Restoration of a 1951 RCA-Victor Model X-711 AM

Restoration of a 1951 RCA-Victor Model X-711
AM/FM Radio
Daniel Vickery
May 13, 2010
1
Introduction and Historical Context
In 1929, the Radio Corporation of America purchased the Victor Talking Machine Company,
which was at the time the largest manufacturer of phonographs and phonograph records in
the world [1]. This created a new subsidary known as RCA-Victor, which was the manufacturer of, among many others, the Model X-711 AM/FM radio, the subject of this report.
The X-711 was introduced in 1951, near the end of the “golden age of radio,” when television was growing rapidly in popularity. Just two years earlier, RCA-Victor had introduced
their 45rpm record standard to compete with the popular 33 31 rpm standard developed by
CBS/Columbia. It’s worth noting that the Model X-711 features a phono input jack in
addition to AM/FM reception, a move that could have potentially helped bolster sales in
both the phonograph and phonograph record divisions of RCA-Victor. In fact, RCA-Victor
promoted the combination of its various electronic products through a marketing campaign
during much of the 40’s and 50’s advertising a certain “golden throat” technology in its products [2]. While the term was essentially pure marketing fluff, it did encourage the purchase
of compact “entertainment centers” consisting of a combined AM/FM set like the X-711
with an attached phonograph, as shown in Figure 2.
In 1953 RCA developed the NTSC American color television standard, which quickly
became widespread, replacing the radio as the primary home entertainment medium [3].
Today, “golden age” programming genres like radio comedy and drama are far rarer than
they were at the time of the production of the Model X-711, although they can still be heard
on National Public Radio and local college stations using this same radio.
In this report we analyze the operation of the Model X-711, and describe the restoration process of a particular unit. In Section 2, we give an overview of the Model X-711
architecture, while Section 3 describes individual circuits in detail. Finally, Section 4 goes
through the restoration process of our unit, including component data and performance
measurements.
2
Overview
The RCA-Victor Model X-711 is an AC-DC operated seven-tube AM/FM Superheterodyne
radio. It includes a built-in AM loop antenna, phonograph input, and speaker with a large
1
Figure 1: Original logos of the Radio Corporation of America and the Victor Talking Machine
Company before the purchase of the latter by the former.
Figure 2: RCA-Victor Model X-711 with 9-JY phono attachment. Source: [4].
2
Figure 3: Our RCA-Victor Model X-711.
RF
CONVERTER
IF
AM RF
STAGE
FM IF
STAGE
MIXER
DETECTION
FM IF
RATIO
STAGE
DETECTOR
AUDIO
AM IF
FM RF
STAGE
STAGE
AM
DETECTOR
AF AMP
AM
OSCILLATOR
FM
OSCILLATOR
POWER
AMP
PHONO
IN
Figure 4: Block diagram of the Model X-711.
4 21 ” diameter cone. The plastic case is handsome and compact. A photo of our unit is shown
in Figure 3.
The X-711 covers the standard U.S. AM and FM bands, with a tuning range of 5401600kHz on the AM setting, and 88-108MHz on the FM setting. AM reception is achieved
through the use of an internal loop antenna, while FM is received using built-in line cord
antenna circuitry or an optional external antenna. As shown on the schematic attached at
the end of this report, there are two IF stages; one AM/FM stage and one FM-only stage.
Each band has its own oscillator. There are no RF gain stages. A block diagram of the radio
circuitry is shown in Figure 4.
3
3.1
Circuit Operation
Power
The X-711 power stage is shown in Figures 5 and 6. The external line cord is connected to
the chassis through a second connector, which, strangely enough, is polarized even though
3
Figure 5: Schematic of Model X-711 power stage.
Figure 6: Schematic showing audio power output stage and common RCA “trick” of splitting
B+ across the output transformer.
4
the wall plug is not. Line chokes L8 and L9 filter line noise and L9 facilitates the use of the
line cord as an FM antenna, by making the lower power input terminal a high impedance
node at FM carrier frequencies. For AC use, the 35W4 rectifier tube (V7 ) functions as a halfwave rectifier. C1A and C1B (shown in Figures 5 and 6 respectively) are large electrolytic
capacitors that filter the rectified signal down to DC for the B+ bus.
Figure 6 shows a neat trick commonly used in RCA-brand radios from this era, wherein
the DC bus is split across the output transformer. The center-tap on the transformer is
connected to the same node as C1A in Figure 5. The DC current from the center tap is split
between the plate of the power pentode, V6 , and the B+ bus that follows the low-pass filter
composed of C1B and R27 . Because this current is split, the net DC current in the transformer
coil is reduced, which reduces the amount of iron in the core necessary to avoid saturation.
