Author Topic: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED  (Read 18902 times)

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Offline tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #50 on: December 13, 2017, 08:11:55 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.
This would seem to point to a strong clue.....too much current drain on the EHT.
Components to check are in the EHT section of the PSU and those subjected to the EHT supply on CRT section of TB-Board. You may need a HV meter to check the differential voltages in parts of that circuit.

The voltage differentials between elements of the CRT are crucial for a stable and predictable display.
I had the differential zener fail in a CRO (D312/313) once and it upset things immensely.
Work slowly and methodically through tests and replace any component that you suspect faulty.
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Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #51 on: December 18, 2017, 10:39:19 am »
I spend all the weekend trying to fix this device. I take out ALL transistors and measure them with a transistor meter. All was good.
Then i did resistor measurement by took on leg in the air and i found that one of them (R3019 in PSU page) was 470K instead of 3K92 as you can see in picture.

I 've measured ALL the electrolytic capacitors too. Everyone was OK. Same with the diodes and big nf capacitors too.
I 've checked the pages and mark the components with yellow pen.

After that, i changed the faulty resistor, i did re-assembling all the PCB's, wires, etc.
Now when i switch on the device, i get very low intensity screen and no trace line at all.
The good part is that now the screen don't affected by small hitting the neck of the tube as did before repairing.
Today (i hope) i will do voltage measurements and post the results here.
« Last Edit: December 18, 2017, 10:44:20 am by misterno »
 

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #52 on: December 19, 2017, 07:45:41 pm »
UPDATE: Finally i did fix it. Now this old but very useful oscilloscope, works again. As i 've tested almost all critical components, i found a broken wire that looks well, an open resistor in PSU and finally a small ball of weld which made a short circuit to the HT area.
Component tester works well too. I have a small difference in 1KHz test signal and i would appreciate if you can tell me if i can adjust it well.
Thank you for your replies so far.
 

Offline tautech

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #53 on: December 19, 2017, 09:36:07 pm »
UPDATE: Finally i did fix it. Now this old but very useful oscilloscope, works again. As i 've tested almost all critical components, i found a broken wire that looks well, an open resistor in PSU and finally a small ball of weld which made a short circuit to the HT area.
Component tester works well too. I have a small difference in 1KHz test signal and i would appreciate if you can tell me if i can adjust it well.
Thank you for your replies so far.
Well done.

How do you know the 1 KHz is out ? Have you put a counter to it ?
Some CRO's can be adjusted/trimmed to 1 KHz but for most use it doesn't matter.
The fast edges are what matters and the approximate frequency and amplitude are only needed as sanity checks.
Near enough is good enough for the probe cal output.
Avid Rabid Hobbyist
 

Offline misterno

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #54 on: December 20, 2017, 09:41:47 am »
In this case i will prefer to close the cover and start using the device. Thank you again for info's and help.  :D
 

Offline GreyWoolfe

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Re: Repairing an Old Hameg HM203-6 (not 203-7) - No trace: FIXED
« Reply #55 on: December 20, 2017, 02:25:30 pm »
Congratulations.  Take pride in your repair and enjoy the scope. :-+ :-+
I am of the age that my brain no longer says "maybe I shouldn't say that" but "what the heck, let's see what happens"
 


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