[Kenwood] Ts-940 No pwer out in AM... Help needed

Sat Apr 2 19:01:31 EST 2005

NO RF POWER OUT AM MODE.
Symptoms: IN AM mode:
With meter in Power mode Push PTT and the needle does not move: No
carrier: This is with Carrier VR5 adjusted to full scale 10 (or less).
The transmit red light comes on, but no power out. Also have external
power meter to confirm no power out. Also when listening with monitor no
sound from microphone comes out the monitor. [Compared with USB, LSB,
FM or FSK where monitor sound works OK].
-ALC meter reads mid scale,
-SWR Meter works
All other functions appear OK: 
Puts RF out on SSB, CW, FM and FSK. 
Could anyone suggest how I could fault find IN AM mode? Is there some
where in AM transmit mode I could take voltage readings to indicate what
you should be happening in a normal operating transmitter?
WHAT I HAVE DONE TO FAULT FIND SO FAR?
Took out individually and swapped spare IF board and spare Control
Boards Both boards believed to be working OK and it makes no difference.
Put originals back. This probably eliminates those major areas.
WHAT I HAVE DONE BEFORE THIS:
I have been undertaking a major service and fault repair:
The following have been carried out:
IF Board:
- removed and replaced many times to fault find in AGC,
- replaced R 157, R153, D50, R151, D49, C128, C127 which were not
faulty,
- swapped R149 and R150 per Kenwood agreement that components being
mistakenly in wrong place. Made huge difference to AGC performance,
-reset IF VR3 to get s meter to S0
- increased VR 13 monitor volume,
-Undertook Amateur Service Bulletin 913 replacing a resistor,
- Undertook Amateur Service Bulletin 910: C75 changed to 0.047Uf 50 v
ceramic, 
-Fitted YG-455-C1 filter
- turned Q4 around to put circuit back as Kenwood intended.
- inspected back of board resoldered a few suspicious joints,
-replaced C31 with 10uF 63V per Kenwood circuit value. Someone had put a
strange value of 3.3uF 25V. [Cannot understand why someone would put a
lower value capacitor there.] - Reset IF board tx control [ALC-6-1 1]
voltage to 3.20.
RF Board:
- Undertook Amateur Service Bulletin 910: JP& C75 changed to 1uF
electrolytic and 1s1555 and 15ohm resistor,
- inspected back of board resoldered a few suspicious joints,
- Undertook Amateur Service Bulletin 917: fitted R154, R155 C193, C194 
Digital A Board
-Removed for inspection, and checked all board pin connectors with a
meter back to next circuit component. 
On back of board:
-Remelted solder on all connection pins:
-Soldered extensive bad joints probably 80% of joints.
On front of board: 
-soldered almost pins on IC5 and IC6
_ Spent about 5 hours visually examining entire board via 4X magnifier
and bright light for solder bridges, missing foils and any other faults.
- While working tapped mechanically IC2 to ensure that I did not have
any intermittent mechanical connections problems. None found. 
PLL board:
-Removed and inspected for faults for 3 hours:
Undertook Amateur Service Bulletin 917, which required replacing
resistors, capacitors and shifting capacitors. 
-Soldered a couple of suspicious joints, rest looked OK
-Did Amateur Service Bulletin 900, which is resetting L22, 23 and 24,
using an RF probe at resistor 96 (test point 5), to get a maximum
voltage. Should be only be approx 250 mV. Mine measured 402 mV. Was 220
mV when I started. 
-Took out all wax from two VCOs near L7 and L3. Originally put a
voltmeter between some of the wax and ground and -0.01 volts for VFO
with L7 and near L3 shows -0.01 to - 0.02 volts. 
-Cleaned back of the board where support black foam was, and removed
foam?
-Undertook Amateur Service Bulletin 917: fitted R120, R121,, R124, C184,
C185 and shifted C180, C181 with new components.
VOLTAGE SETTINGS:
- Reset RF board voltage [AGC 4-2] from about 3.8 down to the correct
3.20
- Reset IF board voltage [RV-10-1] from 2.3 down to 2.10.
- Rest the VCO 1 [RF board VC1-9-4] voltage from 13.80 down to 12.00
- Rest the VCO 2 [RF board VC1-9-4] voltage from 13.80 down to 12.00
- Rest the VCO 3 [RF board VC1-9-4] voltage from 13.31 down to 12.00
- Rest the notch by adjusting VR2 and L16 on the IF board,
- Zeroed the S meter by adjusting VR3 on the IF board.
Any suggestions would be most welcomed 
Yours sincerely
Jeff King ZL4AI