Struggling to resurrect my +2 :-(

[zxplusplus]

Hi All

Firstly, apologies for my first post involving me asking for help!

I'm trying to resurrect my grey issue 3 +2

The first thing I did was order a new psu from the tfw8b, I also ordered an RGB scart lead.

However, I'm struggling to get it to fire up - please see attached image.

Things I've tried / tested:-

Based on some other threads on this forum I checked voltages at the following points:-

Voltage Regulator - 5.02v
Pin 8 Ram - 5.01v on all chips
CPU pin 11 - 5.01v
D20 - 12.10v
D22 -5.27v

I've also socketed the ram and tried another set, albeit working MB8264a chips from my 65xe (which I understand are compatible?) but this didn't help.

I've also tried a known working z80 from my 48k model.. the image changes each time I press reset, the horizontal bars switch to a solid colour when the reset is held down.

At this point, I'm running out of ideas and would really appreciate any input from those more experienced than me

Thanks in advance

[Ast A. Moore]

Remove the ROM and the RAM chips and retest. Then replace the ROM IC and retest.

[zxplusplus]

Thanks

I've just tried that, no difference.. is it worth replacing the ROM socket?

Also, I don't have a spare rom and couldn't find them for sale?

[Ast A. Moore]

I don't have a spare rom and couldn't find them for sale?

If you have an EPROM programmer, you can make your own. You can find the appropriate ROM dumps here. You’ll nee a 27C256 EPROM or an SST 27SF256 EEPROM. With a programmer, you can also dump the contents of the original ROM and compare it to the correct dump from the link above. If they match, then your ROM is likely okay.

[zxplusplus]

Sadly I don't have a programmer, although I'd be tempted to pick one up if the process of writing a new rom is straightforward? it's something I've never done before..

Amazon sell a LAQIYA TL866Ⅱ programmer, is this what I'd need?

would you say that the behavior I'm seeing is typical of a bad rom?

[Ast A. Moore]

if the process of writing a new rom is straightforward? it's something I've never done before..

Yes, it’s pretty straightforward, if you know what you’re doing. There are plenty of information online, so feel free to go through it before attempting your first ROM flash.

Amazon sell a LAQIYA TL866Ⅱ programmer, is this what I'd need?

Haven’t used this particular model, but for the most part, they all pretty much do their job.

would you say that the behavior I'm seeing is typical of a bad rom?

128K machines are notoriously hard to diagnose based on the TV picture alone. The startup ROM subroutines are much more involved than on a 48K machine, so a lot of things are not obvious. A good set of diagnostic tools is a must, and a programmer is good for testing the ROM. (If everything checks out, you don’t even have to flash a new one.)

In the meantime, use the schematic and inspect the data and address lines throughout the PCB. Use a magnifying glass and check for trace cracks, cold solder joints, or shorts. Use a digital tester in continuity mode and buzz out the traces. Cleaning the connectors of all IC sockets is also a good idea.

If you have an oscilloscope, it’s a good idea to test the CPU clock as well as any activity on the data/address lines.

There are three custom ICs in the +2: the ULA, the Sinclair Joystick buffer/controller, and the PCF1306P glue logic chip (IC10). If the latter is faulty, finding a replacement will not be possible, save for cannibalizing another +2/128K board.

[zxplusplus]

Thanks for the comprehensive reply

I've just ordered the EPROM programmer and a couple of blank chips, if nothing else I'll learn how to do something new

I've visually inspected the board but appreciate that this is far from scientific! It looks clean and tidy but I've read that they're not the best quality!

There's someone on eBay selling replacements for the PCF1306 chips, perhaps that's the next thing to try if the new ROM doesn't do the trick..

[1024MAK]

Before you start buying more bits, a few words of advice.

With a computer, nearly all circuitry that connects directly, or indirectly to the microprocessor is essential to the normal operation. A single poor connection or a single faulty circuit or component can prevent it (the computer) from working.

At this stage, we can’t eliminate anything yet, let alone tell you that replacing part X will fix it.

So before jumping ahead on a wild goose chase, it’s best to start with the basics.
You have already tested the supply voltages.

Next, it would be wise to do some further tests. But before then, it would help up if you can list any test gear that you have, or have access to.
So for example, do you have a logic probe?
A logic analyser?
An oscilloscope?
A frequency counter?

In terms of the reliability of the components, after power supply system faults, the most common is one or more faulty DRAM chips. EPROMs do sometimes develop corrupt contents, but mask ROM chips are normally very reliable. If the ULA can display a video picture, then most of the time, that indicates that most, if not all of it is good.

What speed rating are the MB8264a chips?

