SERVICE MANUAL
MODEL "ROYAL 500" ALL TRANSISTOR PORTABLE RADIO
CHASSIS 7XT40 CIRCUIT #1
CHASSIS 7XT40 CIRCUIT #2
CHASSIS 7XT40Z
CHASSIS 7XT40Z1
To the Serviceman
GENERAL
These transistor portable chassis are conventional superheterodyne
receivers using an individual mixer and oscillator to produce
the 455 Kc
intermediate frequency. The first and second intermediate frequency
amplifiers are conventional. It is necessary to use neutralization
in
the I.F. amplifier stages as in circuits using a triode tube.
A 1N295
germanium diode is used as the diode detector and AVC voltage
source.
This is then followed by a driver stage and a class "B" push-pull
output
stage. As you can see from the chart, the chassis use matched
transistor
pairs in the final output stage and therefore should one transistor
fail, both transistors must be replaced simultaneously as chances
are
they will not perform properly unless so matched.
Power Supply -- Carbon Penlite Batteries 6 volts D.C. approx.
life
100 hrs. Mercury Batteries 5.36 volts D.C. approx. life 400
hrs.
Frequency Range ......................................... 540
to 1600 KC
Intermediate Frequency ..........................................
455 KC
Sensitivity ......................... Approximately 500 microvolts/meter
for 50 milliwatts output
Power Output Undistorted ................................ 100
milliwatts
Power Output Maximum .................................... 18O
milliwatts
Speaker ............................ 2 3/4 inch P.M. Alnico
V Voice Coil
Impedance 15 ohms @ 1000 cycles
Accessory Earphone ............. 39-20 impedance 2000 ohms @
1000 cycles
Accessory Earphone ............... 39-22 impedance 15 ohms @
1000 cycles
CHASSIS IDENTIFICATION
The "Royal 500" seven transistor portable has been produced with
four
basic chassis. This expedient was necessary to enable us to
produce
sufficient quantities by using transistors from many sources.
All
chassis have the chassis number stamped on them as well as a
color
identifying code on the battery compartment just above the battery
installation instruction label. They are as follows:
Chassis 7XT40 - (Black) code dot
Chassis 7XT40 - (Maroon) code dot
Chassis 7XT4OZ - (Red) code dot
Chassis 7XT4OZ1 - (Green) code dot
The two 7XT40 chassis are very similar with the exception of
the
different coding on transistors. The transistors in these two
chassis
are manufactured by Sylvania.
The 7XT4OZ uses transistors manufactured by Raytheon Mfg. Co.
The 7XT4OZ1 uses transistors manufactured by Texas Instruments Inc.
In addition to this, each receiver has its individual transistor
layout
label and the color of the printing on these labels as well
as the
chassis number on these labels conforms respectively to the
color dot
and chassis number. The accompanying chart gives all the. necessary
information on chassis number, color dot, transistor layout
labels,
transistor numbers, Zenith part number, RETMA part number (where
available), transistor supplier. etc.
You will note that the initial 7XT40 circuit diagram illustrates
an
external earphone connection at the output of the driver transistor.
This is also true of all the other chassis manufactured up to
serial no.
33240. These chassis use an earphone part no. 39-20 which has
an
impedance of 2,000 ohms. Later on in receivers manufactured
after serial
number 33240, the earphone jack will be removed from the driver
circuit
and placed in the speaker voice coil circuit. A new earphone
with an
impedance of 15 ohms, part no. 39-22, will be required for this
circuit.
The previously used 2,000 ohm impedance earphone can be used
in the
voice coil circuit without any discernible distortion, however,
the 15
ohm earphone cannot be used at the output of the driver since
distortion
will be extreme.
RESISTANCE MEASUREMENTS
When making resistance measurements in the circuit, it is most
important
to remove the transistors in the circuit under test other wise
readings
obtained will be incorrect. This is the direct result of a transistor
acting as a diode.
In addition to this, it is important to know the internal battery
voltage of the ohm meter as well as battery polarity of the
meter leads
since incorrectly placing ohm meter leads across an electolytic
condenser with low working voltage may damage the capacitor
due to
excessive reverse current.
VOLTAGE READINGS
It is suggested that a VTVM with an excellent low range scale
be used to
measure all circuit voltages. All voltages indicated on the
accompanying
diagrams have been measured under no signal conditions and a
carbon
battery supply voltage of six volts. Under these no signal conditions,
a
check can be made of the battery packs. With pen light type
carbon
batteries, the total voltage should be 6.0 volts. With four
Mercury type
cells, the total voltage should be 5.36 volts.
COMPONENT REPLACEMENT
When soldering components at the base of the transistor socket,
it is
suggested that the transistor be removed to avoid any possibility
of
excessive heat being transferred through the socket to the transistor.
