Hameg HM303-5 Scope Triggering Problem

[Gaztech]

Hi guys,

Although I can't say that this device is actually "vintage" it's pretty old and I've seen a few posts for similar equipment. Hopefully I'm not breaking any rules by posting this!

I thought I would drop a quick post here to see if anyone can help with an issue I have with my Hameg HM303-5 oscilloscope.

I got this scope from a friend a few months back and I've just dragged it out to see if I can make it work. There was a horrible jitter on the X timebase but this was solved by cleaning all switches and pots. The scope was also full of muck. I cleaned it out and now it looks quite respectable. I found a few leaking electrolytic caps so I decided to change all of them as the unit is an old one. Everything on it now works – that is, except for the triggering.

In auto or manual modes, the thing doesn't trigger at all and won't stabilise any waveforms. I've been using it by adjusting the timebase manually (as I only use if for audio) but I really do need this triggering to work.

I've never taken a scope apart before (never needed to) and I'll be honest, with this triggering fault, I have no idea where to start! I have the service manual and have some schematics but it's not clear how the triggering side works. I wondered if anyone could point me in the right direction as to how I start to diagnose it.

I've done the usual checks. All supplies are up and look ok. Visual checks don’t tell me anything. Cursory component checks don't really yield anything useful either. The service schematics are clear but the text on the board layouts are very difficult to read since this one has surface mount components on the board (ugh...). Identifying components from the schematics isn't easy.

To test it, I'm just using the cal squarewave. On both channels the symptoms are the same. It displays the waveform ok. On auto there is no triggering occurring at all and on manual, moving the trigger level pot causes "some" triggering to take place but it just wont "lock" the waveform. Sweeping the level control back and forward "almost" gets a lock but it never actually holds it for more than a second, then lets it go again.

I guess it’s possible that one of you guys may have had a fault like this so I’m open to any suggestions on how best to tackle this.

Does anyone have an idea on where to start?

[Chrispy57]

Hi,
I have a Hameg HM305-7 which seems to share some of the controls of your Hameg HM303-5 model, and have just had a quick play with it following your description of how you have approached your set-up.

The only way I can duplicate your described fault is by setting the trigger mode selector switch to the ac sinewave symbol position (and this is the only control you have neglected to mention). I guess you have tried the AC, DC, LF and TV positions of this switch, but just in case I thought it worth mentioning since you say you're not familiar with trigger modes?

Good luck
Chris

[Gaztech]

Hi Chris,

Thanks for your reply.

Yes, I failed to mention the switch - but it doesn't work in any position.

I'm familiar with scope operation just not the electronics surrounding the triggering. I'd just like someone to provide some sort of explanation on how the triggering works so that I can begin to diagnose the problem. At the moment it's a bit of guesswork!

[Chrispy57]

Hi,
thanks for clarifying the nature of your technical expertidse and current advice requirements. Since you have had 127 views and no other advice offered from more qualified members I thought I'd offer a couple of suggestions for you to try.

I have downloaded a service manual from the internet, attached here, and copied what I think is the appropriate circuitry Page6, and the adjustment instructions Page39 to point you in the right direction.

I've added a couple of yellow and red blobs on the right of the drawing to indicate what seem to be the trigger outputs TR1 and TR2 from the IC603a that you could check with another scope or DVM to see if they actually change state (active low outputs) to indicate triggering on a low frequency squarewave.

Intuitively, I feel that the same behaviour on both channels suggests a common rather than two separate faults, so suspect the IC to be involved.

If these signals do change state you could follow their progress through the transistor circuits (coloured blobs on the left) in each channel and see where that takes you. Obviously the trigger switches and inputs will have to be set appropriately to reflect the tests you cary out.

The service manual tests on Page39 may help to diagnose the fault.

Good luck
Chris

[Gaztech]

Chris,

Many thanks for taking the time to respond. It is appreciated.

I'll see where we go with this (won't be able to get to it for a couple of days) but will come back here and advise findings. Watch this space!

[Gaztech]

Update: I'm getting some odd readings from some of the chips that don't make sense so have ordered some. Will report back here when I've replaced them.