Grundig MO20 no Signal detected

[Messtechniker]

If thus unit were to have double insulation you would find
on a label or printed on the rear a smaller square within
a larger square. See attachment.
Thus, if not present, then the mains cord you have found
was improperly replaced.

[tillewolle]

Oh I see, I thought that judging from your comments especially to the test meter issues that you was more of a newbie then you actually are, sorry.

Don't worry, I can see your point  I think it's because I try to describe everything as detailed as possible and I'm more firm with the German terms for all of this. So it's more a language barrier because I need to reread all things a lot of times to make sure I'm 100% safe and doing it right

If you turn the timebase knob fully counterclockwise can you see a moving dot instead of a line? Try both positions of the X/Y button. I have a feeling that the thing is broken, maybe just a loose wire inside, maybe worse. If you are ever in the neighborhood of Amsterdam bring your scope and we will fix it

Yeah! It moves from middle to the right! Just when X/Y is pressed.

If thus unit were to have double insulation you would find
on a label or printed on the rear a smaller square within
a larger square. See attachment.
Thus, if not present, then the mains cord you have found
was improperly replaced.

You mean like this? 

[Specmaster]

Yes that is the Double Insulated symbol so may be it is safe after all. I still have my doubts but without getting a full copy of a manual for it I can't be sure, it might be what /02 after the model number denotes?

The case of the scope is made of metal, yes? If yes, grab your multimeter and set it up again for the continuity test and see if the outer part of the BNC socket is connected to the case. If yes, then I'd be extremely careful when using it because while you not in any danger of blowing the scope or the probes (as in the video of Dave's on EEV Blogg). You are then putting yourself in danger of electrocution because if you get the connections wrong on the reel to reel tape recorder, the case could be live (hot) and if you then touch the case and ground (earth) you will be shocked or worse.

With the movement on the screen over the right, is this the dot moving to the right, or is it making a line? If it is making a line then rotate the control in the top line of knobs with double-headed arrow, to the left and see if it fills the screen. Study the attached photo and move the controls to the same settings as shown in the photo and connect a probe (if you have 2 probes connect both as shown) and see what happens to the screen then.

[PA0PBZ]

Yes that is the Double Insulated symbol so may be it is safe after all.

That looks like a piece of paper that someone glued on, and by the looks of it it's ready to fall off.

[tautech]

Yes that is the Double Insulated symbol so may be it is safe after all.

That looks like a piece of paper that someone glued on, and by the looks of it it's ready to fall off.

Exactly, double insulated symbols are always screen printed on or embossed into the case.

IMHO it's a mod.....beware !

[Specmaster]

Yes that is the Double Insulated symbol so may be it is safe after all.

That looks like a piece of paper that someone glued on, and by the looks of it it's ready to fall off.

Exactly, double insulated symbols are always screen printed on or embossed into the case.

IMHO it's a mod.....beware !

Thats what I'm concerned about, if its a mod and been poorly carried out, its possible for the entire scope to become a lethal time bomb just waiting for the right conditions to strike.

[tillewolle]

I still have my doubts but without getting a full copy of a manual for it I can't be sure, it might be what /02 after the model number denotes?

Would nice to get the right one, but where?

The case of the scope is made of metal, yes?

Yes!

If yes, grab your multimeter and set it up again for the continuity test and see if the outer part of the BNC socket is connected to the case.

No it's not. Even if I put both probes on case.

With the movement on the screen over the right, is this the dot moving to the right, or is it making a line?

It's moving the dot for the right.

If it is making a line then rotate the control in the top line of knobs with double-headed arrow, to the left and see if it fills the screen.

It moves the dot from middle to the right.

Study the attached photo and move the controls to the same settings as shown in the photo and connect a probe (if you have 2 probes connect both as shown) and see what happens to the screen then.

like the photo


With Intens down

[Specmaster]

Looking at these results, it is clear that your scope has neither any vertical or horizontal activity which is a real bad place to be and unless you're going to be incredibly lucky, is very likely to mean that it is way beyond economical repair.

