Strange focus error on Oscilloscope Hameg HM1005

Did it gradually or suddenly go wrong? If you feed in a known accurate ,say, 1V pk-pk 1KHz signal in, does it give the correct screen indication of 1V, 1KHz with relevant settings ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I'm sorry I can't answer that. I haven't used it for a couple of years...

I made another 2 photos. Used a HZ 154 probe on the internal CAL.- outlet. Looks weird to me...

give

But does the trace agree volts and usec, X and Y, with the CAL signal? I cannot see the graticule. Is there an optoisolator/optocoupler in the CRT drive circuit?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

"N_Cook" wrote in news:gkasuf$jv6$ snipped-for-privacy@news.motzarella.org:

First,check all the power supplies.Voltage and ripple.

then determine if the sweep circuit is working,then check the unblanking waveform and Z-axis amp. you should be able to vary the amplitude of the unblanking pulse at the output of the Z-axis amp,with the brightness control.(not the CRT bias pot)

I suppose a fault in the horizontal deflection circuitry, because the saw-tooth is not as clean as it should be on the left side. If you have a saw-tooth-generator, can you try that as an X- deflection source? This does, however, not explain the focus problem. So I'd follow Jims suggestion:

Regards, H.

Amplitude is correct, but neither is the frequency nor the duty-cycle.

Couldn't find any.

Maybe there's one in our workshop, if so I'll try it tomorrow.

I'll do that tomorrow, too, as I don't have a second Oscilloscope at home.

Sounds like a good time to replace the electrolytic capacitors.

I once had an old B&K Precision oscilloscope that I bought very cheaply because it had a focusing problem. It turned out to be an open resistor. The focus volage came from the flyback transformer, went through a chain of

Your 'scope may have a cold solder joint in this area.

Your pictures are showing the retrace (which should be blanked). The bright spot at the left is where the beam is stationary awaiting the next trigger edge (this should also be blanked). You can confirm this by free-running the timebase when the bright spot should disappear. Your problem is that either the brightness/brilliance control (front panel or internal preset) is set far too high, or the blanking waveform is missing. The first cause can also be due to incorrect high voltage supplies and the out-of-focus reported may be another effect of this.

I have not worked on this model of scope, but I can maybe give some suggestions.

Check that the main power supply is working to specs. Then check that the high voltage and focus supply are to specs.

Because you are saying that the display is sort of dim on one side compared to the other and there is soft focus, this is a possible indication of voltage regulation, and switching power supply DC filtering problems at its outputs.

Some scopes that I have worked on had high voltage modules that supply the anode and focus supply. I have had the module go defective causing your described fault.

The last alternative if you feel it is worth while, is to call the manufacture and see if they would service the scope. Or, see if you can find a working used one of the same model. This way, you may be able to have some spare parts.

Jerry G.

__

g.

Cal of 1MHz ? , if the screen display geometry agrees with the CAL then unlikely a problem with the EHT/ PDA and focus problem from that

from my repair briefs

Hameg HM103 scope Lack of flyback suppression Replace the optoisolator

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

I've made some first measurements. The power supply seems OK at first sight, but the sawtooth doesn't look that good. I found out that when I set the trigger switch to LINE (~), the sawtooth shows only every 10th tooth, the rest of the time it's GND level. The trace looks like a bright spot on the left and a really dim line to the right. If I use the trigger level switch, the sawtooth is totally missing! In the schematic, on the timebase page, there's a signal called "PDPA". This is high while the trigger level switch is 'used'. In normal operating mode, there's a duty cycle of about 90% to 10%. This clips the sawtooth at the ground level, which could explain the bright spot on the left at the trace. What is this PDPA for and why does the trigger affect the sawtooth?

The schematic is available from hameg.com, look under manuals.

Good description of the hold-off control being in the wrong position.

I looked but the manual I found was only 28 pages with no schematics - url?

In my opinion, seems a blanking problem

Good luck

I used the german version.

url?

Thanks for the heads-up I downloaded the manual for my usual low spec scope HM412-5 and the schema are in there. I've previously repaired it blind (CMOS 4011 failure of all things).

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

There're some 7400 and 74132 in there, should I change them?

I'm not sure wether there's a blanking problem, since the sawtooth is directly affected by the trigger. The hold-off regulator seems to have no effect at all. The second timebase is also affected, seems that both of them influence each other.

Ok - some parts not very legible - but suggest looking at the two monitoring points MP7 and MP6 (page 44 of german manual) for the blanking waveform.

Regarding the hold-off - fig 2 on page 15 of the "englisch" manual shows the effect you described.

The problem is solved!

Unfortunately it was a very... stupid one. The coax-cable for the Z- signal was not correctly soldered to the CRT-board. I asked my father about that (I got the scope from him). He said that it was bought from eBay a while ago. I guess the previous owner messed something up in there.

However, thanks to all of you for your tips and suggestions, I really appreciate that!

Best regards,

Lasse