And are you experienced at desoldering chips? In other words, are you 100% sure that no tracks were lifted and no plated through holes were lost when you desoldered the existing DRAM chips. If you are not sure, please test each pin of each chip to confirm that all the circuitry is complete and present by continuity testing the address, control and data lines to the DRAM chips.

Rather than programming a EPROM or EEPROM with a copy of the ROM, I think you would be better off programming a test ROM. If the Z80 is running and there are no bus faults, this can then provide an indication of what the problem may be.

But first, we need to see what the Z80 is doing... hence the need for suitable test gear...

Mark

[zxplusplus]

Thanks 1024MAK

To answer your questions:-

Equipment - I've got a multimeter but that's it at the moment

The MB8264 chips are 150ns, they were removed from a working system, I could refit them in the Atari and run a memory test just to make sure they're ok.

I've got both a temperature controlled soldering station and also a desoldering station, the old chips came out without any excessive persuasion from me. The solder pads were unharmed and re-soldering to them was quick and easy. This is the one area I'd say I'm proficient in, I can fully desolder a chip in a minute or two without any force needed using the desoldering station.

Could you recommend a test rom? I've ordered several blank eproms so can use one as a test rom.

I've swapped the z80 with the one from my fully working 48k speccy, it's only a month or two old as the original developed a problem with external devices and had to be replaced. I'm open to buying more test equipment though, I quite enjoy this sort of thing, I'm not as advanced as you guys when it comes to repairing them but enjoy learning

Thanks again for your help

[zxplusplus]

quick update on this..

The programmer arrived today and I've saved a copy of the data on my rom chip.

My plan was to compare this to a stock rom file to see if the data on mine looks corrupt but i'm confused, the dump of my rom is 32kb but all of the versions i'm finding on the web are only 16kb.

As such, when I compare my rom to the one downloaded from the web, the first half is identical, but there's nothing to compare the second half to.

I've read that the chip has two halves, one for 128k mode and one for 48k mode, does anyone have a complete 32kb rom file I could use for comparison?

thanks

[Ast A. Moore]

does anyone have a complete 32kb rom file I could use for comparison?

You, most likely, have both 16K ROM images. Any Spectrum emulator (that emulates the +2) will have the appropriate ROM images. Fuse is one of them. The files are called plus2-0.rom and plus2-1.rom.

[zxplusplus]

got it..

one half matches the plus2-0.rom, the other half matches plus2-1.rom

so it looks like my ROM chip is ok..

I'll go ahead and burn the ZX-Diag .bin onto a chip and see if that provides any insights.

[1024MAK]

A suitable test ROM:
https://github.com/brendanalford/zx-dia ... i/Firmware

https://github.com/brendanalford/zx-dia ... s/releases

viewtopic.php?f=22&t=1064&p=15158#p15158

Note that as the test ROM (file “testrom.bin”, a plain binary file) is only 16K bytes long, I strongly suggest you either double it up (make a file comprising of one testrom.bin combined with another copy of testrom.bin making the file a total of 32K bytes long) using your PC before downloading it to your programmer, or download to 0x0000 on your programmer, then copy the block 0x0000 - 0x3FFF to 0x4000. Then you don’t have to worry about getting the test ROM image in the correct part of the EPROM chip (as it will be in both ‘half’s’).

Mark

[1024MAK]

The programmer arrived today and I've saved a copy of the data on my rom chip.

My plan was to compare this to a stock rom file to see if the data on mine looks corrupt but i'm confused, the dump of my rom is 32kb but all of the versions i'm finding on the web are only 16kb.

As such, when I compare my rom to the one downloaded from the web, the first half is identical, but there's nothing to compare the second half to.

The original ZX Spectrum only had a 16K byte ROM. Hence the space in the Z80 memory map for ROMs is only 16K bytes.
However, the 128K machines use either one (or for the later machines) two 32K byte ROM chips. Each 32K byte ROM chip contains TWO 16K byte ROM images.

All the official ROM versions are here.

Mark

[zxplusplus]

Thanks both..

I've just had a go at making a 32kb testrom file as detailed by 1024mak

please see - https://we.tl/t-3TU0vB6VK6

Does this look correct?

[1024MAK]

Yes, that looks fine

Mark

[zxplusplus]

Thanks Mark

I've just baked and ran the rom.

Please see - https://we.tl/t-r9eqxeUZPp

I did manage to get it to run once and actually say "ram fail" on the screen briefly, it looks like all of the ram is failing? but I can test both sets of ram in the Atari and it's fine on that and will run a ram test for an hour plus without any issues.

Does this help with diagnosis at all? what would cause all of the ram to fail once in the Spectrum?

[Ast A. Moore]

what would cause all of the ram to fail once in the Spectrum?

