Sinclair ZX80 / ZX81 / Z88 Forums

Hello all,

I have recently acquired a Timex Sinclair 1000 and just now got around to trying to power it up. It didn't come with the AC adapter (big suprise there) so I connected it to a variable power supply via a 3.5mm TS plug and a couple of test leads. Initially, I had the PSU set to deliver 9v and current limit at max capacity (2a) - I use this PSU for everything and the current limiter never matters. Well, I connected the T/S 1000 and the current limiter immediately kicked in, delivering 5ish volts at 2.2a. This was unexpected behavior for a device that supposedly draws 700 ma. I immediately pulled the current limiter down to 700 ma and the voltage responded by dropping to 2.4v. All my further testing has been done at this 2.4v/700ma level.

Observations:
- The voltage regulator (MC7805) did not heat up noticably over the 15 minutes I had it connected to power.
- The ground pin of the voltage regulator is indeed connected to ground, as is the heat sink. I did not try connecting the heat sink (ie: computer ground) to the ground output of the PSU, but I feel like that shouldn't matter...? The original AC adapter wouldn't have had a ground connection.
- The input and output pins of the voltage regulator are connected (well, 0 ohms resistance) to the power supply.
- The input pin shows 2v to ground and the output pin shows 1v to ground. These numbers are inexact - I measured them with a Fluke T5, which is designed for working with industrial equipment, 480v three phase, you know the type. It only measures to the nearest volt. I may try to find/fix a better multimeter in the future.
- The inputs and outputs of both coils are connected to power.
- The output of coil 12 is connected to ground.
- I cannot for the life of me find capacitors 13 and 14.
-> In fact, I can only find 8 capacitors on the board, which is really interesting when I count 9 on the diagram and the numbering goes up to 14.
-> Is there somewhere I can get a good diagram? I've been using the one at https://archive.org/details/vista-newsl ... ew=theater, but the page keeps flipping on me and I am in Shambles.

All these observations are flying in the dark, because my knowledge of electrical engineering is... limited, to say the least. For all I know, none of this data is useful. However, this is my proof that I am actually trying and not just immediately running to the forums because "oh no my little box is borked". I will be glad to look at/measure anything you suggest, and even more glad to learn *why* you suggest that.

How do you advise I proceed?

span wrote: ↑Tue Nov 26, 2024 9:11 pm It didn't come with the AC adapter so I connected it to a variable power supply via a 3.5mm TS plug and a couple of test leads. Initially, I had the PSU set to deliver 9v and current limit at max capacity (2a) - I use this PSU for everything and the current limiter never matters. Well, I connected the T/S 1000 and the current limiter immediately kicked in, delivering 5ish volts at 2.2a. This was unexpected behavior for a device that supposedly draws 700 ma. I immediately pulled the current limiter down to 700 ma and the voltage responded by dropping to 2.4v. All my further testing has been done at this 2.4v/700ma level.

I'm assuming that you are feeding positive to the tip? And 0V to the sleeve?

A TS1000 on it's own should only need between 300mA and 450mA (the exact amount depends on the version/type of chips fitted). The 700mA is the maximum current rating of the original AC adapter. 700mA allowing for some allowance for the 16K RAM pack.

The DC input voltage can be between 7V and 11V. However, I recommend no lower than 8V if using an AC adapter to allow for ripple and voltage drop.

The TS1000 does not need a true ground/earth connection. The AC adapter can be a double insulated ungrounded type.

Back to your problem. Either the 7805 voltage regulator is faulty or there is a short circuit on the +5V rail. Given that the 7805 is rated at 1A and is suppose to current limit if there is a short on it's output (it has fold back current limiting, so the value varies between manufacturers/exact type, but it should be a lot less than 2.2A), I would suggest that the 7805 voltage regulator is defective. Maybe someone tried using an AC adapter with an AC output at some time in the past or it died on it's own. Any 7805 voltage regulator from a good supplier should do. Don't buy from untrusted suppliers on eBay or other market places, as cheap versions that don't perform to specification are often sold on these sites.

I'll attach a good schematic. Note that the interference suppression / filtering coils are not shown. Some boards have just multiple individual resistors instead of resistor packs. Some boards have an extra electrolytic capacitor connected between the +9V rail and the 0V rail. TS1000 use a single 2K byte RAM chip rather than the options available for the European version (ZX81).

Mark

I am not, in fact, feeding positive to the tip and 0v to the sleeve...

"The Timex Sinclair 1000 needs 9 volts DC, with a 3.5mm connector, with the tip negative."
- https://dfarq.homeip.net/timex-sinclair-1000/

In my strict defense, the specifications of the *power supply* are hard to come by. But, I should have probably done a better job of reading the schematic.

After switching around the leads to my power supply, it suddenly draws a solid 350 ma. I like that far better.

Thank you so much! I like a problem with a nice, simple solution. Likewise, I appreciate the research in your post, and will keep that in my mind for if I ever do fry the 7805. Further, thank you for the schematic, I promise to put it to good use.

I don't have a picture yet, but I'll keep toying with it because that could also be my period correct Sharp TV I found in a shed in north Idaho, or any number of other factors. If there are any other common newbie issues, please do let me know, because I'll probably find them the hard way by blindly following the manual.

Again, thank you!

Span

Yes, the polarity being correct definitely helps

I’m not sure why that article got it so wrong. All three 3.5mm sockets have their sleeve connections connected to the 0V rail. Maybe the author is confused with another computer.

The tape connections sometimes confuse people. The EAR is the tape in from the cassette recorder (or other audio source). Named the same as the output from cassette recorders of the period (“EAR” - earphones). MIC Is the tape output to the cassette recorder. Note that mono plugs must be used. Stereo plugs don’t work.

Note that these computers need a really loud audio signal in order to LOAD a program from modern audio devices. Many modern audio devices can’t produce a loud enough signal. In which case an audio amplifier may help.

If using an expansion or interface, clean the edge-connector on the computer before use. It saves weird random problems. On any expansion or interface check that the locating peg is present. If it’s missing the female connector will almost certainly not line up correctly and hence may short out the signals.

There is no audio on the TV output, so turn down the sound. The stated channel frequency is approximate. The modulator is an analogue design and hence the frequency was never spot on and may well have drifted since it was made. The computer starts up in SLOW mode (compute and display) with a black and white picture. At power up the picture is completely white apart from an inverted K in the bottom left corner.

There were three ULA chip versions. The first two versions don’t always work well with colour TVs. They don’t include part of the video signal known as the back porch. The third ULA chip does include this signal and hence does work with colour TVs.

Many people modify their computers to output a composite video signal in place of the VHF/UHF TV signal.

The single keyword entry system takes a little getting used to if you haven’t used it before.

The other mode is FAST mode. In this mode the computer doesn’t produce a video signal unless it’s waiting for input (for you to press a key).

After running for a while, the heat from the heat sink (for the 7805) and from the ULA will cause the case to feel warm. It’s normal for these to be hot to the touch. The Z80 will also be warm but not as hot.

If there is a fault with the board, because of the way the video picture is generated, many faults result in no video output, either blank black screen or a blank white screen.

I can’t think of anything else, but do feel free to ask questions.

Mark