Tektronix 2213 power supply - [Solved]

[euleken]

Hello together,

I try to repair a Tek 2213 which has trouble in chop mode. In chopper mode the beam starts at the left side and is getting darker. After 1cm the beam is no more visible. If I switch the timebase to lower times, the beam is longer visible but shows mystic curves of the input signal. Alternate mode and every channel vor it's own works.
First I checked the chopper oszillator and the signal to the blanking unit. These signals seems to be ok in timing and voltage. Then I measured the power signals and they are to high:

114V instead of 100V
33V instead of 30V
9,65V instead of 8,6V
-9,7V instead of -8,6V

After reading in the service manual I checked the head voltage of the primary regulator. It should be 41..44V and it is 42,8V - fine! Then I tried to get down the 8,6V line with R946 to the correct value. It's not possible. The smallest value is 9,7V so I replaced a zener diode (6,2V) which has an avalanche voltage of 5,8V but I cannot trim the voltage deeper than 9,6V.

Have someone any idea or had the same problem with a power supply on a Tek 2213/2215?
Thanks for feedback!

Ralf

[oldway]

To work safely, first don't apply any power source...
Instead, use a DC power supply 0-45V 3A (I use an HP6296A 0-60V 3A) to feed C937 (470µF) with 40V DC.... current limitation at about 2A.
You can ajust the voltage of your DC power supply to have -8.6V.
All the others voltages must be right.
You can then check all the scope without worrying about preregulator and power supply...
If everything is right, you can then remove the DC source and apply the 110 or 220V power source for troobleshooting the power supply and the preregulator.

You will need an isolation transformer to work safely.

[euleken]

Thanks for this great tip, oldway!

I connected my Insteg (2 x 30V, 2 x 3A) to the point of head voltage and adjusted the -8.6V line to -8.6V. Therefore the needed head voltage is about 38V and then the scope works fine in all modes.
I can regulate the voltage with potmeter R946 only in a range of +-100mV, but the potmeter itselfs works fine. I have to check the parts after that. Maybe somethings wrong in the voltage feedback line.

Thanks again for the tip. Now I can work and measure more safely. By the way: I have a regulated and isolated transformer lying in my crawl space, but this tool is not safely enough at the moment. It has a panel mount casing and comes from a laboratory. The sides are open (frame system) and before I will use it, I have to close them.

[oldway]

I remember you that preregulator circuits are mains power supply connected and that is very dangerous.
Please read safety procedure of Tektronix service manual before atempting further repair !

You absolutely need to use a good isolation transformer.

Now we know that the oscilloscope functions correctly when the -8.6V voltage is correct.

What is the current with 38V DC?

Normally, the voltage of the preregulator is of the order of 41 to 42V and the potentiometer has a greater adjustment range than +/- 100mV.

There is clearly something wrong!

There are two possible options:
- Or seek what is abnormal, but this seems not beeing very easy.
- Either reduce the voltage of pre regulator to 38Vdc, this seems to me easier.

But in the second case, the current will be higher in the pre regulator and there is a significant risk of overheating, and more particularly overheating of T933 (120-1439-00) that can become defective (isolation failure, short circuit between turns )

I always strongly recommend to install a fan following the diagram of 2236.
I did it on my 2235  to prevent failure of the pre regulator.

To reduce output voltage of the pre regulator, I would try to connect a 470K resistor in // with R922 (86.6K in your diagram).
If output voltage is lower, (as I think it would be), than measure this voltage and report the result here.

[euleken]

I want to solve the problem not at preregulators side cause I don't know what happens in near or far future.
Somethings wrong after the preregulator and I want to solve it anyway. Also its easier and saver to handle with an external power supply instead of srceewing at preregulators head voltage with line supply.

I changed Q458, Q954, Q956, measured all diodes in this circuit. Everything is fine, but I cannot regulate the -8.6V line. At the input of resistor R954 before Q956 I can clearly measure the influence of potmeter R946. Behind this resistor the influence is much deeper (some less 10mV).
For testing purpose I  putted out the zener diode VR951 and nothing else happens, means there's no influence of the zener diode at this moment. Very strange...
My next step is checking/changing all capacitors. I hope, the problem is not the transformer with its windings... 

The power consumption of the scope is on a right way. I measured 0.64A at 38Vdc in normal working state. This is really OK.

[oldway]

There are two solutions, I would have chosen the simplest.

Trying to repair as you do, you may cause another failure, and even a not repairable failure.

I consider that in this series 22xx of Tektronix oscilloscopes, only 2235 and 2236 still worth repair.

So it is better to choose the simplest and fastest solution.

[euleken]

I checked all caps and resistors - all they are fine. Now it's getting really difficult...
The original transistors are also build back and the scope works in same way as before only with 38Vdc head voltage. Why the potmeter R946 has no influence at the -8,6V line remains as a mystery. 

Maybe I have to choose the "simple" way of changing something at the preregulator to get the scope working correct, but I don't feel good with this kind of repairing.

Thanks again for your hints, oldway!

[euleken]

I changed R945 from 2k67 to 4k7 and the potmeter R946 from 500 to 2k5. After that I was able to regulate the -8.6V line in a range of approx. +-1V at a head voltage of 40V. Also I put a resistor with 1M parallel to R922 at the preregulator as oldway wrote and the head voltage goes down to 40V.
I adjusted the scope and tested it for several hours. It's working fine now!