Hameg M203-6 Repair: Jobsa Good'un - VB#5

So I finally fixed it! MINT!

So the opto had gone bad! its very strange how one day its ok and the next its just not working and all because that opto thought: "nah, not today chief, I'm retired!"

I do burble on a bit about high voltage supplies and my experience with them in this vid so, feel free to spool through, but I'm not implying that you should let your guard down when probing high voltage.

Any way I haven't really got anything else to video coming up, so this may be it for a while.

I think in October I might see if I can go mud plugging, pay and play, so I may post a video just showing some off-road footage. See who's open and fancies a play.

In the mean time, I will post up what I did to convert that torch to LED from filament, it isn't neat, it isn't cutting edge, but it will make your charge go further, which is a blessing when your stuck under a vehicle on dark winter nights.

Stay safe, observe care, and keep your peepers on m'blog!

Cheers!

Hameg 203-6 repair: we have a lead! VB#4

So we have found the problem as to why its not working! Turns out my original assumption of thinking that because there was the right potential difference between the -1877V line and the -1900V line that there was indeed those potentials there.

But after thinking about it, I could measure the -1900V line by actually measuring the individual voltages across the potential divider network for the focus adjustment:

Turns out it came out A LOT lower than expected: 95V. That's way lower!

The problem only occurred due to a lapse in concentration. Usually, given that there are are high voltages flying round the timebase PCB and the back of the CRT PCB, I turn off the scope altogether and disconnect the mains whilst I have a think, take notes, and read the schematic. That's a pretty safe way of working. But this time I'd left it on and started to smell burning dust: that smell you get when the radiators are first turned on in your house after summer (ok people in temperate climates might not know what I'm on about).

Turning off the power, disconnecting the mains and then using the finger test found that IC501 - an opto-isolator - was very hot. Pulling it out and diode testing the diode emitter on it found it was very poorly, reading more ohmic than a semiconductor!

With the opto out, I thought why not test to see if I get the right voltages on the focus resistor network, and BINGO! Readings matched the calculations!

So next step is to order a replacement, double check that nothing else could have been taken out, and fit the new component.

Its very odd though - on day the opto's working and the next its dead!

HAMEG 203-6 Repair: More fruitless searching - VB#3

OK; so poking around at less lethal voltages this time and still getting nowhere.

Nothing much to say on this one, but if you trying to do some repair yourself, then feel free to use this as a rough guide of what you should expect and where.

Take care! I mean it, there's high voltages flying round so watch where you put your fingers!

Also: apologies for the lack of definition on this video, using a different camera.

EEVBlog Forum Posts

Busted HM203-6 - VB#2

So this would probably be my second video blog, hooray!

So I hooked up my scope a few days ago and it worked, nice trace appeared. Next day I plugged in some probes I got donated to me, and nothing, sweet FA!

I my haste, I admit, I didn't note down the scope model number. I thought it was a HM203-7 from pictures I searched for, but then realised it was a HM203-6 - DOH!

I noticed after some of the connector numbers did not match those on the schematics!

With the correct schematics in hand, I set about looking at a few high voltage lines - why the high voltage lines? Well I was getting no display yet I was getting the trigger LED to light when I put in the test signal on channel 1, which lead me to suspect the tube was not displaying the signal yet the triggering system was seeing the signal.

I LIKE TO MAKE CLEAR, THERE ARES SOME PRETTY NASTY VOLTAGES THAT WILL BELT YOU IF YOU ARE NOT CAREFUL OR ARE INEXPERIENCED - TAKE CARE OR DO NOT UNDERTAKE IF YOU DON'T FEEL CONFIDENT!

Here are links to relevant things:
Hameg HM203-7 service manual
Hameg HM203-6 service manual
EEVblog forum post