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Measuring clock wave

Posted: 18 Nov 2018, 14:42
by TBeaulieu
Hello,

I'm new to electronics, and trying to learn, hence this kit. I have the clock circuit running and can measure 1.5 RMS. I want to see the signal on my Rigol 1054Z. I've hooked the probes up to the test signal on the scope and calibrated the probe for a clean square wave. When I connect the probe to GND and CLOCK and hit AUTO on the scope I get what I'd consider to be a messy wave of around 35MHZ.

I assumed I'd get a clean square wave. What should I see? Thanks!
Gigatron Clock.png
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Re: Measuring clock wave

Posted: 18 Nov 2018, 20:17
by marcelk
This is unexpected. I would expect to see a 6.25 MHz signal, or 160 ns period (with probably a bit of overshoot/undershoot and ringing). 35 MHz is very close to the switching speed of the inverters, suggesting the crystal and caps aren't controlling the oscillation. I would first check the soldering of those joints.

In the meantime, just to be sure I'm not overlooking something, I can also build up a board to this exactly this stage, and measure with my own Rigol 1054Z. Perhaps somewhere next week already I have time for that.

Re: Measuring clock wave

Posted: 18 Nov 2018, 20:29
by TBeaulieu
Well, I decided to keep moving and finished. The board is strobing the LEDS, implying that it's working, but I can't get any VGS video. Tried two monitors. Going through the "study all points" phase, but here's a current scope reading on the clok point:

Re: Measuring clock wave

Posted: 18 Nov 2018, 20:46
by marcelk
Pfew. That looks like a normal clock. I still keep that action on my list.

The LED pattern is generated by the video/audio software loop. So if those LEDs are scanning back and forth, the software is operational. And with that, practically the entire board works(*). The LEDs should hop to the next state exactly 6 times per second.

If no video is coming out, it would be interesting to put the scope on the /hSync and /vSync signals. Easiest access is on the 68 ohm resistors labeled H and V. They should have a 31.25 kHz (6250000/200) and 59.98 Hz (6250000/200/521) signals on them respectively.

(*) We have seen a case recently where some soldering points in the ROM and RAM area were a bit poor to begin with, but it all worked at first. Then they broke down after upgrading from ROM v1 to ROM v3 due to the insertion force on the board. If that only affects the higher lines of one of the address buses, that can cause partially running software, or a very fast scanning LED pattern in weird cases.

Re: Measuring clock wave

Posted: 18 Nov 2018, 20:54
by TBeaulieu
Thanks Walter. I got it working! I found that applying a small amount of liquid flux and revisiting connections goes a long way. I just enjoyed a kick ass game of VR Snake! Is this Dolby 5.1? he he.

This was a really fun project. I sure hope I stick with it and continue to study the book, watch your videos and continue to learn.

Thanks!!!

Re: Measuring clock wave

Posted: 27 Nov 2018, 08:00
by marcelk
In prepping the VCF slides I found I had already captured the clocks on an otherwise unpopulated board last year. Find attached.

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