Keeping An Old Marti Up and Running
[August 2023 – Not every venue has access to the Internet … and some have only spotty cell coverage. Especially for smaller stations, the old Marti is still the “Go To” for remote broadcasts. Phil has some tips on keeping them going – especially if other options are not working.
Until the 1990’s most stations relied on one or more Broadcast Auxiliary systems for live broadcasts from advertisers’ locations or sporting events and to get program audio to the transmitter.
These have largely been replaced by digital devices, which are smaller, less maintenance intensive, and generally deliver better audio as long as their supporting network can be accessed.
That means the older RPU gear, usually manufactured by Marti Electronics, has ended up in a station closet or out at the transmitter, along with the turntables, cart machines, and other unused gear.
On the other hand, if your station still has its Broadcast Auxiliary licenses, the old gear, and associated cables and antennas, they could be valuable in an emergency.
DO IT ON THE BENCH
Several firms can repair and align non-operable equipment, but if that old Marti was working when it was pulled out of service, getting it back in shape may not be that difficult.
What follows is a general description of what is needed for Marti transmitters and receivers made before 2000. These used circuitry designs pre-dating modern synthesizer-based systems. They may look unfamiliar to younger techs but, in many ways, are more easily repaired.
Detailed procedures are in the manuals. While these are still available online, having the original manual will include the factory test data, which will make things a lot easier.
SETTING UP TO CHECK OUT THE SYSTEM
Here is what you need to get started. Most stations have much of this gear – although it could in the “storage” area where you found the Marti’s!
- A good frequency counter
- A spectrum analyzer would certainly be recommended.
- An RF signal generator with FM capability will allow you to align a receiver, but if its companion transmitter is working into a load nearby, this can be used as a signal source.
- A distortion analyzer will be needed if you want to tweak the discriminator circuits used by the receivers to demodulate the audio.
- A 13.5-Volt DC bench supply used to power the transmitters will allow you to adjust the final PA for the best efficiency (least current).
Many adjustments inside use trimmer caps or adjustable inductors, so metal tools should be avoided in favor of plastic alignment tools. Most of the variable inductors used in Marti equipment were manufactured by CoilCraft, which also makes an accompanying tuning tool.
The CoilCraft 37-1409 “Tri-tuner” has a stepped hex driver on one end and a screwdriver on the other. These are available from CoilCraft as well as many electronics distributors for around $5.00.
Before applying power, it Is a good idea to remove the cover and do a quick visual check.
Ensure the crystals are still in their sockets, all connectors are seated, and the hardware is tightened. The connectors are nylon Molex connectors, and often a bad crimp or excessive current will show up as a brown discoloration on the shell. Another thing to check: receivers and lower power RPU and STL transmitters do not have a cooling fan, making them a quiet and inviting home for mice who can render it irreparable in a short time.
If the unit passes a visual inspection, apply power and check the supply. Marti RPU and STL systems manufactured before 2000 had basic linear power supplies operating at 13.5 VDC. This means that they can also be powered by a vehicle or an automotive battery.
Once powered up, allow at least a half hour for the crystal oven to warm up.
The crystal is located on the modulator board next to the Frequency Control Module. This module is a small, heated block adjacent to the crystal sockets. Inside the block is a thermistor connected to a transistor mounted on the outside of the module used to regulate the current through a resistor that heats the block. These modules were used in transmitters and receivers, with those in the transmitters housing the varactor diode used as a modulator.
The crystals were originally attached to the module with what looked like a rubber band but was a thin section of heat shrink tubing. These usually split as they aged and should be replaced if broken or missing. ICM, the original manufacturer of the crystals, is no longer in business, but if replacement crystals are needed, they are often available from Broadcast Electronics and other vendors.
The modulator board has variable inductors to adjust the frequency and pots to adjust modulation and the bias voltage on the varactor diode. The bias voltage will typically be between 4.2 and 4.7 VDC. This should be included with the factory test data if available. If the test data is unavailable, adjust the bias for the least harmonic distortion in the demodulated audio.
Note: changing the bias also changes the frequency requiring adjusting the fine-tuning variable inductor. The process may have to be repeated to find the combination of bias and inductance for the least THD at the correct frequency.
FROM CRYSTAL TO CARRIER
Since most of the Marti line was designed before the advent of synthesizers and phase-locked loops, they use a series of multiplier circuits to get from the crystal frequency up to the desired carrier.
Each stage of the multiplier is an RF amplifier that outputs a harmonic of its input. The transmitter multiplier board looks similar to an old-style receiver IF strip with pairs of variable inductors used to tune each stage for a peak negative voltage on that circuit’s test point. Input and output connections are RCA jacks, and the output can be connected to a 50-Ohm load through a Wattmeter and a means to get an RF sample to a spectrum analyzer if available.
Once the Voltage is peaked for each stage, the output of the multiplier should be 0.50 to 1.75 Watts on the assigned carrier frequency. It should look reasonably clean on the spectrum analyzer with out-of-band emissions better than 40 dB below carrier.
