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

[finom1]

My friend Old64goat is having a problem with a Tektronix 7633 Scope that he won at a auction!!!
No Trace, No High Voltage?
He explains the issues here in his videos:
1.
2.

Brace yourself, he calls himself The Blind Tinkerer!!! Might take some getting used too, but he has a heart of gold and would help anyone out.

Thank you for any help you can give him!!!

[KWKolb]

You can remove the back panel, transformer, and rectifier/filter assembly as a group by removing three Phillips screws on the left and right sides of the chassis (six screws total).

The assembly will slide out the back.  There should be sufficient slack in the cables, but I had to disconnect one or two on my 7633.

Good luck and please keep us informed.

Kevin

[Electro Detective]

Old64goat YT videos rock! especially before he sold the shop.

Hope he sorts it out asap, good luck and best wishes to him.

Watch out for DRY contacts in the switches on these old school Teks, before getting too medieval.

If enough of them are just not making it, the symptoms can be identical to the usual suspects PSUs, caps etc
and he could be facing an unnecessary troubleshoot merry go round                

[finom1]

Thank you Kevin,
 for sharing your wisdom and helping the Old64goat out!!!

[finom1]

WOW, thank you great advice.

Thank you for sharing your wisdom helping the Old64goat out!!!

[Harb]

Pity the old bugga can't see better.....must be a pain in the but having to use a magnifying glass to look at everything

[KWKolb]

Picture of the rectifier/filter assembly when slid out.  Mine is awaiting a recap.

[finom1]

Thanks everyone for helping this kind gentleman out!!!
He is having a problem with registering, so he sent me this message to post to everyone, this is what Old63goat wants me to share with you:

"If you would do me a favor and mention to them that ALL 5 of the fuses are good, ALL the low voltages are good.
However I am not getting the minus 1,475 volts at the HV test point, all I get is ONE volt positive at that HV test point.
Also I was able to slide out the low voltage power supply BUT I can't get the HV "box" out, the one that has that -1,475 volt test point on it."

Thanks everyone!!!

[finom1]

Fantastic picture, keep them coming!!!

Thank you for sharing!!!

[finom1]

Thank you for helping this kind fellow out.

[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]

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
 

Casinada,
I will send it to him.

Thank you for sharing that link!!!

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

[finom1]

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.

Hi David,
Old64goat is going to make another video to provide more information after reading your post, that you so kindly provided!!!

Thank you for trying to help him, he is a very nice person just trying to get his vintage electronics repair shop up and running again!!!

Thanks again!!!

[dave_k]

In his first video, he says the pilot light for the main power light is not working. This might indicate a short to ground on the 5v supply rail. I had a similar fault in a 7603 scope, which was traced to a shorted cap on the main interface PCB.

[james_s]

Does this use a voltage multiplier to generate the high voltage? I've seen shorted capacitors in those cause a lack of HV while not greatly affecting lower voltages coming off the same transformer.

[finom1]

David,
Here is the video he made for you:
Video For The EEVblog Forum -

Thank you David for all your help!!!

Everyone please leave feedback on his video also, so he can respond to you faster!!!

[finom1]

In his first video, he says the pilot light for the main power light is not working. This might indicate a short to ground on the 5v supply rail. I had a similar fault in a 7603 scope, which was traced to a shorted cap on the main interface PCB.

Thank you for helping him. Please see his latest video!!!

[finom1]

Does this use a voltage multiplier to generate the high voltage? I've seen shorted capacitors in those cause a lack of HV while not greatly affecting lower voltages coming off the same transformer.

Great question, could you please see his latest video and ask that same question again.

Thanks again!!!

[finom1]

Question,
What type of High Voltage Probe does the old64goat need to troubleshoot his scope?

I think we need some urgent help with this one to keep him safe:)

[tautech]

Question,
What type of High Voltage Probe does the old64goat need to troubleshoot his scope?

