Strange oscilloscope screen problem... - Page 1
Forum section: electronics / beginners

author: Tricka90
author_rank: Regular Contributor
timestamp: September 14, 2013, 12:36:56 pm
content: I've bought an used Hameg HM304 oscilloscope. I'm a beginner so that's my first one.I found that the oscilloscope screen has a strange problem when I use "X-MAG X10" function to zoom the waveform a lot.In the video below you can see how the wave deforms in different ways.The more I increment TIME/DIV value the more the wave deforms. But if X-MAG X10 is not active the wave show perfectly (but it's too little to be measured accurately, of course).In this video I'm measuring a 12MHz wave from a PIC external crystal oscillator and moving it around horizontally. The probe is set to 10X.Can I do something to fix this problem?Do you think is an hardware issue or it can be solved just with some calibration?Here's the video:

author: israel
author_rank: Contributor
timestamp: September 14, 2013, 02:23:54 pm
content: Hi, You can check the bridge rectifiers and capacitors in the power supply. Sounds like a leaky rectifier, is a typical problem in such HAMEG oscilloscopes.With the help of a multimeter that has bargraph may see the oscillation of 50 Hz at the output of the rectifiers. Any of them that excessively hot show, is sign of leakage in one of the diodes.If this does not work you will have to go forward with the help of the service manual and diagrams.Regards and good luck in the hunt

author: dfmischler
author_rank: Frequent Contributor
timestamp: September 14, 2013, 02:35:23 pm
content: In the first part of the video, it looks to me like the horizontal sweep ramp is not stable (it's running faster then stabilizes to a slower speed). It looks kind of like my old Tek analog scope on mixed sweep with the knobs unlocked, but the fast part is before the slow part. Where it looks kind of "ghostly" later in the video looks like a triggering problem (i.e. that part of the waveform is not repetitive enough to look solid). Without more information, there is no way for us to tell if the input waveform is unstable there, or if the scope is messing it up.

author: Tricka90
author_rank: Regular Contributor
timestamp: September 14, 2013, 03:22:19 pm
content: Thank you a lot for the help. I will follow your instructions, starting with the ones provided by "Israel".I'm sorry I don't speak english very well. You're saying that I should check the bridge rectifiers outputin the power supply for the X-Mag? Or in thegeneral power supply?I didn't understand what exactly should I measure with the multimeter. You say I should measure theoutputof the rectifier, so why should I find 50 Hz oscillations if it has been already rectified?I'm sorry, my multimeter doesn't have bargraph...maybe I can measure it with the oscilloscope itself?dfmischler, if you need more information I'll happily give them. Please ask me any information you need

author: israel
author_rank: Contributor
timestamp: September 14, 2013, 06:24:05 pm
content: The check would be the electric supply in general. The multimeter with bar graph is useful in the dc current measure, show oscillations of the bar if it contains ac current.In no event Use the oscilloscope itself to make these checks. Otherwise the god Thor and Smoke invade your room, you can use other additional oscilloscope. Giving always miss the measure to be limited only to the transformer secondary and never to domestic ac supply , since otherwise there will also be BANG!I attach the the power supply diagram of HM-604 model which is quite similar to the 304Regards,my English is pretty bad also. although not as much as the Mrs mayor of Madrid

author: Tricka90
author_rank: Regular Contributor
timestamp: September 14, 2013, 06:51:06 pm
content: Thanks a lot for the kindnessI don't have a bar graph multimeter, so could I just measure AC amplitude with multimeter? Would it be useful?And......I was going to measure with the oscilloscope itself but I've suddendly changed my mind!Quote from: israel on September 14, 2013, 06:24:05 pmAhahah well, I can say that also my english isn't as bad as our ex President Berlusconi!I found something that could be interesting, I hope!When I turn clockwise the DEL. POS. HOLD OFF knob, which controls hold off times between sweeps, the problem becomes way less significant! Before the knob was almost entirely turned counterclockwise, near the X1 sign!Can this help you to understand what's the problem?

author: G7PSK
author_rank: Super Contributor
timestamp: September 14, 2013, 09:37:35 pm
content: You could just try some switch cleaner very often all that is wrong with old CRO's is dirty switch contacts.

