Electronics > Repair

HAMEG HM605 trace squishing up on the right of the screen

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dentaku:
I have an old Hameg HM 605 that does strange things when it's moved or the top circuit board is pushed on/flexed only a tiny bit.
The X-POS knob can be pulled out to switch to 10X mag and it's connected to a long rod that connects to pot/switch. This puts allot of stress on the solder joints because it pries it back and forth. Originally the plastic thing that joins the rod to the pot shaft was very badly repaired and it was off center so the pot rocked back and forward a tiny bit by simply turning the knob. I've since fixed that.

Here's what it does...
The trace will move left and right just fine sometimes but eventually it will start compressing along the right side of the screen the more I mess with the X-POS knob or sometimes even when I turn the TIME/DIV. knob all to the way right.
Pulling on the knobs associated with the top circuit board, pushing up or down on the circuit board (the cover's off at the moment) or even just propping one corner of the scope on something so it just slightly twists the whole machine will eventually make it go back to normal. Clicking the X-POS/MAG.10X knob a few times sometimes does the trick.

Here's a video of me moving the X-POS knob while it's working then I put a few mm thick SD card case under the right corner so that it' no longer level and the whole thing goes wonky.
http://1drv.ms/1kkrKXR

It's built very similarly to the one Dave worked on in this video impossible to access the bottom of the boards without allot of fiddly work.

ALSO: probably not linked to the other problem, sometimes when it warms up the slightly hummy transformer starts buzzing louder and I get a trace that jitters around a bit or even sometimes gives me a little downward dip. I've only ever heard a sound like that before, but a bit louder, when I connected a bridge rectifier incorrectly :) Maybe the power supply caps are going bad?
It goes away if I turn it off then back on.

Here's a photo of the bottom of the board where the X-POS pot/X10switch is soldered. The one thicker solder fillet looks weird.
I SO don't want to remove this board. There's a ridiculous amount of rods and even three wires soldered directly to it that would make it difficult to disassemble. Of course if it gets worse I'll have to take it apart.

Could it just be a bad solder joint in the area of the X-POS, the HOLD OFF and the TIME/DIV. pots and switches that does this because pushing in that general area will case or fix that strange behavior?

miceuz:

--- Quote from: dentaku on July 04, 2014, 01:24:23 am ---ALSO: probably not linked to the other problem, sometimes when it warms up the slightly hummy transformer starts buzzing louder and I get a trace that jitters around a bit or even sometimes gives me a little downward dip. I've only ever heard a sound like that before, but a bit louder, when I connected a bridge rectifier incorrectly :) Maybe the power supply caps are going bad?
It goes away if I turn it off then back on.

--- End quote ---

Had similar problem on mine, main DC regulator caps were almost short, I'd suggest fixing this first, just replace the biggest electrolytics.


--- Quote from: dentaku on July 04, 2014, 01:24:23 am ---I SO don't want to remove this board. There's a ridiculous amount of rods and even three wires soldered directly to it that would make it difficult to disassemble. Of course if it gets worse I'll have to take it apart.

--- End quote ---

You'll get used to that after several runs :) It's not as bad as it looks, just get yourself a small hexagon wrench, all those shafts can be removed pretty easily.

dentaku:

--- Quote from: miceuz on July 04, 2014, 08:47:49 am ---
--- Quote from: dentaku on July 04, 2014, 01:24:23 am ---ALSO: probably not linked to the other problem, sometimes when it warms up the slightly hummy transformer starts buzzing louder and I get a trace that jitters around a bit or even sometimes gives me a little downward dip. I've only ever heard a sound like that before, but a bit louder, when I connected a bridge rectifier incorrectly :) Maybe the power supply caps are going bad?
It goes away if I turn it off then back on.

--- End quote ---

Had similar problem on mine, main DC regulator caps were almost short, I'd suggest fixing this first, just replace the biggest electrolytics.


--- Quote from: dentaku on July 04, 2014, 01:24:23 am ---I SO don't want to remove this board. There's a ridiculous amount of rods and even three wires soldered directly to it that would make it difficult to disassemble. Of course if it gets worse I'll have to take it apart.

--- End quote ---

You'll get used to that after several runs :) It's not as bad as it looks, just get yourself a small hexagon wrench, all those shafts can be removed pretty easily.

--- End quote ---

I'm assuming you turn all the knobs fully left or right before disconnecting stuff so you know what position they should go in when you re-assemble it?
The rods aren't terrible but it even has wires soldered directly that will need to be unsoldered. Three of them go to the rear BNCs and two go to the read of the CRT.
I'm thinking I might be able to leave them alone and hopefully flip the board over while they're still connected.

I see there are three 1000uF 25V caps on the bottom board next to the five (five!) little round bridge rectifiers. Of course to access these I need to remove to top board to unsolder them :)
There's also two big 220uF 100V caps that I'm assuming they have more to do with the CRT than the power supply itself?

miceuz:

--- Quote from: dentaku on July 04, 2014, 02:46:17 pm ---I see there are three 1000uF 25V caps on the bottom board next to the five (five!) little round bridge rectifiers. Of course to access these I need to remove to top board to unsolder them :)
There's also two big 220uF 100V caps that I'm assuming they have more to do with the CRT than the power supply itself?

--- End quote ---

No, those are for linear regulation too, in mine unit exactly these were bad.

Measure voltages on a test header first, maybe you will be lucky to access the offending cap via side, but that will require some solder iron bending :) And you are better off changing all of them if it's an old unit.

dentaku:
Well, I'm removing everything. The rods are no big deal but there are screws that are in ridiculously awkward places :-/O There's one last screw I'm still trying to figure out how to get at.

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