I think we need some urgent help with this one to keep him safe:)

Only a DMM capable of measuring the EHT, the voltage will be listed in the SM and you want a little V headroom.
Be aware the EHT voltage is for the CRT cathode and it is negative and commonly in the 1.5 - 2.5kV range. Check the SM.
I use old AVO's for this as they have a 2.5 or 3kV range, depending on the model.

Assume the PDA is working until proven otherwise, it can be +10kV and tricky to measure without a UHV probe.
PDA multipliers do fail but it is rare, for now assume it works.

[finom1]

Can someone please provide screen shots or pdf scans of what the old64goat is asking for?

Here is what he has written:
"This PDF is driving me nuts, I can't find anything on it.
David posted this answer on his latest post:

"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.


"Where is schematic 7?

I can't find schematic 8, ALL fuses test good.

As I had mentioned in the newest video I am getting all the low voltages on that connector
(disconnected) from the Z-AXIS board.
If there are other low voltage test points I can't find them.

What may help is a print out of all the schematics & voltage test points for this no HV problem.
Maybe you can post this for me.
Thanks,
Bill"

Please help provide these documents to help solve his problem if you can. He is very thankful for all the help he is getting!!!

[David Hess]

Going to roughly respond to the video in order:

1. It looks like the triggering is working.  The internal trigger on these oscilloscopes is pretty complicated because the mainframe selects from the left or right trigger sources and the dual trace plug-ins select from their channel 1 or channel 2 sources; that makes 4 internal trigger sources so it is easy to have it configured in a way that it seems like it is not working.

2. It looks like the power lamp is burned out.  It runs on the +5 volt regulated output but since the triggering apparently works, we know the +5 volts is working so the lamp is probably just burned out.

3. The transistors on the horizontal CRT amplifier board run hot; I cannot hold my finger on them when they are operating.  I verified this on my 7603 which is a very similar design.  The load resistors run hot also; that is why they are so large.

3. I figured out the mystery of the CRT heater being active while the high voltage is apparently missing.

The heater for the CRT electron gun is toward the back of the CRT and not easily visible.  What Old64goat  saw is the flood gun heater which is used on storage CRTs to "flood" the screen with electrons as part of the storage capability.  You can tell because the flood gun heater that you see is below the horizontal deflection plates and the electron CRT gun is toward the back of the CRT before any of the deflection plates.  The flood gun heater runs off of the -15 volt supply through an 8 ohm as shown on schematic 10 and will be powered whether the high voltage inverter is operating or not.

4. So that means that the high voltage inverter which supplies the electron gun heater, -1475 cathode voltage, and PDA voltage is not operating.  That leads back to checking Q1195, capacitor C1198, and that the unregulated +15 volt supply is present across C1198.  If those are all good, then the high voltage regulation loop needs to be checked.

In his first video, he says the pilot light for the main power light is not working. This might indicate a short to ground on the 5v supply rail. I had a similar fault in a 7603 scope, which was traced to a shorted cap on the main interface PCB.

The triggering works and Old64goat verified the presence of the +5 volt regulated output at the test point.

Does this use a voltage multiplier to generate the high voltage? I've seen shorted capacitors in those cause a lack of HV while not greatly affecting lower voltages coming off the same transformer.

The high voltage multiplier for the PDA is separate from the cathode voltage rectifier but they share a winding.  It is possible that the high voltage multiplier is shorted but I am not sure what result that would produce.  I suspect it would blow fuse F814.  If fuse F814 is good, then we should still see *something* at the negative cathode voltage test point if the inverter is operating.

Question, what type of High Voltage Probe does the old64goat need to troubleshoot his scope?

I think we need some urgent help with this one to keep him safe:)

His VTV works to 1500 volts which is good enough for checking the cathode voltage and that is all that is needed at this point.  I have a Fluke 40k-6 which I use to measure cathode voltages but it is only good to 6 kilovolts.  It is usually not necessary to check the very high PDA voltage; we should be able to get *something* on the CRT without dealing with the PDA.