Repairing ZX Spectrum issue two

[1024MAK]

1. The computer is unstable. After a while it freezes or resets. At first the problems were very consistent, with thick purple vertical lines, so I stopped them by replacing the bit 0 lower memory DRAM memory. Now the problems occur much less frequently and more inconsistently, so it is difficult to identify which lower DRAM memory as the culprit.

Yes, likely to be a DRAM fault.

• Sometimes R Tape loading error appears. I assume this is related to the tape drive. I suspect that the drive belt is worn and have already ordered a new one that is a bit tighter.

To be fair, almost any problem with the loading process can result in either this error, what appears to be a reset, a crash (locks up/non responsive) or dumping you back in BASIC. The cause can be marginal volume, wrong or inconsistent speed, dirty head, mis-aligned head, worn tape, etc.

• The composite image is not perfect - there are "ghost letters" around the actual letters. Is this normal or can it be improved?

I assume you are using a LCD TV? This is typical on some LCD TVs. Look in the user controls menu/settings, see if there are any noise reduction or picture enhancement settings that can be turned off.

Since it's inefficient to just build a board for the lower DRAM replacement, I am thinking of building other DIY accessories for the ZX Spectrum as well. Do you have any good ideas? For example, I found this tutorial https://github.com/konkotgit/ZX-external-ROM for a DIY ZX diagnostic tool, but there is no information on how to program the ROM. Does anyone know of any other good tutorials for DIY ZX Spectrum accessories?

The project you referenced uses a EPROM. This is a general purpose EPROM (ROM) card, it can use many of the small number of ROM images that are available. EPROM chips have to be programmed (often referred to as “burned”) by a programmer. They cannot be programmed/burned (written to) by a normal microprocessor system. The programming voltage is higher (typically 12V, 12.5V or if an older chip, 21V) and the write time is a lot longer that the write time of a microprocessor. Also, if they don’t have a window on the top of the chip, they cannot be erased (this type is also known as OTP - One Time Programmable). If the chip does have a window, they can be erased with a UV light source of the correct wavelength, normally described as an EPROM eraser.

There are in fact lots of DIY projects for the ZX Spectrum, from back when it was first popular through to now. Including recreating some commercial interfaces and expansions. From joystick interfaces through to hard disk interfaces.

However, finding these does require either going through the magazines of the past, or lots of searching of the web and web sites.
There are some on World of Spectrum forums (which is not indexed by search engines and the forum search is not the best). https://worldofspectrum.org/

Some of the current day diagnostic cards are based on this 2007 project https://www.alioth.net/Projects/Spectrum-Diag/
See also https://github.com/brendanalford/zx-diagnostics/wiki
The EEPROM in this project can be programmed by the ZX Spectrum, however, this chip was discontinued. A different manufacturer did produce a compatible chip, but I don’t know of it’s current availability.

You may want to ask this last question about what DIY projects people know about on the Spectrum Computing Forum as well.

What are you looking for?

Mark

[redgatemodels]

• Sometimes R Tape loading error appears. I assume this is related to the tape drive. I suspect that the drive belt is worn and have already ordered a new one that is a bit tighter.

To be fair, almost any problem with the loading process can result in either this error, what appears to be a reset, a crash (locks up/non responsive) or dumping you back in BASIC. The cause can be marginal volume, wrong or inconsistent speed, dirty head, mis-aligned head, worn tape, etc.

Mark

Indeed, if you have a smartphone why not try one of the tape loader apps to eliminate problems with the tape player? I use PlayZX or ZX Tape Player on Android. One the these cables allows the stereo jack output of the phone to connect to the mono "MIC" lead into the Spectrum

https://www.sinclairzxworld.com/viewtop ... 776#p52776

[Pygmalion]

First I would like to report on the power supply upgrade, which was a mixed success.

I had no trouble removing the old diodes and capacitor, but I had a big problem removing the transformer as it was soldered by two HUGE blobs. The copper traces there have retracted from the board; I have already ordered brass rivets that I will use to repair them.

I replaced the diodes with new 3A diodes and the capacitor with a new 15nF 16V 0.02Ω 105°C capacitor. The old 4.7mF 0.04Ω capacitor was actually fine, proving once again that old electrolytic capacitors are usually fine. (ESR was measured using my signal generator and oscilloscope.)

And what is the electrical consequence? Well, rather meagre. The oscillation of the 9V line dropped from 270mV to 210mV, while the oscillation of the 12V line dropped from 270mV to 240mV. Is this oscillation normal or should I look for the fault in the 12V line?

I still think it was worth the effort. I have a feeling that one of the old diodes was unstable and that this was causing occasional problems with ZX Spectrum. I ran ZX Spectrum for about five hours after the update without an incident. (Apart from the loose 9V socket, which is next on the list for replacement.)

[Pygmalion]

• The composite image is not perfect - there are "ghost letters" around the actual letters. Is this normal or can it be improved?

I assume you are using a LCD TV? This is typical on some LCD TVs. Look in the user controls menu/settings, see if there are any noise reduction or picture enhancement settings that can be turned off.

