Ian's Reference

ZX Spectrum 16/48K

This was my on going interest and research in the ZX Spectrum, partly using it to learn electronics a bit better.
Please use this information at your own risk!

Last Updated: 31/01/2022

I started out with broken issue 2 boards from eBay, but they proved a bit fragile. I moved on to PABB Issue 3B boards from PCBWay. A lot of this information now corresponds to these. I'll really need to update all this for all the varying board issues.

I grew up with Acorn so I go North, South, East and West which correspond to Back, Front, Right and Left in Sinclair.

If anyone thinks any of this is wrong, or potentially damaging, please email me at grandoldian at gmail.com.

ZX Spectrum Components

My attempt at working out the transistors and other parts in a ZX Spectrum 16/48. Many sites list transistor equivalents that aren't necessarily available, or are available in small numbers if available from hobbyist suppliers. That's no good. This has ended up a more sensible list of equivalents and suppliers and part numbers in the UK.

Update: Turns out you can do most of these with Amazon. Amazon links further down.

Position Listed Original Suggestions
(from the web)
My Suggestions Notes (issue 3B) Notes (issue 2)
tr1/tr2 ZTX313 BC549C 2N3904 Amazon
BC549 Amazon
S9014 (Glowy) Amazon
2N3904 CPC SC15978
BC549C CPC SC12757
BC549C Farnell 1574382
tr1 collector east
tr2 collector west
2N3904 tr1 flat side south
2N3904 tr2 flat side north
BC549 tr1 flat side north
BC549 tr2 flat side south
S9014 tr1 flat side south
S9014 tr2 flat side north
Many have been discounted because of horrid glow
tr3 ZTX313 MPS2369 2N4401
See section on tr3 below
CPU clock amp - This isn't totally straight forward
Collector east
Ok, the Motorola MPS2369A seems to be a direct equivalent to the ZTX313, almost identical.
Some seem to be labelled 200mA, some 600mA. The latter is what we want.
I've tried transistors labelled PH2369, presumably Philips, and they don't do anything like. Maybe a bad batch perhaps, but not the same.
tr4/tr5 ZTX650/ZTX213 ZTX651/ZTX751 S8050/S8550 Amazon
BC337/BC327 Amazon
S9013/S9012 Amazon
BC337 CPC SC06661
BC327 CPC SC15365
ZTX651 RS 295-501
ZTX751 RS 295-523
I'm trying to use matching NPN/PNP pairs, though that's probably
not necessary. The S8050/S8550 combination is the top of the list as
they output the highest power.
tr4 collector south
tr5 collector north
S8050/S8550 face away from each other
BC337/BC327 face each other
S9013/S9012 face away from each other
S8050 faces west, S8550 south
tr6 / spider mod ZTX313 MPS2369 2N3904 Amazon
2N3904 CPC SC15978
BC337 Amazon
BC337 CPC SC06661
S9014 Amazon
Bridges A0 and IOREQ for old ULA bug
Perhaps a BC337 is overkill
Collector (1) south as tr6 on issue 3
2N3904 flat side west
BC337 flat side east
S9014 flat side west
tr7 ZTX450   BC550 Amazon
BC550 SC15786
2N3904 Amazon
2N3904 CPC SC15978
S9014 Amazon
BC549 Amazon
BC549C CPC SC12757
BC549C Farnell 1574382
ZTX450 RS 652-702
Drives the speaker in issue 3 or later
Collector east
BC550 sits flat side north
2N3904 sits flat side south
S9014 sits flat side south
BC549 sits flat side north
ZTX450 sits flat side south
I'm yet to calculate the output impedance of these (somehow)
tr8/tr9 BC184 BC547 Amazon
S9014 Amazon
BC237B CPC SC09155
2N3904 Amazon
2N3904 CPC SC15978
Composite/video colour balance
tr8 collector south
tr9 collector north
BC547 face each other
S9014 face away from each other
BC237B face each other
2N3904 face away from each other
Higher power transistors seem to make red fuzzy and bleed
d15 BA157   1N4004
CPC SC17788
d17 BA167   1N4004
CPC SC17788
d19 BZY88   5.1V Zener RS 544-3597    
PWR     DC Jack Socket 5A RS 805-1699    
REG 7805   Switching Regulator 5V DC
RS 193-4004 or
RS 212-4689
Get rid of that heat & heatsink
EAR/MIC     3.5mm Mono Audio Jack
Cliff CL13843G CPC AV28092 (Black)
There's one extra ground pin that can just be tucked under.
There's different colours available
CPC SN36966
Issue 3 onwards only. Unconfirmed as yet. The impedance of this suggestion is 32ohms. Argue amongst yourselves. :-)
I'm still looking into issue 2. The speaker on these boards is 200ohms, which matches the 5C112. The output impedance of a 6C001 appears to be 260ohms.

