Re: Continuing Problems with 7704A Display


n4buq
 

Mark,

Pin 6 of U4110 checks at 10.33V. Pin 2, though, checks at 0.87V, neither of which is correct. Is the presence of that much voltage on the + input enough to cause the overly-high voltage on Pin 6 and, possibly, that's the problem or is the IC still responsible for that as well?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: "tekscopes" <TekScopes@groups.io>
Sent: Saturday, October 9, 2021 2:51:31 PM
Subject: Re: [TekScopes] Continuing Problems with 7704A Display
Barry,

The base of the bottom transistor is too far off making the transistor in full
conduction. This makes me highly suspect of the 741 IC. See if the output
voltage at pin 6 is what the schematic says. If that is not right, replace the
IC. A possible bad Q4105 is possible, open B to C. You can check Q4115 if you
want to confirm it is good or bad.

Mark


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