This not only saves RCA money on iron, but also significantly reduces transportation costs.
3.2
RF Stages
A schematic of the X-711 RF and converter stages is shown in Figure 7. AM RF signals
are received via the internal loop antenna, L3 . The choice of a loop antenna allowed the
designers to carefully select the fixed trimmer and tuning capacitors in A7 for a very high
Q, because the characteristics of the loop would be well known. The RF signal is coupled
into the converter stage through L4 and the AM/FM/Phono switch (not shown).
As mentioned before, the X-711 is configured to use the line cord as an antenna, which
is capacitively coupled through C5 (shown in Figure 5) to the node connected to the lower
screw on the external antenna connector shown in Figure 7. Coupling capacitors C3 , C4 and
C5 isolate the optional external antenna from the line for safety. FM RF signals resonate
with L1 and A14 , and are connected to the converter through the AM/FM/Phono switch
(not shown).
3.3
Converter
The converter is built with a dual-triode tube, V1 (19J6). The triode on the left functions
as the mixer, and the triode on the right functions as the local oscillator, although there are
separate oscillator circuits for AM and FM that are alternately connected to the oscillator
triode through the AM/FM/Phono switch (not shown). The FM oscillator circuit is shown
between the two triodes, and is one of many possible variations on the canonical Hartley
oscillator, tuned by the capacitor coupling the top of the tapped inductor to the triode
cathode. The AM oscillator circuit is shown below the oscillator triode, composed of L4 , A6 ,
and A5 .
RF signals from both the AM and FM RF stages are connected to the grid of the mixer
triode through the AM/FM/Phono switch (not shown). Because there are no extra RF gain
stages, the mixer triode relies on the oscillator to produce a grid voltage large enough to
drive it non-linearly.
5
Figure 7: Schematic of Model X-711 RF and converter stages.
6
Figure 8: Schematic of Model X-711 IF stages.
3.4
IF Stages
A schematic of the Model X-711 IF stages is shown in Figure 8. The first IF stage, made
with V2 , amplifies both AM and FM IF signals. The second, made with V3 , is FM-only.
AGC is applied to the grid of V2 from the wire below the first set of tuned circuits next to
C13 .
In the schematic, AM signals pass through the “lower” resonant tanks, while FM signals
pass through the “upper” resonant tanks. The nature of the two IF frequencies (455kHz
and 10.7MHz) allows for the first stage to be used for both AM and FM frequencies because
elements of the tuned IF resonant tanks of the unused band will have negligibly high or
low impedances at the IF frequency of the desired band. To give an example, the AM IF
signal is coupled into the first IF stage via tuned circuit A3 . At 455kHz (nominal AM IF),
the impedance of the capacitor of tuned circuit A11 is negligibly large, while the impedance
of the inductor is negligibly small. Hence, the AM IF signal passes directly through the
inductor to the grid of V2 .
The AM output of the first IF stage is across tuned circuit A1 . From here it splits from
the FM signal chain and goes directly to the AM detector, V5 . R28 and C16 make up an
RF filter. Interestingly, all three devices in this filter are part of a single discrete compound
component designed to save space.
Since the FM IF signal requires more gain, it continues from A9 onto a second IF stage
made with pentode V3 . To maximize gain, V3 has a cathode-bypass capacitor, C18 , which
is easier to implement on an FM-only IF stage like this, where the nominal frequency is in
7
(a) AM Detector
(b) FM Detector
Figure 9: Schematics of Model X-711 AM and FM detectors.
the megahertz range, because the bypass capacitor can be small enough to implement with
a waxed paper device.
3.5
Detectors
The AM and FM detector circuits are shown in Figure 9. The FM detector (or “demodulator,” to use a more accurate, modern term) is implemented as a ratio detector. This detector
is advantageous as it does not require a separate limiter circuit to remove amplitude variations. The ratio detector accomplishes this by means of filtering amplitude fluctuations
with the combination of R12 , R13 , R14 and C2 , which has a large time constant of about 100
milliseconds. The DC voltage at point A is used as an indicator of signal amplitude and
connects to the AGC network through R17 and C22 .
The FM modulations themselves are represented as the ratio between the voltages on
capacitors C19 and C20 , hence the detector name. As the IF signal deviates from 10.7MHz,
to which A8 and A13 are tuned, phase differnces in the signals on the two diodes of V4 translate
to a deviating voltage at the center tap of the capacitors C19 and C20 . The resulting signal
is sent to the audio stages by way of a low-pass filter made up of R16 and C21 that removes
high-frequency detection artifacts.
The AM detector circuit is made with V5 , which is a dual-diode, single-triode tube.