When soldering the low voltage electrolytics and germanium diodes,
it is
suggested that the wire be held with a pair of long nose pliers
while
soldering. The long nose pliers will act as a heat sink.
TRANSISTORS
At the present time we do not know of any satisfactory commercially
available transistor tester. The only positive method of checking
a
transistor is by direct replacement.
If the transistor is suspected of being defective it can be first
checked with an ohm meter, however, this test will not indicate
the
possible performance of the transistor. Examples illustrate
typical
ohmeter readings.
+--------+
| |
| |
+--------+
| | |
| | |
| | |
Emitter ------>|
| |
Base ----------->|
|
Collector --------->|
Base
-)
) Aproximately 200 - 400 K ohms
Collector
+)
Base
-)
) Aproximately 200 - 400 K ohms
Emitter
+)
Collector
-)
) Aproximately 40 ohms
Base
+)
Emitter
-)
) Aproximately 40 ohms
Base
+)
Collector
+)
) Aproximately 40,000
Emitter
-)
Collector
-)
) Aproximately 20,000
Emitter
+)
If a transistor is suspected of being defective for any other
reason
than a barrier short, the only reliable check is to substitute
a new
transistor and then check performance. There is a possibility
that if
transistors are replaced in the IF or RF circuit, these circuits
may
need re-alignment as the result of slight differences in transistor
characteristics.
SIGNAL TRACING
Past practices used in radio repair commonly known as "Screw
driver
testing" in which the B + at the plate of the tube is shorted
to ground
to check for "clicks" in the speaker, is definitely not recommended.
This practice would be comparable to shorting the collector
of a
transistor to ground which could damage the transistor. Standard
point
to point signal checking with the proper RF, IF and audio signals,
should only be used.
REPAIR EQU1PMENT
The following list of equipment is what we recommend and use
for repair
of transistor chassis.
1. An HF signal generator supplying frequencies front 500
Kc to
1700 Kc to cover the broadcast band, as well
as 455 Kc with
which to align the I.F. stages.
2. An audio generator or an audio signal source to be used
for
signal tracing after the diode detector.
3. A VTVM with a good low range voltage scale and a reliable
resistance scale.
4. M209B Dazor floating lamp & magnifier or equivalent.
5. A set of optometrist tools - pliers, cutters, picks etc.
6. A soldering iron with a very fine tip, not to exceed 35 watts.
ENGINEERING MODIFICATIONS
Since transistor circuitry is new and improvements are continually
being
discovered, Circuit 1 is for the original 7XT40 Chassis. The
engineering
staff has found ways to improve the AGC system as well as other
portions
of the circuitry. Therefore, the following engineering changes
are
design improvements on the original 7XT40 Circuit 1.
The 7XT40 Circuit 2 -
1. Originally the bias bleeder on the first IF stage was lOOK
and 15 K
plus a 2 K volume control from B + to ground. The AGC by-pass
of 3 mfd
was at the junction of the 100 K and 15 K which meant that there
was 15
K in series with the AGC current. During production the amount
of AGC
was found to be insufficient. The bias bleeder was then changed
to 100
K, 4700, 4700 and 5 K volume control. The AGC by-pass of 16
mfd was put
between the two 4700 ohm resistors. This increases the amount
of AGC
applied to the first IF stage and at the same time maintains
approximately the same D.C. operating point.
100K
15K 2K
VOLUME CONTROL
B+ --------/\/\/\------+-----/\/\/\--------/\/\/\
------+
|
^ |
---
| v
--- 3 MFD
| GND
|
v
GND
100K 4700
4700 5K VOLUME
CONTROL
B+ --/\/\/\---/\/\/\---+-----/\/\/\--------/\/\/\
------+
|
^ |
---
| v
--- 16 MFD
| GND
|
v
GND
2. 4700 and 47K in junction with Cl5 and Cl6 were 22OO and 18K
respectively. This change was made to stabilize collector current
of the
driver transistor and slightly increase its gain.
3. In the event you do not wish to modify the circuit as in item
1, it
is suggested that when servicing sets which exhibit AGC and
overload
problems, the bias bleeder of the first IF stage be changed.
The
original circuit had a 100 K and 15 K. These values should be
changed to
47 K and 4700 ohms respectively. This in effect supplies more
AGC
voltage to the mixer and first IF. In addition the lower impedance
bleeder will reduce the variation in current drawn by these
transistors.
4. You will note that C5 and C6 in the revised 7XT40 diagram
have been
terminated at ground and of course then the 470 ohm resistor
from the
1st IF emitter must be by-passed with C23 a .05 condenser. C7
and C8 are
terminated at ground for production convenience.
5. On a receiver that seems to be excessively sensitive and possibly
exhibits some characteristics of 910 or 1365 Kc tweet, it can
be cooled
off in the following manner. The initial run of these units
had a 560
ohm resistor by-passed with a .05 mfd condenser in the emitter
leads of
the 2N94 mixer. Later on a 10 ohm resistor was put in series
with the
560 ohm resistor, only this 10 ohm resistor was not by-passed.
