GR 726-A Vacumm Tube VM, Manual

GR 726-A Vacumm Tube VM, Manual
OPERATING INSTRUCTIONS
FOR
TYPE 726-A
VACUUM- TUBE VOlTMETER
Form 626-A
GENERAL
RADIO
CAMBRIDGE
39,
COMPANY
MASSACHUSETTS
OPERATING INSTRUCTIONS
FOR
TYPE 726-A VACUUM- TUBE VOlTMETER
PART 1
DESCRIPTION
1.1 PURPOSE
The Type 726-A Vacuum-Tube Voltmeter is a high-impedance wide-range voltmeter for
use at both audio and radio frequencies . Its upper frequency limit is in the vicinity
of 100 megacycles.
It is essenti.ally a peak reading instrument , except on the tv10 lowest voltage
ranges~ but the scale is calibrated to give readings of the r-m-s values of sinusoidal
applied voltages.
1.2 USE AS A RADIO-FREQUENCY AMMETER
In addition to its use as a voltmeter, it can be used to measur e current at radio
frequencies with a capacitive shunt. Used in this way, it provides a convenient method
of measuring antenna current.
1.3 CIRCUIT
A diode-condenser rectifier circuit , using an acorn tube, is built into a small
probe which is made of low-loss bakelite . A cable , which also supplies heater voltage
to the tube in the probe, carries the rectified voltage to a d-e amplifier and indicating meter in the cabinet of the instrument .
The d-e amplifier is of the degenerative type using but one tube. The rectifie<i
voltage is applied directly to the amplifier control grid, rather than through a voltage-dividing network, and the change from one voltage r ange to another is accomplished
by varying the degeneration fact or.
The schematic circuit diagram shows this circuit in elementary form. A complete
wiring diagram , with parts list, is given in Part 4.
c,
H~ t-.....------........,MAN\1\/'.......--,
INPUT
LOW
PLATE SUPPLY
-1-
GENERAL RADIO COMPANY
PART 2
OPERATING CHARACTERISTICS
2.1 RANGE
0.1 to 150 volts ac, in five ranges (1.5, 5, 15 , 50, 150 volts, full scale).
range can be extended to 1500 volts by means of the Type 726-Pl Multiplier .
The
2 .2 ACCURACY
~2%
of full scale on all five ranges, on sinusoidal voltages.
2.3 WAVEFORM ERROR
The instrument is essentially a peak voltmeter calibrated to read r-m-s values of
a sine wave, or 0 .707 of the peak value of a complex wave. On distorted waveforms the
percentage deviation of the reading from the r-m-s value may be as large as the pe rcentag e of harmonics present.
2.4 FREQUENCY ERROR
At high frequencies resonance in the input circuit and transit-time effects in the
diode rectifier introduce errors in the meter reading. The resonance effect causes the
met er to read high and is independent of the applied voltage . The transit-time error,
on the other hand, is a function of the appl ied voltage and tends to cause the meter
to r~ad low. The accompanying curve gives the frequency correction for several differ1.0 4
- --:::::--- -~
I.00
~
~~~ ~
~~
~ \
~~
~ \\
........ :---,
........
150
0
w
·""
I;
15
W(!)
c.!><t
<ti-
l- ....J
....JO
0 .9
o>
>o
ow
wt-<t
....Ju
a.._
~
0 .88
l\
0.84
~ ~\
0.8 0
i\\
0..0
<[z
0.761
\
2
10
20
FREQUENCY IN Me
-2-
100
200
1000
GENERAL RADIO COMPAN Y
ent voltage levels. It will be noted that at ~ow vo~tages the transit-time and resonance effects tend to cancel, while at the higher voltages the error is almost entirely due to resonance.
At the low audio-frequency end, this voltmeter may be used at frequencies as low
as 20 cycles with an error of less than 1%. Typical measured errors are as follows :
Frequency
Error
20 cycles
-0.5%
14 cycles
-2%
10 cycles
-4%
2 .5 INPUT IMPEDANCE
The input circuit is equivalent to a resistance of 6 megohms in parallel with
6.6 ~~f. At the higher frequencies the effective parallel resistance is reduced by
losses in the shunt capacitance. The accompanying plot gives the variation of Rp and
Xp with frequency.
