Author Topic: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED  (Read 11920 times)

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Offline tron9000

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Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« on: August 15, 2015, 01:45:33 AM »
Finally got the dust off  my Hameg I bought from a car boot sale about 2 years ago and forgot about it. I didn't have the space, then moved to a place where I had a garage, set up my little lab, plugged in the scope and checked there was a trace - which there was: good all still working!

closed up for the night and came back next day with some probes donated to me. Hooked it up to channel 1 and to the test signal - power on: nothing.

Tried to see if I could find the trace again, switched the triggering into AC, twiddled the trigger level till the Trigger LED came on yet no trace on screen. checked all button's like X10 mag, EXT trigger, all the daft things and still no trace.

Remove the probe, knocked the coupling on CH1 to GND, which should produce a flat line and adjusted the X & Y positions to see if i could find a trace - alas: fruitless!

The fact that the trigger LED comes on when I adjust the trigger level means that the signal is there. So I suspect that the display driver could be shot?

I have found a service manual (English, kindly shared on another forum):https://docs.google.com/file/d/0B87O0bRBiC6fUS1lUkM4cGVUNmc/edit?usp=sharing
users manual:http://www.manualslib.com/download/563302/Hameg-Hm-203.html

and have found these posts:
EEVBlog - http://www.eevblog.com/forum/testgear/old-analog-hameg-20mhz-dual-channel-scope-died-on-me-where-would-i-start/ - no conclusion to if he fixed it or not, have PM'd the OP.
Badcaps.net - http://www.badcaps.net/forum/showthread.php?t=28509

Guy on Badcaps say that the tube may be FUBAR'd - is this likely from what I've described? It may not be a problem as I have a Kikusui Scope which is just a CRT with X, Y adjust. It works but all that appears is a dot - it needs some signals in the back (i'll post pics if anyone's interested?)

I've added a pic attached of when it was working and stuck a sine wave into it.

So has anybody else got any advice or had experience they can share fixing this scope? Would save some time.

Cheers
« Last Edit: September 07, 2015, 06:10:19 PM by tron9000 »
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Offline oldway

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Re: Repairing an Old Hameg HM203-7
« Reply #1 on: August 15, 2015, 05:42:40 AM »
Quote
Guy on Badcaps say that the tube may be FUBAR'd
For sure, it is not...its very rare :-+
Look for bad/cold solders, that's very common on these scopes.
Check the power supply .
Be carefull with high voltage !
 

Offline tautech

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Re: Repairing an Old Hameg HM203-7
« Reply #2 on: August 15, 2015, 06:52:33 AM »
Quote
Guy on Badcaps say that the tube may be FUBAR'd
For sure, it is not...its very rare :-+
Look for bad/cold solders, that's very common on these scopes.
Check the power supply .
Be carefull with high voltage !
+1
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Offline tron9000

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Re: Repairing an Old Hameg HM203-7
« Reply #3 on: August 15, 2015, 07:54:54 AM »
phew well thats good to hear. Cos I really wouldn't like to break up the kikusui, its a nice tube and would like to turn it into a single channel scope as a project. Its got half the functionality, just needs a some x & Y inputs and its a scope!

Yep will be careful around the HV, thats a given. What voltage we looking at? 1000V?
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Offline tautech

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Re: Repairing an Old Hameg HM203-7
« Reply #4 on: August 15, 2015, 08:09:12 AM »
What voltage we looking at? 1000V?
No.

You must work carefully through the scope, first checking ALL PSU voltages are to spec for both voltage and ripple.

From the low voltages the CRT voltages are derived, often by a switcher PSU, a common point of failure.

This is not a repair to hurry, let us all have a chance to download the Service manual and do some study.
You too.
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Offline tron9000

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Re: Repairing an Old Hameg HM203-7
« Reply #5 on: August 16, 2015, 12:57:44 AM »
What voltage we looking at? 1000V?
No.

You must work carefully through the scope, first checking ALL PSU voltages are to spec for both voltage and ripple.

From the low voltages the CRT voltages are derived, often by a switcher PSU, a common point of failure.

