Fixing the OS12 scope

The tube driving the horizontal deflection is V5. The X shift is drien by a variable internal resistor, in series with a 440K resistor. In that configuration, The beam never went far from the center-right of the screen with the variable resistor set to the minimum, and in the maximum the beam went far to the left side. I changed R32 to a 150k resistor so the beam could go a little bit more to the right, but it wasn't necessary as I found out later.

I was not aware of RC resonators so the circuit looked kinda odd to me, but after making some google searchs I found a circuit very familiar, a Miller integrator. With the switch S1, different capacitors are used, so the frecuency changes too. A variable resistor (P5) is used to adjust the time base even more. Something is clearly wrong here, the oscillator is here, but it doesn't work. I proceeded to leave it working alone, without any other part of the circuit. I disconnected R24 which feeds the triode of V5, which then connects to the pentode. The oscillator works! I could set the frecuency with the dial and the selector, pretty neat if you ask me.

So there is something wrong with the triode or its circuit, maybe with V4, but wait a second.

You see, the selector also switches into two capacitors and a internal variable resistor which also looks like a oscillator, but with V4 being the tube driven. I see a resistor that doesn't look original (R46). I remove it and voilà:

The thing just worked. It was maybe a weird unfinished mod. The variable resistor is now doing nothing. It is marked on the board as "stability", kind of ironic. I tested what to sustitute that resistor for, and even with a 10Mohm resistor the scope stops working, a capacitor doesn't affect the behaviour, so I just let it be.

TO-DO: fix that part. I'm almost sure the variable resistor doesn't hast to be connected to V-, maybe to 0V.

Lastly, the ouput of V5, a really neat sawtooth waveform.

I hope this work helps someone fix their scope. Thank your if you've read this.

Source: Miller integrator. www.r-type.org

The tube driving the Y shift is V2, more specifically the pentode on said tube. With the help of a resistor divider made with R10 (kinda), P3 and R14||R15. P3 is the Y shift dial, and they drive the grid of the pentode of V2. At first glance I was shocked for the high value of R10. In VCC I have 440V and after that resistor, in test point 23 I had 40V. Just so teh grid coould get a higher voltage, I put a 680k reisstor in parallel with R10, rising the voltage to 60V, and voilà, Y shift was working "again". That is strange, no matter how you look at it. I think it has to do with that resistor in paralel with R15. It dosen't look original, could be a mod from the previous owner? A weird mod if you ask me.

First step was knowing the insides. The scope has 7 tubes. 2 of them are 12AT6 (double triode), 3 are ECF80 (triode, pentode), a EZ80 (full wave rectifier) and the last one is the cathode ray tube, a D6 7-131 (equivalent to the D6 7-31 I presume).

It has a transformer with various windings, compatible with 115 and 230VAC changing some jumpers. It has 6.3V output for the heater of the board tubes and a dedicated 6.3V output for the cathode ray tube. It also has a 400V output with common winding and an 85V one. The scope has two silicon diodes for rectifying that voltage.

I first obtained the wiring for the transformer and the front and back pannel. The numbers you see are the numbers marked in the board for every wire connected to it, very convenient.

The more complicated step was getting the schematic. Now we can see how the scope is doing its magic.

With the schematics obtained, now it's time to see what could be happening. I can say for sure the tubes are OK. Changing the position of tubes of the same kind makes no difference at all, and all of the tubes look pretty neat. I also measured 440V out of the rectifier tube, so the ZF80 works. The cathode ray tube also looks okay. The focus and brightness "work" and there is some kind of deflection in both axis. 

I measured all filtering caps, at least capacitance, and they all looked ok. With (another) scope I could see no major AC component in the power rails. If it's not the power nor the tubes, maybe some other component went bad. 

Going to the point here. Every resistor and capacitor looked ok, maybe not in the tolerance limit, but there were not open circuits or short circuits. Every tube was getting its voltage.

There are two major problems: Y shift and X deflection. First one could be easier to get into, so that's the one I'll be focus on.