2.6 TEMPERATURE AND HUMIDITY EFFECTS
Over the no rmal range of room conditions (65° Fahrenheit to 95° Fahrenheit; 0 to 95%
relative humidity) the accuracy is substantially unaffected by temperature and hum1dity
conditions.
10
100
-~--
r----.
"
'"~"' ""'""'
'\.
--
....._ .........
Rp
r-.
~'--....
..........
~
'\.
'\.
' r\.
'
"" '\
"'
' "'\.
r-..
""'
'\1\,
'\
'\.
.I
""' ""
~
~
........
10
............
(.)
"I\,
Xp
I'- .......
~
'
'\
X
10
"
r-.r--...
"' 1'\.
:--.....
~""--..
............
'\
--
.I
I
""
r'-.
FREQUENCY IN Me
-3-
0
!;i
a.
(i)
(/)
0
\.
"' ~
-v '
r-.'\.
.01
"1\. 1\
'
1"1\.1\,
.01
z
~1\.
'X.D
........
Li:
~
~
'\.
a:
~
/
~'r-.. 1\
r-.
.I
10
100
GENERAL RADIO COMPANY
2.7 POWER SUPPLY
100 to 130 volts ac, 60, 50, or 42 cycles and 200 to 260 volts, 50 cycles. The instrument incorporates a voltage regulator to compensate for supply variations over this
voltage range. The power input is less than 30 watts . Line voltage and frequency are
engraved on a plate under the POWER INPUT socket .
2.8 TUBES
One Type 955, one Type 6Q7-G, and one Type 1-V are used; all are supplied .
2.9 ACCESSORIES SUPPLIED
A seven-foot line connector cord , spare pilot lamps and fuses.
2.10 MOUNTING
Black crackle-finish aluminum panel mounted in a shielded walnut cabinet.
2 .ll DIMENSIONS
(Width) 9-l/2 x (depth) 14 x (height) 8-l/2 inches , overall.
2.12 NET WEIGHT
17-l/2 pounds.
PART 3
OPERATING INSTRUCTIONS
J.l
PO~~R
SUPPLY
Connect the voltmeter to the a-c line by means of cord and plug provided. Be sure
that the voltage and frequency of the power line correspond to those engraved on the
plate under the POWER INPUT socket .
3.2 ZERO SETTING
Short circuit the HIGH and LO · terminals, and vary the ZERO ADJUST knob until the meter reads zero.
3.3 MEASUREMENTS
Voltages may be measured at the panel terminals with the probe in place at the rear
of the cabinet or at the probe terminals wi.th the probe removed from the cabinet. The L0\'1
terminal is grounded to the panel through a 0 . 02 ~f condenser and may be directly grounded
at the panel terminal "G" if so desired.
The instrument indicates the voltage actually appearing at the cathode and plate of
the 955-type rectifier tube in the probe . At high radio frequencies , therefore , care
should be taken not to place the cable too close to the voltage source, as standing waves
may be set up in the shield, resulting in voltage being transferred backward to the input
terminals.
The d-e voltage between the LOW input and GND should not exceed 400 volts.
PART 4
MAINTENANCE
4.1 VACUUM-TUBE REPLACEMENTS
Replacement of either the Type 955 or Type 6Q7-G Tube will, in general , require readjustment of the movable contacts Al, A2, AJ, A4 and A5 on resistors R12, R14, and R24
to make the zero settings coincide on the different ranges . To make this ad jus tment the
instrument must be removed from the cabinet and the respective contacts adjusted until
the zero reading is the same for all 5 ranges. Then after the instrument is replaced in
the cabinet, the external zero adjustment will bring all ranges to zero at the same poJnt.
-4-
GENERAL RADIO COMPANY
Replacement of the Type 955 or Type 6Q7-G Tube will cause no substantial change in
the calibration except possibly for the 1 . 5-volt range where a readjustment of resistor
R18 will correct any error. This is a s crew-driver adjustment on the card moun~ed on tne
meter. If either tube is abnormally gassy, however , the calibration may be affected appreciably.
