Re: 7904 - Very Dim Trace


On 14 Feb 2016 20:04:24 -0800, you wrote:

No joy. The Service Manual Parts List calls out a 10 ohm series resistor in the heater, and that's what's in there. It measures 9.8 ohms. Re-checked deflection. Both X and Y are within 2%.
I'm still not convinced the 14kV is the only problem. This scope is new to me, and it could well have been 'mal-adjusted' to make up for the low PDA voltage. Still noodling on this. Continued comments welcomed...
I am not convinced either so I ran another test ...

I was considering how I could operate my 7904 with a 14 kilovolt PDA
to see how the CRT would behave but I lack a suitable power supply. I
do however have a 7603 sitting right next to the 7904 on the same desk
and it uses a 12 kilovolt PDA and the connectors are the same ...

So I reversed the 7603 to allow the anode lead to reach and connected
the PDA lead from the 7904 CRT to the 7603 power supply and powered
the 7904 up and then the 7603. It worked! I could even turn the 7603
off and watch the effect on the 7904 CRT as the PDA slowly discharged.

At 12 kilovolts, it was not as bright or sharp as when the 7904 is
using a 24 kilovolt PDA but it was close. Deflection was down about
10% versus being halved with no PDA at all.

So reducing the PDA to half of its nominal value had a minor but
noticeable effect on brightness and deflection. I think you have a
different problem causing a very dim CRT.

You have already verified that the output from the z-axis amplifier is
good and that the resistor in series with the heater is probably good
so I suspect the z-axis DC restorer. Did you make a measurement of
the actual grid to cathode voltage? This can also be done by floating
your Fluke...

The way the DC restorer works is that the difference between the
z-axis amplifier output and the grid bias voltage is subtracted from
the cathode voltage making the grid voltage more negative. Maybe
R1675 at the grid bias adjustment is open or the grid bias
potentiometer is bad.

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