Re: Help with a Tek 7A26

 

Hi William,
The bigger the problem the easier it is to find.
This is a biggie. There are only a few possibilities since both channels have the same problem, the cause must be common.
Power supplies are one possibility but power comes from the mainframe and if that was bad you would see it in lots of other places.

* You should start at the Display Switching and Output Schematic (#4).
* There is a small possibility that the power is not right. This entire section of the 7A26 appears to use only +15V and -15V so check these voltages first.
* Next set the CH 1 and Ch 2 inputs to 5mV/Div so there is no attenuation of the incoming signal.
* Place a small signal on the inputs or use the 40mVcalibrator signal.
* Look at the signal where it comes into schematic #4 in the upper left, from schematic 2.
* Schematic 4 shows the waveform you should get on each of the channels. You will have to look elsewhere to figure out what you should have input to the plugin to get that waveform. There has to be some overall gain from the input to the point you are measuring.
* Q820 and Q840 are in a common base configuration. The square wave going into their emitters is about 26mV pp on a DC level of 10.6V. * The signal coming out of their collectors is 200mV pp on a DC level of 0V AFTER it passes through R820 and R840.

Compare these readings to the good 7A26. That should narrow down the problem considerably.

The trigger circuit works exactly the same so it is possible you can swap transistors from the trigger circuit to the signal circuit to determine if a transistor is to blame.

Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of k6whp
Sent: Thursday, February 06, 2020 2:32 PM
To: TekScopes@groups.io
Subject: [TekScopes] Help with a Tek 7A26

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.
--
William, k6whp
"A friend told me to cheer up, things could be worse.
So I cheered up and, sure enough, things got worse!"





--
Dennis Tillman W7PF
TekScopes Moderator

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