tektronix 475A trigger slope not working consistently on startup

[crazyhog]

Hi,

I have just brought my first analog scope, a Tek 475A!  The scope has a slope switch to select trigger on raising edge (positive slope) or falling edge (negative slope).  However, on cold startups (e.g. early morning), the trigger fires on both raising and falling edges regardless of the slope switch.  After running for a 15 minutes, the scope would work properly and trigger according to the slope switch setting. 

Any idea how to fix this?  Is it normal to warm up these analog scope before use?

I've attached a picture of my scope with an input from a function generator into ch2.  The slope switch is set to positive slope (green oval).  However, notice I am getting 2 traces with both positive and negative slope.  The scope is firing on both positive and negative slope of the input signal.

Thanks

[barry14]

Two possibilities come to mind.  One, the trigger slope switch may be dirty.  This will happen with older equipment. Try switching it back and forth a number of times.  You may  have to open up the scope and spray some contact cleaner on the switch contacts.  The other possibility is that the trigger level control needs to be adjusted. Yes, analog equipment, even solid state, usually needs some warmup for full normal operation because the power dissipation of the internal circuitry will cause various components to increase in temperaure until equilibrium is reached.  Most analog equipment manuals call for some warmup time, on the order of 5 to 15 minutes, for the device to operate within all of its specifications.

[David Hess]

The 475A uses tunnel diode trigger circuits which sometimes drift enough with age to require adjustment.  The detailed procedure is in the service manual.

[crazyhog]

Cool, thanks all!

Even though my Rigol has a lot more features, I have to I admit I like this scope more!   Love all the mechanical switches and knobs!  No confusing layers of menu is a joy forever!