No video out on Timex Sinclair 1000

[jason21]

I've just picked up a Timex Sinclair 1000, a computer I used to have many years ago, and I'm not getting any video out

Based on similar posts here are the things I've checked:

- 5.1v at regulator, ULA, and CPU
- ULA Clock in pin 35 (2.79mhz: pic)
- ULA Clock out pin 14 (1.7mhz: pic)
- CPU Reset pin 26 (5.000v)
- CPU A15 pin 5 (activity: pic)
- CPU Clock Pin 6 (2.26Mhz)

The CPU, ULA, and ROM all get warm after being powered on

I tried RF via a VCR and directly via an old 1999 CRT TV via the a few switchboxes and an adapter (I believe my ULA has backporch 2C210E), and after those didn't work. I've since installed a composite mod, same result, pics of RF output on scope

The original adapter was putting out 15v no load, I never used it

Doesn't seem right the clock varies so much, but I've only just gotten my first scope and still learning how to use it, basically just hit the autosetup button, I included screengrabs. I'm a novice to electronics repair

Seems like the ULA is a common failure point, but seems like mine is perhaps good

Any suggestions on what I can check next?

[1024MAK]

I’ve not much time, sorry. But the clock on ULA pin 14 should be a 3.25MHz frequency. As should the clock on pin 6 of the Z80A.

What are the specifications (primarily the analogue bandwidth) of your oscilloscope?

It’s normal for the ULA to run hot, the Z80A to run warm. The ROM is normally the coolest of these three.

Mark

[jason21]

Thanks, it's a Siglent SDS1202X-E 200 mhz scope.

I'm showing 2.26Mhz on pin 6 of the CPU. Basically the clock seems to be different each place it should be that I test, I'm not sure if that's the issue itself or I'm not testing it correctly

[1024MAK]

Here’s what I get on one of my ZX81s



NOTES:

• The actual wave form shapes of the 3.25MHz clock signal will depend on the type of ‘scope probes used, their calibration if X10 type, as well as the bandwidth/sampling rate of the ‘scope itself.
• The digital section of the ZX81 and TS1000 are the same. However the positions of the components varies between ZX81 issue 1 boards, TS1000 issue boards and ZX81/TS1000 issue 3 boards.

Mark

[1024MAK]

- ULA Clock in pin 35 (2.79mhz: pic)

I normally don’t recommend that people test on pin 35, as the test equipment can affect the operation of the oscillator circuit.

The original adapter was putting out 15v no load, I never used it

That’s normal. The PSU is an unregulated type with a small mains transformer. The U.K. version can output up to around 16V when off load…
When on load, the voltage at the 7805 voltage regulator input typically drops to between 10V to 12V.

Can you tell me which ‘scope picture is which signal.
Also, have you calibrated your ‘scope probes (assuming you are using X10 type)?

Are the waveforms more stable if you use manual settings (50ns/div for horizontal time base, use the trigger set to AUTO and adjust the trigger level to the middle of a waveform (around 1.5V to 2V).

Mark

[jason21]

1st is ula clock out pin 14
2nd is ula lock in pin 35
3rd is the RF jack
4th is cpu clock pin 6
5th is cpu a15 pin 5

I did the built-in calibration, ie connecting to the 1khz calibration terminal and adjusting so a nice square wave, I'm using the 10X setting on the probes.

I'll try your settings, this is the first scope I've ever used so I basically just hit auto-setup and went with what it gave me

[jason21]

Upon further testing I am getting 3.2 MHz connecting directly to CPU pin 6 and ULA pin 14. I think perhaps my other ones were messed up as you say when I was testing ULA pin 35, I notice that does mess it up. Not sure what to check next if clock is good

ula clock out pin 14 :

cpu clock pin 6:

[bola_dor]

You can see the actual video output at ULA pin 16
See /RFSH at Z80 should be pulsing no matter what.
To test the /reset must see it rising slowly from zero at start. If not just make a warm reset shorting it briefly to gnd (can do it at the electrolytic legs is just beside).. if /RFSH is not pulsing after reset Z80 is not working at all.
If you don't have a signal downpulsing at 15.6Khz at ULA pin16 and assuming the resonator is OK then the ULA IS NOT generating the video output. It doesn't need any ram rom nor even a CPU to make a white screen.
You said it was your machine in the past but just in case check for the presence of r30 in case you are using a 60Hz TV (NTSC or PAL-M) or it's absence in case you are using a 50Hz TV (PAL-B/N)

[jason21]

I does look like there is pulsing on rfresh / cpu pin 28

I tried checking ula pin 16 but I'm probably not testing it correctly

It does have the r30 resistor, here is a pic of my board

[bola_dor]

I am afraid you have a faulty ULA, at least partially as it seems to drive the CLK OK. best way to confirm that is trying a known good ULA. Luckily yours is on a socket.
Your machine is configurated to 60Hz TV.