No trace on Tek 475A

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
The odd thing here is that more than one voltage is involved, see if they're derived from the same initial supply source or if there's a circuit board somewhere that uses mainly the ones that are affected.
Manual says the following:

The +50 volt supply is the reference for all other
supplies and must be repaired first. The +15 volt supply
and the +5 volt supply should be working properly before
repair on the –8 volt supply or the –15 volt supply is
attempted.



So I guess it makes sense that if the +50 is so blasted high the rest of the voltages will be high too.

If the +50 was high would that mean the -8 would be taken toward the positive? It is +1 V.

There is supposed to be a +50 adjustment but I wanted to wait for advice on if it should be adjusted and where it is!

I am guessing that it is not just a simple adjustment. How would it get so far out all of a sudden?
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Caps are cheap, dirt cheap. Maximum $5 each.
Have not bought the really big ones in a number of years. I just figured they would be expensive. Glad to hear they are not too bad.

But replacing them is another thing. They are really buried. It looks like I would need to remove 2 boards. The one looks like it will require desoldering to remove it from the front panel.
 

tom66

Joined May 9, 2009
2,595
Manual says the following:

The +50 volt supply is the reference for all other
supplies and must be repaired first. The +15 volt supply
and the +5 volt supply should be working properly before
repair on the –8 volt supply or the –15 volt supply is
attempted.



So I guess it makes sense that if the +50 is so blasted high the rest of the voltages will be high too.

If the +50 was high would that mean the -8 would be taken toward the positive? It is +1 V.

There is supposed to be a +50 adjustment but I wanted to wait for advice on if it should be adjusted and where it is!

I am guessing that it is not just a simple adjustment. How would it get so far out all of a sudden?
If the caps become leaky the voltage might rise. Adjust the pot until the voltage drops a bit, but don't go all the way.

As far as I know, there is no desoldering involved in removing boards. They are usually connected together and screws attach them to the case, but I could be wrong. The manual will be the best place for information on this.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
If the caps become leaky the voltage might rise. Adjust the pot until the voltage drops a bit, but don't go all the way.

As far as I know, there is no desoldering involved in removing boards. They are usually connected together and screws attach them to the case, but I could be wrong. The manual will be the best place for information on this.

I found the +110V. It is at +135V. I also found the -15. It is at +.7.


The desoldering would be to remove the one board from the front panel. It appears that a resistor is soldered between the 2 BNC connectors( trigger inputs). The connectors appear to be removed from the front panel out.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
If the caps become leaky the voltage might rise. Adjust the pot until the voltage drops a bit, but don't go all the way.

As far as I know, there is no desoldering involved in removing boards. They are usually connected together and screws attach them to the case, but I could be wrong. The manual will be the best place for information on this.
I adjust the +50 adjust and got it down to +58V before I stopped.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Sorry I guess it is just too late. Time for sleep. :) Anyway I finally got on the TekScope yahoo forum and someone sent me the manual with the schematic. Thanks for your help.

I might be asking again on another thread. :) Bu the folks at Tekscopes are very helpful so far and I am on my way to fixing this thing. I think I already figured out one of the issues but I am waiting on a reply on Tekscopes to verify my hunch.

Good news!!! My scope is now working! It was a bad op amp in the -15 / -8 power supply.

I want to thank everyone in this thread that encouraged me to not give up and thank all of those that helped.

When I first popped the hood of the scope and looked around, I thought there was no way I was going to be able to fix it. But thanks to all of the wonderful people on this forum (including the people in this thread) I was able to do it! :)

If I had this forum 30 years ago, I do not think I would have ever given up the hobby.
 

gootee

Joined Apr 24, 2007
447
Excellemt! Congratulations!

How did you find it? (And how long did it take?)

Did you adjust all of the power supply voltages?

Do you have any way to verify the calibration, at least for the vertical voltage display accuracy and the basic timing accuracy? You can probably do a "poor man's calibration" of those, even without any specialized equipment.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,830
Excellemt! Congratulations!

How did you find it? (And how long did it take?)

Did you adjust all of the power supply voltages?

Do you have any way to verify the calibration, at least for the vertical voltage display accuracy and the basic timing accuracy? You can probably do a "poor man's calibration" of those, even without any specialized equipment.
Once I got the schematic I saw that the op amp was in common with both -8 and -15 but I still took a bunch of measurements at the direction of folks here and at yahoo tekscopes.

Once oi was pretty certain, it was some where in the opamp area, a forum member instructed me on setting up a test circuit. I first tested with a "known good" (nothing is really known good :) ) op amp and that worked, so I tried the op amp in the scope and it failed. Popped the new op amp in and the scope came right up.

I would be interested in a "poor man's" calibration.
 

jpanhalt

Joined Jan 18, 2008
11,087
Leave it on and go run some errands. See if it is working when you get back. You may still have some power supply/capacitor problems. Otherwise, great news.

Speaking for us old guys, when something fails that is that old, there's gotta be a reason.

John
 

gootee

Joined Apr 24, 2007
447
Simple calibration checks are straightforward. For example, make a DC voltage, check it with your multimeter, then display it on the scope and check the amplitude. Find a known-frequency signal, display it, and check the timebase accuracy.

Read the calibration procedures. Some of them call for specialized equipment but you can often substitute. If I get time I will read through them (It's been a while.). The TekScopes people should have some great ideas, for that type of thing.

By the way: I have a page on my website that describes a method for cleaning the attenuator switch contacts, and one or two other remedies, in case your traces are too fuzzy. It's at http://fullnet.com/~tomg/tekclean.htm .

Cheers,

Tom
 

Rbeckett

Joined Sep 3, 2010
208
Glad ya got her going. Those guys in the Tek forum on Yahoo do know a good bit about the old scopes. That coupled with the knowledge here and anything is definately possible. I too am learning and the explanations of some of the concepts are priceless. I can read and reread something a hundred times, but once I get the concept explained where I understand it then I am good. Glad to hear ya got it fixed.
Bob
 
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