ZX81 generates high pitched sound on CRT

[Tiger]

If the sound results from high-tension then it is not the location at the picture. But in the upper right corner at the tube-connector-PCB there is a solderpoint marked as C1 which I'm not sure that it looks damaged - if the wire isn't correctly soldered it may be the reason.
At the monitor there must be a high-tension area (mostly capsuled with a metal-plate) in that area you could see the sparks making the sound. Be careful to prevent for an electric shock - don't touch the PCB's . Wait 30 minutes or more after disconnecting the monitor from power before you handle any inner parts of the monitor!
With power of you can search for little marks (dark stripes) anywhere at the high-tension area PCB or solderpoints. But if you didn't find such marks, forget it, it is really dangerous to handle at the opened CRT.
You could be sure that the ZX81 isn't the reason of the noise. Anything at your CRT makes the pitched sound, don't know why it only occurs with the ZX connected ...

[dessony]

Mrtind,

Real cool to look at a few pictures of your monitor's CRT neck and its printed circuit board! However, how is your monitor's all of the electrolytic capacitors' health?

Cheers,
DesSony

[1024MAK]

Given the stated history, and no symptoms showing up in the displayed picture, it doesn’t appear to be a faulty electrolytic capacitor. Of course there is no harm in carefully inspecting all the electrolytic capacitors.

The typical location for the high voltage (EHT) to flash over is in or on the line output transformer. This can be identified as the item having the single rubber like lead that connects to the middle of the tube. If the monitor was stored in a damp area, or had been used heavily, flash-over can also occur in or around where this cable connects to the tube.

The 90 to 100V (HT) supply can sometimes leak, sometimes at the line output power transistor. But this normally causes symptoms to be displayed.

The other thing, is that if there was / is a serious problem with the monitor, it should make the offending noise when displaying images other than from the ZX81.

My feeling is that, as this monitor was designed for television video use, there may be a limitation in the design that is not happy with the non-standard sync signals and the non-standard video timing from the ZX81.

Testing it with a wider range of computers is my next suggestion, assuming you have any others to test with.

Mark

[mrtinb]

It works with the composite output of a Raspberry Pi, without the noise.

My plan is to use my Lambda 8300 with the custom ZX81 ROM. This will work without the noise.

Maybe I can make a modified ROM for the ZX81, that does not produce the sound. :-/

[Tiger]

I don't think that it is a software based fault. In my opinion modifying the ROM will not help. A reason can be the composite mod of the ZX81. There are so much possible ways to make the composite output. Maybe it is made with an capacitator it could be damaged - but then the noise must be heard in the video connection area between monitor and ZX81 somewhere at the jacket of the monitor or ZX81. Possible that the UHF-signal of the ZX81 is not "cut off". I think all your other parts have composite outputs which are not "selfmade".

[mrtinb]

As mentioned in the first post, there is no sound when software that uses WRX graphics is used.

So it is software affected. And remember: Most of the hardware in ZX81 is software.

[Tiger]

Sorry that I tried to help you -

[rmzalbar]

I don't have a scope right now or I would take a look at what the ZX81 signal looks like just before, during, and after vblank. Is it a flat line, or are there spikes in it? Out of interest.

[mrtinb]

I'm grateful for all the help you all give me. I havn't had time to test all suggestions yet.