Re: Help with a Tek 7A26

Harvey White

Just some things I'd think about.  If the gain is the same for both channels, then either both channels have the same problem, or the problem is after where the switching happens (or may be something in the switching).

I'd put a standard signal into one channel, go for one channel only.  I'd look for the signal that Tektronix shows as a reference input.  Tek ought to show you the voltage readings (DC and AC) for a particular input value.

As an example, say that at a particular point you're supposed to see 10 volts p-p with the gain controls setup a particular way and with a specific input.  If you see 6.6 volts p/p, then I'd suspect the gain of the stage immediately before.  If there's an overall gain, I'd look at those parts specifically after you see if you can narrow down where the gain goes wonky.

I'd also look to see if the positioning controls have the proper range, although you can get much the same result with DC inputs or AC inputs.

In this case, I'd take one channel, follow it through from the beginning to the switching and then to the end.  While I suspect that the gain is bad somewhere after the switching, I'd still run a test to see if it's where it's ok.  You can run a divide by test, where you pick halfway through the chain (approximately) and then check the gain.  If it's ok, then after that point, if not, then before.  Continue to divide by  2 until you get to one, then play one stage at a time.


On 2/6/2020 5:32 PM, k6whp wrote:
Gentlemen (and ladies),

I have a sick 7A26 which I would seek advice on. The symptoms are as succinctly as follows:

(1) The vertical measurements are inaccurate; they are approximately 2/3rds of the correct value.
(2) The problem is common to both channels.
(3) The calibration of the unit does not provide sufficient range to correct the problem.

I have gone through the manual and have run the troubleshooting flow chart cursorily (he shamefacedly admits) and have scoured the schematics but would request help. I am NOT looking for a solution to be suggested (although, if something occurs..) but would appreciate a pointer or two on where to begin. I am going to borrow a friend's good 7A26 and "ohm the rails" as well as take comparative measurements on both. (I have a 7704A and can put the 7A26 in the left most slot to do the probing.) I would be grateful of some recommendations as to where to start snooping.

I am more interested in learning than ONLY fixing.

Thank you for your patience and suggestions.

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