SUBJECT: SERVICE MANUAL RX SECTION LEVEL DIAGRAMS 10/28/88
INTRODUCTION
This bulletin will explain the development and use of Kenwood Service
Manual receiver section level diagrams. Since they list typical RF and
audio levels, the diagrams provide the most efficient means of locating
a faulty RX circuit in a transceiver.
In the past, Kenwood has recorded RF levels in volts (V), decibels (dB),
and more recently dBu and dBm. Conversion tables and an equation are
provided at the end of this text so you can change the RF levels to the
form of your choice.
Using the level diagrams require basic test equipment and a couple of
test fixtures. One fixture is an 8 ohm non-inductive load that replaces
the internal speaker. The other is an injection probe. It is used to
inject RF from your signal generator into specific sections of the receiver.
Both fixtures are discussed at the end of this text.
HOW THE RX SECTION LEVEL DIAGRAM IS DEVELOPED
This section will describe how the level diagrams are made for CW and
SSB transceivers. It will then discuss the different methods used for FM
transceivers.
A working transceiver is tuned to its center frequency. For HF rigs, this
would be 14.200MHz. A Standard Signal Generator (SSG) set to the same
frequency is connected to the antenna terminal of the transceiver. The
SSG is set to a level that will not allow the AGC circuits to function
(no S-METER reading). For HF rigs, this would typically be 0dBu (0.5uV).
An 8 ohm load is inserted into the external speaker jack and the AF gain
control is adjusted to obtain 0.68V across the load (measured with an
AF VTVM). The AF gain setting is not changed from this point on. The SSG
is replaced with a 50 ohm load to terminate the antenna input.
An injection probe is connected to the SSG. The ground clip is attached to
the chassis of the transceiver. The probe is placed at the first point
listed on the level diagram just past the antenna input. The SSG level
is adjusted until the audio output is again 0.68V. The SSG level is then
listed on the diagram as a typical reference for that point. This process
is repeated until all measuring points are assigned a reference level.
NOTES
After each mixer stage, the SSG must be adjusted to the respective IF
frequency. As an example, after the first mixer on the TS-940S, the
frequency would be set to 45.05MHz. After the second mixer, the frequency
would be set to 8.83MHz.
The levels in the audio stage of the receiver are measured with an AF VTVM.
These measurements are made when the SSG is connected to antenna terminal
(just after the AF gain is set).
FM RX SECTION LEVEL DIAGRAMS
FM transceiver level diagrams are made in the same fashion as CW/SSB
diagrams. The transceiver is tuned to its center frequency (or close to it).
A low level signal is applied to the antenna terminal. The audio output is
held constant while the injection method is used. The differences are as
follows:
On some FM transceivers, the injected signal is modulated (typically a
1KHz tone at +/- 3.0KHz deviation is used). Other transceivers are tested
without modulation.
On some transceivers, the constant output is measured at 12dB SINAD.
On others it is measured at 20dB of noise quieting (NQ).
Each Service Manual level diagram contains footnotes that describe the
method used. Due to the variations and available test equipment, it may
be necessary to change the initial set up and then write your own level
diagram.
USING THE LEVEL DIAGRAM TO TROUBLESHOOT A FAULTY RECEIVER
It is not practical to set the audio output to a specified level by
injecting a signal into the antenna terminal of a transceiver that has
low sensitivity. In many cases, the input signal would be so strong that
it would attack the AGC circuits. This would cause all measurements in
the receiver to vary from the typical levels in the diagram. The best
approach to this situation is to start at the end of the receiver circuit
and work towards the antenna.
Connect a 50 ohm load to the antenna terminal and an 8 ohm load to the
external speaker jack. Inject a signal at the end of the IF stage. The
IF frequency and required level are listed on the Service Manual level
diagram. Adjust the AF gain control to obtain the specified audio level
at the 8 ohm load. Do not change the AF gain setting from this point on.
Inject the next signal in the middle of the receiver chain (again refer
to the level diagram for the frequency and level). If the audio comes up
to the specified level, the faulty component is somewhere between the
antenna terminal and the middle of the receiver. If the audio does not come
up, the bad component is between the two injection points.
The next injection point would again cut the chain in half in order to
work towards the faulty component. For instance, if the problem is between
the antenna terminal and the middle of the receiver, inject the next signal
in the middle of those two points. Continue in this manner until the problem
can be isolated to a small portion of the circuit.
NOTE
Do not forget that D.C. measurements around an isolated circuit are
necessary. For instance, an amplifier will not have any gain if an open
resistor cuts off the supply voltage.
INJECTION PROBE
An injection probe is a coax that connects to your SSG, has a 0.01uF
capacitor on the center conductor of the coax, and a tip connected to
the other end of the capacitor. The shield of the coax connects to a
ground clip. Many electronic supply shops sale injection probes. If you
decide to purchase one, make sure it contains a 0.01uF capacitor between
the tip and center conductor of the coax (used to block D.C.). If you
decide to fabricate your own cable, it can be done with a few simple parts.
8 OHM LOAD
An 8 ohm non inductive load is used to substitute the internal speaker on
the transceiver. The load is connected to the external speaker jack for
convenience. For the purpose of troubleshooting a faulty receiver, it is
not necessary to purchase a precision 8 ohm resistor (also, precision
resistors are inductive). Six carbon composite 47 ohm, 1/2 watt resistors
connected in parallel make an adequate load.
DECIBELS
As mentioned in the introduction, Kenwood Service Manuals have listed
levels in V, dB, dBu, and dBm. Unless otherwise specified in the footnotes
on the level diagram, dB and dBu levels are equal.
Aside from the conversion tables, a conversion equation and examples are
provided.
V(out)
EQUATION: dB = 20 log --------
V (in)
In the equation, solving for dB would yield a value that is the difference between the two voltages. In other words, suppose you already know that 2uV = 12dBu and 15.8uV = 30dBu. You can see that the difference is 18dBu. Plugging these uV values into the equation would yield the same result. With this in mind, one can plug in a known voltage from a conversion table, plug in a voltage not listed in the table, and find the dB difference. To make the answer simplistic, use 0.5uV for dB/dBu and 224mV for dBm. Since they equal 0dB/0dBu/0dBm respectively, the difference is the actual value of the unknown voltage.
EXAMPLE: Given 0.5uV = 0dBu, | EXAMPLE: Given 224mV = 0dBm,
find the dBu value of 6uV. | find the dBm value of 6uV.
|
|
-6 | -6
6 x 10 | 6 x 10
XdBu = 20 log -------- | XdBm = 20 log ----------
-6 | -3
.5 x 10 | 224 x 10
|
|
XdBu = 20 log 12 | -6
| XdBm = 20 log 26.8 x 10
|
XdBu = 20 (1.08) |
| XdBm = 20 (-4.6)
|
XdBu = 21.6 |
| XdBm = -91.4
|
X = 21.6dBu |
| X = -91.4dBm
-6
REMINDER: 6uV = 6 x 10 = 0.000006
-3
224mV = 224 x 10 = 0.224
Suppose you are given a dB value from the level diagram and you want to know its equivalent voltage:
EXAMPLE: Given 0.5uV = 0dBu, | EXAMPLE: Given 224mV = 0dBm,
find the uV value of 55dBu. | find the uV value of -55dBm.
