Hameg HM-604 intermittent triggering problem - Page 1
Forum section: electronics / repair

author: darthtony
author_rank: Contributor
timestamp: December 05, 2015, 08:02:31 pm
content: Hi i have this scope, and it has erratic triggering behaviour. When it starts up, it wont trigger(sometimes it will trigger off the external trigger), as it warms up it will trigger from the external trigger, then from the main channells(but with no auto trigger) and finally after 10++minutes it triggers off the main channel with auto trigger. What could be the problem?I checked the power supply voltages and they seem spot on.Here is its service manualhttps://drive.google.com/file/d/0B4lU9OhfONg6cDc1VjRMNXQzSlk/view?usp=sharing

author: oldway
author_rank: Super Contributor
timestamp: December 05, 2015, 08:20:33 pm
content: First look for bad contacts of trigger source and slope +/- switches.

author: darthtony
author_rank: Contributor
timestamp: December 06, 2015, 12:02:24 am
content: This was my first thought, however i caqnt see any bad contacts/cold joints. Also the behaviour doesnt look like a bad contact(it sometimes manifests on the external trigger as well) and it really seems like warm-up solves the problem. I had the scope sitting with the trigger set to channel A(auto trigger), and of course it wasnt triggering, and i checked it out ~20min later and it had triggered!Could it be the electrolytic caps?However the power supply voltages seem ok

author: oldway
author_rank: Super Contributor
timestamp: December 06, 2015, 12:16:23 pm
content: Check also for cold solders, that is a very common problem on Hameg scopes.Have you another scope to repair this one ?Check the trigger signal on pin 3 of IC309 (74F74)...Is it correct ?If it looks to be a thermal problem, you can try to use a hair dryer and a cold spray to localise the faulty component.

author: darthtony
author_rank: Contributor
timestamp: December 07, 2015, 02:49:28 pm
content: I am pretty sure the problem is warm up related. When the scope starts only external manual trigger works(sometimes) then as time passes it will auto trigger off the external, then manual trigger off the main channels, and finally auto trigger from the main channels(after >20min operation).I checked pin 3of IC309 and i can see a squre wave whose duty cycle is dependent on the trigger level, only when the scope is triggered, otherwise it is grounded.(My input is a ~1Khz sinewave)I also checked pin#3 of IC306(NE529) which is labeled trig-IN on the schematic. When the scope starts, i can only see a (heavily) clipped sinewave when external trigger is selected and no signal(+5V) when CH1 trigger is selected. Then as the scope warms up a sinewave will appear there(significally smaller than the external trigger signal). When the sinewave appears, it will manual trigger normally, but not auto trigger. Then after some more time passes it will auto trigger.Edit: After taking a closer look, it is looking like it is some sort of amplification issue. When the scope is cold, the signal amplitude to Trig.IN (pin #3 on NE539) is small to nonexistent, then as the scope warms up it starts getting bigger. The external trigger works first, because it has a larger signal amplitude, and the auto trigger works last, because i suppose it requiers a bigger signal than the manual trigger.

author: oldway
author_rank: Super Contributor
timestamp: December 08, 2015, 10:10:00 am
content: Triggering signal on pin3 of IC306 (NE529) is coming from S305 (ext trigger switch) and should be 50mVpp/div.If this signal is too weak or not existing, it seems that there is a problem with the trigger signal preamp.Check voltages and signals of T318 (MPS3640 sel) and T319 (MPS918 sel). Try to heat them with hair dryer...is anything changing ?Are the signal amplitudes and voltages of collectors of T317 and T318 (MPS3640 selected) the same (with 6 divisions pattern on screen)?

author: darthtony
author_rank: Contributor
timestamp: December 08, 2015, 06:36:46 pm
content: I tried fiddling around with a hairdryer and a freeze spray, and when i heat the board up the trigger signal stabilizes, but i cant seem to localize a component(after heating the freeze spary doesnt replicate the problem easily).I am not exactly sure what you mean by 50mVpp/div. The signal amplitude changes with the vertical setting. For a 2Vpp sinewave and the scope @0.5V/div the TRIG IN pin is a sinewave @3Vpp(When stable). I probed the collector of T317 and T318 and the signals are identical, for the above settings a 0.2Vpp sinewave with ~3.5V DC offset. It also seems stable regardles of the warm up condition(It is there even when the scope is not triggering). What componets do exibit such temperature related behaviour? Thank you very much for your time, you are helping me a lot

author: oldway
author_rank: Super Contributor
timestamp: December 08, 2015, 07:23:05 pm
content: Reading all your posts, it seems that transistor T319 (MPS918) is faulty.Check also R3098 (33R2), R3101 (1K21) and resolder all those components.

author: darthtony
author_rank: Contributor
timestamp: December 11, 2015, 08:01:41 pm
content: Thank you very much! I removed the board(the transistor was at a very hard to rech place with the board on, and the board couldnt be removed easily) and i reworked the joints, one of which probably was bad. I did a thorough clean of the board and now it triggers nicely!