Still
later, a 22 ohm resistor was substituted in place of the 10
ohm
resistor. In the circuit presently being used, the 560, 10 ohm
and 22
ohm resistors have been eliminated and in place of them we are
using a
3900 ohm resistor by-passed with a .05 mfd condenser in the
emitter lead
of the mixer.
The 7XT4OZ has not had any modifications up to the present time.
The 7XT4OZ1 -
1. The 2200 ohm bias resistor in the 121-17 second IF transistor
was
4700 ohms. It has been changed to 2200 ohms to reduce tweet,
by slightly
lowering gain.
2. 1 K resistor in the emitter of the 121-16 mixer was
470 ohms. This
change was made to reduce tweet and noise.
3. On the patent & transistor layout label 7XT4OZ1
(Green Printing) the
mixer transistor was marked 121-6. This was a typographical
error and it
should be 121-16.
ZENITH ROYAL 500 HAND WIRED CHASSIS ALIGNMENT PROCEDURE
|======================================================================|
| |
| Connect Inner | |
| |
| |Input Signal |Conductor From
|Set Dial |
| |
| Step | Frequency | Oscillator To |
At | Trimmers | Purpose |
|======|=============|===============|=========|===========|===========|
| |
|
| | Adj. T1, |
|
| |
|
| | T2,T3 for | For I.F.
|
| 1 | 455 KC
| One | 600
KC |max output | Alignment |
|------|-------------| Turn
|---------|-----------|-----------|
| |
| Loosley | Gang
| |Set Osc
to |
| 2 | 1620 KC |
Coupled | open |
C1C |Dial Scale |
|------|-------------| To
|---------|-----------|-----------|
| |
| WaveMagnet |
| |Align Loop
|
| 3 | 1260 KC |
| 1260 KC | C1A | Antenna
|
|------|-------------|
|---------|-----------|-----------|
| |
|
| Gang | Adj. slug |Set Osc to |
| 4 | 535 KC
|
| closed | in T6 |Dial Scale |
|----------------------------------------------------------------------|
| 5 Repeat steps 2,
3, 4
|
|----------------------------------------------------------------------|
ZENITH ROYAL 500 HAND WIRED CHASSIS DIFFERENCES
|====================================================================================================================|
| |
|TRANSISTOR| |
| |
| |
|
| |
| | CHASSIS
| LAYOUT |
| |
| | CRYSTAL |
|
| |
| | COLOR
| LABEL |
| |
| 1ST & | DIODE |
|
| |
| CHASSIS | DOT | COLOR
|PART NO. | MIXER | OSC. |2ND I.F. |DETECTOR
| DRIVER | OUTPUT |SUPPLIER
|
|=========|=========|==========|=========|=========|=========|=========|=========|=========|=============|===========|
| |
| 102-1809 | Zenith | 121-22 | 121-21 | 121-6
| | 121-7 |
121-8 | Sylvania |
|7XT40 #1 | Black | Black |
RETMA | 2N94 | 2N94 | 2N94
| 1N295 | 2N35 | 2N35
| |
| |
| | Type
| NPN | NPN |
NPN | |
NPN | NPN |
|
| |
| |
| |
| |
| |Matched Pair |
|
|---------|---------|----------|---------|---------|---------|---------|---------|---------|-------------|-----------|
| |
| 102-2263 | Zenith | 121-22 | 121-21 | 121-6
| | 121-7 |
121-8 | Sylvania |
|7XT40 #2 | Maroon | Maroon | RETMA
| 2N194 | 2N193 | 2N216 | 1N295
| 2N35 | 2N35
| |
| |
| | Type
| NPN | NPN |
NPN | |
NPN | NPN |
|
| |
| |
| |
| |
| |Matched Pair |
|
|---------|---------|----------|---------|---------|---------|---------|---------|---------|-------------|-----------|
| |
| 102-1997 | Zenith | 121-14 | 121-9 | 121-10
| | 121-11 |
121-12 | Raytheon |
| 7XT40Z | Red | Red
| RETMA | |
| | 1N295 |
|
| |
| |
| | Type
| PNP | PNP |
PNP | |
PNP | PNP |
|
| |
| |
| |
| |
| |Matched Pair |
|
|---------|---------|----------|---------|---------|---------|---------|---------|---------|-------------|-----------|
| |
| 102-1998 | Zenith | 121-16 | 121-15 | 121-17
| | 121-18 |
121-19 | Texas |
| 7XT40Z1 | Green | Green |
RETMA | |
| | 1N295 |
|
|Instruments|
| |
| | Type
| NPN | NPN |
NPN | |
PNP | PNP |
|
| |
| |
| |
| |
| |Matched Pair |
|
|--------------------------------------------------------------------------------------------------------------------|