Fri Oct 06, 2017 8:41 am
Thanks Mark. What are the signs of flux?
Typical flux residue from this period is brown/dark brown in colour and may be slightly sticky to the touch if the surface is disturbed.

5% tolerance, 1/4W (250mW) carbon film is the minimum requirement in most circuits (including the ZX81) unless otherwise specified. But 1% or 2% or 5% 250mW metal film are fine as well. In the ZX81, carbon film is good enough.

Mark
Thanks Mark. I had wondered about the brown stuff previously!

Would a resistor like the following be suitable for R7 to R14 (470Ohm)?

http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03799

And this one for R16 to 27 etc. (1K)

http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03803

Lardo
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
1024MAK
Posts: 1822
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Oct 06, 2017 6:48 pm

Yep. They look okay.

Mark

Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 10:32 pm

Resistor part list: -

Resistors
R1 10K (Brown Black Orange)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 10K (PK 100) - MCF 0.25W 10K
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03815

R2, R28 680Ohm (Blue Grey Brown)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 680R (PK 100) - MCF 0.25W 680R
£0.72
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03801

R4 18K (Brown Grey Brown)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 18K (PK 100) - MCF 0.25W 18K
£0.66
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03818

R5 330OHM (Orange Orange)
Brown Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 330R (PK 100) - MCF 0.25W 330R
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03797

R6 2K2 (Red Red Red)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 2K2 (PK 100) - MCF 0.25W 2K2
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03807

R7 to R14 470OHM (Yellow Purple Brown)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 470R (PK 100) - MCF 0.25W 470R
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03799

R15 220K (Red Red Yellow)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 220K(PK 100) - MCF 0.25W 220K
£0.85
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03831

R16 to R27 1K (Brown Black Red)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 1K (PK 100) - MCF 0.25W 1K
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03803

R29 1M (Brown Black Green) (4th band may be yellow)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 1M (PK 100) - MCF 0.25W 1M
£0.72
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03836

R33 4K7 (Yellow Purple Red)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 4K7 (PK 100) - MCF 0.25W 4K7
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03811

R34 220OHM (Red Red Brown)
Pack of 100 CPC Farnell RESISTOR, 0.25W 5% 220R (PK 100) - MCF 0.25W 220R
£0.68
http://cpc.farnell.com/unbranded/mcf-0- ... dp/RE03795

If anyone feels like double checking these are right... ;)
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 10:37 pm

Resistor networks: -

RP1 8 * 10K 10KΩ 9 leads / Resistor network
(min 5) CPC Farnell RESISTOR NETWORK, 10K, ± 2% - 4609X-101-103LF
£2.16
http://cpc.farnell.com/bourns/4609x-101 ... dp/RE05350

RP3 5 * 10K 10KΩ 6 leads / Resistor network
(min 5) CPC Farnell RESISTOR NETWORK, 10K - 4606X-101-103LF
£0.66
http://cpc.farnell.com/bourns/4606x-101 ... dp/RE04745
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 10:41 pm

Capacitors (the ceramic ones are a guess): -

C1, C7, C12 47pF 47 Ceramic disc
(min 5) CPC Farnell CAPACITOR, 47PF, 50V - K470J15C0GF53L2
£0.78
http://cpc.farnell.com/vishay/k470j15c0 ... dp/CA06814

C2, C4, C8, C9, C11 47nF 473 Z Ceramic disc
(min 5) CPC Farnell CAPACITOR, 47NF 50V - MCRR50473X7RK0050
£0.54
http://cpc.farnell.com/unbranded/mcrr50 ... dp/CA05519

C3 22uF 22u Electrolytic 16V min
CPC Farnell Electrolytic Capacitor, 22 µF, 35 V, M Series, ± 20%, Radial Leaded, 5 mm - ECA1VM220
£0.42
http://cpc.farnell.com/panasonic-electr ... dp/CA07275

C5 1uF 1u Electrolytic 5V min
CPC Farnell Electrolytic Capacitor, 1 µF, 63 V, MCMR Series, ± 20%, Radial Leaded, 4 mm - MCMR63V105M4X7
£0.30
http://cpc.farnell.com/multicomp/mcmr63 ... dp/CA08191

C6 100pF 100, 101, n10 Ceramic disc
(min 5) CPC Farnell CAPACITOR, 100PF, 200V - MCRR50101COGJ0200
£1.26
http://cpc.farnell.com/multicomp/mcrr50 ... dp/CA06299

C10 10nF 10n, 103 Ceramic disc
(min 5) CPC Farnell CAPACITOR, 10NF, 50V - MCRR25103X7RK0050
£0.90
http://cpc.farnell.com/multicomp/mcrr25 ... dp/CA06295
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 672
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Oct 06, 2017 10:47 pm

Other bits: -

Transistors -
TR1/TR2 - ZTX 313 (another guess)
CPC Farnell TRANSISTOR, NPN, TO-92 - BC182B
£0.06
http://cpc.farnell.com/cdil/bc182b/tran ... dp/SC09311

Diodes -
D1-D8 - 1N4148 (Yellow Brown Yellow Grey)
Pack of 50 CPC Farnell Small Signal Diode, 100 V, 150 mA, 1 V - 1N4148
£0.46
http://cpc.farnell.com/unbranded/1n4148 ... dp/SC07296


To be continued...
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
1024MAK
Posts: 1822
Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sat Oct 07, 2017 9:08 am

The transistor in position TR2 has to be a type that has a fast switch off characteristic. Most plastic small signal through-hole types are too slow.

Bipolar transistors can continue conducting current from the collector terminal through to the emitter terminal for a time after the base voltage and current has fallen to zero.

When used in a circuit like the Z80’s 3.25MHz clock circuit, the result is that the transistor does not switch off. The collector voltage will be found to be zero or near zero and so the Z80 CPU will just sit there doing nothing.

So far, I have not had to replace TR2 or the transistor that performs the same function in a ZX Spectrum. The remakes of the ZX80/ZX81 that I have use logic chips and not a transistor for the Z80 clock signal.

I have though seen the problem with a recreation of the Jupiter Ace computer. That required a lot of experimenting with different values of resistors, capacitors and head scratching while probing with an oscilloscope... Eventually I found a combination of parts that worked, but I would not recommend it!

So although I have had a look for a suitable replacement for the ZTX312 / ZXT313 transistors, for use in position TR2, I have not found a plastic cased TO92 transistor that I am happy with yet.

Mark

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