Siemens oscillar MO7192 repair

[grusus]

Hi.
As me and rf+tech were trying to fix Tek 2246 scope, It turned out that my second scope is broken also.
To not make garbage in 2246 topic we decided to start new one.
Here is our thoughts about this one.

Unfortunately there are no schematics, service manual, nothing for this scope. I found only some Pics and some past sale offers.

Repair is for fun not for profit. Just to learn something new.

grusus:

When you input signal for ch1 trace is there. Also for ch2. Problem is that there is only one trace at same time and channels interacts to each other (you can change position of trace for ch2 with ch1 knob and vice versa) Also changing volts/div for ch2 changes position of ch1 trace and vice versa.

rf+tech:

The problem you have described with the M07192 appears to be with the channel switching circuit.

ALTernate setting (A&B) should sweep Ch 1, then switch to Ch 2 on the next sweep. Ch 1, Ch2, Ch1, etc.

CHOP setting (A+B) switches very rapidly between Ch 1 and Ch 2, during every sweep.

When the channel switching circuit works correctly, the two vertical position controls should not interact.

Forum member w2eaw has a video that demonstrates this behavior.

https://www.youtube.com/watch?time_continue=1&v=zsy-pxpfB4o&feature=emb_logo


What effect is observed with the norm. invert. button in and out?

When invert button is in and out, trace is moved up and down on the screen. See video.
Video: https://photos.app.goo.gl/TeZtw1fYbZQDkF5KA

Some pictures of the guts:
Front right side view:
https://photos.app.goo.gl/Zmy6YfBLbnt4BpzC8

What I think is trigger and time base board:
https://photos.app.goo.gl/Ja2o6uYxLwajYPAcA

Other side:
https://photos.app.goo.gl/H46BzDguwxrpEM5m6

I think this is Horizontal and vertical amplifier board:
https://photos.app.goo.gl/s6QR4gwte6VXyxtH9

And continuation of board obove as it is L shaped. Here PSU + HV:
https://photos.app.goo.gl/7wfGVXXeEQbG7yRm6

Another front left side view:
https://photos.app.goo.gl/hw5bjk3yopdGmCZN9

This is what I think Horizontal and vertical preamp board:
https://photos.app.goo.gl/o7wVsLXFBYiN3syv9
https://photos.app.goo.gl/MuHEDTd6MxuJH3BU8
https://photos.app.goo.gl/BbXAgrBEgnSupqao9
https://photos.app.goo.gl/qyqXioT9hy8dm9457

Other side:
https://photos.app.goo.gl/UbSxo6d631ecmiD67

[rf+tech]

Thanks for posting the video. So, one trace is all that can be obtained. To my eyes, both traces appear to be sum or difference, no channel alternation.

A&B versus A+B.

& is a logical operator implying both arguments, A, B are true, or present.

A+B should be "Sum"; with Inv pushed in, the result would be "Difference".

Have you tried contact cleaner, preferably DeOxit, in this channel mode switch? Is this switch one, two, or more pole, four position?

[grusus]

Have you tried contact cleaner, preferably DeOxit, in this channel mode switch? Is this switch one, two, or more pole, four position?

We don't have deoxit here in Poland. I am using another contact cleaner that worked for me in the past.
Yes i have sprayed this channel mode switch, also other switches, pots and interconnections between boards. Sprayed everything again today - no luck. That will be too easy.
This is 4 position 2 pole switch.

https://photos.app.goo.gl/rAY2VHPfpjyLKTpw8

[rf+tech]

From the simplicity of construction, I would guess the scope bandwidth to be 20 MHz. The vertical channel switching on these classic analoge scopes typically consist of a two-transistor multivibrator, acting as a flip-flop, and a handful of steering diodes. The yellow horizontal box looks to be where the multivibrator is located and the vertical yellow box shows the steering diodes at the delay line input.



An additional photo with the delay line and metal panel lifted, would be of benefit.

As a reference and guide to interpreting the M07192 vertical board, I have selected a subset of the Tektronix 422 schematics, attached below. The block diagram on pages 2 and 3 provide an overview. Pages 11 and 12 show the vertical switching and output.

[grusus]

Thanks for schematic - that will be helpfull.
You are talking about Q265 and Q275 from tek schematics and D201-208?

Some pics:

https://photos.app.goo.gl/YcHnfsUhHY99c5pa9
https://photos.app.goo.gl/vkaMD3VByyNt8EDC7
https://photos.app.goo.gl/nWbJGf7iKMVzRsDT8

[rf+tech]

You are talking about Q265 and Q275 from tek schematics and D201-208?

Yes and yes.

[Chris56000]

Hi!

Will this manual for the Siemens "Oscillar" M07107, which is a analogue storage version of the M07192, be of any help to you?

http://www.rainers-elektronikpage.de/SM-SIEMENS/SIEMENS-M07107.pdf

Altho' all the text is in German there are voltages and waveform samples marked on the circuit diagrams, and you might find the non–storage parts of this a better guide to your circuits than a Tek manual would be!

Chris Williams

[grusus]

Hi!

