Oscilloscope ICEL OS-10 (similar Voltcraft AO 610)

[coquinho]

Hi. my father found this oscilloscope on a dumpster visit, and it was working, just dirty. I decided to do a "retro bright", with worked first time that I did it on my life. The problem now that its unstable, was almost unusable until yesterday that I removed and cleaned a "serial precision resistor" it seems (see on attachments), don't know how its called. I work with reballing GPUs and such, so no so much my area but I like to learn.

I found the Voltcraft AO 610 schematic and some photos, they are very similar, maybe mine is a cheaper clone:

https://drive.google.com/file/d/1CImIbdkhb2JpTng-lvpxPIfpUUmWDoW4/view?usp=share_link
https://www.mikrocontroller.net/attachment/497563/4.jpg

Sources:
https://www.mikrocontroller.net/topic/515171
https://www.eserviceinfo.com/downloadsm/234286/Voltcraft_AO-610.html

The "serial precision resistor" its not present on the Voltcraft schematic, it seems that in replace they used a opamp circuit based on CA3140. Don't know if the resistors are better than this method. Its visible on the 4.jpg, that I linked above and in the schematic (search by 3140)

In attachments there are some photos that I took from my oscilloscope.

The problem is, from my understand, when I change voltage divisions value, the line should stay on same place, but it goes up or down, sometimes stuck so low or so high that I cant even scroll more on X position.

I was thinking on make a replica from schematic of Voltcraft, and disactivate this block where they used this presion resistor... What do you guys suggest? I cleaned everything, multiple times, changed the pots, still the same. 

[wasedadoc]

Have you tried adjusting pots R41 and 1R81?

Isn't JR1 the precision resistor network?

[coquinho]

Have you tried adjusting pots R41 and 1R81?

There is no R41, because this circuit with de 3140 is no existent on my "version" of this osciloscope, maybe (I think) instead they used this resistor network... found no schematics for this.

And 1R81 ajust something but is global, so offset all volt/div settings.

Isn't JR1 the precision resistor network?

JR1 has 9 terminals, I think that is the switch key, because the resistor network has only 6 terminals.

[wasedadoc]

JR1 has 6 terminals as does the blue item in your photos.  One signal input from the emitter of V4, one ground and four outputs to one gang of the volt/div selector.

You are misreading the schematic.  JR1 is not part of the switch.  JR1 is definitely doing signal attenuation.  The scope has ten volt/div scales.  R1, R2 and R4 paths provide the coarse 1:1, 10:1 and 100:1 attenuations and JR1 provides 4 finer attenuations.  The two in tandem could give 12 scales but only ten are used.

With no signal input, from V6, v7 onwards to the tube's deflection plates all the voltage pairs of the "upper" and "lower" paths should be close to being equal.

[coquinho]

Oh, I see, 6 terminals, sorry. But any of those terminals goes to ground, I check yesterday.



I even removed the transistors to see, and yes the signal stayed in place, but without any input, as expected.

Oh and R18 does not goes to ground too. I think that is there when the schematics begin to not matches with my board. Everything from V6 and V7 and forward are equal, even the names on the board.

[wasedadoc]

The Voltcraft does not have the 6 pin blue resistor network but it has a functionally equivalent item.  You can see the small daughter board with 6 pins in the same position on the PCB underside in https://www.mikrocontroller.net/attachment/497565/6.jpg.

Possibly the Voltcraft in the photos is later production when the blue module was no longer available and they made a replacement using SMD resistors.

I suggest you trace your board to find out where the end of R18 goes.  And where does the "ground" end of the resistor network go?

Update: I just now noticed there are two R18.  1k at the base of V5 and 100 at the base of V7.  Which one did you mean when you mentioned not going to ground?

[coquinho]

Possibly the Voltcraft in the photos is later production when the blue module was no longer available and they made a replacement using SMD resistors.

Yes I see the little board, very nice!

Still does not explain why mine is missing the CA3140 opamp...

Update: I just now noticed there are two R18.  1k at the base of V5 and 100 at the base of V7.  Which one did you mean when you mentioned not going to ground?

The V7 one, does not go to ground.

EDIT:

[wasedadoc]

Is that a break in the track immediately beside (towards the bottom of your photo) the transistor lead under your yellow V7 text?  If yes, that would be a reason for the no connection to ground.  Also if yes I suspect someone else has tried to faultfind this scope.  Your earlier photo shows solder on the track to the left of your yellow 100R text.  Possibly the same person cut that track and then bridged over it.

[coquinho]

Is that a break in the track immediately beside (towards the bottom of your photo) the transistor lead under your yellow V7 text?  If yes, that would be a reason for the no connection to ground.  Also if yes I suspect someone else has tried to faultfind this scope.  Your earlier photo shows solder on the track to the left of your yellow 100R text.  Possibly the same person cut that track and then bridged over it.

I have the full resolution photo on my smartphone, I just zoom it, its a shadow, I will attach below.

[wasedadoc]

What component is the wire in my red box?

What was or were in the holes in my yellow box?

[coquinho]

What component is the wire in my red box?

R16, a 3K resistor

What was or were in the holes in my yellow box?

V5 and V4, both are 2SC9013 NPN transistors


I took a lot of pictures, measuring to ground, and some from the board, and one with light from below. Is hard to take pictures directly from above because the tube is on the way, and I'm afraid to break it if I take out, that's why I shot the photos this way. I will link to drive, because its too large for the forum, it will be in a zip file.

EDIT: here https://drive.google.com/file/d/1wRDKXZyrTTVmpgBscTPAg1vYlIcMEKEL/view?usp=sharing