ZX48 Issue 2 Ula unsocketing -> blue border and paper, no text

[tridy]

Hi.

I need some help with the diagnostics of the Issue 2 ZX Spectrum 48K.
After replacing the voltage regulator with 78S05, and swapping the capacitors, I verified that it is working, and everything looked OK.

After that, I decided to add the heatsink to the Ula. Ula was socketed, so there was not enough space for the heatsink. So, I removed the socket and soldered Ula directly to the board. I expected the contacts with the magnifying glass that there are no bridges and it looks good.

I have another working 48K following the ZX Spectrum (48K & 16K) Initial Tests video, I compared the values from the voltage regulator, resistance, and then voltage on RAM 13-6 ICs, TR4, and TR5 resistance, and the only thing that is different is that:

- the voltage regulator on the left pin shows 65.9KΩ, and the suggestion is that it should not be lower than 100KΩ.

When I start it with the Composite out, I get the blue screen, blue border, and no text on the screen.

When I run ZX Diag, It does not show the data lines working:

Youtube Video

(here is the comparison to what it should do and how it works on a working 48K)


How do I go further with diagnostics? Is it that I have overheated Ula when I was soldering it? Should I desolder this Ula and try in a working ZX48 or can I test it with the multimeter further?

The questions:
- should I replace the voltage regulator 78S05
- is there a way of testing Ula while it is on the board
- what kind of further diagnostics can I do to figure out the blue paper and blue border with no text?

Thanks!

[1024MAK]

- the voltage regulator on the left pin shows 65.9KΩ, and the suggestion is that it should not be lower than 100KΩ.

By left pin, do you mean the input pin (see picture)?

If yes, then that is nothing to worry about. Testing circuitry that has semiconductors in it on the resistance range of a multimeter may give different results as there are countless different models of multimeter, some of which give slightly different results.

When I start it with the Composite out, I get the blue screen, blue border, and no text on the screen.

That is not good news.

The data lines will only show activity if the system is using the data bus. If there is no data bus activity, it may be that the Z80A is not running. The Z80A gets it’s clock signal from the ULA.

How do I go further with diagnostics? Is it that I have overheated Ula when I was soldering it? Should I desolder this Ula and try in a working ZX48 or can I test it with the multimeter further?

The questions:
- should I replace the voltage regulator 78S05
- is there a way of testing Ula while it is on the board
- what kind of further diagnostics can I do to figure out the blue paper and blue border with no text?

If the +5V rail is within specification (4.75V to 5.25V) then there is no need to replace the voltage regulator.

It’s possible, but unlikely that you have overheated the ULA. How quickly did you solder each pin? I don’t recommend desoldering it at this stage.

Before doing anything drastic, the first thing to do, is very carefully examine all the pads, tracks and solder joints for the ULA. Look for solder splashes (which can be strands of solder as thin as a human hair), poor solder joints, and broken tracks. Use a good light and a magnifying glass. If you don’t find anything, use the resistance range to test each pin to the adjacent pins, and also test to the +5V rail and to the 0V/GND rail.

How did you remove the socket? Is there a possibility that one or more of the plated through holes was damaged? If yes, then you need to use a copy of the schematic and prove that each ULA pin connects to all the other nodes (chips, resistors, diodes or transistors) as shown on the schematic. Use the continuity range on your multimeter.

If none of the above leads you to the problem, then fault finding gets more difficult without more test gear.

Incidentally, for the benefit of others reading this topic, I’m definitely NOT in favour of fitting heatsinks to chips (including the ULA) unless they were originally intended to have heatsinks fitted. The ULA has an operating temperature range up to 125°C. It’s actually designed to run hot. And the inside of a ZX Spectrum never gets anywhere near 125°C.

The most common cause of ULA faults is edge-connector abuse.

Mark

[tridy]

Thanks for the reply!

How did you remove the socket? Is there a possibility that one or more of the plated through holes was damaged? If yes, then you need to use a copy of the schematic and prove that each ULA pin connects to all the other nodes (chips, resistors, diodes or transistors) as shown on the schematic. Use the continuity range on your multimeter.

I desoldered it from the bottoms, then used the special U shape puller to carefully pull it out. Some pins from the socket stayed on the board, so I had to remove them manually.

I will try to ping the nodes connections with the multimeter continuity range.

I am learning about the schematics and here is something that I cannot figure out:

Connected to the power, I get 5V on pin 20, not on pin 40. What am I doing wrong?

Thanks!

[1024MAK]

The pin-out for the ULA you have there is my diagram for the ULA from a ZX81, not a ZX Spectrum!

The original upload of the ZX81 ULA is here.

You want the “5C and 6C ULAs” diagram here.

There is a copy of the issue two schematic here. Unfortunately, like most of the ZX Spectrum schematics, there are numerous errors and mistakes.

Pin 20 on the ULA, is one of the keyboard input lines. Each of these has a resistor to pull it up to +5V if no keys are pressed. Pin 40 on the ULA is 0V/GND

Mark

[tridy]

Thanks, Mark!

The first thing I tried was to measure the voltage on pin 13 and pin 14.
pin 13 gives 3V
pin 14 gives 5V

the working ZX gives 3V on pin 13 as well, so I assume it is OK, even if the doc says it should be 5V.

I will start with trace pinging, document it in a spreadsheet, compare with the working ZX and then report it here.

[1024MAK]

The first thing I tried was to measure the voltage on pin 13 and pin 14.
pin 13 gives 3V
pin 14 gives 5V

the working ZX gives 3V on pin 13 as well, so I assume it is OK, even if the doc says it should be 5V.

The supply to pin 13 goes via a resistor to reduce the supply voltage. Inside the chip, this goes to an internal voltage regulator circuit, which in turn supplies the logic circuitry. Hence around 3V is normal.

Mark

[tridy]

The supply to pin 13 goes via a resistor to reduce the supply voltage. Inside the chip, this goes to an internal voltage regulator circuit, which in turn supplies the logic circuitry. Hence around 3V is normal.

Thanks, got it the R34

I have tested the neighbor pins for shorts and did not find any.
I checked the following traces, and all of them sounded good:

What would your suggestion to be for the next step?
I have a working 48K with socketed Ula, I can try to desolder the non-working one and then try it there.

There is one thing that I have found. when I slightly press on the board, it clicks. I have not figured it out what it is that clicks, but I removed RAM modules from sockets and trying to hear and spot where it comes from:

https://youtu.be/EtBvkGpJQcw

it could be related to the problem, I think.

[tridy]

I have socketed ULA back
Tested ULA another working 48K, and it was OK.
Then I put it back into non-working 48K and this time got black paper white border. But, I could run ZX-Diag properly this time.

then I replaced IC15 with the working one and it works ok now. tried a game, all looks good! SOLVED

the clicking of the board from the previous post was from several sockets contacts producing it, so no big deal there.

thanks for the tips!