Yes, likely to be a DRAM fault.Pygmalion wrote: ↑Wed Nov 01, 2023 10:43 am
- 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.
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.
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.
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.Pygmalion wrote: ↑Wed Nov 01, 2023 10:43 am 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?
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