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From: Zentronics42@...
To: "tekscopes" <[email protected]>
Sent: Monday, November 15, 2021 6:55:51 PM
Subject: Re: [TekScopes] 7704A - Grid Bias

Barry,
I have tried for a while to get the setting copied here but PDF's just wont
cooperate with me at the moment. The YouTube video is the correct procedure
based on the service manual. You could easily do this same procedure based on
X-Y mode as well however you will be stuck when you get to step 6 transient
response. This will need a moving sweep of about 10 Mhz. What they mean in the
manual by free running sweep is the time base in auto untriggered. This means
that you do NOT need the calibrator a 7aX set to GND on the input would be
fine. But this does lead me to another question. If step 6 is transient
response, do you have brightness issues with A sweep at faster time base
settings? Relevant pages in the service manual are 4-7 and 4-8

Then manual just does the grid bias with a moving untriggered sweep and them
moves right in to transient response. This set up allows you to go both
adjustments with out having to reconfigure the test set up. The digital scope
used in the video was to get the +4v set point as well as the transient
response. Sorry about the confusion. This is what I get for trying to do things
from memory at this point.

Zen

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of n4buq
Sent: Monday, November 15, 2021 9:46 AM
To: tekscopes <[email protected]>
Subject: Re: [TekScopes] 7704A - Grid Bias

Zen,

Regarding that calibration video, I notice the fellow is setting the timebase in
the "test" oscilloscope to 0.2s/div. That doesn't make sense to me and I
presume it's incorrect. He switches to a digital scope to measure the +4V
delta but the image on the 7704A is a line, not a dot. I don't suppose it
really matters exactly what's displayed but I presume the timebase in the 7704A
is not set per the instructions(?).

Thanks,
Barry - N4BUQ

----- Original Message -----

From: Zentronics42@...
To: "tekscopes" <[email protected]>
Sent: Sunday, November 14, 2021 8:06:21 PM
Subject: Re: [TekScopes] 7704A - Grid Bias

Barry a question about the setup. Do you have any plugins in the frame
at all when you are attempting to do the grid bias setting? No plugins
will not give the tube any drive signals so there will be no dot. The
Easiest way to get a dot on a 7704A is to use 2 vertical plugins one
in vert one in horz. This will put the frame in to X-Y mode and should
give you a display that is adjustable from the controls. That is how I
would do grid bias with out the standardizer, but your milage may vary.

This might also help if you have not seen this it is a full
calibration on a 7704A

Zen

-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Jeff
Dutky
Sent: Sunday, November 14, 2021 8:34 PM
To: [email protected]
Subject: Re: [TekScopes] 7704A - Grid Bias

Barry,

I have not been able to find my notes on the grid bias problem I saw
in the 475A, but here is the thread on the subject, which will be
nearly as good as my
notes:

/g/TekScopes/topic/grid_bias_adjustment_on_475a/79348
510?p=Created%2C%2C%2C20%2C2%2C120%2C0&jump=1

The problem was that I could barely get the spot to show up during the
grid bias adjustment process, and the grid bias trim pot was cranked
all the way over to one end of its range, so there was no room for
further adjustment, and adjusting in the only direction available to
me would completely quench to spot, so it wasn't useful.

The solution, after various investigations, turned out to be the HV
precision resistor that was used to regulate the HV multiplier output.
Replacement of that resistor fixed the problem (though it may have
been possible to fix the problem merely by thoroughly cleaning and
resoldering the resistor, but I never went back and tried that). The
main difference between my problem and yours, however, was that I was
NOT seeing high CRT brightness. If anything the CRT brightness was a little low.

-- Jeff Dutky