Hi Arthur,
It isn't really what I believe that counts here. I was just repeating what
Tektronix has written in the service manual for 555 required equipment. I
think they specify an Iron vane movement because the saturable reactor
distorts the waveform as it regulates the filament voltage. We had iron
vane meters (0-10 VAC) specifically to make this adjustment in each
Tektronix Service Center and they have been assigned the Tektronix Part
Number 067-0514-00 so some engineer at Tek must have thought it was
important. That was many years ago and maybe modern true reading rms
voltmeters are up to the job today.
Stan
_____
From: arthurok [mailto:arthurok@...]
Sent: Sunday, December 31, 2006 6:36 PM
To: 'coresta'; Stan and Patricia Griffiths
Cc: TekScopes
Subject: Re: [TekScopes] Re: Bringing up a 555
you believe the crest factor is so high that a modern true rms
multimeter wouldnt do the job?
an iron vane voltmeter is true rms responding.
a chinese engineer told me the best true tms voltmeter for non
super accurate measurements is a scope
read by a trained eye.
the best rms converter i know of is a thermal converter
"thermocouple vacuum junction"
----- Original Message -----
From: Stan and Patricia <mailto:w7ni@...> Griffiths
To: 'coresta' <mailto:coresta@...>
Cc: TekScopes <mailto:TekScopes@...>
Sent: Sunday, December 31, 2006 8:28 PM
Subject: RE: [TekScopes] Re: Bringing up a 555
Hello Pierre,
If I remember right, the big iron potted case contains a device is called a
"saturable reactor" (or something like that) and is used to regulate the AC
voltage applied to the vertical amplifier tubes. I think this was an
attempt to minimize the effects of cathode interface which looks like a
spike on the leading edge of a fast-rise square wave. Actually, it is a
DECREASE in low frequency gain of the vertical amplifiers.
In a scope without regulated filament voltage, cathode interface is always
worse at low line voltages which means lower filament voltage and therefore,
lower filament temperature. If you observe a fast-rise square wave while
you lower the line voltage on a scope with bad cathode interface, the
leading edge spike seems to grow over about one minute of time. If you look
very carefully, it is actually the trailing edge, or flat portion of the
square wave, that is DECREASING in amplitude over time as the filaments cool
down. You can verify this by simply plugging in the scope calibrator and
watching the gain of the vertical change as the calibrator signal appears to
decrease in amplitude. No spike seems to grow on the leading edge because
there are no high frequency components in the scope calibrator signal.
There is actually a filament voltage adjustment in the 555 power supply and
the list of equipment needed to calibrate a 555 includes an iron vane AC
voltmeter to accurately set the AC filament voltage. I think the 517 is the
only other scope Tek made that has adjustable filament voltage . . . but I
could be wrong about this since it has been years since I looked at my
517's.
Stan
_,_._,___