author: israel
author_rank: Contributor
timestamp: September 14, 2013, 09:45:10 pm
content: Quoteis also very good advice. I have encountered many problems in old scopes, bad contacts. especially in the switches of "AC-GND-DC" and magnifiers.Regards,

author: Tricka90
author_rank: Regular Contributor
timestamp: September 14, 2013, 10:06:16 pm
content: Thank you for the adviceBut how should I clean it? With some particular solvent?

author: G7PSK
author_rank: Super Contributor
timestamp: September 15, 2013, 09:08:08 am
content: Switch cleaner, in an aerosol can such as that made by Servisol or any other reputable manufacturer or supplier.http://cpc.farnell.com/servisol/100005000-200ml/aero-klene-50-200ml/dp/SA00001http://uk.rs-online.com/web/c/?sra=oss&r=t&searchTerm=switch+cleaner&x=18&y=10Any of these would do but as it is an oscilloscope and the switches can be awkward to get at I would go for any that comes in an aerosol with a straw, this makes it much easier to apply without a lot of dismantling. You don't want to disassemble more than you have to, you have to put it all back together again such that it works.

author: Unknown
author_rank: Unknown
timestamp: September 15, 2013, 09:36:23 am
content: My guess would be dried out caps causing excess ripple on some of the power rails.

author: Tricka90
author_rank: Regular Contributor
timestamp: September 15, 2013, 01:07:47 pm
content: Thank you, first of all I'll try to clean the circuit!Quote from: alm on September 15, 2013, 09:36:23 amHow can I check if there's a dried out cap? Should I measure it in some way?

author: hlavac
author_rank: Frequent Contributor
timestamp: September 15, 2013, 03:07:58 pm
content: Bad cap may be bulging or leaking.Or run hot because of the increased ESR.There are tools that can measure caps ESR even in circuit (using very low AC voltage)If you had an oscilloscope already, you could check waveforms on filter caps, they yould be discharging too deeply

author: Unknown
author_rank: Unknown
timestamp: September 15, 2013, 11:57:42 pm
content: Checking ripple on the power supply rails (service manuals would often give a maximum ripple level) would be the usual trick. That's a more direct and easier measurement than ESR. You can even perform it in circuit. Ideally you would use a scope for this, but a decent multimeter in VAC mode may also be helpful. You may even be able to use the scope to test itself (use a 1X probe with the scope set to AC coupling).Measuring ESR is useful for a SMPS which fails to start up, but not so useful in a linear supply in my opinion. I generally measure the ESR if I desoldered the cap anyway because it was suspicious or for some other reason.

author: Tricka90
author_rank: Regular Contributor
timestamp: September 16, 2013, 06:23:18 am
content: Thank you a lot for all the precious advicesI'm going to measure VAC on power supply rails with a multimeter! Could you please tell me how can I find power supply rails? I'm not expert in oscilloscopes, that in fact is my first one...I don't have a circuit diagram to show you, I'm sorry, but userIsraelsaid it shold be similar to the one I'm attaching

author: Unknown
author_rank: Unknown
timestamp: September 16, 2013, 06:52:30 am
content: The various nets labeled +140V, +68V (hard to read because of bad scan quality), +12V, etc. The filter caps are often the easiest components to find, and they conveniently connect to both the power supply net and to the power supply ground.

author: miceuz
author_rank: Frequent Contributor
timestamp: September 16, 2013, 07:11:08 am
content: On the "XY board" there is a "test strip" - something that looks like a connector, but nothing attached to it. It's there for power rail testing. You have to put your black probe to chasis and red probe to contacts in a strip.At first, check that there are voltages within +-2% of -5 +5 -12 +12 +68 +140 volts. After that use another oscilloscope to look at the waveforms.In the given schematics capacitors C2001 to C2008 ar the suspects - it's electrolytics just after the rectifiers.Just yesterday I was debugging a HM 605 - an electrolytic cap on a +140V rail was dead short. So as long as you open the scope up, it's a good idea to replace all of them to something like Nichicon.

author: Tricka90
author_rank: Regular Contributor
timestamp: September 16, 2013, 08:06:00 am
content: I found that HM304 is very similar to HM303, it only has some digital parts added.Here you can see the complete schematics of HM303http://monitor.espec.ws/files/s_hm303-5_119.pdf