This is the Harris Radio Section of
The Broadcast Archive
Maintained by:
Barry Mishkind - The Eclectic Engineer
Tech Tips on the SX Series
by Tom Osenkowsky and others
(If you have additional tips that you would like to
share on the SX series, or any other transmitter, please send them to us.
The following is a troubleshooting and maintenance guide for the Harris SX
series of AM transmitters.
I'll start with a tip for proper Loading control adjustment. Especially in
higher frequency SX transmitters there may be two different locations on the
Loading control where minimum reflected power can be obtained. The correct
location for the Loading control is where you experience an increase in current
with lessening inductance or turning the Loading control to your right produces
an increase in current.
Here is a listing of some problems and cures based on my experiences:
Problem: Transmitter randomly turns itself on or off.
Cure: A noisy optoisolator is the cause. Replace appropriate chip.
Problem: High power interlock appears to be engaged when it is not.
Transmitter acts weird. Transmitter will not engage in high power mode. Low
& med power lights on, red Fault light on.
Cure: Replace U42 on S&M board. Note: This chip is *NOT* included in the
Harris spare parts kit.
Problem: Unable to clear overloads.
Cure: Check/replace U45 and U46 on S&M board. U46 is *NOT* included in the
Harris spare parts kit.
Problem: TX inoperative. No 5 volt supply.
Cure: Check 5V fuse on AC controller board. Fuse is in 6V switching supply.
Also, the 7815 and 7915 regulators have been known to fail.
Problem: No PA drive.
Cure: Check small .1 uf bypass caps for shorts on RF Oscillator board. Also
check Q11 and Q13 on IPA board. Ensure int/ext jumper on osc board is in correct
position.
Problem: No PA Drive.
Cure: from Ron Huckaby, of Butte, Montana. He worked on this obscure
problem, and it turned out to the the 8-fold coax from the IPA power divider to
the PA input. Cable tests 100% good, but will not drive the PA. New cable,
testing equally as good, solved the problem. The solution came from one of the
old SX-1 design engineers, I am told, who knew of this quirk.
Problem: Main AC contactor chatters, tx will not engage, erratic symptoms,
display readings. Symptoms may disappear when top Controller board is
disconnecting by removing top blue connector.
Cure: Replace the MSW-5101 U12 chip. Socket this chip. Intermittent thin PC
board solder traces and connections have also been known to cause erratic,
intermittent operation. Especially check the solder jumpers E-1 to E-4. Also
check battery for proper storage voltage. Ensure charging resistor has been
soldered to pad when tx was installed.
Problem: TX will not turn on.
Cure: Check auxiliary L4/T4 contacts on K1 and K2 AC contactors. Gnd switch
logic may be affected by oxidized contacts providing false information to
controller.
Problem: Out of band emissions. Erratic problems with RF Oscillator.
Cure: CR1 on RF OScillator board can oscillate around 3 kc. Newer SX tx's have a
5 watt CR1. Be sure drive pot on RF Osc high enough so that square wave becomes
less like a sine wave. Freq can double causing major drive problems if drive is
too low.
Problem: PDM amplifiers fail.
Cure: Ensure PDM amps have correct size fuses installed. SX-5's have a 4 amp AB
fuse, not a 2 amp AB. Sometimes engineers confuse the 2 amp AB used on the RF
Amp boards with the 4 amp AB used on the PDM amps. Ensure tight ground lug
connections on all the nearby 1500 uf caps. The U11 chip on the PDM amp can
sometimes oscillate or generate noise on the PDM signal. U11 is *NOT* included
in the Harris spare parts kit. The diodes on the PDM Pullup board can fail
causing PDM amp transistors to fail. Arcing underneath the PDM amp heatsink can
cause failure. PDM amp damper diodes can fail. Older diodes are no longer
available, Harris makes a retro kit to fit newer style diodes as replacements.
Diode CR1 is critical to module protection, check and replace if necessary.
Problem: Multimeter #26 reads appx 234, PA volts very low, PA amps high,
power output about 300 watts on SX-1.
Cure: CR13 and CR19 shorted on PA RF Output board.
Problem: Output flicks on/off with no faults indicated. Problem clears
after doing a #99 reset. Problem can also be cleared by inducing a fault.
Cure: Older SX tx's R211B and R221B on the RFI board should be 910 ohms, not 510
ohms. R212 and R222 should be 51 ohms, not 510 ohms. Older schematics may not
reflect these changes.
Problem: Display garbled, with the exception of #40-44.
Cure: A/D converter chip bad. Replace.
Problem: Pinned PA voltmeter. PA fuse finder reads appx 2.
Cure: Shorted diode(s) CR41 and/or CR21. This will also shows as bad MOSFETS
when ohmmeter tested. Two PDM amps read zero on multimeter. Two phase volts read
appx 260. Problem caused by burned up PA output network capacitors. When this
happens, numerous PA MOSFETS and fuses will have to be replaced.
Problem: Upon turn on, tx produces fault #2. PA volts & amps pulsate
in fault condition. Can reduce pwr on low or med position. Power varies rapidly
and does not correspond to reading #44.
Cure: PDM Generator board pot R28 is bad.
To be certain L1-C1 output network components are properly set, be sure PA
tuning somewhat coincides with peak in PA current.
RF PA Output Modules.
The back-to-back Zener diodes that clamp ringing on the small Rf transformers
usually OPEN in failure mode. This is contradictory to the usual Zener failure
mode of shorting. Never operates tx with bad PA MOSFETS removed from socket.
Leave shorted MOSFETS in socket. Diodes on PA modules may tend to fail. Check
diodes whenever MOSFETS fail. Also check for shorted transformers. Very early
production runs had turns twisted too tightly causing premature failures.
Be sure E-2 on PDM pull-up board is mechanically secure or else damper diode
may function intermittently or not at all. When troubleshooting system
controller boards, open a door to prevent tx turn on at power to prevent damage.
If any output network components are replaced, it is necessary to retune
using a Vector Impedance Meter.
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