Since adding a Sinclair 16K RAM expansion, a horizontal bar travels constantly up my screen. The 1K computer doesn’t do this.
I have tried three different TV sets with the same results.
What can I can do to eliminate this horizontal bar? Keeping both the TV contrast & brightness control at a minimum diminishes this effect but doesn’t eliminate it.
I suspect that the computer and the 16K pack overload the power supply but I cannot be sure. I have the 650 MA power supply.
Luis A. Roman, Ponce, PR
That’s right–your power supply has excessive ripple. Sam Porter suggests you increase C3 in the computer from 22uF to 220uF (Apr.82 p.18), but see the following letter and answer for alternatives.–KO
Sam Porter’s fix for charger ripple (April 82) treats the symptom, not the disease. Horizontal “hum” bars on the screen do result from a weak charger, but the power supply is weak because a component has failed. The most likely culprit is one of the two diodes soldered to the transformer windings inside the plastic case. If one diode opened, supply capacity drops by 1/2 and more hum results. Adding capacitance reduces the hum to a workable level, but replacing the diode is a better fix.
To get inside the sealed plastic case, use a sharp knife to cut the plastic welds along the crack between the case top and bottom.
The power supply already has a 1000 uf, 16V cap in it.
William Bosley, Valparaiso, FL
Chargers can also develop excess ripple if the transformer is badly wound. Voltages would be within specs, but ripple would be high, especially after warmup. You cannot easily repair bad windings, so get a new unit. ZX80/1 owners should write to Sinclair Research, 1 Sinclair Plaza, Nashua, NH 03061. Use this address for all parts, manuals, and ROM upgrades.–KO
If you added a big keyboard and need to label 8K functions on the keys, here’s a suggestion from Chuck Ludinisky of Chelmsford, MA. Find a Sinclair ad in the computer magazines; color in Kilobaud or Byte, b/w in SYNTAX. Cut out the keyboard picture and glue the labels to your keytops with household (airplane) glue. Cover with tape or clear nail polish. The labels are small, but readable.
Cliff Danielson, Chelmsford, MA
Sometimes pressing on the top of my ZX81 case in certain areas causes a crash. Static from my hand isn’t causing it. Do you know what might be causing the problem and what the solution is?
Hugh B. Amber, Mississauga, Ont.
Check the insulation between ground strap & circuit board–sometimes the tape punctures. Cracked traces & solder joints give similar symptoms.–KO
In Apr.82, Don Shank, Palo Alto, CA, wrote about problems with his ZX81’s power jack connection. I had the same problem with my power connection. (Sinclair should fix this!) I wouldn’t put up with this problem nor would I put my computer on the shelf. Replacing the existing power plug with an RCA type plug would solve this problem, so I got the necessary parts from a local electronics store and installed them. Problem solved.
I am by no means an electronic repairman but this plug can be installed with a little imagination and care. You need only a screwdriver, a relatively inexpensive soldering iron and about 2″ of 16 to 18 gauge wire.
I hope Mr. Shank reads this letter and takes his ZX81 back off the shelf. He’s missing a lot of pleasure and an enjoyable teaching aid for himself and his family.
David L. Isabel, Farmington, MO
I am just starting on the ZX81 and am looking for info:
1) Where to get parts to build add-on RAM
2) Switch to color
3) Printer interface
4) List of owners in my area (especially amateur radio operators & electronics builders, computer technicians)
James R. Copeland, Crozet, VA
Check electronics surplus houses for inexpensive parts for hardware projects. They often advertise in the back of computer magazines like Byte. We have bought from BNF in Peabody, MA, and John J. Meshna in Lynn, MA.
Several people are working on color boards for ZX81s. Watch SYNTAX for details.
Printer interfaces with ASCII conversion are available from Byte-Back, Rt. 3, Box 147, Brodie Rd., Leesville, SC 29070; Data-assette, 52 S. 3rd St., Oxford, PA 19363; and Memotech Corp., 7550 W. Yale Ave., Suite 200, Denver, CO 80227.
To reach amateur radio users, contact Marty Irons, K2MI, 46 Magic Circle Dr., Goshen, NY. He runs a free newsletter only for amateur radio people with ZX81s. To find local ZX users groups, see p.3.–AZ
My problem is that prepared tapes LOAD satisfactorily; however tapes SAVED from the computer do not LOAD. The audio signal from the tape is much weaker than the prepared tapes (obviously what is going out isn’t strong enough).
The output from IC1 pin 16 exits via R29 (1M) and C12 (47pfd) to MIC jack with R27 (1K) and C11 (47nfd) to 0 volt bus.
Is there any obvious reason why I cannot SAVE programs? Should I disconnect R30?
A.R. Newhouse, Potomac, MD
Do not disconnect R30; it creates the US TV scan and plays no role in SAVE and LOAD. Try a borrowed tape recorder, and use a short (C-30 or smaller) tape. Next, check the component values or substitute others, and inspect the solder joints and printed circuit traces. Measure the output from the MIC jack on your computer–you should get 1-5 millivolts.
If you find no problem, we recommend you exchange the unit or send it for repair.–KO
I think I understand “phantom memory” (addresses above 37767), but I don’t understand why some 8K ROM code references locations in the phantom memory. For example, there is a CALL PO,58085 (hex E2E5) at 171 (hex 00AC) and a JP PO,57310 (hex DFDE) at 180 (hex 00B4). Or is my disassembly all wet? Similarly, more appear to be many references to locations between 8192 and 16383 (hex 2000 to 3FFF). I can accept this more readily but still have some trouble with it.
Why don’t I have 17K of RAM with my 16K RAM add-on?
James H. Canfield, Dayton, OH
Addresses 00AC and 00B4 contain text or tabulated values, not commands. Locations 007E to 01FB contain the key tables. Unless you like doing your own disassembly, you can use Ian Logan‘s, available direct from Ian in England or from several North American suppliers.
Adding 16K external memory disables the internal 1K that comes with the ZX80/81.–KO