ZX81 generates high pitched sound on CRT

Discussions about Sinclair ZX80 and ZX81 Hardware
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mrtinb
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ZX81 generates high pitched sound on CRT

Post by mrtinb »

Hi :-)

As mentioned in another thread, I bought a B/W 10" CRT security monitor.

It produces a great picture for my ZX81. However when the ZX81 is connected to it, it has a hight pitched sound mostly like a lot of grashoppers.

I've made a video of it https://youtu.be/mRqWEro4Wdg but you have to turn the volume up to hear the sound. The sound is more irritating than what you can hear on the video.

However the sound is only there in text mode. If I load a program that uses WRX graphics, the monitor is pleasantly silent. In the video clip I first load MAZOGS, but since this is a text mode game, the sound is still there. After that I load DSTAR WRX version which is made with z88dk and when the program is run it changes to WRX graphics mode, and the monitor is silent.

So the high pitched sound is tied to the display routine in ROM, but the sound goes away with other display routines.

I've also tried the monitor with my Lambda 8300 (rom version 2), and here is not high pitched sound. If I remember correctly, the Lambda 8300 rom uses different timings in it's text mode in the rom.

Any ideas how to remove this annoying sound? Maybe a custom ZX81 ROM is the only solution. :-(
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
rmzalbar
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Re: ZX81 generates high pitched sound on CRT

Post by rmzalbar »

Well....

I put it in Audigy and found that the "chirps" are 6500 hz tones at 50 times per second. This 50hz repetition must be related to the retrace at vblank. A total guess but there may be some regular pattern of black scanlines drawn offscreen in the overscan area at the beginning or end of the screen refresh, that is different between the graphics and the hires modes. Your particular monitor is just sensitive to the frequency of voltage swings that occur when drawing those and you are hearing a coil vibrate at that frequency.

If you can adjust V.SIZE to squish the screen down where you can see the normally offscreen portions, you may see if there's a visible difference in those regions when switching between text and hires modes.

This is just out of interest, to determine the cause. It doesn't really help you solve the problem itself.. which might be noise coming from the vertical deflection yoke, in which case wedging it tighter and/or adding some hot glue across it may help.
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1024MAK
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Re: ZX81 generates high pitched sound on CRT

Post by 1024MAK »

Not sure if anything here will help, but...

Can you tell where the unwanted sound is coming from in the monitor?
Is it coming from the loud speaker, from an inductor/coil/transformer on the main board, or from the scan coils (the yoke on the CRT neck)?

The ZX81 does not produce a standard broadcast video signal, and the signal level is only ever one of three states (white, video black and sync black). Sharp signal level changes can cause unexpected side effects elsewhere.

So either some part of the video signal is leaking into the sound circuitry, or a magnetic component (e.g. scan coil) is vibrating causing sound.

Mark
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mrtinb
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Re: ZX81 generates high pitched sound on CRT

Post by mrtinb »

Well the volume is turned totally down. So I think the sound is comming from somewhere else.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
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mrtinb
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Re: ZX81 generates high pitched sound on CRT

Post by mrtinb »

I havn't opened up the monitor.

I don't feel so comfortable with an open monitor turned on, and sticking my head near, trying to listen, when it's turned on. I think of the high voltage of the CRT.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
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1024MAK
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Re: ZX81 generates high pitched sound on CRT

Post by 1024MAK »

It’s actually difficult to get an electric shock from the very high voltage used by the CRT. The greater danger is from parts that are connected to the mains supply...

If you are not happy operating it with the cover removed, I fully understand.

Mark
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mrtinb
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Re: ZX81 generates high pitched sound on CRT

Post by mrtinb »

I'll have no time looking into it tonight, but thank you for your replies. I'll post back in a few days.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
dessony
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Re: ZX81 generates high pitched sound on CRT

Post by dessony »

Mrtinb,

It is a good chance that noise might come from an arcing cracked flyback transformer. To confirm it, dim your room light, turn on your monitor and check into the monitor's ventilation slots. If you see a blue arc dancing around the monitor's board then it clearly is a cracked flyback transformer.

DesSony
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Tiger
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Re: ZX81 generates high pitched sound on CRT

Post by Tiger »

@mrtinb
vielleicht kannst Du ja deutsch verstehen, denn in englisch kann ich es leider nicht so ausdrücken:
Bei Röhrengeräten (CRT) kommt im Hochspannungsteil ein Zeilentrafo zum Einsatz der eine Hochspannung zur Bildröhre erzeugt. Im laufe der Jahre ziehen diese Trafo's irgendwie Feuchtigkeit und dann wird die Isolierung der Trafospulen brüchig. Dadurch kommt es dann zu Hochspannungsübersprüngen, die sich oft in den von Dir beschriebenen Geräuschen äußern. Die Funken, die dabei überspringen kann man manchmal auch sehen: es sind kleine blaue Lichtbögen, die mit diesen Geräuschen auftreten. In solchen Fällen sollte der Zeilentrafo alsbald ersetzt werden, denn über kurz oder lang wird die Hochspannung zusammenbrechen und Dein CRT wird kein Bild mehr liefern ...
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mrtinb
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Re: ZX81 generates high pitched sound on CRT

Post by mrtinb »

Thank you for your replies about a faulty flyback transformer.

There are maybe 2 things that indicates this is not the problem:
  • I don't know the age of the monitor, but I have just bought it, and has never been used before. I unwrapped the package.
  • The sound does not come, when I connect
    • a camera
    • a lambda 8300
    • load a program with WRX graphics.
    It only comes when using the ZX81s standard text ROM routines.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube
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