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The regulated voltage powers the oscillator directly and
the transformer drivers through a 200 ohm locking 10 turn pot, which
replaced the single turn front panel "I Adj." pot. I had to replace the resonating capacitor as it
was shorted resulting in almost no heater voltage. A shorted
cap is apparently a common failure with this design as one of the other units I obtained using this approach had had its cap replaced in the past.
The NV-100 does one thing differently though:
It maintains the DV-100 heater at a constant temperature (within a limited current range) rather than using constant voltage or current.
The simpler approach of using a pair of blocking diodes would have dropped the battery voltage by 0.7 V which would greatly reduce its useful life; using the switched contact on the adapter jack
would not permit selecting between the regulated 9 VDC and battery.
Others I've obtained used, but working, include a pair of
DV-4AMs, a DV-5M, and another DV-3M.
With the resistor bypassed, this readout appears to work correctly with the DV-3M.
The DV-6M uses a much lower power heater, has slightly better sensitivity below 20 milliTorr, and a somewhat faster response than the DV-3M (which is no longer manufactured), but is otherwise similar.
The manufacturer of this system uses a Hastings meter but has apparently removed the 115 VAC input circuitry and powered the low voltage electronics directly which includes
a zener regulator and a 500 Hz oscillator (two transistors,
toroidal transformer) to drive the tube from a 9 V battery
(pushbutton activated so as not to run the battery down too quickly!) - I added an input for
an AC adapter since portability isn't a major concern for home-built lasers and replaced
the pushbutton with a good old toggle switch!
DV-3 gauge tube current (by measuring the output as described above) since this tube requires the most current.
However, there is a resistor of about 15 ohms in series with
the DC output of the TC gauge tube (pin 7) which results in the 10 mV meter movement actually requiring
approximately 14 mV for full scale deflection.
In addition, for the DV-5 and DV-8, the op-amp circuit will need to be modified to have
a peek at this web-site 5 times the gain since the high vacuum output
of these tubes is only 2 mV rather than the 10 mV for the
DV-3, DV-4, and DV-6. The calibration curve between atm
and high vacuum is probably a bit different (the DV-6 is probably more
stretched out at the high vacuum end) but since I haven't yet painted the scale, it won't matter.
The heater circuit can power the DV-3 at its spec'd current (though at the low end of
its adjustment range). This provides enough range to accomodate the DV-3 and DV-6 (as well as the DV-4, though
the meter scale would be totally different). When I acquired this unit,
I thought it used a DV-3 tube because that's
what the SV-1 uses according to my chart from Hastings.
So, either it uses a different TC gauge tube (perhaps the very obsolete DV-1 or something even more ancient).
This unit uses a small Sola constant voltage (ferro-resonant) transformer
for regulation rather than zener diodes or a transistor oscillator fed from a regulated DC supply.
Since SG-TC2 outputs a more or less constant voltage for the TC gauge tube heater instead of a more-or-less constant current as with SG-TC1, it
will work for other Hastings TC gauge tubes having the same internal wiring except the DV-5 without any circuit changes to the heater
current supply.
It turns out that the design of SG-TC2 is quite similar to that of the Hastings NV-100
TC controller for the DV-100 TC gauge tube. CAUTION: Do not install a TC gauge tube
other than the one that SG-TC2 (or SG-TC1)
has been set up for without making sure the heater current is
less than the spec'd value for the new tube. Failing to do this may result in instant destruction of the tube
by blowing the heater! However, the DV-5's voltage is much lower so the adjustment range may not be adequate (but a 1k ohm "I Adj." pot or additional
series resistor would take care of that).
This is because the heater voltage specs are relatively close to each
other. Approaches include the use of a dummy load for the heater
of the proper resistance and/or powering the control unit with a Variac and starting low
while monitoring AC heater current or voltage. 5.1 VDC
sources for the op-amp since the oscillator voltage was somewhat marginal.
An IC regulator (LT1084) provides stable 9 VDC from an 11 to 15 VDC source.
However, the internal zener circuit isn't that stiff
so operating from the regulated AC adapter is more stable. |
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