Re: 7904 - Very Dim Trace


 

On 14 Feb 2016 16:10:25 -0800, you wrote:

Now I'm confused even more than usual. As I understand from the messages to date, some conflicting info is apparent, so I'm trying to get it straight:

1. The problem scope seems to have the right sized display, just very dim, working at 14 kV instead of 21.
That is my understanding.

2. I thought the PDA made the deflection sensitivity relatively immune to variations in anode voltage.
It does make it relatively immune compared to a change in cathode
voltage and I think this depends on the design of the CRT. Going from
full PDA to zero PDA only halves the deflection sensitivity of the
7904 and 7603 but I remember in a past discussion thread here that in
a 465, it has very little effect.

3. Jochens disagrees with that, and David's actual experiments seem to show that the deflection sensitivity is affected, the same on both a 7904 (which I believe has PDA) and a 7603 (which I believe does not).
The 7603 has a cathode voltage of -2975 volts and a post deflection
acceleration voltage of +12 kilovolts.

See the problem here? Statements 1 and 2 can fit together, but not 2 and 3, so therefore not 1 and 3. am I missing something again?

If 1 and 3 are true, one possibility is that someone adjusted the gains to compensate for the reduced anode voltage. BUT, I thought this just happened recently, and the scope was working normally before.
I suspect such a minor change in PDA voltage from +21 kilovolts to +14
kilovolts is not enough to affect the deflection sensitivity
significantly. That is why I think another explanation is needed for
the very dim display. Maybe the low PDA voltage was a long standing
but unknown issue with this particular 7904.

Ed

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