Replacement Transistor for TR3

I'm constantly redoing this section, it's daft. The first table will be with keeping R25 as 180R as per the schematic.

These are values based on my observations with transistors found to be appropriate for tr3. I'm trying for standard values, honest.

Table with R25 remaining at 180R:

Model Orientation R25 R73 Notes
MPS2369A Flat side south 180R 1K As schematic
There's something funny about this.
2N4401 Flat side south 180R 1K Different makes seem to vary, needs more testing

The following presumably need more power to do what we need, and so R25 is being reduced.
I'm going to end up doing these all again, but this is what I have so far.
There doesn't seem to be an absolute requirement of having a centred clock, it's just if things are being going to be change - might as well try to get a good balance.
For some of these a 1/4W resistor for R25 is going to get warm. Make sure it's a good one, or up rate it.
Not really 100% sure this is a great way of doing things, but if you can't do the above, here were go:

Model Orientation R25 R73 Notes
2N2222 Flat side south 47R 47K on 6C001E
5.6K on 5C112
3.3K on vLA82
S9013 Flat side south 47R Remove on 6C001E
7.6K on 5C112
5.1K on vLA82
BC337 Flat side north 39R 22K on 6C001E
5.6K on 5C112
3.3K on vLA82
2N2219 Pointing thing north west 47R Remove They look cool.

Components from Amazon UK

TBH you can probably get most of these from anywhere, Amazon is just an example.

Transistor Kit 1


tr1/tr2 - BC549
tr3 - BC337 (see notes above)
tr4/5 - BC337/BC327
tr6 - BC337
tr7 - BC550
tr8/tr9 - BC547

Transistor Kit 2


Free wire!

tr1/2 - 2N3904
tr3 - 2N2222 (see notes above)
tr4/5 - S8050/S8550
tr6 - 2N3904, BC337
tr7 - 2N3904
tr8/9 - 2N3904

Transistor Kit 3


tr1/2- S9014
tr3 - S9013 (see notes above)
tr4/5 - S9013/S9012
tr6 - S9014
tr7 - S9014
tr8/9 - S9014

Spider Mod (tr6) on an Issue 2

Where do you put the spider mod on an issue 2 after you've socketed the Z80? Answers by email. By ic4? I can understand not wanting to drill holes and doing everything component side. Not straight forward when there are sockets there.

This is a 2n3904. Collector to ic3 pin 14 (5V). Base to ic4 pin 6 (A0). Emitter to ULA pin 33 (IOREQ).

Other Spectrum Components and Notes


Mod for compositeless (totally) RGBtoHDMI YUV changes - 1K for Y to ground - 1K9 to U to 5V (right of C68, C69 are good), 470R over R56 (U to 0) and 1K over R63.

AliExpress Spectrum Parts

Some luck has been had for parts from AliExpress for the ZX Spectrum. These links will probably stop working.

4116 RAM


Keyboard Connectors

RAM Tester

My RAM tester I used to test my ZX Spectrum RAM chips is here. Credits to Noel Llopis of Noel's Retro Lab and PCB Geeks.com for the board I've very slightly repurposed. Please see the header for more information.

ICL OPD / Merlin Tonto

To follow, just notes right now....

PSU & Monitor Adapters

Started with an ATX PSU with an 7905 to take -12v to -5v to get things going. That was a bit big. Latest version uses a buck converter and a 7805 (or substitute)7 with a standard 12V power brick, much simpler.
Using a standard monitor just takes inverting R, G, B and Vertical Sync. A 74LS04 is fine for this. I'll do something in paint whilst I work out KiCAD (?).