For those of you having problems with LOADing and SAVEing tape programs on your 2068 here’s some thoughts from Don Lambert:
After having some problems verifing LOADed programs on a tape he was making, Don cleaned his 3 tape recorders and while waiting for the solvents to dry he read his mail from a friend in Canada. He found a paragraph in the letter which read: “Loading with the TS2068 often will not be successful in difficult cases when the computer has been on for a while and is hot. The solution is to turn it off for an hour or two and try again when it is cold. Also the TS2068 cassette system seems prone to problems with glitches, brown-out (power voltage drop) and too many accessories mounted on its power supply bus. This is particularly a problem with the cantankerous ZX-81 upload programs which load ZX-81 tapes at regular speed to convert the programs to TS2068 BASIC.” This was from Bill Harmer, Ottawa, Ontario and his manuscript “Tips, Tricks and Techniques of the User Group Masters For the Sinclair ZX-81 / Timex TS1000”.
Don goes on, “What this did was jolt my memory about a little known fact about the TS2068 and that is the cassette routines are effected by heat, or I should say the chip that does the routine is effected by heat. Normally, the voltage supplied to the computer is about 16 volts and inside the computer is a chip that reduces the voltage to an exact 5 volts, the difference is eleven volts and since it has to go somewhere it goes into heat and since the computer is poorly ventilated it gets hot inside and heat makes the SCLD chip get erratic. One cure is to cut the input voltage to 8.5 to 9 volts with the only loss being color. You need at least 12 volts for color. The problem is complicated by having the computer loaded off the rear expansion port with RAM boards, disc interfaces, modems, printer interfaces and what else might show up. They all use a little of the 5 volt power supply.”
Using the tape that gave him problems, he loaded successfully in to spare 2068 with no addons on the expansion buss.
Another hint from Harmer, was that an audio transformer can be used to increase the voltage of a poor quality tape that sometimes a non LOADable tape can be LOADed. While he mentioned an 8 ohm to 1000 ohm transformer, I found a 500 to 1000 ohm unit and tried it out. Without the transformer and the recorder volume set at 8 it put out a signal of known strength and with the transformer hooked up, I had to cut the volume down to 5 to get the same signal level. So now I can increase the volume of weak signals on a tape.”
I am also trying to find all the parts I picked up for the powersplitter project to keep the load off the 5 volt switching regulator inside the 2068. I have bigger heatsinks to put on the 7805 regulators…how big of a heat sink is required to dump about 7 volts at 570ma?
Other bits: if anyone has a Royal Letter Master printer they want to hook up to their 2068 with an Aerco printer interface -DONT- I tried it and it would not work, as the 7805 was shorting to ground. I made up a sh rt cable to go between the interface cable and the printer and cut the last line that Aerco had. It works now but it is S L O W.
Does any one have the instructions for a ZAD board, that Zebra used to make and sell? Also the Zebra C-130 Talker board and the Zebra VS101 board. Got these at the CapitalFEST and was promised doc, sent Zebra a SASE and still nothing. Will pay for copies if not excessive.
Don Lambert
[address omitted]
Cedar Rapids, IA