ZX81 Comp. Mod, no Video

[p5s]

I’m seeking some advice and guidance as to why I am unable to get any video from my ZX81. Here’s some info about the system:

• USA issue-1 board, red solder mask/straight tracks.

• From the state of the soldering, it was clearly a USA kit built by someone. I have inspected and reflowed the entire board.

• I implemented the “basic” composite mode (transistor + 2 resistors), per these instructions: https://www.bytedelight.com/?page_id=3560

• Confirmed I see a composite video signal on pin 16 on the ULA (yes, I understand it’s a 2C184E which is not preferred but from all my research should work just fine with the composite mod).

• Confirmed same signal present on the center pin of the RCA output.

• I am using a known good Apple IIc 9” monochrome composite monitor.

• I’ve verified all voltage present on ROM, ULA, CPU, RAM and 5v into the composite mod.

• I’ve verified activity on the CPU data lines. But with more info, perhaps may need to recheck.

The result of all the above is that the video is blank/black regardless of brightness adjustment on the monitor. While it could be:

• The transistor I’m using, I’m doubtful given I see a signal of some sort coming out the RCA center pin (same as is visible on ULA pin-16).

• Some idiosyncrasy with the Apple IIc display? But it’s just a basic composite in monitor.

• Bad ULA — but then why would I see a composite signal on the scope on pin-16? Although concede, not sure what the correct signal should look like. I have TS1000 on the way, so can do a ULA swap soon and verify.

• Bad CPU?

Finally, the blue bodge wire is not connected to anything, it was connected to R5 for some reason. I disconnected it to see if it would make a difference — which it did not.

If anyone else has ideas, or thoughts on other items to check the would be appreciated.
Thanks -Alan

[timsenecal]

I cannot answer any of your other questions, but using a more complicated Composite output board that generated the back porch signal in a 1982 era timex sinclair 1000 worked for 2 years with an Apple IIc 9" monitor.

[1024MAK]

You talk about seeing a composite video signal. What test equipment are you using?

Mark

[TMAOne]

I only suggest this because I've made this mistake. Double-check that you've got the transistor wired the correct way around. (I was amazed at how well the circuit works when I finally discovered my flub.)

[p5s]

Thanks for the advice. To answer the questions asked:

1. Using an oscilloscope to determine signal present on ULA #16, and video output at RCA.
2. I did check the transistor orientation. However, have since removed the basic comp. mod and created and installed this one http://zx.zigg.net/misc-projects/ZX81_V ... ioning.pdf (quicker, less expensive and more fun that waiting for someone to ship me a pre-made version). Installed and verified it today.

Attached is the output of the scope showing the signal at ULA #16 and the new back-porch comp.mod installed.
- yellow = ULA #16. FWIW, with the prior basic comp.mod the signal at the RCA looks liked this too (i.e. identical to ULA #16)
- blue = zigg.net back-porch comp.mod. which I believe you can now clearly see.

Still no video on either on a composite vintage monitor or a comp--> hdmi converter on modern LCD display.

Will continue to troubleshoot -- including swapping the URL for one I just received in a TS1000.

Opinions, and guidance most welcome.

[1024MAK]

Here's a screenshot from my 'scope showing the video waveform one of my ZX81 boards showing the output of pin 16 of the ULA (in this case, a 2C210E on an issue one board, this ULA includes the back porch circuitry). For UK and other PAL ZX81 models, note that the output of pin 16 of the ULA also goes to the modulator input. This is not the case in other versions.

This board is using the modulator for it's video output.

ZX81 video line screenshot (with modulator in circuit)

As you can see from the second screenshot, when the modulator is disconnected, with no load, the video waveform levels are rather different.

ZX81 video line screenshot (with modulator disconnected)

Both screenshots are for one video line and both show part of the inverse K.

A normal composite video signal to a TV/monitor/capture card should not exceed 1V peak to peak.

Mark

[1024MAK]

Looking at the image from your 'scope, the ZX81 does appear to be working.

Some HDMI converters don't like the non-standard video signal from a ZX81, and some LCDs also don't like the signal. But a CRT "vintage" monitor (assuming that it's a 15625Hz "PAL" type) monitor or CRT TV should work okay.

And although the levels in your 'scope image could be better, the display should be attempting to show an image.

Mark

[p5s]

Thanks 1024MAK. I adjusted the circuit, bypassing one of the diodes on the way to the video out. This improves the back-porch further -- see below:

To your point the CPU appears to function, as you can see the same video signals below w/CPU removed and w/out all the CPU activity.

This is an NTSC converted ZX81, so will pursue that next -- removing a resistor to revert back to PAL. All this said, the video still refused to appear either not the HDMI converter or my vintage (working) apple Iic 9" green screen. In the end it could be my two available display choices are not going to cooperate. Will keep at it...

[p5s]

So...a little more information. I noticed the CPU activity dropping out occasionally while troubleshooting the video issue. This led me to (a) change the two electrolytic caps and (b) check voltages across the main ICs, and discovered the following:

When all ICs installed:
-- 7805 = 4.971v & 10.63v
-- ULA = 2.439v
-- CPU = 2.148v
-- ROM = 2.266v
-- C4 = 4.968v

When all ICs removed:
-- 7805 = 4.991v & 10.63v
-- ULA socket = 4.985v
-- CPU socker = 0.120v
-- ROM socket = 4.985v
-- RAM sockets = 1.8v
-- C4 = 4.9992v

Clearly issues on this board, likely also contributing to video issues, or perhaps video is fine but the voltage(s) are too low to enable the display of anything. I'll continue to troubleshoot, and would appreciate if anyone can help point me in the right direction while I check for shorts.

Questions:
- If anyone has any thoughts about which of the ceramics are prone to fail?
- Is this the ULA or issue with one of the other ICs -- how would this account for the low CPU socket voltage when all chips removed?
- If someone can share the values of: i1, i2, R30, R31 & R32 that would be appreciated (unrelated, I think, to all the above)?

Request:
- If someone has a ZX81 Issue ONE USA board (red solder mask, straight lines), and can take a could clear pic of the component side board for me that would be appreciated.

Thanks.

[1024MAK]

On ZX81/TS1000 boards (all versions) none of the capacitors are prone to failing.

The electrolytic types are non-critical hence IMHO do not need to be routinely changed. One decouples the (nominal) +9V supply to the edge-connector (the computer will work with it missing), one is part of the reset circuit for the Z80 (the Z80 will not work correctly if this one is faulty, but if it gets to the stage of generating the inverse K video picture, the Z80 must be running), and on some boards there is an additional decoupling capacitor for the (nominal) +9V supply.

The ceramic capacitors are very reliable. They only normally fail if physically damaged.

With regards to the +5V supply voltages to the various parts, that looks like an open circuit fault rather than a short circuit. If it was a short circuit, the +5V points would be low everywhere including on pin 3 (output) of the 7805 voltage regulator.

I don’t think there is an issue with any of the chips.

Mark