ZX Spectrum Issue 1 and 2 boards
Extra resistor near the CPU.
This is R73, a 1kΩ resistor connected between +5V and ULA pin 32 / R24 / C67 (D14).
From the service manual:
service manual wrote:This modification is required for efficient operation of certain machine code software and should be implemented on all units. It has been implemented in manufacture for all ULAs 5C112-2 and later.
D14 replaced by C67 (100pF)
R24 changed from 3k3 to 1k
R27 changed from 680R to 470R
R73 (1k) added between IC1/32 and +5V
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
I picked up some 100nF (I think) capacitors like that from a surplus supplier (Greenweld) many years ago.
Anyhoo, the machine should still work with it missing. I do however recommend you fit a modern 47nF or 100nF multilayer ceramic as a replacement.
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
And it doesn’t hurt to combine several capacitors of different values in parallel, say a 47nF and a 1µF one. Each capacitor will operate at a different frequency, because their efficiency is very frequency dependent. In some cases, however, it’s mostly gilding the lily.
Every man should plant a tree, build a house, and write a ZX Spectrum game.
Morpheus wrote: ↑Sat Dec 28, 2019 6:56 pm
Did anyone ever fit the 220nf capacitors on the lower RAM to help with the “jailbars” ?
On a 16K/48K/+ replace C5 to C8 with 470nF or 1uF multilayer ceramic capacitors
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Hello people, I know what broke the ZX spectrum. The power supply i mentioned earlier should be outputting 9 volts. I took out my multimeter to test the voltage. And guess what? THAT POWER SUPPLY WAS ALREADY OUTPUTTING SOLID 14 VOLTS!
Slauri wrote: ↑Tue Jan 21, 2020 6:07 am
The power supply i mentioned earlier should be outputting 9 volts. I took out my multimeter to test the voltage. And guess what? THAT POWER SUPPLY WAS ALREADY OUTPUTTING SOLID 14 VOLTS!
If it’s an unregulated power supply, it’s normal for it to show a higher voltage with no load. Even if it is regulated and outputs 14V under the rated load, that alone shouldn’t have killed your Speccy. The 7805 voltage regulator inside the Spectrum can easily handle input voltages ranging from about 7–8V to 36V. So, no, a 14V input DC voltage wasn’t the primary cause of your computer’s fault.
Every man should plant a tree, build a house, and write a ZX Spectrum game.
A 48K ZX Spectrum draws between 600mA and 700mA (with no expansions connected). 700mA is half the rated capacity of the Sinclair ZX Spectrum 9V 1,4A PSU.
The output of the PSU is required not to drop below 9V at the full load current of 1.4A. The output voltage is not specified at other output current levels.
When connected to a ZX Spectrum, the PSU output voltage is usually between 10V and 12V. You can measure this on the left hand leg / pin of the 7805 voltage regulator chip. Use the metal heatsink tab as the 0V/GND/ground point for your meter.
If the PSU you have been using has similar characteristics, then it is unlikely to have damaged your Spectrum. However, issue 2 boards really don’t like an input voltage of 11.9V or greater (unless they have had their on board DC/DC converter circuitry upgraded).
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
The power supply might actually be the culprit, because the picture is much different after swapping out the entire lower ram and the cpu. It still shows garbage but is it normal for the ULA to heat up in the center in under 30 seconds?
Slauri wrote: ↑Thu Jan 23, 2020 12:15 pm
Is there any way to test the ula?
Sure, in a known-good machine. Keep in mind that ULAs are usually only backward compatible, so a later-issue ULA will work in all the previous Spectrum models, but that might not be the case the other way around.
Every man should plant a tree, build a house, and write a ZX Spectrum game.
If the machine is generating a video output /even if it’s full of different coloured squares and other ‘garbage’), that’s most of the ULA working (the main function of the ULA is to generate the video display, produce the CPU clock, deal with the keyboard and interface to the speaker ear and mic sockets.
If the area of the ‘lower’ RAM (4116 or equivalent) contains garbage data, the ULA does not care, it will read and then display this ‘information’. It’s the code running from the ROM that clears the memory and sets up black text on a white background.
Mark
Standby alert
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
Standby alert 
