Hameg HM303-5 Trigger Issue

[gaztech]

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.

[StillTrying]

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.

https://www.opweb.de/english/company/Hameg/HM303-5

[BreakingOhmsLaw]

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

[gaztech]

Hi.

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...

[StillTrying]

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.

[gaztech]

Thanks for the input.

Thinking about it, what you're saying appears to make sense. I'll dig the scope out again and put together a small truth table for all the switch permutations. Maybe then we will have something to go on.

If anyone else knows what the actual outputs per switch position "should" be, that would be very useful.

All output pins appear to be "logic 0" outputs so I would tend to agree that Ch1 / T1 should probably both go low with Ch1 set ON. 

Will revert back when I've got the info. 

What do you think the chances are of getting this chip if it's faulty?  I'm thinking that I might have to design a small logic board to do the same job... 

[StillTrying]

If the outputs CH1 and TR1 are permanently low and CH2 and TR2 permanently high with CH1 set as the trigger I'd consider IC603 working, especially if the 4 ouputs swap over with CH2 set as the trigger.

Of course check its nearby 12V to 5V emitter follower regulator while you're there, I don't know why they haven't just used a 5V6 zener on it's base.

[gaztech]

Ah... It seems like IC603 is working ok.
The way the schematic is drawn is a bit confusing. There  are two sets of numbers. I got tbe pins mixed up with the data line numbers. It seems to be doing what it is supposed to.

Im having trouble getting my head around the first stage of the trigger circuit. If an external trigger works then I am assuming that everything to the right of the ext trig switch shoukd be working. Make sense?

So this would indicate that there should be a sizable signal at P3100 on SY1 / SY2. There isn't so this seems to be problem. On the XY Channel Switch board there look to be two amplifier stages and two switching stages, the signals deriving from the respective input. I'm thinking that the two amplifier stages are probably ok as I'm pretty sure that if they weren't then I'd get no signal displayed on the crt at all.

This leaves the switching circuit on the board whuch is a real pig to chase through on the board as everything is smd and really tiny. 12v and 5v supplies are where they should be but i cant think of a reason why BOTH channels went off simultaneously. It doesn't make any sense as we have two completely separate circuits here for Ch1 and Ch2.

Any thoughts on this?

[StillTrying]

CH1's Y11/Y12 and CH2's Y21/Y22 seem to come from the input attenuation board, and then are output as SY1/SY2 where you say there's no signal.

I suppose on this bit there are some components/connections that are common to both of the channels triggers.
I think you'll have to see if there's anything on Y11/Y12 and/or Y21/Y22.

[gaztech]

Problem is, and this is typical... I need a scope to see these signals... 

I don't have another one other than a really crappy small Chinese one that doesn't work that well. I'll have to use that.

I think you're right though - Y11/Y12 is certainly the place to start except for one thing:  The triggering works with an external signal. 

Therefore I would say that probably Y11/Y12 are ok.  If they weren't then I'd have no Y deflection and the ext trigger wouldn't work (I think).

This is what is confusing me here...  However, the lower part of the circuit only deals with the feed to the Trigger board so I' going to start there. I'll update here as soon as I get chance to measure some stuff. I think we are getting closer to this.

Many thanks for staying with it here. 

[StillTrying]

I think you're right though - Y11/Y12 is certainly the place to start except for one thing:  The triggering works with an external signal. 

The last CH1 and CH2 bit I posted is before the EXT/INT switch, EXT trigger doesn't use that bit. (I spent ages tracing it across the pages. )

Which is probably this bit.
It looks like CH1's SY1 and CH2's SY2 are combined into 1 = TSX by the 5 top left transistors, which then goes to the EXT/INT switch, where after that the triggering works.

Everyone fault-finds in different ways, most are terrible at it.

"Therefore I would say that probably Y11/Y12 are ok.  If they weren't then I'd have no Y deflection and the ext trigger wouldn't work (I think)."

I agree Y11, Y12, Y21, Y22, are going all the way down the Y amp route fine, that's why I'm trying to post the bits where they've been separated off into the CH1/CH2 trigger path.

[gaztech]

Hi guys,

I got there in the end. Thanks to everyone for all the advice and tips.

I found some very odd readings around transistors T601/T602 on the YP Board. They were very different to readings from T603/T604. I would expect them to be the same or at least very close.

I had some MPS918s so I thought just to eliminate them I'd replace all four.

Turned back on and although readings were still odd, they were different to before! Maybe I did have a bad transistor in there... Everything else checked out ok so I took T601/T602 out again and measured around them with the scope turned on. It turned out that the voltage on the junction between R625/R621 was virtually non-existent. Big clue there...

Further investigation revealed a partial short between W601 pins 1 and 2. Tracing this back via the connecting wires to the Timebase Board (TB1), I found that the board connector at the other end had a solder blob between pins 4 and 5. It must have always been there but over the years gradually moved and eventually made contact between the pins!  The resulting short was shorting out the SY1 line and messing up the signal on Channel 2 as well.

I removed the connector, cleaned everything up and replaced it - now all is well!       

Just goes to show, it's not always faulty components that give us grief. WIthout measuring the connector, this short was invisible. 

Onwards and upwards!