Repairing Hameg HM 205-3 (analog time base not working) [almost SOLVED] - Page 1
Forum section: electronics / repair

author: rentier
author_rank: Newbie
timestamp: March 25, 2020, 01:33:19 pm
content: Hi, I have two of these HM 205-3 storage scopes.Interestingly enough both have the same error.In both cases the classical analog time base does not work. In digital storage mode they work fine.The problem is as follows.1) In analog mode (storage button not pressed) I don't see any trace. But there is a flickering in sync with the time base (repeatedly from bright to dark), so I can adjust the flickering from slow to fast. y position control seems to work, as it affects the flickering towards up / down the screen. Cannot judge if x-control has any affect on the flickering. (Image D and E.)2) XY-Mode does not work either in the sense that there is no focused dot on the screen, just a dim diffuse light towards the right of the screen. Also here, the diffuse light moves accordingly when I rotate the y-position control. However it doesn't seem to be affected by x position control. (Similar to image E.)3) component tester is able to produce a trace that can be shiftet in horizontal direction by rotating x-pos knob. (Image C.)4) In digital storage mode, everything works as expected, also I can shift towards + or - x. (Image A and B.)With the schematics at hand (https://cdn.rohde-schwarz.com/hameg-archive/HM205-3_english.pdf), what i did so far was checking power supply voltages, +12V, -12, +5 V, +150 V, +130 V, on the "XY-04". They all seem ok, except 4,89 V instead of 5 Volt but that might be ok anyway.Also, the 5 Volts for supplying the time base are present. I checked at the emitter of T3911.With this background, I suspect that the error is located somewhere around the generation or, more probably, in the amplification of the analog time base. Where could I look next? Is that a common fault? After all, this affects two of two devices which I have here.EDIT: While writing this, both of the scopes just magically recovered from half-broken to fully alive. I think there was a faulty contact at the X-Y-switch. After pressing/releasing it a couple of times, it's almost back to normal.thanksregards, rentier

author: rentier
author_rank: Newbie
timestamp: March 25, 2020, 02:06:24 pm
content: Now there accurs a second problem. In analog mode, it's not possible to get both traces focused at the same time. I.e. when I focus trace 1, trace 2 is out of focus, and vice versa. And, related to this, the input stage / amplifier of channel two is not working. Does not accept any signal. Neither in analog nor in storage mode.In storage mode I can focus both traces at the same time (it's the focus setting of channel one that seems correct), but still, as expected, no signal from input channel two is displayed.Ok I think this is easier so solve, I'll dig into the schematics around channel two input stage.However, any suggestions are welcome :-)regards,rentier

author: shakalnokturn
author_rank: Super Contributor
timestamp: March 25, 2020, 04:52:47 pm
content: Your focus problem is peculiar.What are the different symptoms relating to that when displaying CH2 only, both alternate, both chopped?Any differences depending on TB setting?Considering you've had other switch problems start with the coupling switches on CH2 for the lack of signal.

author: rentier
author_rank: Newbie
timestamp: March 25, 2020, 05:57:14 pm
content: When displaying CH2 only, it keeps its focus, be it in focus or out of focus.So changing from CH1, CH2, dual does not affect the respective traces' focus.Neither is focus affected by TB setting.But I noticed that chop mode does not work - it's just like dual mode where both traces are alternatingly written on the screen one after another.The "dual" swith and the "add" switch seem to work in there respective functions. But both together do not bring the oscilloscope to chopped mode.