Will this manual for the Siemens "Oscillar" M07107, which is a analogue storage version of the M07192, be of any help to you?

http://www.rainers-elektronikpage.de/SM-SIEMENS/SIEMENS-M07107.pdf

Altho' all the text is in German there are voltages and waveform samples marked on the circuit diagrams, and you might find the non–storage parts of this a better guide to your circuits than a Tek manual would be!

Chris Williams

Hi.
I have seen this manual few days before. I didn't know how channel switching is done until rf+tech did some explanation.

Yes and yes.

And 3rd Yes from me.
Scope is working as it should. Need some calibration. (I have calibrated ch 1 and 2 gain, and step att balance for both channels only)

Sine
https://photos.app.goo.gl/nr1RWXS8B3hRCpt96

Triangle
https://photos.app.goo.gl/m2pzZHPd67CECxiK9

Square
https://photos.app.goo.gl/5nYt6NHXumG2s7u6A

I have some undershoot on ch1 and overshoot on ch2.

Culprit was one transistor leg (under delay line stand) which looks like it broke off, or was even not soldered correctly from factory.

Many thanks!!!

[rf+tech]

Fantastic find!

As Dave would say "winner winner, chicken dinner." 

[grusus]

Yeah.
That was too easy.
However I spent more than an hour checking diodes and transistors - all ok. Then i moved under magnifying glass and started to wiggle elements, looking for bad solder joint. Suddenly one transistor bend on 2 legs and third was lifted. Resoldered. Results are as you've seen.
It's a pitty that there are no markings on the board for calibrating pots and variable capacitors. I must address this overshoot and undershoot.

[rf+tech]

Have both scope probes' compensation been adjusted for the respective channels?

A good starting point to understand which adjustments are which is to refer to the Tek 422 vertical preamp schematic and calibration procedure. [1] These classic analog scopes are all the same in this respect. Jumping into the middle of a calibration procedure can lead to undesirable results due to interaction of adjustments.

So, we shall start with the usual preliminaries.

First step is to adjust the step attenuator balance, for channel 1.
- Set sweep speed to 1ms/div.
- Select Auto trigger, set Trigger Level for free-running sweep.
- Set channel 1 input to ground, position trace on horizontal center line of display.
- Rotate the Volts/Div switch from 10mV to 20mV to 50mV and back. Ideally, the trace should not move up or down.
- R21 on the Tek 422 is adjusted to null the shift. On the M07192, the trimmer will likely be under the step attenuator switch.

* Tip: with the power off and before making any adjustments, measure and record the resistance from the wiper of each trimmer, to its ccw or cw end. This will facilitate resetting a control in the event it is not the intended location to be adjusted.

- Observe that adjusting the step attenuator balance trimmer causes a vertical shift of the trace.
- Re-center the trace with the vertical position control and again step from 10 to 50 mv/div and back, readjusting the balance trimmer as needed.
- Three or four repetitions may be required to reach the goal, due to interaction. The sensitivity of this adjustment can test one's patience.
- Once the balance is optimized, check the 5mV/div for minimal vertical shift.

Second step is to adjust the variable attenuator balance, for channel 1.
- Same preset conditions as above.
- Rotate the variable Volts/div control throughout its range and back. Ideally, there should be no vertical movement.
- R35 on the Tek 422 is adjusted to null the vertical shift. Again, on the M07192, the trimmer should be close to the back end of the Volts/div switch and connected to the variable attenuator potentiometer.

Repeat these two procedures for channel 2 adjustment.

A closeup side view photo of the step attenuators and variable controls can help identify the adjustment locations.

Before moving on to adjusting the gain and compensation of each channel, let's get these first two steps and controls sorted.

[1] Tek 422 service manual (135 MB) can be found here:
https://elektrotanya.com/tektronix_422_r422.pdf/download.html
Refer to pages 85 & 86 paragraphs 8 & 9.

[grusus]

I have adjusted step attenuator balance for both channels. Its on front panel. It is named sym.
No trace movement now.

I cannot find pot for var att balance, but movement is about 0,4 cm ( It looks like pot for step att is influencing var att balance)

I will post some pictures in the evening

Edit:

Here are photos:
https://photos.app.goo.gl/Wr28uCBGAXCXnrjF8

[grusus]

In case anybody will search schematics and manual for this scope:
Right name is Siemens oscillar MO-7192 MO7192

It was also released as Schlumberger CRC 5041/5043

Manual/schematics here: https://drive.google.com/file/d/1xSEd64FWdFMlail6zve46SnYoJvqsXur/view?usp=sharing

I know it's french, but I think that anybody who is interested will manage to understand it.

[grusus]

And it failed again today.... In the same way.

Trace was jumping (shrinking) vertically - then was normal for some time. Today when I powered scope - fail was there permanently. Again ch1 and ch2 are mixed together.
Checked same transistor out of circuit - ok. checked all transistors on that board and diodes - all seems ok. Voltages spot on. I don't see broken solder joints. Tried to wiggle elements on boards no luck.

Any hints?

Edit:

Sometimes it fixes itself, but for short time(1-2sec). Argh... Another broken solder joint or trace. Or maybe intermitent component like cracked resistor?

[grusus]

Yeah solder crack. Fixed.