The diode on pin 5 is an AGC connection, while the diode on pin 6 serves as a simple
envelope detector for the AM IF signal. The rectified AM signal is filtered or “smoothed”
by the combination of R23 and C26 , which are connected to the triode grid pin. The triode
functions as an audio amplifier, which drives the coupling capacitor to the power amplifier
stage as a load.
8
(a) Original Configuration
(b) Modified Configuration
Figure 10: Safety modifications to the X-711. Note the “fat” pin on the plug that indicates
a polarized connector.
3.6
Audio Stage
A schematic for the audio power output stage is shown in Figure 6. This is a fairly simple
stage where the objective is clear: lots of gain, high power output. To achieve this, a power
pentode, V6 , is used, and its cathode is bypassed with a large 20µF electrolytic capacitor.
4
4.1
4.1.1
Restoration and Measurements
Repair Description
Safety
The RCA-Victor X-711 radio is a “hot chassis” model, which means that one side of the
utility line is connected directly to the metal chassis of the radio, indicated by the “ground”
symbol on the schematic. Because the original line cord of the radio has an unpolarized
plug, there is a 50% chance that the chassis will be connected to “hot” line voltage. This
creates a safety hazard wherein simple internal wiring faults can cause screws, knobs, or
antenna terminals on the exterior of the plastic case to likewise become “hot”. To remedy
this problem, the original line cord was replaced with a new one of similar color that has a
polarized plug. To reduce the risk of shocks from contact with filament pins or various parts
of the power stage while the radio is off, the on switch was moved to disconnect the “hot”
wire instead of the neutral one.
Figure 10(b) shows an updated schematic reflecting the changes described above.
9
4.1.2
Component Replacements
The radio we obtained originally contained only one modification: electrolytic capacitor C1A
had been replaced with a more modern, but still relatively old 50µF electrolytic capacitor,
placed in parallel with the original. This capacitor was in relatively good shape, but of
course did not perform as well as the modern electrolytics in the lab, so it was replaced.
The new replacement was not wired in parallel with the original “can” electrolytic, to avoid
any problems it could cause. Instead, the new electrolytic, as well as similar replacements
for C1B and C1C , was wired in with “flying joints”. The original electrolytic can was left in
place for historical accuracy, though unconnected to the radio circuit.
All waxed paper capacitors were replaced with extreme prejudice. While some tested
better than others, none could compare with the modern capacitors in the lab. Specifications
for all original and replacement capacitors can be found in Table 1. Images of 10 of the
replaced capacitors that were removed from the radio are shown in Figure 12.
4.1.3
Other Repairs
After all component replacements had been completed, the radio was powered up through an
isolation transformer and variac to be tested. Both bands worked fine and sounded decent.
Subsequently, all AM and FM tuned circuits were aligned according to the instructions in
[5]. For the most part, little adjustment was necessary.
The one other repair made was to fix a damaged AM IF tuned circuit, A3 , shown without
its steel casing in Figure 11(a). The tunable screw in the shaft was very tight, and the shaft
was loosely connected to its base, so that during an alignment attempt, the entire shaft
would be turned instead of the screw inside. This led to a break in a solder joint connecting
a fine wire from the shaft to a pin on the base, as shown in Figure 11(b). After fixing the
joint, the shaft was securely attached to the base with hot-melt glue.
4.2
Measurements
Fourteen total capacitors were replaced in the radio: four electrolytics and ten paper capacitors. Measurements of these capacitors are given in 1 and photos of the ten paper capacitors
are shown in Figure 12
A plot of the measured AM frequency response of the Model X-711 is shown in Figure
13. This measurement was taken using an 8.2Ω power resistor as a substitute load in place
of the speaker. It was found that the maximum output power just at the onset of distortion
is about 0.56 watts.
DC plate and screen grid voltages were measured with the internal loop antenna shorted.
The results are shown in Table 2. In general, these were found to be about five to fifteen
volts above the specifications in [5], likely due to component variation and differences in line
voltage. Measurements were also taken of DC voltages in the AGC network for very large
modulated and unmodulated AM RF inputs. The results are listed in Table 3.
10
(a) IF tuned circuits. Right: A11−12 . Left: A3−4 .
(b) Solder joint break.
Figure 11: IF tuned circuits A3 , A4 , A11 , and A12 with steel case removed. The shaft on the
left (A3−4 ) had to be glued to the base.
Capacitor
C1A
C1B
C1C
C2
C13
C18
C21
C23
C24
C25
C28
C29
C30
C32
Original
40µF/150V Electrolytic
80µF/150V Electrolytic
20µF/150V Electrolytic
2µF/50V Electrolytic
0.05µF/400V Paper
0.005µF/100V Paper
0.002µF/200V Paper
0.01µF/100V Paper
0.1µF/400V “B.B.”
