Saving programs on cassette from my ZX81 gives a buzzing sound at the beginning and end of my programs. This caused problems when reloading programs. Can I eliminate this buzzing sound?
Ho Tiap Fen,
Bandar Seri Begawan, Brunei
Ground-loop interference causes this problem. Unplug the EAR cable during saving and the MIC cable during loading. Also, use short tapes–C-30 maximum. Longer tapes can’t be pulled smoothly.–EO
Last March I bought a ZX81 kit and 650 mA power supply. This Oct., I purchased a 16K RAM pack. I can’t SAVE/LOAD a program because of the hum during the 7-sec. silent portion. Also, when using the 16K RAM, the screen fills with scrolling horizontal bars 15 minutes later. When I SAVE or LOAD in 1K, everything works fine–no hum, no bars. Does this indicate a defective RAM or power pack?
William Mitchell, Ontario, Canada
Sinclair RAM packs contain a power converter to get the voltages needed to drive their chips. This sometimes puts transients on the power for the ZX/TS, and can cause loading and saving problems. Put noise suppression on the 9V supplies in the RAM, or get a new RAM pack. Fuzzy horizontal bars on the screen indicate power supply overload, perhaps from the RAM pack power converter. Memotech RAMs typically do not show these problems, because they do not have a power converter.–EO
Like many Sinclair users, I dislike using the difficult flat membrane keyboard and various overlays for the keyboard are not any more promising. I decided to add my own typewriter style keyboard but the only way to connect it was to force a 13 conductor wire between the two halves of the case.
Can I connect a keyboard through the output port in the rear of the ZX81 and does it require additional hardware?
Lance Fong, Oakland, CA
Connecting a keyboard through the back edge connector needs more than just wires. Memotech’s keyboard will connect this way. Until then, use flat 13-conductor ribbon cable and run it out the slot used by the membrane keyboard or between the case halves.–EO
Is there any way to get COBOL on the ZX/TS computer?
Here’s a tip for ZX80 owners who want to adapt ZX81 adventure games. Change LET V$=INKEY$ instructions to INPUT V$ and delete the IF statement that checks for “” (i.e., IF V$=”” THEN…). Then hit ENTER after each entry.
Noel Pepin, Quebec, Canada
No COBOL for ZX/TS yet.–EO
It seems that machine code programs begin on line 1 (REM 1). How, then, do you SAVE them, since the program is not “named”?
Max R. de Arellano,Santurce,PR
Specify program name after the SAVE command, i.e., SAVE “NAME”. Line 1 need not contain the name.–EO
I’d like to do some modifications on my ZX81 but I’m unsure whether they can be done and if they’re worth the time or effort.
First, internal memory expansion. I understand you can increase the present 1K to 2K RAM. With a little effort, possibly more can be added by stacking or ribbon-connecting from the 2K socket to an unlimited RAM board. Past SYNTAX articles mentioned that external RAM packs bypass or cutoff the internal RAM units. Is it possible to connect the internal and external RAMs to create more memory space? If so, I could plug an external 16K RAM into an internal 16K RAM to equal 32K only when I needed it.
Second, I see by ads and bulletins that EPROMs can be used with ZX81s. How can I interface either an internal or external EPROM socket and what is needed to facilitate faster and easier programming?
Rex Ward, Upper Darby, PA
Adding RAM inside the ZX/TS probably costs more than buying a RAM pack. But, some people remap their 1K or 2K internal static RAM to the 8K-16K area. EPROMS are easy to use and complete circuit diagrams and boards for burner and read cards will appear in SQ 2/83.–EO
Here’s a low-tech solution to ZX/TS overheating. Do what the mainframe computer operators do–refrigerate the computer. Buy one or two “blue ice” packets that are sold in markets and keep them in your freezer. When you are ready to set up operation, wrap them in a plastic bag and place them under the computer, then plug it in.
This method successfully eliminated heating problems for me. I’ve used it for up to 10 hours of continuous operation with the 16K RAM attached. There must be other users like me unwilling to void their warranties by opening the case to perform a more permanent fix of the problem.
Does anyone have a low-tech solution to the loose-connector problem? There are many of us out here who lack confidence in our skill especially with soldering irons and wiring diagrams.
Jim Giddings, Providence, RI
Here’s a couple of suggestions to help you correct your loose connector problems: try taping your RAM pack on or put Velcro on the front of the RAM pack and the back of your ZX/TS computer.–EO
I would like to clarify and commment on the VOTEM hardware review (Oct.82).
With the machine language routine supplied, VOTEM can sample analog data at a rate of about 1Hz with a resolution of roughly 1 in 2000. Since VOTEM uses a voltage-to-frequency conversion technique, there is a trade-off between sampling rate and resolution. Changing two bytes in the machine language routine decreases the conversion time at the expense of resolution. Decreasing the conversion time to 50 ms still yields approximately 10-bit (1 in 1024) resolution. Compared to other techniques, this method is slow, but is fast enough for many applications and offers easy interfacing and low cost.
I am not sure I understand the reviewer’s fear of software calibration. It is actually required for high accuracy and precision.
VOTEM’s jack arrangement can be confusing. The diagram in the manual does not clearly illustrate the best way. But, there is a neat arrangement, using the dual cable supplied with the computer and the single cable supplied with VOTEM. We intend to change the manual in the next edition.
Finally, I don’t understand how VOTEM can cause television interference. The highest frequency generated is under 30KHz unless higher harmonics or Fourier components are also present. Still, all of the cables are shielded, or should be. We are interested in hearing from users experiencing problems with VOTEM.
Alger Salt
Down East Computers
PO Box 3096, Greenville, NC 27834