Hameg HM203-7

[percival007]

Hi Folks,

my Television theory was never great, I have always been an Audio Engineer so I need some help with a fault on a Hameg 'scope I have.

It's a HM203-7, the Service Manual can be found here:-
https://www.eserviceinfo.com/downloa...g_HM203-7.html
(Free to Download. I have no affiliation to the site supplying the Manual)

Please see the attached Picture for the fault symptoms.
Some sort of 'flyback' line problem? I have no idea.
It doesn't stop the 'scope being useful but it is just a little annoying.

Many thanks,

James.

[Pinörkel]

Hello James,
I am still kind of an oscilloscope noob myself, but my first guess would be an issue in the unblanking circuit. When the sweep reaches the right side of the screen it is blanked to hide the flyback period and when it reaches the left side again and the scope is triggered, the display is unblanked, which seems to happen too early on your scope. However, unblanking and the start of the sweep(the linear ramp-up motion of the trace from left to right) should start simultaneously when initiated by the sweep start gate. Your trace seems to unblank way before the start of the sweep. So I would also have a look at the blanking circuit. This stuff seams to be located in the HV section on page 11 of your manual. The unblanking seems to be realized by a 23V delta on the -1913V line. So be very careful there and only fiddle in the HV section, if you know what you are doing.

Maybe you could check on which height your scope actually triggers. Is it at zero height or at the height where the still flying back trace appears? This could hint at which circuit to blame.

Also, there is a small little dot at the top right of you display. As far as I remember, this an indication of the blanking circuit working correctly. What startles me is that the trace seems to be visible beyond that point to the right, which, I think, should not be the case. Usually this point should be located at the far right of the trace, being only visible when pressing the beam finder button.

If you want to learn some basics, a great start is this: Troubleshooting your Oscilloscope

Denis

[Pinörkel]

Just had a closer look at the manual. The actual blanking/unblanking control seems to be on the CRT section on page 10. At the lower right side you can see the voltage graph (0.8V to 4.4V) with a marking for the blanking level. Now if you imagine the switching threshold of the blanking level in this graph being too low, that could IMHO lead to the issue, your scope has. Maybe you could try to unplug your scope long enough, so no high voltages are present anymore (check with a suitable multimeter first!) and then measure the values of the components around that area.

[RogerEvans]

This scope produces the blanking / unblanking signal in two parts. The 'fast' component is via T304 / C315 to the base of T307 on the HV part of the circuit. A 'slow' component is via T304 / R3024 to the opto-isolator IC302 and then to R3026 and T307. The opto-isolator takes around 1usec to switch and the capacitive coupling fills in the missing switch-on and switch-off fast transients. This extra complexity is one way of getting low voltage circuitry to control the voltage on the CRT cathode from DC to several MHz.

Having said that it is not easy to work out what sort of failure would cause the effect you see where the beam is permanently brightened up and only blanked for the first part of the retrace. If the opto isolator is socketed I would say just swap it for a new one. You may get better advice from someone with more hands on experience of this scope.

Try searching here for any similar problems, I am fairly sure someone else had a problem which was the opposite of this, the beam would only appear for the first part of the sweep and the length of the sweep varied with sweep speed. Solution was a new opto-isolator. Take great care, one side of the opto-isolator is around -1900V.

Regards,

Roger

Roger

[Pinörkel]

If it helps: The complete HM203-7 manuals are also still available from the original manufacturer:
German
English translation without diagrams

[See_Mos]

I had the same symptoms on one of my Hameg scopes. The fault was intermittent and every time I took the case off the fault would disappear. Put the case back on and it would be OK for several weeks. A sharp tap near the back of the case would sometimes clear the fault.

The cause was a cracked joint on the common pin of one of the voltage regulators bolted to the metalwork at the back of the circuit board. The crack wasn't visible but removing the regulator and cleaning the pins before refitting cured the fault. The clue was an increase in the regulated voltage, the +15v rail I think.

[ajgriff]

I always think it's worth checking the supply rails at an early stage when troubleshooting an older scope. At least you then know that the roots of the tree are in good order before moving up the trunk. The Tek guide highlighted by Denis is well worth studying before diving in.

Alan

[TonyDuell]

I think that's a good piece of advice in general. Electronic devices need the right power supply voltages. As you know I work a lot of vintage computers, it's amazing the number of ways that TTL logic can misbehave when run from 4V (rather than 5V).

[WME_bill]

Hameg HM203-7.
Your screen shows the very typical symptoms of the failure of the bright-up circuit. I think the trace disappears on the right hand part of the screen because it has speeded up. I suspect that altering the brilliance will reveal the missing bit.
The Bright-up circuit, described as "ZUP" in the manual, is quite straightforward.
As RogerEvans says, the first part of the bright-up pulse is fed through a small capacitor direct to the cathode driver transistor T307, and the slower DC component is fed through the opto-isolator IC302.
To understand the circuit, as it is not drawn as easily as it could be, sketch in a line dividing the EHT lines and the low voltage 12V parts of the circuit. The opto-isolator and the three high voltage capacitors are on this dividing line. Also makes it clearer where you should not poke your testmeter probe.
T307 and T308 are at the EHT voltage levels between -1913v and -1890v and operate on 22v, set by the zener diode D313. They drive the CRT cathode.
The grid is fed through the Intensity control from a cut off maximum of -1975v.
I suggest you check these transistors first. Simplest to substitute with another. T307 is a high voltage TV video driver type, and the other is a standard PNP type. I usually find these high voltage ones or the opto-isolator tend to fail first.
Then measure the voltages around the sweep gate blanking transistor T304. Usual rule, 0.6V between E and B. Much greater between C and B.

If you want voltages readings with any meaning around the timebase / bright-up. you must be sure to stop the TB working or switch to X-Y. Otherwise, a sweep with variable mark space will give you a fine variety of meaningless voltage readings. A second scope makes fault finding very much easier.
If you are still looking for the manual, as Pinorkel says, the English version on the Hameg site is complete except for missing circuit diagrams.
And I endorse what Ajgriff says. If in doubt about the fault, check the voltage supply rails. Admittedly here, it looks pretty clear.
wme_bill

[MeerKat]

I've had this fault on my Hameg, it turned out to be D318 and T307, a BF199. These were killed I think by excessive HT caused by a poor connections on CN3-9 and a duff IC301. There were some fearsome cracks coming from the scope as it flashed over!!!.

I think I still have some BF199 transistors if you need one, PM me and I'll drop one in the post

[percival007]

Thanks chaps.

It's my work 'scope so obviously can't look at it at the moment.

When I'm allowed back in i'll take a look and report back.

Thanks again,

James.