Author Topic: Hameg HM 203 double trace problem (Read 29633 times)

BFX · « **Reply #25 on:** July 29, 2014, 06:36:49 pm »

It's -1.9kV on check strip.

David Hess · « **Reply #26 on:** July 29, 2014, 07:07:35 pm »

That is the unblanking pulse which goes to the CRT grid and has nothing to do with CRT deflection. I would expect that level of ripple on it which is really just the unblanking waveform.

BFX · « **Reply #27 on:** July 30, 2014, 07:07:31 am »

Quote

oldway: Have a look for ripple upon C817 ! What is the frequency of this ripple ? 50 or 100 Hz?

I made this measurement on power lines and here is result (the similar is on each).

David Hess: I think, unblanking pulses is on pin 10 but I made this measurement on pin 9.

Andy Watson · « **Reply #28 on:** July 30, 2014, 08:06:41 am »

What voltages are on the pins of IC801 ? Is it trying to regulate?

BFX · « **Reply #29 on:** July 30, 2014, 07:38:39 pm »

Voltages on the IC801 pins are:
1 -9.4
2 2.2
3 0.84
4 -9.4
5 -9.4
6 2.14
7 4.95
8 -2.1

but here is measurement by oscilloscope on pins 3 and 6.
I tried to replace IC801, but without visible change.
I think something can be wrong in voltage multiplier circuit.
I'll try to replace diodes D801 - D808 (I think 1N4007 will be enough).

BFX · « **Reply #30 on:** July 30, 2014, 07:57:53 pm »

Quote from: Andy Watson on July 28, 2014, 11:09:12 pm

Quote from: BFX on July 28, 2014, 10:40:38 pm
measured by HP 3478A

I don't know how to measure ripple lower than 20 mV to get right accuracy using my equipment.
The HP 3478A has a 300mV AC range. Set the meter to measure AC and use the capacitor (as you did previously) to block the DC. But connect the meter and capacitor before you power-up the Hameg - otherwise you risk damaging the meter.

Quote
I try suggestion by David Hess "Try setting the trigger to line input and sweep to about 5 ms/div to see if the noise is line related."
Yes it litle betther but I'm not sure with result.
Your video shows that the trigger is working - it is locking to your test waveform. Something else is superimposed on your test waveform and it is drifting across the screen because it is at a slightly different frequency to your test waveform (or, actually, an integer frequency divisor of your test waveform). What David is suggesting is that if setting the trigger to "line" causes the problem waveform to lock and your test wave to drift, then the problem is most likely due to mains/line related issues (like excessive ripple).

Edit: It might be clearer if you move your test waveform frequency away from being an integer multiple of your mains frequency - say 1005Hz.

Thank you when I set trigger to line waveform is clear

But now I don't know what's the problem

Andy Watson · « **Reply #31 on:** July 30, 2014, 08:22:43 pm »

Looks like it is trying to regulate but doesn't have the necessary voltage for part of the cycle. Either there's a fault with the quadrupler, or the EHT supply is over loaded.

Edit: this was a reply to the post - 2.
With regard to triggering on the line, what you have done is freeze the "problem" - indicating that the problem is very likely caused by something that is mains related - e.g. excess ripple.

tautech · « **Reply #32 on:** July 30, 2014, 08:23:49 pm »

I have in the past had to rework this area of a CRO.
This Hameg is unusual in that the -2kV is derived from the mains transformer and not a separate transformer.
The diodes may be similar to 1N4007 and as they are working at mains frequency they may be OK, but cross reference to the listed diode.
The caps are part of what is often called the DC restorer and I have had to replace them in the past.
That you have 4 of the same value is good for checks against each other and less types to get for replacement.

BFX · « **Reply #33 on:** July 30, 2014, 08:41:24 pm »

Quote from: Andy Watson on July 30, 2014, 08:22:43 pm

Looks like it is trying to regulate but doesn't have the necessary voltage for part of the cycle. Either there's a fault with the quadrupler, or the EHT supply is over loaded.

Edit: this was a reply to the post - 2.
With regard to triggering on the line, what you have done is freeze the "problem" - indicating that the problem is very likely caused by something that is mains related - e.g. excess ripple.

It's not freeze

Only I just set trigger level and tune my generator to waveform move slower to be possible take good photo.
Here is litle video https://www.youtube.com/watch?feature=player_detailpage&v=LuXJr5GrI8c

BFX · « **Reply #34 on:** July 31, 2014, 03:16:01 pm »

I tried also external trigger and the same problem as internal.

Can be replaced 120pF/600V using 200pF/1000V on the position C807 of -2kV circuit? (150pF is impossible to buy)

Andy Watson · « **Reply #35 on:** July 31, 2014, 03:42:08 pm »

Quote from: BFX on July 31, 2014, 03:16:01 pm

Can be replaced 120pF/600V using 200pF/1000V on the position C807 of -2kV circuit? (150pF is impossible to buy)

I would not. C807 forms part of the frequency compensation network within the regulator loop - changing its value will change the loop response. In fact, unless it is known to be faulty, I would not replace.

BFX · « **Reply #36 on:** July 31, 2014, 05:35:07 pm »

Thank you Andy Watson I try to find some distributor of similar part but int difficult in central europe. (I think distributors are crazy)
I found only one in mouser but price is 61 euros

its impossible , its crazy dream

BFX · « **Reply #37 on:** August 02, 2014, 01:45:46 pm »

Only this types of cpacitors is possible to buy 120pF/500V (it's lower voltage as currently used 600V) or 150pF/3kV
http://www.tme.eu/en/details/cch-120p/500v-tht-ceramic-capacitors/sr-passives/
http://www.tme.eu/en/details/cc3k-150-5/3kv-tht-ceramic-capacitors/sr-passives/
Which to use?

I think better diodes to quadrupler will be http://www.tme.eu/en/details/by2000-dio/tht-universal-diodes/diotec-semiconductor/by2000/
Because original diodes is EM513 / 2kV

But I'm still not sure that ripple of 2kV output is cause of this problem, does have anyone any other hypothesis ?
Why it's happens in internal and external trigger mode and not in line mode?

Andy Watson · « **Reply #38 on:** August 02, 2014, 06:27:07 pm »

Quote from: BFX on August 02, 2014, 01:45:46 pm

Only this types of cpacitors is possible to buy 120pF/500V (it's lower voltage as currently used 600V) or 150pF/3kV
http://www.tme.eu/en/details/cch-120p/500v-tht-ceramic-capacitors/sr-passives/
http://www.tme.eu/en/details/cc3k-150-5/3kv-tht-ceramic-capacitors/sr-passives/
Which to use?

For what purpose? Unless you have evidence to the contrary, I would leave C806 and C807 alone. C802/3/4/5 could be replaced with slightly larger values - if necessary - but find the fault first.

Quote

Because original diodes is EM513 / 2kV

EM513 are still available, e.g. http://www.rapidonline.com/electronic-components/diotec-high-voltage-rectifier-diodes-526299

Quote

But I'm still not sure that ripple of 2kV output is cause of this problem, does have anyone any other hypothesis ?

It is difficult to be certain, however, your earlier measurements show that you have 140-200V peak-peak ripple on what should be a regulated supply. I believe somebody has already posted a link to specifications of the tube which will indicate the effects of EHT voltage variation on the geometry (sensitivity) of the CRT display. The waveforms that you measured around IC801 show that it attempting to regulate for part of the mains cycle but is being prevented from regulating for 100% - presumably because it runs out of input voltage. As I said above - either the quadrupler is faulty, or the EHT supply is overloaded. (At least that is my reading of the evidence presented to date.) Checking the quadrupler would, to my mind, be the next, easiest step to take.

Quote

Why it's happens in internal and external trigger mode and not in line mode?

It is still happening in line mode. Triggering on the line frequency has frozen the problem on the display - I bet if you put the graticule back in front of the CRT you will find that your square waves are not quite as square as the should be

BFX · « **Reply #39 on:** August 02, 2014, 07:23:55 pm »

But I measured all diodes in quadrupler and all seems to be ok, also capacitors

Only one is suspected, under diode D409, D408 is dark BCB as overheated.

Also on another place is signs of breakdown of EHT to ground, I made a gap in PCB.

David Hess · « **Reply #40 on:** August 02, 2014, 08:31:55 pm »

One problem with testing high voltage diodes and capacitors is that they may fail by having high leakage only at high voltage and most testers use a low voltage.

If they cannot be tested under realistic operating conditions and if other causes are eliminated, then it may be easier to just replace them.

Andy Watson · « **Reply #41 on:** August 02, 2014, 08:40:06 pm »

Quote

.... then it may be easier to just replace them.

and clean the PCB whilst you're there! What is the history of this instrument - looks like it has been dipped in the sea!

BFX · « **Reply #42 on:** August 02, 2014, 09:30:06 pm »

Yes will be best to replace all diodes and capacitors.
Therefore, I asked which be better to applied.
I don't know history of that , it's looks like has been in a humid environment.

BFX · « **Reply #43 on:** August 09, 2014, 04:00:33 pm »

Ok all parts of EHT was changed ... let's go testing

Andy Watson · « **Reply #44 on:** August 11, 2014, 10:58:59 pm »

Quote from: BFX on August 09, 2014, 04:00:33 pm

... let's go testing

... and ?
Or are you waiting for the magic smoke to clear

BFX · « **Reply #45 on:** August 13, 2014, 06:59:40 pm »

OK

Testing testing is complete and oscilloscope works again as new

I replace all diodes in quadrupler and also capacitors c806 and c807

I used BY2000 (2kV/3A) diode as replacement of EM513
http://www.tme.eu/en/details/by2000-dio/tht-universal-diodes/diotec-semiconductor/by2000/
and those capacitors
http://www.tme.eu/en/details/cc3k-10n-7.5/3kv-tht-ceramic-capacitors/sr-passives/#
http://www.tme.eu/en/details/cch-120p/500v-tht-ceramic-capacitors/sr-passives/#
Now is time to calibration , clean and put together.
Front panel is already clean. (img. 2)

Thank you

tautech · « **Reply #46 on:** August 13, 2014, 07:29:14 pm »

Well done.
Nice sharp trace.
Yes that PCB looks grubby and wanting a good clean.
Hope she is trouble free for you now.

BFX · « **Reply #47 on:** August 13, 2014, 07:36:44 pm »

Quote from: tautech on August 13, 2014, 07:29:14 pm

Well done.
Nice sharp trace.
Yes that PCB looks grubby and wanting a good clean.
Hope she is trouble free for you now.

Thanks , it was good experience in troubleshooting of analog oscilloscope.
And also very good relax for me because I'm software developer.

BFX · « **Reply #48 on:** August 17, 2014, 06:07:10 pm »

Ok boys ,

good news first

here is some pohotos after litle callibration process.

generator signal from owon

The same signal from hameg

Front panel cleaned and completed

And now somethink bad

Chanell B was broken input FET transistors and I used BF245B. Also T204 was bad and replaced by same type.

But now is aplitude on the screen from channel B about 10x lower as on channel A.
Is possible change some pasive or is needed to change FET?
Better replacement of TIS69 I think like 2N5199, 2N5046, BFS21A.

Or can be something other wrong? Signal after attenuator is the same as from channel A.

tautech · « **Reply #49 on:** August 18, 2014, 02:09:09 am »

For T201 & 204 to have failed, I would suspect a single over-voltage event.
If that is the case, expect IC201 to have failed also, along with damage to diodes and caps.

It will just take a little time to work through them.


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Author Topic: Hameg HM 203 double trace problem (Read 29633 times)

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