On Mon, 09 Jul 2012 13:53:45 -0000, "keithostertag"
<keitho@...> wrote:
The service manual says to adjust TP1625 for +55 volts plus-or-minus 25 volts. What exactly does that mean? I've never seen such a wide range given for a single adjustment. Does it mean that any voltage between 20-80 volts is OK? Is it implying that once you adjust it to 55 volts that you can expect it to vary between 20-80 depending on temperature/load? Or maybe that if all other voltages are correct then you can leave it at any voltage between 20-80? Or?
The voltage at TP1625 is controlled by U1635 to regulate the -3kV
supply and indirectly the CRT acceleration voltage. It will vary with
the total load on the power supply. As long as it is within range,
the -3kV supply will be directly held in regulation. Tektronix was
conservative here since TP1625 can actually swing from about 128 volts
to -48 volts when controlling the -3kV supply.
The -3kV supply and CRT acceleration voltage have to be tightly
controlled because they directly affect the horizontal and vertical
deflection. The 7903/7904 is a little different from my other 7000
series oscilloscopes since these voltages are fixed while in my others
they have a separate adjustment built around U1635.
R1292 controls the output of the 130 volt supply and indirectly all of
the other unregulated outputs including the high voltage supply. It
might be better to adjust it until the 130 volt supply is dead on and
then check TP1625 to see if it is within range. If not, then adjust
R1292 until TP1625 is within range. It might be informative to know
the voltage of the 130 volt supply when TP1625 is adjusted which is
actually mentioned in the manual. It should be 130 volts +/- 5.2
volts.
I'm finding (so far) that adusting it to 55 when cold, it then proceeds to slowly decrease (as low as 30 volts) as it warms up. After warming up for an hour and re-adjusting it to 55 it still fluctuates slightly. Though -50V TP is steady. Since I am (evidently) not finished replacing caps I expect to revisit this adjustment several times...
Since the -50 volt supply is separately regulated, it is not going to
reveal much unless the -54 volt supply is low.
David Hess