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By Robert W. Meister WA1MIK
The MSF5000 station provides several modular jacks for metering purposes. These jacks accept a standard 8P8C modular plug, the same type used on computer network cables. There are two metering panels: the Radio Metering Panel (RMP) TLN2418A that has a meter, meter select switch, speaker, amplifier, and two toggle switches, and the Diagnostic Metering Panel (DMP) TLN2419A that has all of the above plus an array of 64 LEDs to monitor the MUXBus and additional switches to allow individual MUXBus bits to be manually set. Of the two, the DMP is preferred and much better to have, but either one can be used for metering purposes.
Meters, Cables, and Jacks:
The meter in these test sets is an ordinary 50 microAmp (uA) DC meter. You could use any 50 uA meter with an appropriate selector switch and one end of a network cable. You could even make an adapter cable to use some of Motorola's other portable test sets. The metering panels also provide a 10V and 25V metering position to be used with straight meter probes, however I've never used these positions or the probes, which usually get lost; I'd rather use my trusty Fluke digital multi-meter.
The modular meter cable is wired straight through, with pins 1-8 at one end going to pins 8-1 at the other end. See the MSF5000 Photo Tour article for more details; click on the Metering Sets and Tuning Tools link. It doesn't matter which end of the cable goes where.
On the front of the RF Tray there is a metering jack for the receiver and another one for the transmitter. You plug the meter cable into the appropriate jack, set the meter for positive (+) polarity, and select the position you want to view.
There are metering jacks inside the Power amplifier (PA) as well. You need to flip the PA down and remove its rear cover with a T20 Torx wrench. When measuring the PA, set the meter for negative (-) polarity.
In the text that follows, all meter values are in microAmps (uA).
|1||This position displays the output of the quadrature detector, which demodulates the FM signal to extract the audio. The reading will vary as the received carrier frequency is changed. This is similar to the discriminator meter position on older equipment. On 900 MHz stations it moves 6-7 uA for a 2.5 kHz change; On all other stations it moves 6-7 uA for a 5 kHz change in frequency.||20-25 idle|
|2||This position displays the IF signal level, an indication of signal strength. The reading will increase as a signal is applied. The meter may move a lot for a slight increase in signal level but it will eventually limit itself to 40-45 maximum and the upper end will be compressed. When aligning a station try to keep this meter indication no more than 10 uA higher than the resting level for best results.||10-15 idle|
|3||This position displays the mixer output level, which is quite high. While the receiver may hear signals if this position shows zero, the receiver is not aligned properly or not working properly in this situation. Tuning the mixer coils is somewhat broad but if the receiver is being moved more than 5 MHz you may not get any indication at first, and you'll have to adjust each coil bit by bit to find the slightest meter increase. A received signal will not affect this meter indication.||25-35|
|4||This position displays the output level of the station's internal reference oscillator. On 800 or 900 MHz stations with high stability oscillator trays, this position may indicate zero, as the reference oscillator is not installed in the RF Tray. A received signal will not affect this meter indication.||25-40|
|5||This position displays the receiver's VCO control voltage. A higher voltage causes the VCO to generate a higher frequency. On UHF and higher stations, the alignment procedure tells you to adjust the VCO for a value of 38 uA +/- 2 uA on the highest receive frequency, but any value between approximately 12 and 42 uA is fine, so long as it stays within that range on the lowest frequency as well. The VCOs are not adjustable on VHF stations so you don't have to align them, but the indication should still be between 12 and 42 uA on all receive frequencies. A received signal will not affect this meter indication.||12-42 VHF,
|1||This position displays a value proportional to the forward output voltage (power) of the PA. If the station has a Driver PA and a Final PA, this displays the Final PA's output power. The indication will vary as the output power is adjusted.||15-40|
|2||This position is unused on stations with just one power amplifier, but on stations with a Driver PA and a Final PA, it displays a value proportional to the reflected output voltage (power) of the Final PA.||0-2|
|3||This position displays a control voltage proportional to the output power of the station. The indication will vary as the output power is adjusted.||15-25|
|4||This position is unused on stations with just one power amplifier, but on stations with a Driver PA and a Final PA, it displays a value proportional to the forward output voltage (power) of the Driver PA. The indication will vary as the output power is adjusted.||15-40|
|5||This position displays the transmitter's VCO control voltage. A higher voltage causes the VCO to generate a higher frequency. On UHF and higher stations, the alignment procedure tells you to adjust the VCO for a value of 38 uA +/- 2 uA on the highest receive frequency, but any value between approximately 12 and 42 uA is fine, so long as it stays within that range on the lowest frequency as well. The VCOs are not adjustable on VHF stations so you don't have to align them, but the indication should still be between 12 and 42 uA on all transmit frequencies.||12-42 VHF,
I won't go into the details at this time. The metering positions and number of metering jacks varies with the number of modules installed in the PA. Most of the metering positions measure the voltage drop across balancing resistors to indicate whether an individual PA module is working or not when compared to the others. The "Power Amplifier Deck section from The MSF5000 Instruction Manual for the analog station only" has extensive metering values for the UHF PA. Click here to download that manual as a 7.5 MB PDF file. Despite the manual's name, the PA is the same for analog and digital-capable stations.
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.
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This article first posted 26-May-2014
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