Hameg HM604-2 Oscilloscope: Help: Trig level very temp dependent and unstable

[geekyoyd]

Hiya
I have been working on this fault on my HM604-2 Oscilloscope for a while without success so I thought I'd swallow my pride and ask for help!
The symptoms are that the trigger level (set on auto or manual) takes a while to settle down after switch on and varies over a wide range as the scope warms up.  When it has warmed up, the trace is not very stable which is due to noise on the trigger signal which I can see with another scope connected to IC304 output on pin 1 or [trig out to NE529 comparator].  I am guessing the faulty part will be in the analog section of the timebase triggering circuits, the filters or the amp.  I am hoping that an expert tinkerer on here will be able to suggest a stressed component or two that are likely candidates for failure.  It is difficult to diagnose a circuit like this with many stages within a feedback loop.

Thanks in advance.
Dan

[Ringmodulator]

Hi Dan,

I can only offer some generic hints. I own a HM 705, which is somewhat older.

At fist, I would check the power supply lines with another scope. Problems with the rectifiers and electrolytic caps are quite common. I have experienced also problems with microcracks at solderjoints at leads that interconnect boards giving strange problems, that looked like temperature dependent.

Next, I suggest using a plasic toothbrush to brush around to locate the area of the problem.

Next step is using freezing spray on single components.

Use of an isolation transformer should be mandatory.

Good luck,
Chris

[bd139]

Agree with Ringmodulator.

Almost all the problems I've had with Hameg scopes are either the electrolytic capacitors giving up (and leaking), IC sockets or dry joints.

[BFX]

Do not forget to cleanup switches, pots and check discoloration of resistors.