For our “Hardware Tips” department we have asked Robert Hartung to comment on some letters from our readers. Since he had only the information given in the letters to work with, he cannot guarantee his answers. Rather his responses should be regarded as suggestions to help look for the answers. In most cases more details would be required in order to give a fuller answer. Any hardware changes are undertaken strictly at the reader’s own risk. We welcome comments from readers on these problems also.
LOADing Problems
Recorder Output
I have been unable to load with a GE 3-5105 recorder or a new Panasonic even after trying all the suggestions in Sinclair’s “Loading and saving with your ZX81 Computer.” According to Sinclair’s technical note, the recorder must output 5-6 volts. The GE output had 2-3 volts, and the Panasonic 2.5 volts. I believe this is the problem. However, the TV screen does go through the loading sequence.
C. D. Tuttle
St. Louis, MO [address omitted]
Comment:
I have had good results on three ZX computers using a very ancient Lafayette as well as Radio Shack CTR-41, CTR-57, and GE 5005 cassette recorders. So the fault may not lie in the recorders you have tried.
An LED connected across the two leads of the LOAD cable at the EAR plug lugs should be lighted when a program tape is played back for LOADing, with only occasional flickers.
If this checks out OK, then it is possible there is inadequate filtering of the DC power to the computer, or that your particular ZX81 is sensitive to the voltage drop imposed by the additional load of the RAM pack.
SAVE a program to tape, then play it back at normal volume through the cassette player speaker. If in the 5-second silent lead-in portion you hear a high-pitched whistle, similar to the horizontal-sweep frequency of a TV, this oscillation may be masking out the program signal or over-riding the auto-level circuitry used in most portable recorders. If you can SAVE and LOAD all right with the RAM pack detached, connecting a 2200 uF 35 WVDC capacitor between the DC power cord leads, observing proper polarity, might raise the effective RMS voltage enough to stop the oscillation interference.
LOADing problems can also occur with program tapes which have been overmodulated in recording or in copying to the extent that a sharp definition is lacking between the rise and fall of signal pulses. Too high a playback level will also cause LOAD defaults. An LED shunted across the LOAD cable leads will show this as a steady brightness with little or no flickering. Tape dropouts are indicated by an interval of reduced brightness as compare with the preceding and following portions of the same program SAVE.
Recorder Leads
Initially I had to disconnect one or the other cassette lead when LOADing or SAVEing. I got rid of the interference problem by carefully slitting the molded plastic buttons joining the leads and separating them.
Andreas Rainwater
Coyle, OK [address omitted]
Comment:
Some cassette recorders create an audio-feedback groundloop with both LOAD and SAVE cables plugged in. A 4P4T slide switch in a small metal box with plugs and jacks between the cables and cassette recorder and an external mike is a great convenience.
Keyboard Problems
The BREAK/SPACE Key
Is there a hardware modification to remove the BREAK command from the SPACE key? This combination is extremely inconvenient at times, especially when using the INKEY$ function.
Mike Swanson
APO San Francisco [address omitted]
Comment:
The only hardware required for the following routine is a dime over the SPACE/BREAK key (or a penny if you are broke). A conditional transfer or a RETURN line may be added after line 40 if this is a part of a main program or is a subroutine.
10 REM USE ENTER/NL FOR SPACE
20 PAUSE 40000
30 IF INKEY$=CHR$ 118 THEN PRI
NT "#";
40 IF INKEY$=CHR$ 118 THEN PR
INT INKEY$;
50 GOTO 20Line 40 can also be:
40 IF INKEY$=CHR$ 64 THEN PR
INT INKEY$;This prevents unintentional printing of token words although it is interesting that the INKEY$ function treats the image quotation mark (SHIFT Q) the same as the quotation mark (SHIFT P).
Shift lock
I made a shift lock using a Radio Shack SPST soft feel push on/push off switch (275-1565). I bored a hole through the left edge of the keyboard just above and to the left of the shift key. There is a small key free area there. I located the proper pins on the keyboard ribbon connector and soldered a wire to the underside of each. Two ribbon cables come out of the keyboard. The left consists of 5 ribbons; the right, 8. I soldered one wire to the rightmost of the left group and the other to the third from the right of the right group. Knowing this beforehand will bypass the need to peel off the keyboard for a look. After putting the switch in the hole, I soldered the wires to it.