This is not a repair to hurry, let us all have a chance to download the Service manual and do some study.
You too.

no hurry, just wondered if anyone new off the top of their head.
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Offline oldway

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Re: Repairing an Old Hameg HM203-7
« Reply #6 on: August 16, 2015, 01:12:02 AM »
Power supply is very simple, low voltages of + and - 12V and +152V.
HV: -1975v....be careful, that is too high for multimeters, you must use an HV probe.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-7
« Reply #7 on: August 17, 2015, 06:26:52 PM »
OK, so I've fibbed: Its a 203-6 not 203-7  :-[

somewhat embarrassing  :palm:, but still managed to get hold of a schematic and there's really not a huge amount of difference to the general block diagram. I googled my scope and looked a the pictures and picked the one that looked closest to mine and took the PN, rather than note the model number down before I left it the other night. :palm:

HAMEG HM203-6 service manual - most of its german, but all the schma is there.

Anyway I tested some of the voltages the other night, I've made a video, just editing to length.

But the jist of it is that I've tested the heater filament voltage, the intensity control voltage, and the filament to cathode (K ?) voltage and they all appear to be as expected.

(If you look at page 13 of the PDF) I've been measuring them differentially with a cheapo-meter on the PCB on the back of the tube . So measuring between -1877V and -1900V you would expect a 23V difference - which is what I got. Along with 6.3V AC on heater filament and about 33V from intensity control pot. The focus contol is a bit tricky and need to think a bit more carefully as the control pot is part of a large string of resistors connected to -1900V and GND.

So it think the best way of measuring this is to work out the voltage expected across the focus pot and measure the wiper from one end of the pot.

your also probably thinking: "why the hell are you looking at the HV stuff first!?" - well why not, gotta start somewhere, plus it was the most easily accessible place to start! which is actually somewhat concerning: you'd expect the HV to be the LEAST accessible!

Once I get the video done I pop it up here.

cheers
« Last Edit: August 17, 2015, 06:34:41 PM by tron9000 »
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Offline firewalker

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #8 on: August 17, 2015, 06:30:26 PM »
I had the same problem on my HM605. It was just a voltage regulator.

Alexander.
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Offline firewalker

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Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #10 on: August 17, 2015, 07:16:48 PM »
Cheers firewalker, will check for that.
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #11 on: August 18, 2015, 04:58:42 PM »
First ajust intensity pot to max !
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #12 on: August 18, 2015, 06:00:49 PM »
First ajust intensity pot to max !
did you watch the video?

I'll give it a try, would have to hack or fashion a new spindle for it. But I measured the intensity signal and it was as expected or is that not enough to test it?

Firewalker (Alexander) - I could not find that regulator and I also couldn't see any cracked joints on components that were in that area.

« Last Edit: August 18, 2015, 06:03:33 PM by tron9000 »
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Offline tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #13 on: August 18, 2015, 06:15:04 PM »
First ajust intensity pot to max !
Good spotting.  :-+

I'll give it a try, would have to hack or fashion a new spindle for it.
And you must, it is essential to have the Intensity control working.
Better still, replace the Intensity pot.

Not watched your vid yet, will wait until my broadband is under less load.
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #14 on: August 18, 2015, 10:14:31 PM »
Measure output voltages of horizontal and vertical amplifiers (or vertical and horizontal plates voltages)....beam is perhaps almost out of the screen and this can be a reason why don't see nothing on the screen.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #15 on: August 18, 2015, 11:42:04 PM »
That would be another place to check.

Would you say points D1 through D4 on page 13 of HM203-6 schematic?
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #16 on: August 19, 2015, 04:13:18 AM »
Yes, D1 to D4 on the crt board but be carefull because there is more or less -2000V on this board.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #17 on: August 19, 2015, 07:48:03 PM »
I measured points D3 & D4 last night and fitted a spindle to the intensity pot (I removed the old spindle and just stuck a self tapper in the hole where the spindle was! Does the job!) and checked the intensity voltage levels whilst adjusting the the pot as well as testing the focus voltage levels whilst adjusting the focus pot.