To open the probe for replacing the 955-type tube, remove the two flat-head screws
and the plug terminal with its hexag ona l insert on the LOW side of the probe. Do not
attempt to remove the insert on the HIGH side.
4.2 METER
4.21 Sticky Meter: Extremes of temperature a nd humidity may cause the pivots to
stick in the rueter bearings. A pe rson skilled in meter work may open the case and loosen
either bearing. Otherwise, the mete r should be returned to the Service Department for
repair or replacement.
4.22 Changes in Meter Sensitivity: Although this trouble is rare, ageing might cause
changes in sensitivity. The meter should be returned to the Service Department.
~
The voltmeter tube will occasionally affect the meter linearity.
~ When the shield of the shielded cable that connects the probe to the instrument
becomes broken (this might happen where it is soldered to probe), the meter will read full
scale with the probe removed and the zero will not remain set with the probe in place.
~When the cathode circuit of the voltmeter tube is open, the meter will stay at
extreme full scale, and the source of the open circuit can easily be located by observing
which scale is affected. (Resistors R-1 to R-6 and R-8 to R-22. most usually open circuit).
4.26 Inability to Set Zero on 1.5-Volt Scale , or Wandering of Meter Needle:
probe.
4.261
After trying a new voltmeter tube, try changing the 955 acorn tube in the
4.27 Meter Backing Off Scale
4.271
An open in the plate circuit of the voltmeter tube.
4.272
Burned-out 955 tube in probe.
!:r.d:1l_
No filament on 955 tube, or
.!:r..d:1l± Open 10-megohm resistor, R-16, in probe.
4.28 Needle Vibrates on the l. ;-volt Scale 1 as i f A-C Were Getting Into Meter
4.281
Panel light or filament of 1-V (rectifier)tube grounded to panel.
4.282
Filter condenser, C-1 (1
~f)
leaking between terminals or to ground.
4.29 Meter Reading Up Scale or Appearing Erratic on the 1.5-Volt Scale
4.291 This is due to the 50-megohm resistor, R-17, open circuiting.
located inside the probe.
This is
4·3 ERRORS BECAUSE OF D-C IN VOLTAGE BEING MEASURED
~
Condense rs C4 and C5 located in the PROBE might possibly develop leakage resulting in changes in reading because of direct current in the input voltage.
4.32 These condensers should be replaced if their leakage resistance is less than
20,000 megohms . Instruments tested in our laboratory give less than 0.3-volt deflection
when 45 volts u-c are applied across the HIGH and LOW terminals (+ to LOW) for a period of
about one minute.
-5-
GENERAL RADIO COMPANY
4.4 VOLTAGE ACROSS CONDENSER C-1
~
This should be between 325 and 375 volts d-e.
4. 5 WIRING AND CONNECTIONS
1t..:..21 The wiring may become displaced and should be checked if any difficulty with the
instrument develops.
~
Connections should be inspected and, if necessary, resoldered.
4.6 VOLTAGE BETWEEN LOW AND GND.
If one side of condenser C1 short circuits to the case, a voltage will be present between the LOW terminal and GND. The condenser should be replaced.
A·7 TU BE DATA
4.71 Foreword: These data were measured in the Calibrating Laboratory of the General
Radio Company using a Model 772 Weston Analyzer for the a-c and d-e measurements of all
tube voltages and currents. Similar instruments may be usea for checking in the field, but
care should be taken to see that voltme ters used have a relativ ely high impedance, say 1000
ohms per volt or better.
Values as tabulated were obtained from standard instrument s that were beir,g calibrated
for stock. Variations between instruments as great as 10% or 15% are normal .
4.72 Operating Conditions: Line voltage, 115 vol ts (230 for 50-cycle model); frequency either 42, 50 or 60 cycles, depending on the model; RANGE switch set to 150; no in-put signal.
Type of
Tube
Heater
Plate to
Cathode
*Grid to
Cathode
-.5 v d-e
Tl
RCA 75 or
RCA 6Q7-G
5.0 v rms
100 v d-e
T2
RCA 1-V
6.4 v a-c
370 v d-e
T3
RCA 955
5.5 v rms
Plate
Current
.25 ma
5 .8 ma
*Measured with 200,000-ohm voltmeter.
