Author Topic: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED  (Read 19945 times)

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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #25 on: August 21, 2015, 02:19:50 pm »
Ok, if you see the heater glowing inside the tube, that's allright, you don't need to measure the heater resistance.

The  Hameg scopes don't have a beam find button.

You could try to tweak the VR501 trimpot and see if you have something on the screen.
Be careful, use a well isolated screwdriver because this trimpot is at high voltage (-1900V)
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #26 on: August 21, 2015, 02:42:21 pm »
Best crack out the VDE set I bought!  ;D
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Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #27 on: August 25, 2015, 08:28:55 am »

Why do you waste your time with useless and not interesting videos? |O

I asked you:
Quote
You could try to tweak the VR501 trimpot and see if you have something on the screen.
Why have you not done this ?

Want you really to fix this oscilloscope? :-//

We are trying to eliminate one by one all possible causes for which you have nothing on the screen:
- The supplies have been checked and are OK
- The high voltage seems to be OK (no way to measure it)
- Heating of the CRT looks OK
- The spot is at the center (D1 = D2 and D3 = D4)

Other voltages you measured have no interest because it can not help to have something on the screen.

It is almost impossible that the CRT suddenly become defective.

The only possibility would have been a defective open filament, but it glows so it's OK

Then, only the cathode voltage / Whenelt may still be responsible for the problem.
You measured this voltage and it is - 20V at max brightness
This is correct but without the blanking pulses, it is too much negative voltage for having a spot on the screen.
That's why I asked you to vary the trimpot VR501 with brightness potenciometer at maximum intensity so  to reduce the cathode voltage / Whenelt and see if you can see something on the screen.
If it's ok, that's mean that you have a blanking pulses failure.

Why do you not do what you are asked to do ? :box:
 

Offline TheBay

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #28 on: August 25, 2015, 11:17:00 am »
Totally agree with this Oldway!


Why do you waste your time with useless and not interesting videos? |O

I asked you:
Quote
You could try to tweak the VR501 trimpot and see if you have something on the screen.
Why have you not done this ?

Want you really to fix this oscilloscope? :-//

We are trying to eliminate one by one all possible causes for which you have nothing on the screen:
- The supplies have been checked and are OK
- The high voltage seems to be OK (no way to measure it)
- Heating of the CRT looks OK
- The spot is at the center (D1 = D2 and D3 = D4)

Other voltages you measured have no interest because it can not help to have something on the screen.

It is almost impossible that the CRT suddenly become defective.

The only possibility would have been a defective open filament, but it glows so it's OK

Then, only the cathode voltage / Whenelt may still be responsible for the problem.
You measured this voltage and it is - 20V at max brightness
This is correct but without the blanking pulses, it is too much negative voltage for having a spot on the screen.
That's why I asked you to vary the trimpot VR501 with brightness potenciometer at maximum intensity so  to reduce the cathode voltage / Whenelt and see if you can see something on the screen.
If it's ok, that's mean that you have a blanking pulses failure.

Why do you not do what you are asked to do ? :box:
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #29 on: August 25, 2015, 12:29:42 pm »

Why do you waste your time with useless and not interesting videos? |O

Why do you not do what you are asked to do ? :box:


right I'm trying to create a log of what I've done here and also practice a bit of video editing.
Quote
This was just looking at the voltages, which I already posted a few days back. Its just to show where they were and what I measured. nothing exciting!
^^ right there, this is not a progression, this is just a record!

How many people search forums for a solution to there problems? I'm doing this off my own spare time and trying to contribute here, pay a little back in the form of some findings, instead of just lurking! That such a bad thing? :box:

secondly, cos I got lead down a different avenue which actually lead to the problem! not that I don't appreciate the help, but where's the fire? does it look like I'm in a rush to fix this? does it bother you that much that I'm figuring some things out on my own? :box:

I decided to actually TEST the -1900V by measuring all the voltage drops across each resistor in the chain for the focus adjustment: and low and behold, came out at 94V ish!, which meant my original assumption was flawed.

Checked the 480V~ from the transformer: was present and was only after I left it on for 10mins I found that I could smell burning dust. I found that the opto: IC501 was really hot, took it out and tested the diode: reading came back as 0.133V both forward and reverse bias. Powered it on with the OPTO out and re-measured all the voltages between R507 to R521, guess what: 1900V!

