Hameg 208 Input attenuator problem - Page 1
Forum section: electronics / repair

author: mavu
author_rank: Contributor
timestamp: October 27, 2017, 08:45:15 pm
content: HiI 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

author: modmix
author_rank: Contributor
timestamp: October 27, 2017, 10:28:27 pm
content: Have you checked the manuel (eg.https://cdn.rohde-schwarz.com/hameg-archive/HM208_deutsch.pdf)?hthUlli

author: mavu
author_rank: Contributor
timestamp: October 28, 2017, 09:09:03 am
content: Quote from: modmix on October 27, 2017, 10:28:27 pmYes, 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.

author: carl_lab
author_rank: Frequent Contributor
timestamp: October 30, 2017, 11:00:05 pm
content: 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

author: mavu
author_rank: Contributor
timestamp: November 01, 2017, 09:59:55 am
content: 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 brokenI 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.

author: carl_lab
author_rank: Frequent Contributor
timestamp: November 03, 2017, 11:24:52 am
content: 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.

author: mavu
author_rank: Contributor
timestamp: November 04, 2017, 09:49:33 pm
content: Quote from: carl_lab on November 03, 2017, 11:24:52 amI 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.