From the multiplier board, the next step is the power amplifier which usually has two stages for a 10—15 Watts output. Trimmer caps on the PA board are used to tune the amplifier for the cleanest signal with the best efficiency. This is best done with a bench supply rather than the transmitter’s internal supply to easily measure the current.
If the supply has adjustable current limiting, this can be used to prevent accidentally pushing the output device beyond its safe operating current, which is usually around 3.5 Amps at 13.5 VDC.
ADJUSTING THE MODULATION AND CHECKING THE AUDIO
Frequency deviation (modulation) is adjusted using the pots on the modulator board after the bias Voltage and operating frequency are set.
Using a known good receiver or analyzer connected to a distortion analyzer, THD at 100% modulation should be 0.5% or better. Changing the bias on the modulator board can lower the distortion but keep in mind it will also change the frequency, so it will need to be readjusted as well.
Set the deviation for RPUs by feeding a 400 Hz tone into the high-level AUX input and adjusting the gain so that the front panel meter shows 3 dB of compression. For the STL transmitters, set the deviation at 100% with a +10 dBm, 400 Hz tone driving the audio input.
Something to be aware of is that some of the assigned channel bandwidths were changed by the FCC around 2000. This was to match the bandwidths used for land mobile channels adjacent to or shared with some of those used for broadcast auxiliary. Check the current FCC Rules to ensure you use the correct deviation for your channel.
Pre-2000 Marti RPU receivers (CR-10 & AR-10) were single conversion types that used a 10.7 MHz IF.
They were crystal controlled and could switch between two channels. Like the transmitters, the RF connections between the stages inside used RCA connectors so that each circuit could be isolated and checked on its own. STL Receivers and export or specially ordered receivers operating above 460 MHz had an additional 74 MHz IF stage.
Like the transmitters, receivers used a 13.5 VDC power supply and could be powered directly from a vehicle’s electrical system.
Checking the receiver alignment should start at the end of the RF path at the detector board.
Pre-2000 receivers, like their transmitter counterparts, used designs from before the advent of frequency synthesis and phase-locked loops, meaning that demodulation utilized a discriminator circuit with a 10.7 MHz input.
Using an RF generator connected to the detector input, set it for 10.7 MHz at 100 µV (-67 dBm). Adjust the input tuning coils for a peak on the front panel meter when switched to indicate the received signal. If the meter calibration is within specs, it will be 100% (0 VU).
The discriminator, which is a comparatively large component, has two adjustable ferrite slugs that can be adjusted for the lowest harmonic distortion in the demodulated audio.
Depending on the bandwidth required, the IF filter may or may not be adjustable.
If it can be tuned, connect the RF generator to the IF input with the same settings used for the detector board and tune for the peak received signal.
The first converter assembly mounted on the back panel of the receiver contains a gain stage and a local oscillator and mixer to down-convert the input signal to the IF downstream.
These had two crystal sockets adjacent to a frequency control module almost identical to those in the transmitters, using a strip of heat shrink tubing to hold the crystals against the heated module.
A chain of multiplier circuits similar to those used in the transmitters up-converted the crystal frequency to drive the local oscillator input of the mixer.
Adjusting the multiplier stages uses the same procedure as the transmitter. Adjustable coils are used at the first stage to obtain a peak negative voltage at test point one. Successive stages use variable caps to peak their test point to a peak negative Voltage. At some point, an indication of the received signal will be visible on the front panel meter.
An RCA jack on the circuit board is provided to check the frequency. For VHF receivers, the signal at this jack will be the desired channel frequency minus 10.7 MHz. For UHF receivers, this will be half of the desired channel frequency minus 10.7, and for 950 MHz STL receivers, it will be one-quarter of the channel frequency minus 74.
Once the multiplier stages are peaked there should be between 80 and 100% indicated on the front panel meter when switched to the Mixer position.
Now the input signal can be peaked using the variable caps on the input pre-selector. These are under the small brass screw-on covers on the top left side of the converter assembly. When properly aligned, the receiver will indicate a 100% received signal with an input of 100 micro-Volts with the 10 dB attenuator turned off. With the 10 dB attenuator on it should indicate 100% with an RF input of 316 micro-Volts.
LAST CHECKS READY FOR USE IN THE FIELD
Before testing a system on the air, check to see if the station’s broadcast auxiliary license is still valid.
It would also be a good idea to check with your area’s SBE frequency coordinator, especially if it has been out of service for several years.
After all this, you may well have a known working system ready, and it will all be worth the extra effort in the event of a storm or some other emergency.
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A former contract engineer, before and after his time at CCA, Phil Langston worked at CCA Broadcast Electronics as an RF Customer Sup-port technician and then Harris Corporation as an Applications Engineer. He currently is at the Wisconsin Educational Communications Board or ECB.
You can contact Phil at: firstname.lastname@example.org
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