0.01µF/100V Paper
0.001µF/100V Paper
0.02µF/400V Paper
0.01µF/100V Paper
0.05µF/400V Paper
Leaky? Capacitance DC Resistance
150V
49.5µF
Open
100V
100V
No
100V
100V
100V
No
100V
100V
50V
9pF
69.6nF
6.58nF
2.76nF
13.6nF
112nF
12.6nF
1.36nF
27.0nF
12.8nF
51.6nF
Table 1: Replaced Capacitors.
11
57kΩ
365kΩ
1.64MΩ
270kΩ
48.6kΩ
397kΩ
4.6MΩ
137kΩ
250kΩ
118kΩ
Replacement
47µF/160V
100µF/160V
22µF/50V
2.2µF/50V
0.047µF/630V
0.0047µF/630V
0.0022µF/630V
0.01µF/630V
0.1µF/630V
0.01µF/630V
0.001µF/630V
0.022µF/630V
0.01µF/630V
0.047µF/630V
(a) C2
(b) C13
(c) C21
(d) C23
(e) C24
(f) C25
(g) C28
(h) C29
(i) C30
(j) C32
Figure 12: Photos of ten of the replaced capacitors alongside their replacements.
Model X−711 AM Frequency Response
2
10
Volts Peak to Peak (Absolute)
550kHz Carrier
1000kHz Carrier
1610kHz Carrier
1
10
0
10
−1
10
1
10
2
3
10
10
Modulation Frequency (Hz)
4
10
Figure 13: Measured AM frequency response of Model X-711.
12
DC Plate Voltages
Tube
Specified Voltage Measured Voltage
V1 Mixer Triode 90VDC
97.4VDC
V1 Osc. Triode
90VDC
97.0VDC
V2
90VDC
103.5VDC
V3
90VDC
100.5VDC
V5
33VDC
37.1VDC
V6
110VDC
118.3VDC
DC Screen Grid Voltages
V2
90VDC
103.9VDC
V3
90VDC
100.6VDC
V6
90VDC
104.4VDC
Error
+7.4V
+7.0V
+13.5V
+10.5V
+4.1V
+8.3V
+13.9V
+10.6V
+14.4V
Table 2: Plate and screen grid DC voltages, measured in AM mode with loop antenna
shorted.
UNMODULATED AM
Measurement
Result
DC AGC Voltage -86mVDC
V2 Plate Current 1.7mADC
V3 Plate Current 5.0mADC
400Hz 100% MODULATED AM
DC AGC Voltage -10.9VDC
V2 Plate Current 2.1mADC
V3 Plate Current 5.0mADC
Table 3: AGC bus voltages and plate currents for large modulated and unmodulated AM
RF inputs at a 1MHz carrier frequency.
13
5
Conclusion
In this report we have described the operation of the RCA-Victor Model X-711 AM/FM radio
in detail. Our unit is restored to working order, realigned, and updated for modern safety
standards. It now crackles back to life, as functional as it was the year it was manufactured.
To take a broader perspective on the “rebirth” of this particular radio, the fact that it
can now be powered on and receive AM and FM transmissions just as well as it could sixty
years ago is an incredible testament to the power of standardization. Both AM and FM
bands continue have such a broad and diverse listener base that the basic principles of how
we encode, broadcast, and receive public radio signals have not undergone any fundamental
transformation since their inception long before the production of this unit. Given the quality
of the replacement components now inside the radio, and the permanence of such broadcast
standards, this X-711 will continue to be as functional as ever for decades into the future.
References
[1] “The Rise of RCA Victor,” Thomson, Inc., 2002, [Accessed 12-May-2010]. [Online]. Available: http://home.rca.com/en-US/PressReleaseDetail.html?Cat=RCAHistory&MN=8
[2] AntiqueRadios.com, “The Ubiquitous ‘Golden Throat’ Decal,” 2004, [Accessed
12-May-2010]. [Online]. Available: http://antiqueradios.com/gallery/v/RCA/RCA
Golden Throat Decal.jpg.html
[3] Wikipedia, “RCA,” 2010, [Accessed 12-May-2010]. [Online]. Available:
//en.wikipedia.org/wiki/RCA
http:
[4] AntiqueRadios.com, “RCA model X-711 radio with model 9-JY phono attachment,”
2005, [Accessed 12-May-2010]. [Online]. Available: http://antiqueradios.com/gallery/v/
RCA/X711.jpg.html
[5] “RCA-Victor Model X711 Photofact Folder,” 1951.
14