  • Intensity range (measured between -1900V and pin 5 on back of CRT PCB) - 22V to 42V  DC approx
  • Focus Range (Measured between -1900V and pin 6 on back of CRT board - 5.4V to about 12V DC
  • D4 to 0V - 88V DC approx
  • D3 to 0V - 64V DC approx

NOTE: on the schematic, pg. 12, y final amplifier - points D4 and D3 are noted below each trace as ca +75V. so does this mean they should be in the region of 75V (as in circa 75v)? If so, then that's a bit ambiguous!

also another vid in the pipeline btw
« Last Edit: August 19, 2015, 07:50:15 PM by tron9000 »
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Offline tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #18 on: August 19, 2015, 10:37:37 PM »
DC values on a schematic usually are referenced to a specific scope setup for those values to be valid.
RTFM
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #19 on: August 20, 2015, 02:02:48 AM »
Adjust vert. position pot. to have D3 and D4 more or less 75V
And D1 and D2, how much voltage ?
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #20 on: August 20, 2015, 05:42:33 PM »
Adjust vert. position pot. to have D3 and D4 more or less 75V
And D1 and D2, how much voltage ?

Yeh I just read the users manual and it says to have the adjust vert pot to mid way, so I'd have had it just off centre probably

D1 & D2 - unfortunately, I haven't had time, got called away for Daddy duties. Should only take a couple of seconds to test, so will probably find out tonight.

Interesting thing though, Iread both manuals and only the HM203-7 has a section on operating voltages and Trouble shooting:
HM203-6 User manual
HM203-7 users manual

in the 203-7 manual, trouble shooting section I found a single sentence that may be relevant:
Quote
An absent trace in spite of correct plate voltages means a fault in the CRT circuit.

So if I do get the correct plate voltage on D1 & D2 and re-test D4 & D3 after adjusting the vert adj pot to centre, then that's probably the next best place to look.
« Last Edit: August 20, 2015, 05:49:29 PM by tron9000 »
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Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #21 on: August 21, 2015, 06:05:04 PM »
OK Measured D3&D4 again - set the vert adj to mid way and yep: both were about 75V DC. Further twiddling and managed to get both to be 75V

D1 & D2: set the CH1 hor. adj. to mid way and both were about 68V DC.

So this does lead to a possible problem on the CRT circuit as the user manual says.

I've noticed on in the 203-7 schematic and manual it mentions a 22V blanking signal (supplied as a difference between the -1900V and -1877V). It looks like a delayed sawtooth or slow rising square wave form what I can gather.

without another scope, I think I'm stuck!
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Offline tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #22 on: August 21, 2015, 06:53:12 PM »
Quote
without another scope, I think I'm stuck!
Not yet, but often the case with CRO's you need another to fix them.

Will the CRT show a flash of a trace, dot or any signs of life when powered off and on, Intensity at max?

Does this Hameg have a Beam Find button?
Does that work?
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #23 on: August 21, 2015, 07:23:52 PM »
OK Measured D3&D4 again - set the vert adj to mid way and yep: both were about 75V DC. Further twiddling and managed to get both to be 75V

D1 & D2: set the CH1 hor. adj. to mid way and both were about 68V DC.

So this does lead to a possible problem on the CRT circuit as the user manual says.

I've noticed on in the 203-7 schematic and manual it mentions a 22V blanking signal (supplied as a difference between the -1900V and -1877V). It looks like a delayed sawtooth or slow rising square wave form what I can gather.

without another scope, I think I'm stuck!
Have you tried in XY mode ?
With spot centered (D1 = D2 and D3 = D4), with -20V between cathode and wehnelt and I think you have the -1900V accelerating voltage (even if you have no way to measure it...you need a HV probe or a Avometer analog multimeter to measure that voltage), you should see a spot !

The only think you did not check, it is the heater continuity of the CRT.
Descharge first the HV capacitor, desolder on the crt board one of the 2 heating's wires of 6,3V coming from the transformer and measure the resistance between this wire and the place where it was soldered.
You must find a low value resistance, more or less 18R.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #24 on: August 21, 2015, 09:39:22 PM »
When powered on I can see a glow inside the tube -does that count? probably not!

I can disconnect the connector from the CRT PCB from the main PCB altogether, rather than desolder and check it.

Will try it in XY mode and see if that yields anything interesting.

I'm sure this scope does not have a beam find button.

There is sweet Fanny Adams when I turn it on and after it warms up, even with the intensity all the way up. No flashes or dots or anything.

To measure the HV, could I just build a big pot. div using say 10M 0.5W resistors?  I know its a bit Heath Robinson...
« Last Edit: August 21, 2015, 10:38:44 PM by tron9000 »
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