Yes I have LCD TV and yes the picture can be improved marginally by changing TV settings. But the picture is never crisp. I guess this is the best one can get.

Since it's inefficient to just build a board for the lower DRAM replacement, I am thinking of building other DIY accessories for the ZX Spectrum as well. Do you have any good ideas? For example, I found this tutorial https://github.com/konkotgit/ZX-external-ROM for a DIY ZX diagnostic tool, but there is no information on how to program the ROM. Does anyone know of any other good tutorials for DIY ZX Spectrum accessories?

The project you referenced uses a EPROM. This is a general purpose EPROM (ROM) card, it can use many of the small number of ROM images that are available. EPROM chips have to be programmed (often referred to as “burned”) by a programmer. They cannot be programmed/burned (written to) by a normal microprocessor system. The programming voltage is higher (typically 12V, 12.5V or if an older chip, 21V) and the write time is a lot longer that the write time of a microprocessor. Also, if they don’t have a window on the top of the chip, they cannot be erased (this type is also known as OTP - One Time Programmable). If the chip does have a window, they can be erased with a UV light source of the correct wavelength, normally described as an EPROM eraser.

Some of the current day diagnostic cards are based on this 2007 project https://www.alioth.net/Projects/Spectrum-Diag/
See also https://github.com/brendanalford/zx-diagnostics/wiki
The EEPROM in this project can be programmed by the ZX Spectrum, however, this chip was discontinued. A different manufacturer did produce a compatible chip, but I don’t know of it’s current availability.

As far as I learned by searching, all can be programmed with a microprocessor, but you need either ultraviolet light or high voltage to erase it. And all ICs I came across were erasable. I think making DIY ZX diagnostic tool would be a good project, but I would also need instructions how to program the EPROM, which makes the project a bit difficult. What if I used flash memory instead?

There are in fact lots of DIY projects for the ZX Spectrum, from back when it was first popular through to now. Including recreating some commercial interfaces and expansions. From joystick interfaces through to hard disk interfaces.

What are you looking for?

I don't have any particular idea. And I already have a joystick with an adapter for ZX Spectrum. I would just like to populate my 10cm x 10cm PCB with nice projects before I send it to JLCPCB.

Indeed, if you have a smartphone why not try one of the tape loader apps to eliminate problems with the tape player? I use PlayZX or ZX Tape Player on Android. One the these cables allows the stereo jack output of the phone to connect to the mono "MIC" lead into the Spectrum

https://www.sinclairzxworld.com/viewtop ... 776#p52776

I already have this cable at home. I tried using PlayZX and the sound is so quiet the programs sometimes do not want to load. I tried with another smartphone and it is even worse.

[redgatemodels]

I already have this cable at home. I tried using PlayZX and the sound is so quiet the programs sometimes do not want to load. I tried with another smartphone and it is even worse.

That's really odd, as with my Xperia 10 iv I don't need full volume, 3/4 works fine, every time

[Pygmalion]

I already have this cable at home. I tried using PlayZX and the sound is so quiet the programs sometimes do not want to load. I tried with another smartphone and it is even worse.

That's really odd, as with my Xperia 10 iv I don't need full volume, 3/4 works fine, every time

Tried three different Samsung smartphones and Huaweii tablet, too quiet on all of them.

[Pygmalion]

Hello all,

I would just like to add a follow-up post. In the power supply, I replaced the (nonoriginal) 1.5Ω resistor used to reduce the voltage at the input of the ZX Spectrum with a cheap DC-DC Step Down 9V 5A.

Now I have a flawless 9V output. The pictures below show the situation without load (with very low ripple) and with the ZX Spectrum connected (without voltage ripple).

The project to upgrade the power supply is now complete. (Well, apart from the transformer and PCB there is no original electronic part left )

[redgatemodels]

Good result

as an aside, I bought a cheap later styled PSU (that matches the plus and 128K cases) that had an alarming hum to it when powered up. Turns out that some of the laminations on the transformer were loose and vibrating. I ran some UHU glue down between the gaps and now no more humming.

The cable was also broken at the end of the strain relief grommet, so I cut open the grommet lengthwise with a scalpel and released the cable from it, cut it back and resoldered to the PCB, ok a bit shorter but now sound. the original strain relief grommet was fitted back around the cable and a spot of superglue made it whole again.

[Pygmalion]

I am currently testing my repaired ZX Spectrum and I have noticed something strange. When loading programs, the loading fails if the sound is either too quiet or too loud. If the sound is very very loud, loading will not start at all.

I can understand that a sound that is too quiet can be a problem, but a sound that is too loud seems strange to me. Is that normal?

[redgatemodels]

I am currently testing my repaired ZX Spectrum and I have noticed something strange. When loading programs, the loading fails if the sound is either too quiet or too loud. If the sound is very loud, loading will not start at all.

I can understand that a sound that is too quiet can be a problem, but a sound that is too loud seems strange to me. Is that normal?

yes, you have to get the sweet spot. When I was a kid a tippex mark on the volume control was applied once the optimum level was found