So all I need is a new CQY80N to see if that fixes it and find out what else could have been taken out and why it caused the -1900V to collapse, if that is the culprit.
« Last Edit: August 25, 2015, 02:26:50 pm by tron9000 »
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Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #30 on: September 06, 2015, 09:08:14 pm »
Right, just an update:

IT'S FIXED!! :-+

It was the opto-isolator, IC501!

Cheers to oldway & tautech.

As promised, the finer details, if your interested:
Entry #1 - Beginning & diagnosis
Entry #2 - Further Diagnosis - more fruitless searching
Entry #3 - Culprit found!
Entry #4 - Fixed!
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Offline BFX

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #31 on: September 06, 2015, 10:55:48 pm »
Good Job   :-+
It's nice unit I've also repair similar one recently. But it's not nice documented as yours.
https://www.eevblog.com/forum/repair/hameg-hm-203-double-trace-problem/msg484236/#msg484236

I have also next two from ebay for 50euro for repairing. It's relax for me  :D
« Last Edit: September 06, 2015, 11:06:33 pm by BFX »
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #32 on: September 07, 2015, 08:08:21 am »
Good Job   :-+
It's nice unit I've also repair similar one recently. But it's not nice documented as yours.
https://www.eevblog.com/forum/repair/hameg-hm-203-double-trace-problem/msg484236/#msg484236

I have also next two from ebay for 50euro for repairing. It's relax for me  :D

cheers! Your problem seemed a little more complicated than mine. Notice you mentioned that you used a home made pulse generator: what's it based on?
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Offline BFX

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #33 on: September 08, 2015, 01:05:57 pm »
tron9000

It's very old function generator around XR2206 chip.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #34 on: September 09, 2015, 08:04:32 am »
It's very old function generator around XR2206 chip.

I'm thinking about building my own round one of those chips. Just something I can whip up and stick in a box.
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Offline BFX

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #35 on: September 09, 2015, 10:39:55 am »
tron9000

I think better one is MAX038, but it's also vintage chip.
 

Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #36 on: September 09, 2015, 03:12:32 pm »
Jeez maxim..I'd have better luck finding a lesser spotted flying pig! :-DD
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Offline Stephan_T

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #37 on: September 11, 2015, 07:42:10 pm »
Best crack out the VDE set I bought!  ;D

Hi Chris,

are you aware, that the bare metal screw you use as a shaft for the intensity pot, is no where near VDE approval? According to the schematics, that pot also operates at high voltage (-1900V).
In one or two of your videos you showed how you found vagrant solder on the board. This could have likely been the root cause of the problem. Can you imagine what happens, if such a piece of vagrant solder finds its way between one of the contacts of the intensity pot and your conductive shaft screw?

Watching your videos gave me the impression that you underestimate the danger of high voltages.

Before you end your repair session, you may also check some of the adjustments. Especially the trim pods (VR501/VR502) around the intensity control may need some attention. The service manual should contain a procedure of how to set them.
 


Offline tron9000

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Re: Repairing an Old Hameg HM203-6 (not 203-7)
« Reply #39 on: September 14, 2015, 08:50:35 am »
Best crack out the VDE set I bought!  ;D

Hi Chris,

are you aware, that the bare metal screw you use as a shaft for the intensity pot, is no where near VDE approval? According to the schematics, that pot also operates at high voltage (-1900V).
In one or two of your videos you showed how you found vagrant solder on the board. This could have likely been the root cause of the problem. Can you imagine what happens, if such a piece of vagrant solder finds its way between one of the contacts of the intensity pot and your conductive shaft screw?

Watching your videos gave me the impression that you underestimate the danger of high voltages.

Before you end your repair session, you may also check some of the adjustments. Especially the trim pods (VR501/VR502) around the intensity control may need some attention. The service manual should contain a procedure of how to set them.

Far from it - I fully understand the dangers of high voltages, if I didn't I wouldn't be posting this, I don't think the afterlife has internet! :P

The mod I admit was on the fly and am relying on the insulation of the pot shaft, to which I am now compromising by jamming the screw in there. This will be replaced by super-gluing some plastic dowel in there instead.

I do note in the videos that there is high voltage and that you should proceed with caution and I always power off, make my test connections and then power back on and hardly even measure my test points directly with the probes.

I'll have a look at trimming those pots, but to be honest I think the intensity is unchanged, I'll have a read of the manual.

Thanks BFX for those links!