My gut reaction at this stage is to dump it and buy another that you are able to see working before you part with cash for it. The logic behind this is I do not believe this left the factory as a double insulated device, which means that someone has been inside and meddled with it and what you are seeing is the end result where they did things that screwed it up, had no idea how to fix it so sold it on pronto hoping that the next buyer would not know any better until it was too late.

Why do I think it has been messed about with? Is simple to understand, if the double insulation was a factory option then why was the symbol printed on a paper label and stuck on, why was it not printed on like the rest of the information on the back of the scope?

Without a genuine workshop manual on the scope to work from, you are batting blind, even down to what voltage is supposed to be where? I know that you said that you would leave the CRT high voltage area well alone, well it is not quite that simple, because sometimes there are areas of the PCB that are connected with the CRT but not the EHT, but voltages there can be in the realms of 2 to 4 times the line volts. Yes these areas should be shielded but not always so or the shielding may have been left off by the person doing the modifications, and poking around without that all important and elusive manual, you are poking in the dark.

You may disagree with me, but thats my considered opinion with many years of experience and you already have, and I'm sorry but I have to say it, made some school boy errors, as most people on this forum would have walked away from that scope because the visual clues that are available, 2 pin plug and 2 core cable, paper stuck on label with the double insulated symbol and also of course, the fact it was not working as intended when you saw it.

[Specmaster]

You might like to check out this video, it elaborates on the video that Dave did with actual demonstrations, including using an isolation transformer which while can be an improvement in safety, can still under the right (or I should say WRONG) circumstances still expose you great danger.

[tillewolle]

Yeah, guess I'm keeping my eyes open for another one. Any recommendations to start?

[Specmaster]

Anything should be OK around that bandwidth should be pretty good and cheap, but make sure its working before buying it this around. I would have thought either a Phillips or Hameg should be plenty full in your neck of the woods.

[tillewolle]

Not giving up...

Found someone selling the same model with the same Sticker on it, same cable... So mine seems not to be unique.

https://www.ebay-kleinanzeigen.de/s-anzeige/grundig-elektronik-oszilloskop-mo22-2-20-mhz-2-kanal/1331890265-168-6214

[Specmaster]

Not giving up...

Found someone selling the same model with the same Sticker on it, same cable... So mine seems not to be unique.

https://www.ebay-kleinanzeigen.de/s-anzeige/grundig-elektronik-oszilloskop-mo22-2-20-mhz-2-kanal/1331890265-168-6214

So it would seem, but it is possible that the same person may have had a few of these, maybe a laboratory somewhere had them modified for a specific purpose etc? Without a proper manual for the scope it is still going to be a hard uphill battle for someone such as yourself with an admitted very limited knowledge. Hence why I said that it would be better to source another scope of similar bandwidth but working to begin with, they can be picked up if you look around for very little money. Once your knowledge level increases, especially if you manage to locate a full and proper manual for that scope, you could then use the new one as a tool to assist you repairing the other. It is always a good idea to have more than one scope as old ones do tend to break quite often being as they are going to be around 40 years old now.

[MarcX]

So it would seem, but it is possible that the same person may have had a few of these, maybe a laboratory somewhere had them modified for a specific purpose etc?

No this variant was available from Grundig too (Schutzklasse II), and is mentioned in the manual. But you should pay attention by measuring voltages with more than 50VAC over earth because the grounding is connected with the chassis.

For all people with german language skills, their is a copy of the manual on the german mikrocontroller.net bulletin board. (Page 5, Chapter 4)

https://www.mikrocontroller.net/attachment/360138/MO22_AN.pdf

There was also an option for Z-Modulation (INTENS-MOD). All Grundig MO20, MO22, MO30, MO32, CO30 and CO33 (CO = Branded for Conrad Electronics Store) and also the Siemens branded versions, Oscilloscopes can be equipped with this option by adding a BNC connector which is connected to X-Deflection PCB when i‘m remembering correctly. (AUFHELLSCHALTER, page 4)

https://www.mikrocontroller.net/attachment/360139/MO22_SM.pdf