This could mean one of the following:

1. The RAM ICs are okay, but the wrong time (timing)
2. The RAM ICs are okay, but the CPU can’t access them properly.

It is thus possible that the problem lies somewhere in the data/address lines. In which case, it is likely that the CPU likely can’t communicate properly with the ROM (although, unlike the RAM ICs, the ROM is connected to the CPU directly). That is not to say that the PCF1306P, if faulty, couldn’t wreak havoc on the same address lines.

Check IC15 (an LS04 hex inverter). Check and recheck all the pertinent PCB traces for all the data and address lines.

Not sure what else we can do without an oscilloscope.

[zxplusplus]

Thanks

Since posting the video the symptoms worsened, I'm not getting the initial beeps anymore, just a screeching sound and psychedelic bars on the border!

I've ordered a brand new CPU just incase the one I'm using has been on its last legs.

I've also ordered the ls04 chip you mentioned as these are cheap.

Beyond that, I have to admit that the repair services offered by people like mutant caterpillar are starting to look like good value!

I'd like to learn more but think it would help to have a good working system initially so that I can take readings off that and understand normal behaviour before trying to diagnose faults on a broken system.

[Ast A. Moore]

I have to admit that the repair services offered by people like mutant caterpillar are starting to look like good value!

Uh, he’s a good guy, I don’t have anything against him personally, but methinks snatching another +2 off eBay would look like an even better value. You get to keep your old machine and can use a your known-good specimen for testing (provided it works, that is).

[1024MAK]

Since posting the video the symptoms worsened, I'm not getting the initial beeps anymore, just a screeching sound and psychedelic bars on the border!

That sounds like the ROM code is failing it’s self check. On start-up it does a checksum of the whole 16K bytes of the ROM. If this checksum test fails, it just sends a count to the ULA register that controls the border colour and the beeper sound.

If this is what is happening and it’s not a problem with the EPROM or the CPU, then you have either an address, control or possibly a data bus problem.

Mark

[1024MAK]

The other thought that has gone through my mind, have you tested the +5V supply AND the 0V/GND for the important chips. By this, I mean, put the red meter probe on the +5V/VCC pin AND at the same time put the black meter probe on the 0V/GND pin of the same chip. The idea is to confirm that both the +5V power network AND the 0V/GND network are both intact. But be careful that neither probe slips...

With the large range of results, I strongly suspect a cracked PCB track, or a dry joint somewhere is helping with the cause of at least some of your troubles.

I had not tried the current release of the test ROM that I pointed you to (but had used an earlier version). So I gave the latest version a run on a Harlequin board I had to hand. That worked fine. But when I reinstated the normal EPROM chip in it, which contains the ZX Spectrum BASIC code, it got to the white screen, but instead of displaying the Sinclair copyright message, the characters were black squares! But sometimes, I got a few characters displaying properly. Other times it would not start up at all. And every time, it crashed shortly afterwards.

After looking for a bent pin, or dry joint, I found a wire that had not been properly soldered. The intermittent connection caused by the wire just touching, or not touching the plated through PCB hole caused a very confusing array of symptoms.

Mark

[zxplusplus]

Thanks Mark

I'm experiencing similar symptoms to those you've described, one boot it'll beep and get as far as the menu, another boot I'll just get random colours on the border, if I then remove the rom and cpu to reseat them I might get it back to having the 8 beeps and maybe the "ram fail" message. There's just no consistency to it which is making it far harder to understand.. the ROM is still using the original socket, but I did run along all the solder points and reflow them just incase there was a dry joint but this didn't seem to help.

The new CPU should arrive today, so I'll see what that does.

I'll also test for a functioning ground on each of the chips as you've described.

[zxplusplus]

just another mini update on this..

New CPU arrived, I put it straight in the +2 and fired it up, diagnostic rom loaded fine, I was even able to go through all of tests, ram passed, ULA passed..

And just as I was getting my hopes up it started playing up, ten minutes after fitting the new CPU and it started breaking down again, now I'm back to square one!

It seems to be eating CPU's, it killed the one from my 48k machine and looks like it's broken another brand new CPU in ten minutes or so.

What I don't understand is that the voltage readings were fine, and the CPU's not even getting hot!

I'll check to make sure there's a working negative on all the chips next..

[zxplusplus]

sorry, just another mini update

I've just been testing some of the voltages on the chips

11 + 29 on the CPU = 4.9v

8 + 16 on the ram = 4.9v

But.. when I initially tested the ram I tested the wrong pins, I used 8+9 and got a consistent 4.9v on the inner bank but only 4.14v on the bank closest to the edge of the pcb, I'm not sure if this is relevant but thought I'd mention it