Electronics > Repair

Hameg HM303-5 Trigger Issue

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Hi guys,

Hopefully someone can point me in the right direction. I have a Hameg HM303-5 scope. Had it for years and it's done a lot of work.

Suddenly the trigger has stopped working. So far, I have been unable to get it working again. Won't trigger anything.

I took it apart and found that there were a load of very "crusty" electrolytics in there. To be expected I guess in a scope of this age so I decided to go on a mission an replace them all. I used correct types and made sure they were all high quality Panasonics. I fired up the scope after putting it all back together. It works ok as expected but the trigger still doesn't function.

I know that Hamegs are susceptible to issues with the switches so I gave all the wafer switches a thorough clean with a deoxit agent  - but still no joy. Cold joints are also an issue with these but I found none. All solder joints (on these awful SMD boards) look good. I've tried heating the trigger board and then freezing components with freezer spray -  still nothing. Not temperature related then.  :--

There is a history of -12V rails fluctuating on these and causing all sorts of problems so I obtained a new NE529 regulator and replaced it even though I didn't measure any fluctuations. Still nothing. There must be something fundamental that is stopping the trigger from working.

What I really need is a test procedure that I can go through to ascertain which parts of the board are actually working and which aren't. I'm not familiar with this circuit at all so some guidance of where to start and what to look for would be a tremendous boost to moving this forward.

If anyone has had a fault like this and managed to fix it, I'd like to know what you found. This particular scope is not easy to fault-find as all the parts in it are SMD and it doesn't lend itself to removing components and testing them!

Someone on another forum suggested checking it on external trigger. I've not yet tried that but it does make sense to do that as it eliminates a few things. I will dig it out again and try it and report back.

Can anyone help?  I really don't want to buy another scope - this one does everything I need but I need it to work...

Thanks in advance.

Can you safely get to the trigger parts while it's on.
I've only had a very quick look, I'm sure these triggers have been fixed - eventually.


Check if you see a trigger signal on Pin 8 of IC603A to narrow down the defect to a board.


Yes, I can get to the top board ok. The XY Amp is underneath it though - not so easy to get at..

I did try to input an external trigger today though - and it works. That should eliminate a few things...

As for IC603, I am assuming that this outputs DC voltages based on what the switch combinations are. I find that some things appear to switch and some not. It's not very logical - so this might be at the heart of the problem. Can anyone explain how the outputs of this chip should work? If so, then I can test it properly.

I find that I don't see any signal at input plug P3100 (Sheet TB1 - Timebase Trigger Filter). I think I should see something that looks like the input to the channel here.  There seems to be a load of switching circuits before it which turn stuff on and off. Could be bad switching I guess but I need to know how this works to fully fault-find it.

Does this lot add any input to your thoughts guys?  The mystery is why BOTH channels have the same problem. I would have thought if it's a component failure only one channel should be affected. This is why I need to work out what IC603 is doing...

With the EXT trigger working I'd agree IC603 is worth a look, it's a PLD.

Most of its inputs look like 0V or 5V DC depending on the switches. The diagram suggests that with CH1 set as the trigger the outputs CH1 and TR1 should be low, etc. I wouldn't worry about ALT trigger for now. :)


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