For the different positions of the RANGE switch, the voltage and current conditions
for the Type 75 or 6Q7-G Tube change as follows. Other operating conditions are as above.
Ran e
1.5
5.
15.
50.
150.
to
Cathode
Plate to
Cathode
330
320
290
255
100
v
v
v
v
v
-~:-Grid
d-e
d-e
d-e
d-e
d-e
-.5
-.8
-1.1
-1..3
-.5
*Measured with 200,000-ohm voltmeter.
-6-
v
v
v
v
v
d-e
d-e
d-e
d-e
d-e
Plate
Current
.42
.42
.42
.25
.25
ma
rna
ma
ma
ma
PROBE
r------------1
HIGH
@
LOW
0
GND
+
I
C4
V3
VI
I
~~~~~~
I
Rl7
NOTE : EXACT VALUES OF RIB TO R22 ARE
DETERMINED WHEN INSTRUMENT
IS CALIBRATED
I
I
I
G')
m
I
L _____ - - - - - - - .J
SHIELD LEAD
,1\ 1\l\ l\l\ .......::.4.
I
Rl6
·c2
zm
C6~
;;IC
>
r-
-J
;;IC
R23
>
0
...J
I
0
n
0
.,~
~
>
z
jf'------____J
~I Cl
r
~'-"-"-"-"
2
000000
3
Wirin g Diagram for Type 726-A Vacuum-Tube Voltmeter
':1
rr
~
6 .3v
+
1-<
GENERAL RADIO COMPANY
r' A!\'1'0
LIST
F'O:R
TYPE 726-A VACUUM- TUBB: VOLTMETER
Rating and Tolerance
Symbol
Manufacturer
Mfr 1 s . Ty pe No .
Generc:.l Radio
T:v oe ur Dwg. No
Re s is tors
R-l
R-2
R-3
R-4
R-5
R-6
R-7
R-8
R-9
R-10
R-11
R-12
R-13
R-14
R-15
R-16
R-17
R-18
to
R-22
R-23
R-24
~
400 0 Q + 0 . 2~c
20,150 Q + o.;::%
59,000 Q + 0.2%
22o , ooo Q I o.2%
630,000 Q ~ 0 . 2%
50 , 000 Q ~ 1%
2245 Q ~ 1%
990 Q + 1%
0.5 MQ-+ 5%
20 , 000 Q ~-10%, va riabl e
1100 Q ~ 1%
14,900 Q ~ 2%
15 ,ooo ~""2 ~ 1%
20,000 Q ~ 2%
9000 Q ~ 1%
12 MQ + 20%
6o MQ I 2o%
Values determined at
time of calibration.
10 MQ
500 Q ~ 2%
~vW-3
I RC
I RC
I RC
I RC
I RC
I RC
IRC
I RC
I RC
GR
I RC
GR
I RC
GR
I RC
I RC
I RC
\vW-3
'NW-3
WW-4
WW-5
WW- 3
WW- 3
WW-3
BT- l
301- A
WW- 3
154-401
WW-3
301-411
"vil'v- 3
FX-l/2
FX- l / 2
GR
726-370
I RC
GR
BT-l
301-427-2
726-4
Ca2aci tor s
C-1
C-2
C-3
C-4
C-5
C-6
C-7
l. 0 microfarad
0.02 microfarad
Same as C-2
0.01 microfarad
(leakage resistance
must measure 20,000 M.Q
or greater on G-R Type
544-B Megohm Bridge)
Same as C-4
100 micromicrofaraos
Same as C-6
139-1111
Dub ilie r
Dubi li er
Type 4
Dubilier
Type 3L- 5Sl
Mi camold
Type S
Tubes
V-1
V-2
V-3
Amplifier
Rectifier
Diode
RCA
RCA
RCA
6Q7G
1-V
955
Sw itches
S-1
S-2
Range Switch
Power Switch
Oak
3955-E2
SWT-323
Met er
0-200 microamperes,
special scale
Wes t on
643
Fuse s
Line Fuse - 2 amp.
Plate Fuse - 0.1 amp.
Bussmann
Bus sma nn
7AG
7AG
Pilot Lam2
2LAP-330
Pilot Lamp - 6.3 volt
-8-
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising