Author Topic: Hameg HM203-5 repair (bad trigger?) (Read 3446 times)

pi-amp · « **on:** January 23, 2020, 08:05:33 pm »

Hello!
this is my first post here

i'm a french hobbyist and tube amp tech.
I have a trusty Hameg HM203-5 scope that began to behave strangely some days ago.
There seems to be something wrong with the triggering, i get a vertical line on the left and i can see the "return line" (see included picture)
I get the same results with different function generators. On both channels. At every trigger settings...
And sometimes (couldnt get it on picture) there is a intermitent inverted trace added to the signal...

What's wrong with my old hameg?
Thanks!

shakalnokturn · « **Reply #1 on:** January 23, 2020, 08:42:13 pm »

I can see the vertical line to the left of the screen on your photo. That seems to indicate that blanking and start of sweep aren't in sync. Sweep begins too long after un-blanking.
I can't see the "return line" in your picture though, if you can see it that would also point towards a problem with the blanking.
What makes you think the trigger is the problem? Does your trace jitter or roll horizontally on the screen?

Kleinstein · « **Reply #2 on:** January 23, 2020, 08:53:42 pm »

For the relatively slow 500 µs / DIV mode the return trace would be weak. It gets more relative time (an thus more visible if the blanking is bad) with a faster deflection.

If the return line is also visible (with fast deflection) one may have the blanking not working at all and not just a delay problem.

shakalnokturn · « **Reply #3 on:** January 23, 2020, 08:57:14 pm »

A couple of topics on other versions (should be similar) of HM203 that I have repaired if that can give you a clue for things to look at:

https://www.eevblog.com/forum/repair/hameg-hm203-6-blanking-glitch-189069/

https://www.eevblog.com/forum/repair/(solved)-blanking-issue-on-hameg-203-7-oscilloscope/

pi-amp · « **Reply #4 on:** January 24, 2020, 07:18:07 am »

Thanks for your help!
here's another picture at a faster rate (a 4khz sine) i can see the return line
so it would be the blanking ?
Any idea where to look at ?

goaty · « **Reply #5 on:** January 24, 2020, 07:31:51 am »

https://elektrotanya.com/hameg_hm203-5_oscilloscope_sch.pdf/download.html

Look at CRT-Board first. Maby Optocoupler CQY80 is weak and you need to adjust VR95 ?

pi-amp · « **Reply #6 on:** January 24, 2020, 08:53:02 am »

thanks,
i'm used to HT (tube amps, 500v) but never messed with CRT (2kv...) any advice?

goaty · « **Reply #7 on:** January 24, 2020, 10:02:46 am »

Maby first try switched off to localise the VR95 and clean it.
It could be potentiometer wiper contact problem also.
There is some "contact spray" that improves contact. I think it is called "Tuner" something.
Or simply move VR95 a few times and return to the same spot.
Then try oscilloscope again.
This way you can try without HV.

If switched on, I would use good isolated screwdriver to adjust VR95 and maby also gloves. But I´m no expert in HV.

shakalnokturn · « **Reply #8 on:** January 24, 2020, 05:45:21 pm »

At 2kV the same safety rules apply for you as at 500V. On top of that you'll need to take extra precautions to not damage your test equipment while troubleshooting. The poor man's solution is a set of high voltage high value resistors to use in series with the instrument.

Check power supplies first.
Then out of experience I'd be looking at: IC903, T933, C933, C934, C935.

pi-amp · « **Reply #9 on:** January 24, 2020, 05:58:20 pm »

i don't see a lot of big H filter caps so i guess there's no remaining HV once disconnected from main voltage and powered off ?

so i opened the scope
It's not looking very good near VR95 on the TB board, as it had been very hot or something, but i don't see burnt component. For now i'm not sure i want to unsolder things as i would have to dismantle all the switches...
but i located the optocoupler (CQY80N) and tested the diode pins (1 and 2) and i get 0,4V reverse voltage one way and 1,3 the other way! so i think it's fryed

another strange thing :
on the schematics T932 and T933 are "head to head"
on my scope they have the same orentation? T933 seems on the wrong orientation ? (cf picture)

and.. isn't IC930 upside down

so strange, as the scope use to work quite good and i never opened it !!!

I tested T933 and i get a strange reading... is it dead ?
so many questions

shakalnokturn · « **Reply #10 on:** January 24, 2020, 10:25:10 pm »

It is best to never assume a capacitor is discharged. Although the film capacitors used for -2kV here are of smaller value than the electrolytics you may be used to seeing in amplifiers they also have less self-discharge.

The black residue on your photos is smoke particles / dust attracted there by the high voltage. Surely you've seen similar things in tube amplifiers, maybe not as bad?
Best cleaning it, as long as it is dry no problem, once it gets damp enough (moving from cold to warm environment) trouble shows-up.

If diode side of the opto is cooked it could just be old age, more likely is that it got zapped by the -2kV arcing across (there your dust can be a cause) either the opto itself or one of the 3 ceramic capacitors previously stated.
I don't usually write things like this, but: Considering their cost, the ability to test them correctly and the extra time it will take if they cause another fault I'd just replace the 3 capacitors.

Assuming there has been an arc or other isolation fault, you ought to take some time to check the other semiconductors in that area: D930-932, T930, D933-935, Z931, T932.
Replace whatever looks bad and try again.
If you're not in a rush check the high value resistors are not open.

