Heath-Schlumberger SO 4550 scope

[John Farrington]

At 07:52 PM 10/02/2002 you wrote on the Heath list:

>Hi.  I have been given the above scope, of course it does not work.. I am
>sure the power transformer is bad.  (smells and arcs-hv winding probably.
>Would it be worth it to try and get a transformer or would one be available?
>Along with a diagram.  Any suggestions appreciated.  Don Voigt, W4HQF


About 1976 the Heath model number was not SO- but IO-4550, and it cost $400
then. It's a 10 mHz dual-trace, with reduced response to near 20 mHz if the
signal is strong enough. Vertical sensitivity is 10-20 mv/cm, so you need a
preamp for smaller signals;  max input V= 400V peak, rise time is 35 ns,
V-input Z is 1 megohm shunted by 38 pf, power consumption= 70 W. They are
kind of drifty, so they have to be re-calibrated periodically if you want
to come close to accurately measuring voltages.

It's worth fixing if you're not going to look at higher frequency signals
and don't have to spend a lot; there are 2 manuals, operation and assembly,
so if you're going to work on it ask around for originals, as copying the
entire manuals would cost about as much. I don't know if the SO-4550
circuitry is the same as the IO-4550, so ask about that.

Don't just assume the transformer is bad, it may be something else that's
gone - you first need to check that the primary switch has been wired
correctly and set for the apropriate 120 or 240 VAC, then disconnect all
its outputs and check them with a meter, preferably with resistor loads
on them that won't draw a lot.  There are 5 secondaries, 2 of them
center-tapped, one tapped off-center for the CRT filament.

An endemic problem with Heath equipment is the carbon composition resistors
which drift with age, humidity, temperature and high voltage. I have a 4550
in which the resistors in the regulator circuits drifted so far off that it
wouldn't focus, so I had to isolate and check each one, resulting in a lot
of replacements. Old electroytic caps should also be checked, especially
if it has been sitting around unused.

If yours has been sitting around in storage you should also check inside
for insects and dirt which may have become carbonized. I don't recall if
the rotary switches are open, but those and the slide switches will surely
need a dose of DeoxIT.

73

John   KE5ZB

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