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Strange focus error on Oscilloscope Hameg HM1005

P

[email protected]

Hi,

I'm currently having trouble with my good-ol' HM1005. It has some
problems focusing correctly.
When it's cold, you first see nothing on the screen. Increasing
intensity results in an almost completely glowing screen, looking like
an EL-backlight. Leaving it on for some minutes shrinks the glow more
and more to a spot (in X/Y mode) or to a horizontal line (in normal
operating mode). Now it's possible to use that thing, but it's not
completely working correct.
On the left side of the line the beam is more intense. I've made a
photo of that, please have a look at http://www.preamp.org/dinge/hm1005.jpg.
Using a time base of 50ns shows some kind of overshoot next to that
bright spot. The trace is moveable to X and Y as it should (I did that
on the photo, so the bright spot is not on the grid).

I'd appreciate any kind of help that can lead me to the source of this
strange behaviour.


Thanks in advance and with best regards from Germany,

Lasse
 
N

N_Cook

Hi,

I'm currently having trouble with my good-ol' HM1005. It has some
problems focusing correctly.
When it's cold, you first see nothing on the screen. Increasing
intensity results in an almost completely glowing screen, looking like
an EL-backlight. Leaving it on for some minutes shrinks the glow more
and more to a spot (in X/Y mode) or to a horizontal line (in normal
operating mode). Now it's possible to use that thing, but it's not
completely working correct.
On the left side of the line the beam is more intense. I've made a
photo of that, please have a look at http://www.preamp.org/dinge/hm1005.jpg.
Using a time base of 50ns shows some kind of overshoot next to that
bright spot. The trace is moveable to X and Y as it should (I did that
on the photo, so the bright spot is not on the grid).

I'd appreciate any kind of help that can lead me to the source of this
strange behaviour.


Thanks in advance and with best regards from Germany,

Lasse
Click to expand...


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 ?
 
P

[email protected]

J

Jim Yanik

..


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
?
Click to expand...

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)
 
H

Heinz Schmitz

I made another 2 photos. Used a HZ 154 probe on the internal CAL.-
outlet. Looks weird to me...
Click to expand...
http://www.preamp.org/dinge/hm1005.jpg.
http://www.preamp.org/dinge/hm02.jpg
http://www.preamp.org/dinge/hm03.jpg
Click to expand...

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:
First,check all the power supplies.Voltage and ripple.
Click to expand...

Regards,
H.
 
P

[email protected]

But does the trace agree volts and usec, X and Y, with the CAL signal?
Amplitude is correct, but neither is the frequency nor the duty-cycle.
Is there an optoisolator/optocoupler in the CRT drive circuit?
Click to expand...
Couldn't find any.
If you have a saw-tooth-generator, can you try that as an
X- deflection source?
Click to expand...
Maybe there's one in our workshop, if so I'll try it tomorrow.
This does, however, not explain the focus problem.
So I'd follow Jims suggestion:
Click to expand...
I'll do that tomorrow, too, as I don't have a second Oscilloscope at
home.
 
M

Matt J. McCullar

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
22-megohm resistors, and then to the picture tube. One of those resistors
had opened up, causing the won't-focus-properly problem.

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

Geo

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...

http://www.preamp.org/dinge/hm02.jpg
http://www.preamp.org/dinge/hm03.jpg
Click to expand...

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.
 
J

Jerry G.

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.

__
 
N

N_Cook

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.
Click to expand...


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
 
P

[email protected]

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.
 
G

Geo

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.
Click to expand...
Good description of the hold-off control being in the wrong position.
The schematic is available from hameg.com, look under manuals.
Click to expand...
I looked but the manual I found was only 28 pages with no schematics - url?
 
P

[email protected]

CMOS 4011 failure of all things

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.
 
P

[email protected]

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
 
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