Hameg 208 Input attenuator problem

[mavu]

Hi

I got my new old Hameg HM208 today, and it had a slight problem:

The input attenuator for channel 1 was loose inside, and the movement (possibly in shipping) had broken some wires that connect the module to the rest of the scope.

I took it out and shortened the ribbon cable by a millimeter, and resoldered it.

This seemed to fix channel one, with one problem:

If i turn the fine adjust (small knob on top the big knob) it acts like a vertical adjust knob.
in fact, i have to turn this small knob almost all the way to the left (max decreases Y-sensitivity by factor 2.5) to move the waveform down into the visible area of the display.

This should normally increase the vertical size of the waveform, and not shift it up or down.

Any Ideas where i would have to start looking for a problem?

Please tell me if I can add some information, I'm quite new to this

[modmix]

Have you checked the manuel (eg. https://cdn.rohde-schwarz.com/hameg-archive/HM208_deutsch.pdf)?
hth
Ulli

[mavu]

Have you checked the manuel (eg. https://cdn.rohde-schwarz.com/hameg-archive/HM208_deutsch.pdf)?
hth
Ulli

Yes, and after someones suggestion I performed the DC-Balance adjustment described in the manual.
This was way off, and I can now leave the fine adjust in the CAL (base) position and move the waveform into the visible area with y-adjust.

But the problem is still the same, changing fine adjust moves the waveform down instead of compressing it.

[carl_lab]

Back in 1977 or 1978 I gave my HM512 to Hameg service to solve a similar problem.
They replaced the matched and thermally coupled input stage FET pair of that channel:



Because the circuit is different to your HM208, you could try to replace IC101 (CA3127) first:



BTW, this is a working link to HM208 service manual:

https://ia600200.us.archive.org/19/items/hameg_hm208/hm208_text.pdf

[mavu]

Thank you, But as it is, the only thing not working is the fine adjust of the v/div an I rather live  without that than risk repairing it more broken

I got another suggestion from a different forum, to play around with the adjustment pots for variable DC balance, and I'll try that first when i get the time.

[carl_lab]

DC balance and y-variable are interdependent. That's clearly described in user/service manual.
If DC balance is not adjusted correctly, turning y-variable potentiometers cause y-shift.
That's your problem, isn't it?

If DC balance and y-variable potentiometers are OK (measure them!) try replacing transistor array or input transistor pair. You can use sockets because HM208 bandwidth is not too high.

[mavu]

DC balance and y-variable are interdependent. That's clearly described in user/service manual.
If DC balance is not adjusted correctly, turning y-variable potentiometers cause y-shift.
That's your problem, isn't it?

If DC balance and y-variable potentiometers are OK (measure them!) try replacing transistor array or input transistor pair. You can use sockets because HM208 bandwidth is not too high.

I did adjust DC-balance and now I can use the Y-adjust normallly, but the v/div fine adjust still shifts the waveform vertically.
the coarse v/div works fine, and does not cause shifting.

I have one more pot I can try but have not got around to it yet, mainly because I have not needed the v/div fine adjust yet.