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Tektronix 7633 Scope, Can anyone please help Old64goat!!!

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David Hess:
These oscilloscopes are very repairable however the 7633 is the most complicated of the 76xx series and finding faults may require an electronics engineer's ability to understand the schematics.  The failure is probably simple if you can find it.

I am puzzled that you saw the CRT heater operating but measured no cathode voltage.  The CRT heater supply comes from the high voltage inverter so usually both are present or neither are present.

The high voltage connector for the PDA (post deflection acceleration) pulls straight apart.  You might have to twist it a little bit if it sticks do to friction.  But there is no reason to test the PDA voltage because the CRT will work *without* the PDA connected and without any PDA voltage although the CRT will be dim, poorly focused, and have the wrong deflection.

The exposed tip of the PDA connector is the CRT side so there is no high voltage there when it is disconnected.

Tektronix did not use security screws.  Your magnified views show stripped Philips heads which is a common problem.  I use small vice grips to remove them and then replace them with appropriate machine screws.

The low voltage test points are exposed on the opposite side of the PDA connector or they can be accessed from the rear after the cover is removed.  Page 4-9 and 5-32 discusses them and figure 8-17 at the back of the manual shows where they are on the z-axis board.

The 7A26 is a 200 MHz dual trace vertical amplifier and operates at 100 MHz in the 7633.  It is nicer than the 100 MHz 7A18 (75 MHz in the 7633) which would normally be installed into the 7633 because it has the switchable 20 MHz bandwidth limit.

How do you know that the 7B53A timebase works?  You might swap it with one of the 7A26 vertical amplifier to see if that unblanks the beam.  The beam is only unblanked when a working timebase or vertical amplifier plug-in is installed into the horizontal (right most) slot.  A vertical amplifier would normally be installed into the horizontal slot for X-Y operation although the 7B53A timebase also supports this.

I think you will have to gain access to high voltage board A10 for measurements but I am not sure how to do that.  The oscilloscope will operate without the readout board installed which may help.  There is a test point for the z-axis output which I would take a close look at.  If the CRT is unblanked, then the intensity control should change the voltage level there.

The long cover in front of the PDA connector is just the CRT cover.  There is nothing under it except for the CRT.

finom1:
David,
WOW, thank you for all that detail.

casinada:
In his videos he mentions the User manual. Does he have the service manual?
http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%207633%20%20Service.pdf
 :)

David Hess:
The available free scans of the 7633 service manual are actually pretty good and the one casinada linked is the one I have.

1. If the -1475 cathode voltage is missing, then check that Q1195 is good, capacitor C1198 is good, and that unregulated +15 volts is present across C1198; these are all shown in the upper left corner of schematic 7.  Fuse F814 located on the rectifier board and shown on the left side of schematic 8 supplies the unregulated +15 volts to the high voltage board but I think Old64goat checked it.

I know this means gaining access to the high voltage board but what are you going to do?  Unfortunately, I am not sure how to do that.  Someone else here are on the TekScopes@yahoogroups.com email list should know.

2. When you pull the reduced scan switch out on the front which I saw Old64goat messing with, then the -1475 cathode voltage is doubled to about -3000 volts while the PDA voltage stays the same.  This reduces the CRT deflection to 1/2 of its normal value and makes the display sharper and brighter.  This function makes the high voltage circuits more complicated on the 7633 than the 7623A which lacks this function.

3. Once the high voltage is working and if there is still no display, monitor the output of the z-axis amplifier to see if the beam is being unblanked and if the intensity control is working.  There is a procedure in the service manual for adjusting the transient response of the z-axis amplifier which describes how to monitor this signal.

4. That readout board we see in the video is one of the later analog/digital hybrid readout boards and produces the second best display in my opinion.  (1) The earlier boards used analog (!) ROMs and the hybrid board replaced them with a digital ROM so this 7633 is one of the later ones.  The service manual casinada linked only shows the earlier all analog readout board but that should not matter for this.  For those who are interested, the details of this hybrid analog/digital readout board can be found in the 7904A service manual.

(1) There is *another* hybrid analog/digital version which is even better (or at least as good) but this one is pretty good.  My favorite 2232 use a similar character generation method and looks great.  Vector is always better than raster.

finom1:

--- Quote from: casinada on May 02, 2017, 05:00:38 pm ---In his videos he mentions the User manual. Does he have the service manual?
http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%207633%20%20Service.pdf
 :)

--- End quote ---

Casinada,
I will send it to him.

Thank you for sharing that link!!!

Yes he has this manual now.
Thanks again!!!

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