Hi there,

Introduction:
I bought a second hand oscilloscope a couple of weeks ago. It had an issue, the one mentioned in the topic, but at least it did display something so I thought I would probably be able to fix it. It now became somewhat of a challenge for me and I spent a lot of time with it and learned tons about that old art of analog circuits. Amazing engineering!

I did some research and came to the conclusion that the error would probably come from the later y-amplifiers because it would affect both channels in the same way. I opened it and noticed that someone has already worked on it, and not very diligently. One of the bridge rectifiers had been replaced by an ugly huge one which didn't fit at all aka "3D-legsbending". I also noticed some modifications on the Z-board: some resistors have been added that did not only have a different value than given in the datasheet, but also were connected in a completely different logic. Also, an additional capacitor was added somewhere on the back side of the PCB.

First research attempts:
After some research on this specific model, I checked the checkpoints and all except one were okay (ChP 7, see in the picture below). I also measured the voltages (those less than 2kV) at the CRT and they were fine except for the y-deflection plates (~55V but should be 85) More general information on how to troubleshoot an oscilloscope lead me to the idea to short the bases of the final y-amp's transistors (yellow line in picture). I hope I managed to isolate the fault to the second stage of transistors within that circuit (red area in picture). Lo' and behold, it was the same one where all that tinkering had been done!

However, while working on it the error magically disappeared so I suspected a loose connection, which made perfect sense considering the looks/dust of them and how hard it was to get them apart and together. The display was also indicating that the knobs and potentiometers were very noisy so I decided to do a complete restoration of my "lucky cheap repair".

What I did:
- Replaced all electrolytic capacitors
- Restored original parts wherever I saw some hacks (green resistors in picture)
- Cleaned the heck (!) out of it. Yes, it took tens of hours. However, I did not touch the trimmers.

Result:
Okay, so now I got a nice shiny oscilloscope with smooth potentiometers and so on, but the error came back :'( I still notice somewhat of an improvement, though in terms of signal distortion. So the problem that I can only use the upper half of my scope remains. I can use y-Pos and put my signal above the center to work around it but it is still kind of distorted/nonlinear. When I put 2 identical signals on top of each other they almost look the same but when they have different y-position they will also have a different y-size. Everything that is lower than 0 will be displayed as 0 so I can use the calibrator signal and make it a flat centered line by simply reducing the y-position enough.Further investigation:
So now that I know where the tinkering had been done I became a lot more suspicious about the Z-Board. Especially since a checkpoint voltage was wrong (~5.2V but should be 3,75V) coming from between the y-final amp rails. Only then I found out that the faulty section does not even contain the original transistors anymore but some other ones with different specs (red in picture). No wonder it's behaving strangely...

The picture shows the display in dual mode. Both probes are connected to the calibrator but one channel's y-position is turned down. The line is my "absolute zero" and it's all that is left of the square wave.



Future plans / call for support:
The next step is that I wanna replace the transistors with the original models. But I somehow got the idea that it is not the only thing going wrong in here... that's why I am asking for your help. Soeone with more experience can probably help me out here. I will try to provide you with all the information I can get so just ask if you have an idea.

What I don't wanna do:
I don't want to measure anything in the kV range (but then again I don't think that section is where the problem is located anyway). Even more important, I don't want to buy a cheap digital one instead because "it's not worth repairing" or "they are better anyway". I know that and I don't care about it. I already paid more for the probes and parts, shipping, DeOxit and so on. I am not poor so I could just buy a new one but I am doing this for fun and to learn more about electronics. I don't expect to have a 100% accurate Hm 203-4 in the end that works until 20MHz. All I wanna measure with it are audio signals so 20kHz in a quantitative fashion so it's fine for me to sacrifice some accuracy for nostalgia.

Any help would be greatly appreciated =)

 

While randomly playing with the knobs I managed to flip the "display error"... now I wasn't able to see positive y-values. But it was in a random button spree and I can't really reproduce it. So while I really would like to get the scope back in its original state (at least the plate voltages should imho be within specs again) I think that still there is some loose connection. Guess I will keep on searching... ;-)

I only get like 30% of the signals' amplitude when in dual mode. I guess this is what comes mainly from those badly done z-Board "mods"!? And I still wonder how they broke that rectifier...

 

With the scope set to channel 1 and with the channel coupling switch set to GND, adjust the vertical position until the displayed line is in the centre of the screen. Measure the Y1 and Y2 deflection plate voltages and also pins 4 and 6 of the board shown in your schematic above.

