Elecraft K2 Owner's Manual - Manual (Page 166)

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sheet 3, lower right-hand corner). To test VPWR, set the DMM for DC
volts, then measure the DC voltage on pin 2 of U8 when
T U N E is
pressed. Expected: 0.7-2.5 VDC. Actual: _____.
4. If VPWR reading is high (> 4.5 V): The ALC software will set VPWR
to its highest level (about 5 V) if the transmitter cannot be driven to the
requested power level. This happens for one of two reasons: (a) the
transmitter gain is low (or transmitter isn’t working at all); (b) the RF
detector has an incorrect component. Check all component values in the
RF detector. If you can’t find a problem with the RF detector, continue
with the next signal tracing section (transmit mixer, etc.).
5. If VPWR reading is low (< 0.4V): VPWR can be too low because: (a)
the ALC software is being “fooled” by a signal from the RF detector that
says the power is higher than it really is; (b) because U8 on the control
board is defective or has a pin shorted to ground or not soldered. Check
all component values in the RF detector (RF, sheet 3). If these appear
correct, check DC voltages on U8 (control), as well as resistance to
ground on all pins.
Transmit Mixer, Buffer, Band-Pass Filter, T-R Switch (RF, sheets 2-3)
Note: The measurements in this section and the next may vary widely,
especially if you do the measurements on a band other than 40 m. However,
the ratio between any two back-to-back measurements should remain fairly
constant, and is a good indication of gain or loss of a stage in the transmitter.
For example, the ratio of measurements in steps 3 and 2 below is about 12.
1. Connect the RF probe to the DMM. Set the DMM for a 2 or 3-V DC
volts range.
2. Xmit Mixer Output: Measure the key-down signal at U10, pin 4.
Expected: 0.016 Vrms. Actual: _____.
3. Buffer Output: Measure the key-down signal at U9, pin 6 (LT1252).
Expected: 0.200 Vrms. Actual: _____.
4. Band-Pass Filter Output: Measure the key-down signal at W6.
Expected: 0.030 Vrms. Actual: _____.
5. T-R Switch #1 Output: Measure the key-down signal at the anode of
D1. Expected: 0.029 Vrms. Actual: _____.
Pre-driver, Driver, and PA (RF, sheet 4)
1. Pre-Driver Output: Measure the key-down signal at the case (collector)
of Q5 (2N5109). Expected: 0.120 Vrms. Actual: _____.
2. Driver Input: Measure the key-down signal on the base of Q6 (2SC5739
or 2SC2166; pins are labeled B, C, E). Expected: 0.026 Vrms. Actual:
_____.
3. Driver Output: Measure the key-down signal at the tab (collector) of
Q6. Expected: 1.8 Vrms. Actual: _____.
4. PA Input (Q7): Measure the key-down signal at the base of Q7
(2SC1969 on bottom of the board; pins are labeled on the top). Expected:
0.38 Vrms. Actual: _____.
5. PA Input (Q8): Measure the key-down signal at the base of Q8.
Expected: 0.38 Vrms. Actual: _____.
6. RF Detector Input: Measure the key-down signal on the anode (non-
banded end) of D9 (1N5711, middle of the right edge of the board).
Expected: 2.0 Vrms. Actual: _____. (This voltage should be fairly
constant regardless of the band used.)
7. PA Transistor Tests: If the PA input voltages were higher than
expected, but the RF detector input was too low, one or both PA
transistors could be defective. After checking DC voltages and
transformer leads, turn off power to the K2 and use your DMM’s
diode/transistor test range to test the transistors. With the DMM’s
positive lead on the base of Q7, you should measure about 0.6 k to the
emitter or collector. With the DMM’s negative lead on the base of Q7,
you should measure about 1.3 k to the emitter and > 3 k to the collector.
These also apply to Q8.
DC Voltage Tables
The tables on the following pages provide DC voltages for all ICs and
transistors on each of the three boards, as well as the diodes in the T-R switch
(RF board). Typically, your readings will match these within 10%.The
voltages were measured using a high-impedance DMM (10-11 Megohm).
The K2’s internal voltmeter can also be used for most measurements.
Receive-mode voltages are listed except as noted. Most of the Control board
measurements were made with the Front Panel module removed for easier
access. Exceptions are indicated by (**).
Equipment Setup: Supply voltage 14.0 V; no antenna; LCD = NITE; GRPH
= DOT; receive mode; no headphones or speaker connected; RF GAIN mid-
range; AF GAIN minimum; OFFSET mid-range.