Sommerkamp FT-277 Instruction Manual
Franziska Alcini
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I recently picked up a FC757-AT tuner to compliment my FT757Gx. I got it for an absolute steal, and it turns out that it was because it was faulty ? (You have to love estate sales on eBay). This story outlines the journey that I went on to get the tuner working.
Not being one to let something being faulty worry me, as soon as it arrived, I dived right into it. I started by making a custom cable following instructions on Youtube. Essentially, there are 4 band inputs between the FT757 and the tuner in addition to one ground, one 13.8V supply and one Inhibit line to allow the tuner to tell the radio to reduce its output power.
The FC-757 is an automatic impedance matching device utilizing a
modified pi-L network to do the actual matching. At the input to the unit, there is a CM coupler that measures forward and reflected power continuously that the microprocessor in the unit uses to determine whether to make tuning changes.
In the tuner, the Tune and Load capacitors are microprocessor controlled, and the Inductance (L) is a switched inductance based on the band being tuned. There is also a stepped capacitance which is changed with bands (i.e. the input C and band L are changed together with band changes)
As mentioned above, there is a directional coupler on the transmitter side of the network and an RF pickup at the antenna side allow the microprocessor to identify any mismatch, and adjust the Load or Tune capacitances accordingly. According to the manual, whenever the SWR is measured at greater than 1.5:1, the tuner automatically tunes to correct the impedance mismatch.
The CM coupler outputs a DC voltage, on the FWD and REF outputs that is proportional to the power being driven through the unit. An output of approximately 9.6V is present when 100W of RF is passed from input to output with a 50R load.
The microprocessor implements a backup facility that automatically sets the Tune and Load capacitances at the settings that resulted in the lowest SWR reading as the band is changed. Subsequent tuning adjustments are automatically stored in the microprocessor for later use.
It turns out that the entire device uses a mask programmed microprocessor, a 4 bit cpu (7505) made by NEC. I downloaded the datasheet for the CPU, and discovered that it needed 5v (+/- 10%) to run. A quick measurement of the supply voltage showed that it was being fed by 7.5v. I located the 78L05 voltage regulator and immediately replaced it. This bode poorly for the micro, so I dropped a message onto Facebook to see if anybody else had had a similar issue. Sadly there was not much help.
An interesting design feature of the Yaesu FC 757 tuner is that the position of the Load and Tune capacitors is reported to the microprocessor via a set of potentiometers that are directly connected to the drive shaft for the caps. This connection uses a series of flexible couplers, and time has not treated these nicely. The ones on my unit were cracked. This cracking is caused by a combination of the vinyl shrinking over time, coupled by the stresses exerted by the grub screws that secure the couple to the shaft. Fortunately, the cracking allowed the capacitor drive to rotate without causing damage to the potentiometer. I will repair the shaft couplers on a rainy day when I am not working the bands.
A significant amount of worry was caused because I believed that the microprocessor, which is made from Unobtanium, was faulty. I should have pulled the CRO out much earlier to look at waveforms, which would have confirmed that the micro was operating, and it would have also reminded me that DC multimeters are only useful for measuring supply voltages when working with computers.
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