This is the Harris Radio Section of
The Broadcast Archive
Barry Mishkind - The Eclectic Engineer
Tech Tips on the MW-5
by Tom Osenkowsky
(If you have additional tips that you would like to
share on the MW-5, or any other transmitter, please send them to us.
The MW-5 uses three PDM filter coils, L1, L2, and L3. Although these may
appear identical physically, they are NOT electrically identical. Here are the
specs for each of the three coils:
L1 (916-5109-004) 120 turns per pi section 55' of wire 16 ohms R
L2 (916-5109-002) 82 turns per pi section 50' of wire 12 ohms R
L3 (916-5109-003) 90 turns per pi section 53' of wire 17 ohms R
The R value indicates the measured DC resistance using a digital meter. To
test L1 and L2, I devised a simple test procedure. Using a 100 picofarad 1%
capacitor in series with L1 on a wooden test bench, connect an oscillator
(Potomac, etc.) across this series circuit. Place a scope with a x10 probe
across the circuit as well. Sweep the circuit until a voltage minimum occurs.
With L1 this should be at 100 kc. Substituting L2 should produce a minimum at
152 kc. Use a frequency counter to verify the frequencies.
Problem: TX power output low, HV will not stay on unless Plate On button is
held in. DISS and ARC OL lamps lit, cannot clear them.
Cure: OL Reed relays are contaminated. Engineer washed PC board with cleaner
which contaminated relays. Relays replaced, problems solved.
Problem: TX has to be pushed to achieve 125% modulation. While TX barely
makes THD specs, it does so with PDM freq set to over 100 kc (slug removed from
L1 on PDM osc board). Aux Current reads zero on FP meter.
Cure: Zener diode CR15 on 1A1A3 Audio Driver shorted. This does not allow Q2 to
operate causing the above conditions. Step start resistors are staying in
circuit causing low power. Retune transmitter per instruction manual after
Problem: Poor loop closure on AM stereo proof at 5 Kc and above. All
available bulk delay must be used to achieve acceptable closures.
Cure: Change the value of R64 (300 ohms) on 1A1A2 PDM Chassis. Values up to 470
ohms may work. Best solution is to install a small trimpot and adjust for
precise loop closure. R64, C43 and C44 are chosen for flattest freq. response.
These components affect high freq response.
Recommendations: If you do not have the "B" version of the audio
input board, invest in one. It is worth the cost for the features added. The
vacuum variable capacitor Grid Tune mod kit has not been available for some
years. For the older open air variable capacitor models, make absolutely certain
this capacitor is kept very clean. Dust in this high RF voltage area can cause
Solder across the Bypass toggle switch connections for the Modulation
Enhancer. At times these contacts have been known to become intermittent. The ME
can no longer be used if one wishes to ensure compliance with FCC 73.44
emissions rules ("NRSC").
If you have not installed the factory mod kits for HV shielding, make certain
to do so. It is worth the investment in time for the increased safety factor.
For the MW and SX series of Harris transmitters, the best RF load at the
transmitter antenna terminals is a load such that the sideband resistance is
higher at the upper sideband, lower ant the lower sideband and equal negative
reactances at each sideband. Such a load will properly rotate to a fairly
symmetrical load at the PA Final Amp plate. Each transmitter output network is
different and may be tuned differently so this is a general rule. DO NOT attempt
to place a symmetrical load at the antenna terminals. The results at the PA RF
Final Amp plate will not be at all symmetrical.
Problem: Poor PA efficiency and/or poor positive peak capability.
Cure: Ensure that the Third Harmonic Resonator is adjusted properly. The best
way to do this is to employ a signal generator set to the third harmonic and a
scope in the PA Plate circuit. Beware that there may appear to be more than one
combination that produces highest power output. There is only ONE proper
combination of plate and cathode third harmonic resonators that is correct.
Tuning on the incorrect combination can cause excessive harmonic/spurious
radiation and poor PA efficiency. The plate resonator coil may become scorched
There are a number of modifications made to the MW-5, MW-5A and MW-5B to
improve AM stereo characteristics. The list is too long for publication here.
MW-5's at the lower end of the AM band have proven to be more difficult to
optimize than higher band units. Harris did sell an AM stereo mod kit to improve
IPM and other performance, I'm not sure if these kits are still being marketed.
The MW-5 is very integrated in its operation. By this I mean that adjusting
any one control during an audio proof, etc. will necessitate an examination of
all other control adjustments. Every adjustment has an effect on performance.
This is especially true of AM stereo.