Andreas Rainwater
Coyle, OK [address omitted]
Attaching Keyboards and Joysticks
What is the simplest method of attaching a keyboard and a joystick to the ZX81? How does a ZX81 user know whether a source is reliable and will ship merchandise in working condition? Will ZX81 hardware/software fit the Spectrum?
Arthur F. Jenson, Chaplain
Office of the Staff Chaplain
US Military Community Activity Augsburg
APO New York 09178
Comment:
Reputable suppliers of keyboards and joysticks will provide full instructions for their products. If possible, use a charge card for mail-order purchases. If they do not deliver, the bank tells them you do not have to pay until they do. The ZX printer is compatible with the Spectrum, and loaders are being advertised which will transfer ZX programs from tape into the Spectrum.
KBD Signal for TS1000
I have assembled the circuit for the “Repeat Key Option” in George R. Ingle’s article in SYNC 2:5, but I need to know where to pick up the keyboard signal (KBD) on my TS1000.
Paul W. Stuehn
Westland, MI [address omitted]
Comment by George R. Ingle:
Although I do not have a “true” ZX81, the repeater should work on the newer machines since they use the same keyboard scanning hardware/software that the MicroAce and ZX80 used.
To connect the repeater to a ZX81, follow the instructions in the original article with the following exceptions:
1) On the ZX81 the keyboard diodes are numbered D1-D8.
2) The ZX81 does not generate an external keyboard enable signal. This is performed by IC1, the SCL.
3) Connect the 74LS02 as shown in Figure 1. This shows a simple circuit which should generate the KBD signal required by the 74LS44 buffer to operate the keyboard normally for the ZX81 and TS1000.
[Figure: Figure 1. 74LS02 gates A1-A4 generating KBD from IORQ, RD and A0]
Pin 14: +5V (Vcc)
Pin 7: 0V (Vdd)
ICA 74LS02 QUAD NOR, 2 INPUT RD Connect to IC3
Pin 21
IORQ Connect to IC3 (Z80 CPU) A0 Connect to IC3
Pin 20 Pin 30
Power Supply Problems
Overheating and Crashes
In West Germany the OEM adapter we got with the ZX81 becomes very hot within just a few minutes of use. We have to use a 500 watt transformer which in turn plugs into our 220V wall socket. Is this heat responsible for the many program crashes we experience? Is there any way to minimize the heat? What do you recommend to minimize or eliminate constant program crashes? I have looked at QSAVE and the Baby BBU.
Arthur F. Jenson, Chaplain
Office of the Staff Chaplain
US Military Community Activity Augsburg
APO New York 09178
Comment:
The best approach for the power supply problem probably is to build your own. Obtain a 220V/9V or 220V/12.6V transformer of at least 1.5 to 2-A rating. Connect a 4-A 100 PIV bridge to the output of the transformer. Connect a 4400 uF or 5000 uF 50 WVDC filter capacitor across the DC leads of the bridge, observing proper polarity. Run the output of the bridge through a 7805 voltage regulator set for 9V as shown in SYNC 3:2, p. 68 (isolate the regulator from heatsink and ground and insert a 680 resistor in series between the ground lug and ground (-) for 9V only). Include a 1-A inline fuse in the AC input to the transformer.
The most common answers for the problem of crashes are covered elsewhere in this column: Make sure you have good connections with the RAM pack; use a regulated, well-filtered power supply to elminate problems from the power source, unless you have main power outages, of course; preregulate the DC power to the computer to prevent component overheating.
Loose Plugs
The cable plugs and power supply line are always loose. Is there any source of male plugs (with screw threads) that fit the UK jacks?
K. D. Streetman
Oak Ridge, TN [address omitted]
Comment:
Standard mini-plugs, such as Radio Shack 274-286 or 287, will fit the jacks better, in fact, than the originals I received with my computer, which were a bit too long for the jacks.
