ZX81 rebuild

[Lardo Boffin]

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

[1024MAK]

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

[1024MAK]

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

Mark

[Lardo Boffin]

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

[1024MAK]

Yep. They look okay.

Mark

[Lardo Boffin]

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...

[Lardo Boffin]

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

[Lardo Boffin]

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

[Lardo Boffin]

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...

[1024MAK]

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