« Last Edit: September 14, 2015, 09:17:30 am by tron9000 »
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Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #40 on: December 07, 2017, 06:38:48 am »
Dear friends,

i have a Hameg HM203-7 and before 2 days starts rolling the trace.
I did a video just to show the fault. Can you advise me where to start from?
TIA
VIDEO->https://youtu.be/0DVfRPy4IUI
 

Offline oldway

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #41 on: December 07, 2017, 03:08:10 pm »
The failure seems to come from the power supply.

Check the small 1A bridges, they often fail with an open diode .... The rectified voltage is then too low for the regulator with a lot of 50Hz ripple.
 

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #42 on: December 07, 2017, 07:08:18 pm »
The failure seems to come from the power supply.

Check the small 1A bridges, they often fail with an open diode .... The rectified voltage is then too low for the regulator with a lot of 50Hz ripple.

I open the case and i disconnect all the wires. I took both the pcb's in my hand and do measurements. I 've checked all the important capacitors with capacitometer and i found one
electrolytic not connected well. I weld him well now. I also checked all the diodes and transistors. By my fault i 've destroyed one BF472 transistor.
I hope to replace it by late tomorrow.
I didn't check the small bridges you mention though. So i will do it tomorrow and post the results.
 

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #43 on: December 11, 2017, 06:03:13 pm »
UPDATE: I did change the broken BF472, i also measure the diode bridges too. I don't have a HT tester to measure the high voltage but i managed to measure
almost all the voltages from +12V to -12V, 135V, and 541V in the point says 500V. I 'm up to 41V as you can imagine.
What i noticed so far is that when i hit the tube gently near the neck, i get the trace line. When i try to increase the intensity, the line becomes brighter and after disappears.
Is it any possibility to have a damaged tube? Can i do anything more about it?
TIA
 

Offline Calambres

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #44 on: December 11, 2017, 06:42:36 pm »
Reseat the tube connections and/or clean them.

Be very careful with the HT!

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #45 on: December 12, 2017, 06:24:19 pm »
Reseat the tube connections and/or clean them.

Be very careful with the HT!

I did that but no luck. Any ideas how to test if the crt tube is broken?
 

Offline IconicPCB

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #46 on: December 12, 2017, 07:06:17 pm »
Are there any  resistors mounted on the tube connector?
Check them. generally high value resistors can go open circuit and affect bias voltages on the tube.
 

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #47 on: December 13, 2017, 08:02:36 am »
Are there any  resistors mounted on the tube connector?
Check them. generally high value resistors can go open circuit and affect bias voltages on the tube.
That's a good idea. I 'll do it later at evening. Thanks!
 

Online tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #48 on: December 13, 2017, 08:49:41 am »
I once had a CRO that drove me nuts with CRT behaviour so after mucking with it on and off I bit the bullet and reworked most of the EHT area, as with most CRO's it's the area of circuitry that's under the most strain, heat and voltage.
The high value resistors drift and go OC and any more than a M \$\Omega\$ or so are my first suspects. The DC restorer HV caps need be looked at too and they can be a bit hard to find from normal sources as 2-5KV caps are not that common now.
Last I bought were HV blue dipped disc ceramics from Aliexpress.

If the LV and HV (70-200V) rails are clean and to spec go looking hard at components under stress in the EHT (1-3KV) area as they have the greatest influence on CRT performance.
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Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #49 on: December 13, 2017, 07:41:28 pm »
I once had a CRO that drove me nuts with CRT behaviour so after mucking with it on and off I bit the bullet and reworked most of the EHT area, as with most CRO's it's the area of circuitry that's under the most strain, heat and voltage.
The high value resistors drift and go OC and any more than a M \$\Omega\$ or so are my first suspects. The DC restorer HV caps need be looked at too and they can be a bit hard to find from normal sources as 2-5KV caps are not that common now.
Last I bought were HV blue dipped disc ceramics from Aliexpress.

If the LV and HV (70-200V) rails are clean and to spec go looking hard at components under stress in the EHT (1-3KV) area as they have the greatest influence on CRT performance.
UPDATE: I did check the resistors near tube, capacitors, diodes. I did find an open 22ohm resistor R3027 near optocoupler. I did change it but no luck.
Also i did find almost all the voltages corect except HT that i didn't have appropriate probe to measure it.

Is it possible to show me from the schematic, which components should i change too and from in EU i can order them?

Here is the schematic of my HM-203-7  ->https://mega.nz/#!FdZmTa6a!BbGdrKCm1gZLqHX_HqLlNPFyzrR7gzN4L9H-KmAN0uA
« Last Edit: December 13, 2017, 07:43:56 pm by misterno »
 


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