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Thanks for these detailed instructions Jerry.

With everything plugged in as normal, 7 ohms between pins 1 & 2 on P1710.

Unplugging P1710 from the z-axis board and measuring at the board itself- 16 ohms.

Rather than risking removing the crt socket, I measured at the connector (pin 1 goes to pin 1 of CRT, pin 2 goes to pin 14 of CRT)- 13 ohms.

So I think this suggests that the CRT is not bad.

I do occasionally see a brief flickering from the CRT when powering up (similar to the flickering one used to see from first powering on fluorescent lights back when they used ballasts).

Here are some more clues using the outputs on the back of the R7903:

I get no signal of any kind from the Gate Out.
If I put a square wave into the vertical amp (plugin) I do get a square wave from the SIG Out on the back.
Sawtooth Out: using DC coupling I just see a steady -8 volt line. Using AC coupling I get the following trace (note level):



So, where does this suggest I should investigate?

I haven't found an easy way to temporarily unplug the vertical deflection plates, but I could temporarily unplug the horizontal deflection lines. Would that be safe, and presumably give me a straight vertical line on the CRT?

Thanks,
Keith Ostertag

--- In TekScopes@..., jerry massengale wrote:
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> Keith,
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> You are likely faced with a crt with open filaments. With power off, measure the resistance from P1710 pin 1 to pin 2 of the z-axis board. If the reading is very low( 2 or 3 ohms) ease the crt socket off the base of the tube and remeasure on 1 and 2 of the board. It should read about 16 ohms on pin 1 to 2 on z-axis. If it does not read 16 ohms, check R1887 for 16 ohms. If the reading is very high it is possible the filament winding of T1310 is open or a bad connection. The CRT base will have a key or gap between pins 1 and 14. These are the filament connections internal to the CRT. The resistance should be very low. An open or very high resistance means the crt is bad.
>
>
> Jerry Massengale
>
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>
>
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> -----Original Message-----
> From: keithostertag
> To: TekScopes <TekScopes@...>
> Sent: Tue, Jul 10, 2012 9:23 pm
> Subject: [TekScopes] Re: Help with R7903 needed, some success...
>
>
>
>
>
> Some time after my first response to Jim Powell's comment I decided to re-look around, and I discovered that I only needed to remove the bottom panel in order to see the end of the CRT. Not sure why I didn't think of it earlier, duh.
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> But even with all room lights off I can't see any "glow" or other indication that the CRT is working.
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> By dismantling the chasis a bit more I was able to see the vertical sig wires from underneath the vertical amp board (A10) going to the CRT deflection plates, but I haven't figured out yet how to access them. They don't _appear_ to be pins- they look soldered. So I haven't yet figured out a way to temporarily unplug them. The way that board is mounted is fairly complicated due to the how U745 is mounted and wired.
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> By carefully using my probes I was able to measure the voltage at both of those lines going to the vertical deflection plates- both about 34 volts (SM says they should be 34.5 volts).
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> On the horiz amp board (A11) TP4901 measured -7.46, while TP4911 measured -6.51 (schematic calls for -7.07 for both).
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> But also on A11 I measured the +sig to be about 42 volts, and the -sig to be about 61 volts (both should be 54 volts).
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> So I'll be looking into that.... but, are those levels significantly off enough to explain the absence of any CRT activity? No BeamFinder, no Readout, no trace.
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> Twice now I _may_ have seen a very brief and low level flash of light coming from the CRT when I first power on. Hoping to confirm that.
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> Thanks,
> Keith Ostertag
>