Tek RM17 repair update- On ranges 20 / 2 / 0.2 and 20mV / DIV settings, it fails to display the square wave. Instead, it displays sharp negative and positive pulses. That is, the square wave is like going through an RC "differentiator" circuit. a parasitic one of course ! Must be a simple fix I guess... broken connection, bad contact somewhere...
Since it affects only all the 20/2 etc ranges, the problem must be localized to a decade selector, hence should be able to narrow it down quite a bit in the schematic, I would think.
Worked on that issue above.
So I looked at the attenuator schematic, at the x2 section, and wondered what failure mode could potentially turn it into an RC differentiator, which would explain the issue at hand. Looked to me that yes there was a way for it to behave so, if resistor R132, the 500K one, upper leg of the voltage divider, was open circuit. So I checked that and bingo, that resistor was indeed open circuit. Replaced it, fired up the scope and... now the freaking scope won't display a trace at all ! Black screen....
![Banging Head |O](https://www.eevblog.com/forum/Smileys/default/bangheadonwall.gif.pagespeed.ce.8iLbFYV4Bc.gif)
Eventually noticed that there is ONE combination of calibrator amplitude, and attenuator setting, that DOES give me a trace : if I wayyyyy overdrive the input. Set the attenuator to its most sensitive range, 0.1V / DIV (below that, it's AC only and no trace on those no matter what), and set the calibrator output to 50 or 100Vpp, then all by magic I get a trace on the screen.
It's midnight and I am tired so I don't feel like elaborating.. but in short, I can't see how me replacing a freaking resistor in the attenuator, can cause the trace to disappear... and from the clues I gather, it looks like it's the vertical amplifier that's kaput, and likely one of the tubes that just died in front of me, causing a mega offset upwards, way off screen. I am too exhausted and pissed off, to look at that now. Tomorrow.
For now.... good night...
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