Now adjust the vertical position fully one way and measure those four voltages, then repeat with the position control at the other end of travel.
Please report all voltage measurements.

[Edit] Channel 1 is with the four cuttons under the Y attenuator controls in the out position.

 

Hello Albert,

thanks for your reply. Here are the voltages.

Centered
Pin 4 / Y2: 4.66 V
Pin 6 / Y1: 4.73 V
D3- / Y2 : 56.3 V
D4+ / Y1: 56.1 V

Full left (Y-pos min)
Pin 4 / Y2: 4.57 V
Pin 6 / Y1: 4.73 V
D3- / Y2 : 57.0 V
D4+ / Y1: 54.8 V

Full right (Y-pos max)
Pin 4 / Y2: 4.80 V
Pin 6 / Y1: 4.76 V
D3- / Y2 : 47.0 V
D4+ / Y1: 73.7 V

I left Y1-pos on max, activated Dual mode and set Y2-pos to min and took this picture so you can see what "full left" and "full right" would mean on the display.



Cheers,
Domme

 

Please check the 24V and 140V supplies to that board. I suspect both may be low.

 

Please check the 24V and 140V supplies to that board. I suspect both may be low.

Hi Albert, thanks for your input, really appreciate it. The voltages are fine, though (24.07 V and 140.1 V). Do the input voltages for Y1 and Y2 (Pin 4/6) from the measurements above look good to you?

I attached the data sheets from the original (BF199) and replacement (MPS918) transistors. As you can see, they have different specifications and from how the rest of the repairs/hacks looked like I don't think they have been properly matched.

 

The collector voltage rating for the MPS918 is less than the 24V supply and, possibly worse, the pinout is different so unless the leads are bent around to fit the connections to them may be wrong.

If you can get the correct rplacements I strongly suggest swapping them out.

 

The collector voltage rating for the MPS918 is less than the 24V supply and, possibly worse, the pinout is different so unless the leads are bent around to fit the connections to them may be wrong.

If you can get the correct rplacements I strongly suggest swapping them out.

The leads have indeed been bent, which was probably the best idea of the whole hacking process performed on the scope. Fortunately, the original transistor models are easily available so I already ordered 10 to have a chance to get a good match. I hope this will improve the behaviour to a more desired result. I will probably get them on wednesday and report back with the results once I changed them. I will also check the Z-board more thoroughly, maybe there are still more hacks to be discovered but it appears to be fine besides the poor transistor choice...

Kind regards,
Domme

 

I just exchanged the transistors for a 20µV-matched pair of original BF199s but unfortunately it did not really affect the problem at all. The voltages I measured above didn't really change a lot, either. Maybe it's coming from the intermediate y-amps or at least a trimpot on their board. Any other ideas on how to approach this problem? If not, I might as well go crazy with the trimpots... the previous owner probably fiddled around with everything and well I don't have much to lose either ;-)

I bought another used Hameg 203-5. I hope that one works better so I can use it to troubleshoot this one.

Kind regards,
Domme

 

Do you have the rest of the schematic that you could post here?

 

Do you have the rest of the schematic that you could post here?

I attached are just the schematics. More information is available in the PDF attached to the first post. Meanwhile, I acquired another fully-working scope for in-depth troubleshooting.

I think the problem (or at least one of them...) is located on the board 2. ChP 7 is the only voltage that's off (around 5,3 V). From the missing negative half I imagine it might even be a diode with a loose connection. I think I'm gonna try with simply resoldering some of them.

Any help/idea is appreciated

 

While randomly playing with the knobs I managed to flip the "display error"... now I wasn't able to see positive y-values.

So the trace was in the bottom half of the screen?
Does the invert button affect anything (CH1 only).

 

In "broken mode" pushing Invert affects the y-position of channel 1 by a little bit, but the display will still show only the positive value.
Wild guess: it's sending the signal through the negative path in terms of absolute y-position but it's unsigned. Asymmetry is responsible for the change in y-position.

Notes on how I managed to get into "not broken" mode:
- It worked a couple of times after a "cold start" (I guess that was the reason I was referring to when I wrote I randomly played with buttons).
- It worked once after playing with the alt/chop button but will always go back into "positive only" mode after a few minutes maximum.
- It sometimes goes along with "floating" y-positions. Both channels will continuously randomly move up and down by a little bit while it's working. This is usually not happening in "broken mode"

 

I recorded a video repeating the voltage measurements you initially asked for but using a scope this time along with some commentary: http://www.mehrunfug.org/DSC_0219.AVI

Hope this helps
I can do more videos if you explain what you want to see. Sorry for bad audio.