Hameg Osciloscope HM-312 try to make back to life

Thread Starter

enito

Joined Jul 10, 2014
16
Hi,

This is my first post and hope you can guide me ;) I own this old one osciloscope and want to make back to life, if this worth. I powered and no image get, i move and get sometimes a image line and sometime is i move i get some image. Can be some cold solder? I post a set of photos , thx for any tip !

https://postimg.org/gallery/3ixq0ynbe/
 

AlbertHall

Joined Jun 4, 2014
12,345
This may be a bad solder joint, but I suspect that it may also be poor contact on switch terminals or pots.
Does the trace appear/disappear when you tap the 'scope (bad solder joint) or when you operate switches or pots (bad switches/pots). In the latter case switch cleaner will help the switches and you may be be able to get some into the pots.
 

recklessrog

Joined May 23, 2013
985
Hameg are very helpful, if you call or e.mail them they will send you a schematic.
Once you've got that, you should be able to fault find with ease.
Remember that the high voltages present can cause a severe shock so proceed with care.
 

drc_567

Joined Dec 29, 2008
1,156
Sometimes the electrolytic capacitors will not work normally, if they have not seen an electric charge in the recent past. If you simply turn the power on for a period of time, hours ... days ... they will recondition themselves. Sometimes it works, sometimes not.
 

Thread Starter

enito

Joined Jul 10, 2014
16
Hi, thx all for reply, hmmm yep when i tap i get image for few secs and goes, tap and back and goes..i think is cold solder, what tip is for find a cold solder? i view the board and dont see any visualy cold solder.

About Hameg, if you can give me contact? and i see is a display cold solder, what is the board of display (from the photos i posted)

thx again for the time!
 

AlbertHall

Joined Jun 4, 2014
12,345
The bad joint isn't necessarily on the display board, it could be on almost any board. My technique is to tap each board with an insulating 'stick' of some sort and find the part which is most sensitive. That can be extended to find the part of the board or component with the bad joint.

Alternatively, tap until you have no display and then fault find following the schematic in the following order:
Low voltage power supplies;
High voltage power supplis if you have the equipment to do this and take great care while doing it;
Connect a voltmeter between the Y-plates and adjust Y-position controls to get zero volts. If you can't get zero the problem is in the Y-amplifier.
Connect a voltmeter between the X-plates and adjust X-position controls to get zero volts. If you can't get zero the problem is in the X-amplifier or timebase.
Short the grid ans cathode of the CRT (very carefully - high voltages here). If this restores the trace, the problem is in the brightness control circuit.

While going through this procedure you need to keep checking that the trace is still missing as connecting a meter may be enough of a tap to restore the trace and you will need to tap until the trace has gone again before continuing to fault find. You can't find the fault while there isn't one!
 

Thread Starter

enito

Joined Jul 10, 2014
16
Right, i take the idea.. When i tap (with case ) is like back a few secs and goes.. I read too using plastic stuff like toothbrush and touch the board on, for search area when display back. High voltage right when i check powered, because i see crt tube is protected, no need discharge i think like old CRTs .

thx again!
 

AlbertHall

Joined Jun 4, 2014
12,345
When I say high voltage, I mean the cathode supply which will likely be around 1kV, not the CRT anode which will likely be much higher.
Check the schematic for actual voltages and test points.
 

recklessrog

Joined May 23, 2013
985
Hi, thx all for reply, hmmm yep when i tap i get image for few secs and goes, tap and back and goes..i think is cold solder, what tip is for find a cold solder? i view the board and dont see any visualy cold solder.

About Hameg, if you can give me contact? and i see is a display cold solder, what is the board of display (from the photos i posted)

thx again for the time!
They do speak English.
Industriestrasse 6

Mainhausen, 63533

Germany

Founded in 1957

Phone:

49 6182 800 0



Fax:

49 6182 800 100



www.hameg.de
 

AlbertHall

Joined Jun 4, 2014
12,345
Have you tried the tapping method? It is hard to fault find if you are not experienced at it.

Moderator edit: added the word "not".
 

dl324

Joined Mar 30, 2015
16,845
.i think is cold solder, what tip is for find a cold solder?
Good solder joints don't go "bad" at some future date unless they're stressed mechanically or thermally. I'd look for some other more typical problems. Dirty pots, bad caps, or dead semiconductors.

It would be helpful if you posted pictures of the "images" you are or aren't seeing. I take it to mean that you sometimes see the trace sweep, but can't be certain. It would also help to know what the settings are.
 

Thread Starter

enito

Joined Jul 10, 2014
16
Sure, when i arrives to home i tap, for try, and take more photos of board, the better quality i can with a camera. Phone is not the best for details .. i keep in touch, and again thx for the help!!
 

Thread Starter

enito

Joined Jul 10, 2014
16
Hi for a little delay, here i back with a video and photos. In video i show the sympthom, and in one position oscioscope works sometimes, but when i change to other range to display, i need to tap for see something. The beep beep is my multimeter goes standby, no osciloscope. I checked the traces and not found and cold or something.
Any advice, really thx!


and Photos:

https://postimg.org/gallery/hkingjdq/65fdb6de/

thx!
 

drc_567

Joined Dec 29, 2008
1,156
Looking at the various photos, there are several wires that are routed to points on the circuit board. Using a pencil, and keeping hands/fingers away from high voltage, does movement of any of the longer wires produce a definite, corresponding change in the scope trace?
If a particular wire has a significant EM field surrounding it, there could be interference that is causing your problem.
 

AlbertHall

Joined Jun 4, 2014
12,345
Don't use a pencil. Use something completely insulating and long enough to keep your fingers clear.
Tap the wires, connectors, controls, boards and the components on the boards, gently, to find the most sensitive place. That's the place, or component to look carefully at.

 

MrChips

Joined Oct 2, 2009
30,712
What test equipment do you have?
Do you have a DMM, working oscilloscope, function generator?
Do you have a coax cable with BNC plug on one end a two alligator clips on the other end?



or this



There are four push switches (radio switches) above the label "TIMEBASE".
When you were tapping on the oscilloscope, what was the setting on these switches?
What was the TIMEBASE rotary switch setting?

What was the LEVEL setting?
Set the LEVEL knob to AT.

Connect a jumper cable from the 0.2V square wave output to the VERT. INP BNC connector. Set the slide switch to DC.

Set the TRIGGER selector (radio switches) to +/-.
 
Last edited:

Thread Starter

enito

Joined Jul 10, 2014
16
Hi thx for replys , yeah i checked moving cables if effect stop and nop. When i reviewed solder and stuff, now i get better sympthom, now when i poweron osciloscope works like 30 secs or little more , i can change timebase and works , but pass this few secs after i power on, and back again the problem. Only 2 timebase works others need tap for see something a little sec. I have the cables i do test with some source and without nothing (bnc plug). Now i see in X Y area backside board ,
https://postimg.org/image/5r059zwfx/ , goes a bit hot. Dont know if this normal? i check if some resistor is in shortcircuit, and see i get ohms. greets!
 

drc_567

Joined Dec 29, 2008
1,156
If you have the selector knobs set correctly, then maybe you are seeing the effects of aging components. Try leaving the power turned on for 24 hours or so.
 
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