troubleshooting analog oscilloscope

[darthtony]

I was given an old analog oscilloscope(Hameg 604) for free, however i am experiencing some problems(it is my first oscilloscope)
the first problem is that it cant display a signal properly. with all the controls on their normal positions it should have no problem displaying the calibration signal, however, the signal is moving along the screen and i have to play with the trigger hold off knob or use the delay switch to "lock" the signal is there any way to calibrate it?

the second problem is that the second channel's volt/div isnt working properly. between 5V/DIV and 0.2V/DIV the signal isnt displayed as it should.when i go from 0.2V to 0.5V div you expect the signal to have a dicreased display amplitude, however it suddenly overshoots the screen.the same happens until i reach the 10V/div when it seems to work properly.

[w2aew]

Regarding the first problem of the signal scrolling back and forth, this is a triggering issue.  It may not be a problem, it might be that you don't have the controls set right.  You may want to review the video I did on analog oscilloscope trigger controls.

Regarding the second problem, it may be simply some dirty switch contacts on the vertical attenuator assembly.

You may also want to see this thread, and contact the OP for info...
https://www.eevblog.com/forum/reviews/teardown-and-'repair'-of-a-hameg-604/

[darthtony]

Can you link me the video, please(altough, according to the manual, it should have no problem displaying most signals when all the controls are in their normal positions).

As for the vertical attenuator, i have read the topic you posted, but the OP being a guset, i dont know how to contact him. Furthermore, i have opened the scope and cant quite figure how to get to the rotary switch :S

edit: I have checked your video, and it was very helpfull, however, i cant solve my problem. the only way to stabilize a waveform, is to play with either the holdoff control, or the delay control, or to vary the timebase(with the center cal pot not the standard timebases).All of this must me done with the trigger level to auto, if i move it, it wont trigger at any level.

[w2aew]

Glad you found the video.  Here's the link for anyone else that may be interested.  If you've followed the manual's guidelines for triggering, then maybe there is some fault in your trigger circuit.  For a scope of this vintage, be sure to check electro-mechanical issues (switch contacts, solder joints, etc.), as well as the usual starting points (supply voltages, electrolytic caps, etc.).



With your edited description, it sound like the trigger signal is not getting the source from your input channel.  The Auto trigger mode causes the scope to send its own periodic trigger in the absence of a detected trigger.  The auto-generated trigger is not sync'd to the input, which is why it's not stabilizing the waveform.

[darthtony]

I forgot to note that for the first couple times i tested it it seemed to work fine.Also after oening it up, it doesnt seem to be something obvious(i tapped many comonents around the trigger controls, and the signal didnt change).

[darthtony]

Here are some photos in case they help:

http://imageshack.us/photo/my-images/819/20121217181743.jpg/
http://imageshack.us/photo/my-images/844/20121217181926.jpg/
http://imageshack.us/photo/my-images/24/20121217181944.jpg/
http://imageshack.us/photo/my-images/838/20121217182350.jpg/
http://imageshack.us/photo/my-images/18/20121217182355.jpg/
http://imageshack.us/photo/my-images/824/20121217182441.jpg/
http://imageshack.us/photo/my-images/401/20121217182451.jpg/
http://imageshack.us/photo/my-images/38/20121217182411.jpg/

[darthtony]

I decided to start working on this scope again, can someone point me to something?After thorough cleaning, the vertical attenutator seems to works ok.

[madires]

A good starting point is to google for the service manual: http://www.frankshospitalworkshop.com/equipment/documents/workshop_equipment/manuals/Hameg_HM604_Oscilloscope_Usermanual_and_Service.pdf :-)

[darthtony]

i have checked it out but i cant figure out what is wrong.

[edavid]

i have checked it out but i cant figure out what is wrong.

Does the external trigger mode work?  How about line trigger?

[darthtony]

no the external trigger doesnt work either. line trigger i dont know.the only way i have been able to make it trigger is the tv sync switch(i dont know what it does).

[kg4arn]

You may need a second working scope so that you can trace the signals internal to the broken scope.

If in "Normal" trigger mode you cannot get a trace, no matter where the trigger level is set at, then it is possible that the input signal is never even getting to the trigger circuit.  Another possibility is that the trigger level is somehow stuck very high or very low (outside the range of your input signal).

[edavid]

no the external trigger doesnt work either. line trigger i dont know.the only way i have been able to make it trigger is the tv sync switch(i dont know what it does).

If TV sync works, that's great.  Maybe it's just a problem with the switch contacts or pot.

[darthtony]

Cleaning the trigger level pot didnt do anything.After a lot of work, i have been able to remove the timebase/trigger board, and look underneath.There seems to have been a lot of hand soldering involved, and there many suspicious spots, but nothing obvious, can someone please point to some components to look more carefully?I have a second oscilloscope(working) but  to remove the board i had to disconnect and unsolder many cables and connectors, so i cant power it up right now.

[madires]

IIRC there are some voltages noted in the circuit diagram in the service manual. Look for the trigger circuit and check every stage step by step. If you google for triggering problems with the HM-604 you might get some hints for known problems to speed up finding the broken part.

[eak]

I have the same problem with triggering on my hm 604