Spectrum modded / composite question

[redgatemodels]

But without a test/diagnostic ROM or another ULA to swap with, it's hard to be certain.

Mark

I use one of these: https://www.ebay.co.uk/itm/325662918573

I may be able to help you out with a 5C112E-3 ULA, but I'd get it diagnosed first. It may be though that the diag card will not work without a working ULA.....

[Elfish]

If you hear the key click sounds and BASIC appears to be running (try SAVE "P" ), then it's more likely to be a ULA fault rather than a DRAM fault. But without a test/diagnostic ROM or another ULA to swap with, it's hard to be certain.

Mark

Hopefully it's the RAM. ULAs seem harder to come by.
I certainly don't have a spare one.
At least I can't find them on eBay

I guess there's only one way to find out... desolder all the RAM.
I'm going to build this DRAM tester before buying new chips.

https://forum.defence-force.org/viewtopic.php?t=1699

But without a test/diagnostic ROM or another ULA to swap with, it's hard to be certain.

Mark

I use one of these: https://www.ebay.co.uk/itm/325662918573

I may be able to help you out with a 5C112E-3 ULA, but I'd get it diagnosed first. It may be though that the diag card will not work without a working ULA.....

From my understanding I'd need a clear video output in order to even see something.

[1024MAK]

ULAs seem harder to come by.

Original 'spare' ULA chips ran out years ago. Only used ULAs come up occasionally.
However, there are replacement ULA modules. But they are in a limited supply, and are not cheap.
Nebula - Scroll down on this page.
vLA82 - link

From my understanding I'd need a clear video output in order to even see something.

The modern diagnostic programs use sound and the border as well as the screen to indicate progress and faults. There are also more expensive diagnostic boards that have eight LEDs (or more) that also show the progress and faults.


DRAM faults are more common, but ULA chips do occasionally fail.

Mark

[Elfish]

DRAM faults are more common, but ULA chips do occasionally fail.

I'll try that first. Desoldering was easy using a heat gun. I never did that before. I'm quiet amazed lol.
Sockets are already laying on my bench. So fixing stuff like that in the future will be easier.

In case one or more of the chips are bad, which chips/manufacturers should I look for as replacements?

Is it ok to mix manufacturers or models or is it more recommended to use 8 identical chips?

P.S.:
Seems like I can finally post without the need to have my posts approved. Wooohooo!

[1024MAK]

These days, it's whatever you can get hold of that work. And yes, you can mix and match. Sinclair used various makes/brands of 4116 DRAM or equivalents.

Alternately, use 4164 type and modify the board to suit these.

Whatever you use, they do need to have an access time of 150ns or faster. Slower parts don't normally work correctly.

Mark

[Elfish]

The missing parts for the DIY DRAM tester came today.
Only one chip didn't pass the tests. Time to get a replacement for it.

I assume it's not possible to run the machine with just 7 chips to check if there are more broken parts?

[1024MAK]

Well, with only seven 4116 DRAM chips fitted, it will do something, but no, it will not run BASIC.

The ULA should however produce a picture. What you get depends on which chip is missing. The effect being different depending on which one it is that is missing.

Mark

[Elfish]

I recieved the DRAM 4116 replacement chips today.
At least they tested fine. I installed one of the them to have a full set of 8 chips.

It still doesn't work. How ever the screen output looks different now.

I let it run for a couple minutes and the only chip that got warmer and warmer was the ULA.
I turned it off before I wasn't able to touch it anymore.

The voltages on the ULA's pin 13 was 2.88V and on pin 14 4.9V
According to this pinout pin 13 and 14 are supposed to have 5V.

Can anyone point me in the right direction?

[1024MAK]

Some important things you need to know about the type of Ferranti ULA chips used in Sinclair computers:

They use a type of bipolar transistor technology. This is very different to the other large chips used in the ZX Spectrum.

The matrix cells (that provide the logic functions) are CML (current mode logic) and operate at only 0.95V. These are supplied via an internal on chip series pass voltage regulator that outputs 0.95V to the matrix cells.

The ULA chips used by Sinclair in the ZX Spectrum use two independent power pins. Pin 13 feeds the internal series pass voltage regulator that outputs 0.95V to the matrix cells. Pin 14 supplies the peripheral cells with a 5V supply.

Pin 13 is fed from the ZX Spectrum +5V supply via resistor R34 (15 Ohms, a ½ or 1 Watt type). Using a resistor in this way drops the voltage down to pin 13, which helps reduce the amount of heat produced by the chip. Hence it's normal for it to be around 2.8V to 3.2V.

The power consumption per matrix cell is 0.95mW per gate. The power consumption per peripheral cell (that interfaces to the outside world) is 11mW for a 5V supply. Hence if 300 matrix cells are used and 40 peripheral cells are used, that's 725mW. The 5000 series ULA is thought to use over 400 matrix cells.

Combined with the heat from the internal series pass voltage regulator, this means that ULA chips always runs hot. Ferranti specify them up to an internal silicon temperature of 125°C.

Mark

[Elfish]

Thank you for the very detailed info and explanation!

Voltages seem to be ok then and temperature too.

I also measured the DRAM Voltages. They (-5V, +5V, +12V) are okay. Can one assume that the power supply is working as intended?

Current output