Re: Continuing Problems with 7704A Display

Harvey White

To get an idea of what the op-amp is doing, you have to measure the voltage between the + and - inputs.  (If the + input is grounded, then that's fine, but it must be connected).  The output voltage will be close to the difference between the + and - input times the op-amp gain (not the circuit gain, the actual amplifier gain, typically 100,000 or more).   That's the rule here, the op-amp doesn't have a choice.  If the output of the op-amp is too high, then the op amp is doing the best it can, and is being limited by the circuit supply voltages.  If the voltage at the output makes sense, then the op amp is ok.  Some op amps are used as comparators, which means that the inputs are deliberately overdriven, which pushes the op-amp output as far + or - as it can go.  The 741 series can generally get to within 1.5 volts of the supply.

If you have a feedback circuit (such as a gain of 10), if the output isn't being loaded too much, then given the + and - input limit, if that's bad, then the op amp is suspect.  Op amp supply voltages are important, of course.  Can't get more out of an op amp than the supply voltage (and you'd be surprised at how many simulators allow a 100 volt output from an op amp running from + and - 15.

Generally speaking, unless the op-amp is being used as a comparator, you should measure the same voltage on the + and - inputs within a few microvolts or so.


On 10/9/2021 6:04 PM, n4buq wrote:

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?

Barry - N4BUQ

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

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.


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