An in-line switch in the DC power cord, such as the Radio Shack 61-2713 suggested by Andreas Rainwater, will save wear-and-tear on the jacks.
For my own setup, I have the computer power supply, TV, and two cassette recorders plugged into a switched powerstrip, with a lever-type microswitch at the computer as a “panic-switch” when a trial ML routine crashes or takes off to cloud cuckoo land.
Plugs and on/off
The power plug seemed to be wearing rapidly due to the lack of an on/off switch. Installing a lamp cord switch on the line provides one. The power cord is not large enough to be pierced by the prongs for installation; it must be pushed onto them. The switch should be available for less than a dollar at most discount houses. I used a Radio Shack 61-2713A.
Since the 7805 IC (voltage regulator) works fine on any input voltage from 8-20VDC, a special portable or uninterruptable power supply is not needed. The computer can run directly off any 12V battery. At home one can use a battery in conjunction with a small trickle charger as an uninterruptable power supply. The jack plug on the Sinclair is tip positive like most 9V plugs. It is a good idea to add a 3A diode in line for safety’s sake. If you plan to use the computer in such a configuration for any extended period of time, it is a good idea to externalize the 7805 as Stephen Turner did (SYNC 3:1). Running on 12VDC clears up the display so well that it is worth doing for that reason alone.
Andreas Rainwater
Coyle, OK [address omitted]
Comment:
These suggestions are good, but there is one major problem: If the power mini-plug were to be connected to or disconnected from the computer jack with the full amperage of the battery applied across the jack contacts, the results could be disasterous. An in-line 1A fuse with an external 7805 voltage regulator, or at least the fuse, should be added in the power line from the 12V battery for short-circuit protection. The on-board 7805 should be left in place and the out-board 7805 adjusted to 9V by floating (isolating) its mounting tab from ground and inserting a 680 resistor in series between its ground lug and ground (-). See SYNC 3:4, p. 68. While the 7800, 317, and 350 types of voltage regulators do have built-in thermal and short-circuit protection, I have had them fail into a dead short internally along with the filter capacitor of a power supply when subjected to voltage spikes from repeated switching transients. A fuse is very cheap protection.
Regulated Voltage Questions
These questions have arisen in response to my article on “A Regulated-Voltage Power Adaptor” (SYNC 3:2).
Question: Is it safe to switch from one voltage to the other with power applied to the computer?
Comment:
Because the pre-regulator limits voltage input to the onboard regulator to 9V maximum while the unregulated VDC from your AC adaptor may attain peaks of 13V (or higher transient spikes) when the power plug is withdrawn and reinserted, the pre-regulation provides protection for the onboard capacitors and other components which are rated for 16V.
Question: Why is switching to a higher voltage for SAVE suggested when my computer SAVEs and LOADs at the lower voltage?
Question: Is the on-board regulator still used?
Comment:
I suggested the 7-9VDC pre-regulating adaptor for two reasons:
1) Since some users may not want to make any changes inside the computer while using a variety of alternative DC power sources, most of the voltage-regulation heat must be dissipated outside the computer. The 5V on-board voltage regulator must be used with this adaptor when the 7805 or LM-350T regulators are set as described, but the on-board regulator may be removed, subject to the following exception: if the adaptor regulator is set to exactly 5V.
2) If the unregulated power input to the computer jack drops below 8.5-9VDC during the SAVE mode, some 16K RAM packs, such as the Sinclair with voltage-regulation on-board the pack, may develop a high audio-frequency oscillation. This may interfere with the SAVE program signal on some tape recorders by drifting in frequency into the upper range of the audio signals being recorded. Some packs crash or cease functioning altogether.
Interfacing Problems
The ZX Printer and a Vic-20
Is it possible to interface a ZX Printer to a Vic-20?
Brent Myers
Tuscarawas, OH [address omitted]
Comment:
It is possible to construct circuitry to interface any printer to any complete computer. However, the ZX printer is designed to receive output of the actual character dot patterns through the I/O port of the ZX/TS computers while most other printers are designed for standard ASCII codes. I do not have any information on interfacing it with the Vic-20, but perhaps some SYNC reader has done this.