TS1000 Video Issue

Discussions about Sinclair ZX80 and ZX81 Hardware
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mechaniputer
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Joined: Tue Aug 17, 2021 3:40 am

TS1000 Video Issue

Post by mechaniputer »

Hi! I recently purchased a Timex Sinclair 1000 after years of being curious about this family of machines.

The one I got was in untested condition, and most likely never used. I tried it on my modern TV and got no video on channel 3 which is not surprising based on my research. I do not have a CRT anymore.

The machine has the 2C210E ULA, so I performed the composite mod described here: https://www.bytedelight.com/?page_id=3560
I have not added the second resistor, in accordance with the instructions that say to test it for brightness first. So the emitter goes directly to the output right now.

I checked all of the connections to make sure the mod was successful and then tested it. No video. After some repeated attempts, I discovered that I can get very brief video output (often garbled) by gently tapping the coils near the voltage regulator. But this only works once before resetting the machine.

Does anyone know what I should check to hopefully get this working? Thanks in advance
DrVenkman
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Re: TS1000 Video Issue

Post by DrVenkman »

The real Zeddy experts will weigh in here soon, I’m sure, but to set the background: do you have experience generally with vintage electronics hardware diagnosis and repair procedures? Much of the basics will be the same across system architectures.

Since you don’t know of the system was working, start with the basics. Have you verified that that voltage regulator is receiving the proper ~9VDC input and putting out a steady 5VDC? Have you confirmed the +5VDC and GND connections to all the main system ICs? Do you have an oscilloscope or at least a logic probe to verify activity on the clock pins, along with the address and data bus lines? Etc.
mechaniputer
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Joined: Tue Aug 17, 2021 3:40 am

Re: TS1000 Video Issue

Post by mechaniputer »

I've got some experience with both electronic repair and vintage computers, but I've only rarely had to make repairs to the vintage computers I own (several Apple II's and a PDP-11/73 rescued from a piece of scrapped lab equipment, for which I had to improvise a power-on circuit).

I do not own an oscilloscope, but at least I have a cheap multimeter. The voltages all look nominally fine. The input voltage is actually around 10v under load but the regulator is making 5v. Actually, I wonder if the higher input voltage could mean that something is not drawing as much current as it should be. Can anyone tell me if 10v under load is typical?

I do not have a logic probe that I can use at the moment, but I have one from a Z80 kit project from about a decade ago that I can probably wire up later.
Last edited by mechaniputer on Wed Aug 18, 2021 2:34 am, edited 1 time in total.
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mrtinb
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Re: TS1000 Video Issue

Post by mrtinb »

You could try to measure the pins of the ICs like described in this thread

Getting your logic probe to work, and compare how the pulsing should be, would be a good start.
Martin
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ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
bola_dor
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Re: TS1000 Video Issue

Post by bola_dor »

Have you checked the ch2-3 selector at the bottom?
A TS1000 with no image at all, not even a white screen, points to a faulty ULA. It usually runs hot so you can't trust your finger. If it's on a socket the easiest way is to put a known working one. If ULA is OK it could be the 6.5Mhz resonator.
10v under load is the usual voltage.
Ernesto
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1024MAK
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Re: TS1000 Video Issue

Post by 1024MAK »

How well the simple composite modification works (or does not work) does rather depend on the TV. Some are happy with the out of specification video, other TVs not so much.

As said above, we start with the basic checks. You’ve already tested the voltages. Off load (with no computer connected), the unregulated 9V PSU voltage can be as high as 16V. When connected to a TS1000 or ZX81 or other version of the same basic computer, the nominal 9V is typically between 10V and 12V.

So is the centre of the ULA getting hot?

What voltage do you get on ULA pin 16?

If you can find or can’t get your logic probe to work, I can point you to a circuit of a basic logic probe based only on NPN transistors and resistors plus a green and red LED.

Mark
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