Hameg 312 (1978) scope repair

[soldar]

Most of you kids weren't even born yet when I got this scope and it has served me well all these years. It has two channels which can alternate or chop.

Now it has a problem. I went to use it today and the first channel works fine but the second channel is positioned way up there, out of the screen. Using the Y adjustment to bring it as far down as I can it is still up there but if I amplify the signal enough I can get to see the lower part.

I really do not feel like opening up the scope but I guess I have no choice. I have looked at the schematic and I am attaching the part that adjusts the Y position.

The signal enters from the left (EY21, EY22) and exits on the right (30 mV pp)

I guess it should not be difficult to troubleshoot as it has two identical channels and one is working correctly.

Pot R223 sets the Y position so I am guessing I should check everything around T206 and R223.

I would like to go in with as much preparation as possible. 

Anyone have any suggestions?

[soldar]

The signal comes in carried by two green wires from a previous preamp on a separate board.

On the channel working correctly both wires have a DC voltage close to the 2.5 specified in the schematic, the same voltage on both wires.

But on the bad channel the wires have different voltage, about 1.3 V on one and 3.3 on the other.  The average of both would be correct but one is 1V up and the other 1V down.

Now I need to determine whether the imbalance is caused by the preamp on the left or by the circuit on the right.

I cannot think what might be more likely but I will take advantage of the fact that they are on separate boards so I can switch the wires and switch the channels around and see if the problem stays or moves to the other channel.

[soldar]

After some testing I determined the problem was on the preamplifier board which does not have easy access but I saw the FETs and the IC are mounted on sockets so Y jiggled them a bit and pushed them in and that seems to have resolved the problem.  I think most likely one of the FETs was making bad contact.

So, case closed.

[GreyWoolfe]

Hey, some of us 'kids' are greybeards.  I graduated college when that scope was made.  By the way, 2 thumbs up on fixing the scope.  Enjoy.

[soldar]

A friend of my same age likes to say he is not "old"; he is "semi-young".

That scope has given me good service all these years and it is the first time it has given me any problem and, as you can see, it was easy enough to fix. The amount of dust I found inside... that is something else. I am surprised any switches still work.

The metal case is 40 cm (16") long and it sticks out a bit on the shelf. In its time it was "portable" in that I would use it by the car or motorcycle. I fitted some rubber pads/legs in the back and I could stand it vertically on the ground with the screen facing upwards.

I had never really looked at the schematic until now (it is good that I am very meticulous about keeping documentation) and I find it interesting to note the first stage has a differential input but one side is put to ground. It makes me think it might be relatively easy to make it truly differential although probably not worth the trouble.