A real head-scratcher

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kcbooboo
Posts: 437
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

A real head-scratcher

Post by kcbooboo » Fri May 12, 2017 2:09 pm

I'm trying to repair an old Motorola R2001B communications service monitor. It's got a CRT in it that can display text, swept traces, or a mix of both. The text is barely visible while the swept traces are nice and bright. The CRT has deflection plates and works essentially like one does on an ordinary oscilloscope. They position the beam and feed intensity modulation to the grid to make the text dots appear or not, sort of like an old dot-matrix printer.

The intensity modulation is a 0-35V signal that's coupled to the grid of the CRT through a 0.01uF cap. The amplitude increases as the front panel Intensity pot is rotated CW, which also increases the brightness of the display.

Here's the rub. I measured the voltages on the CRT with a Fluke 80K-6 high-voltage probe and Fluke 189 DMM. The cathode is -2,000V. The control grid is -2,018V at minimum intensity and -2,060V at maximum intensity. The focus grid is -1,860V. The 2nd anode is +4,115V.

My vacuum tube theory, at least for a triode, tells me that the more negative the grid, the less the tube should conduct, and the more positive it is, relative to the cathode, the more it should conduct. Does this also apply to the electron beam coming out of a CRT? This unit gets dimmer and actually has NO display when the grid is closer to the cathode voltage, and has a brighter display when the grid is 50-60V more negative than the cathode. This doesn't seem to make sense to me.

From what I can tell, as the intensity modulation signal goes positive, it causes the CRT to get brighter. I'm basing this on the fact that this signal has short narrow pulses on it for the text line on a mixed-mode display, and sits at +35V for the swept trace part of the display. But the intensity control on the front panel seems to contradict this action. NOTE: I may be in error about the intensity modulation polarity; it may be that the positive level dims the display, not brightens it. This has yet to be officially determined.

I've been scratching my head on this for weeks and figure I should share the aggravation with others. Is my theory at fault?

Bob M.

COMMENG
Posts: 78
Joined: Wed Dec 18, 2013 4:30 pm
Location: Midwest

Re: A real head-scratcher

Post by COMMENG » Wed May 17, 2017 2:09 pm

It sounds as if the Character Generator Z-axis output isn't properly modulating the cathode.

Somewhere between the Processor Board's Character Generator output and the Scope Amplifier board there should be an ORing function circuit that ORs the character generator output with other signals to modulate the Z-axis of the scope tube, the SCOPE Z-AXIS signal.


COMMENG

kcbooboo
Posts: 437
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: A real head-scratcher

Post by kcbooboo » Thu May 18, 2017 4:43 am

There is a circuit with several "AND" and "OR" gates that does mix a blanking signal with the character generator dot signal. All of that is done at TTL levels and both signals are solid 0-5V levels, nothing inbetween. This modulation signal gets amplified through one stage that controls the maximum positive level as a function of the front panel Intensity pot. When the pot is set fully CCW, no Z-Axis modulation is present and the CRT is dark. At about 40% rotation, the signal goes up to about 35VDC for the positive level and to ground for the negative level. There's a separate trim-pot to synch the intensity control voltage and the modulation amplitude. The modulation goes through a 0.01uF 3kv cap to the grid of the CRT. The Intensity control operates an opto-isolator that runs a PNP transistor that lets the grid voltage go more toward the -2100V supply as the intensity is increased. The cathode is fixed at -2000V. So everything appears to be doing its thing.

That 0.01uF cap measures 0.011uF. That's from the board that feeds it all the way through to the pin on the CRT tube socket.

I'm still confused in that the grid going further negative than the cathode increases the intensity, and as it gets closer to the cathode voltage the intensity decreases. Perhaps it's just the way CRTs operate, in that the control grid is a cylinder around the cathode with a hole at the front that lets the electron beam through, unlike an ordinary tube where the grid is a fine wire wound around the cathode.

I've got a feeling that the tube is just very low on emission and is on the way out, but I'd still like to verify how the intensity system operates.

Bob M.

COMMENG
Posts: 78
Joined: Wed Dec 18, 2013 4:30 pm
Location: Midwest

Re: A real head-scratcher

Post by COMMENG » Thu May 18, 2017 3:44 pm

What is the CRT's industry tube number?


COMMENG

kcbooboo
Posts: 437
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: A real head-scratcher

Post by kcbooboo » Fri May 19, 2017 3:39 am

Nothing I can identify. The "P/N" numbers in white ink are Motorola part numbers. Nothing else seems like a number I can relate to.

Bob M.
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R2000-series CRT labels

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