For the component orientations, if it was working that way before they souldn't be wrong, this could be due to PCB layout revisions. However if you want to be sure, check component references and pinouts, follow tracks... All sorts happen!

pi-amp · « **Reply #11 on:** January 24, 2020, 10:31:11 pm »

thanks
it seems that T932 and T933 are in the good orientation with C "up" - i read 51r between the 2 collectors , that's correct when reading the schematics. Assuming BF199 and BF440 have the same pinout ?

BUT the IC really seems on the wrong side. reading the schematics, pin1 should be down left, connected to D930. i found the IC upside down with pin 1 pointing up right ! That would explain why it's cooked ? that's strange.
i'll order a new BF199 as it's cooked, and an optocopler. then i'll have to decide which way i put it...

about replacing the 3 caps : that means dismantling of all the board and the buttons... not sure i'm willing to do that !

pi-amp · « **Reply #12 on:** January 25, 2020, 01:04:51 pm »

new parts ordered (optocoupleur, BF199, ceramic caps)

question : assuming the octocoupler was upside down - how would a scope behave ? how does it behave without octocoupler ?

goaty · « **Reply #13 on:** January 27, 2020, 09:51:01 am »

My guess is the orientation in the diagram is wrong. Very unlikely the optocoupler was in there and didn´t work. I´d use the same orientation. Ignore the schematic/PCB diagram.

pi-amp · « **Reply #14 on:** January 27, 2020, 10:27:17 am »

hard to tell...
but i tried to trace the circuit and it seems the diagram is good and the orientation of the chip was bad on my scope

i ordered 5 chips! so i'll try both orientation... but maybe it could do some damage?

goaty · « **Reply #15 on:** January 27, 2020, 10:59:35 am »

Looks like your scope has been repairesd at some earlier time.

It cannot damage scope. Only coupler maby could be dead, but not likely.

pi-amp · « **Reply #16 on:** January 28, 2020, 07:34:49 pm »

so i got the parts !
i replaced the coupler on what i think is the good orientation (= like in the schematics, in the opposite orientation as how the dead coupler was)
changed the dead BF199 for a new tested one

i got a better picture... but still a little horiz line and point at left and the return line.

tried to mess with VR95... and it works !!! picture is ok!
(to be sure, i tried with the coupler on the other orientation, did not work so i know how it shoud sit now.)

for the record, i found an old picture of my scope and i already had that left horiz line and return line... so it was already defective (did'nt notice at that time!)

So it's working better now,
but... i still have another problem (i had it before changing the components) :
With a small signal (some mv) i have 2 signals with opposite phase - its intermitent, sometimes it goes away and comes back... see pictures
it happens on 2 channels with 2 probes and 2 sine generators...
seems like a triggering issue ?

shakalnokturn · « **Reply #17 on:** January 28, 2020, 09:16:48 pm »

Mine will sometimes have similar displays when the trigger settings are not optimum. Have you tried changing trigger coupling or using manual level rather than auto (AT/NORM)?
If you experience the problem despite settings check electrolytics in trigger section. (This is the order of priority I would give them: C501, C436, C441, C503, C504)
Also consider using contact cleaner on all trigger related switches, and internal trimpots VR41, VR42.

pi-amp · « **Reply #18 on:** January 30, 2020, 02:41:08 pm »

will try that.
funny thing is T501 (BF199) seems inoperant. I might have a orientation problem with this one too so i tried the 2 orientations and display stays the same !

shakalnokturn · « **Reply #19 on:** January 30, 2020, 04:38:02 pm »

You'll get used to replacing BF199's. Over the years they have become one of my usual suspects in older equipment. (BC327/337's too.)

pi-amp · « **Reply #20 on:** January 30, 2020, 04:55:05 pm »

it's a new and tested one! but still orientation has no effect on display

shakalnokturn · « **Reply #21 on:** February 02, 2020, 03:17:41 pm »

Schematic has T501 marked as a BC237B not BF199, at least check the transistor pinouts match and that the way that it is fitted conforms to the schematic. (Don't bother with component layout drawings...)

If you have no other oscilloscope to probe with:
Set scope to display both channels.
Send an input to CH1.
Set trigger to CH1 and adjust to have a correct display.
When your trigger problem starts showing use CH2 to probe the scope following the trigger from IC430 pin9, through to T505 and T508.

jdragoset · « **Reply #22 on:** February 03, 2020, 10:54:31 pm »

Very Interesting!
I just acquired the same, missing T 932, T 933 and C 932.
Optical Isolator is oriented the same as the pic, transistor away from the ChP test plug.
Until correct parts arrive, will sub missing components with 2N 3906, 2N 3904 and a 0.1 uf, 400 volt cap.

jdragoset · « **Reply #23 on:** February 04, 2020, 08:53:52 pm »

Sorry, My bad.
Optoisolator in my scope has pin 1 lower left, as shown in the component layout drawing.
The subs for C 932, T 932 and T 933 put the scope to working but trace-to-trace sweep calibration seems to
vary as a sine wave is clean at left side but several traces side-by-side at far right.

shakalnokturn · « **Reply #24 on:** February 04, 2020, 10:18:58 pm »

Quote from: jdragoset on February 04, 2020, 08:53:52 pm

Sorry, My bad.
Optoisolator in my scope has pin 1 lower left, as shown in the component layout drawing.
The subs for C 932, T 932 and T 933 put the scope to working but trace-to-trace sweep calibration seems to
vary as a sine wave is clean at left side but several traces side-by-side at far right.

What variations of this do you get through different timebase settings?
It sounds like a case of check the PS for ripple... I'd start with the plate output amplifiers supply.


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Author Topic: Hameg HM203-5 repair (bad trigger?) (Read 3446 times)

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