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Anyone else in the group using this combo?

I dragged my tuner and FIF-232 out of storage today and along with one of my '980s, set it up for FT8. That rig and tuner ensemble work MUCH better than other contemporaries of the period (and even a few later rigs such as Kenwood's 9x0"AT" equipped models) .

One fly in the ointment: With the tuner set to "auto", it appears that some bands won't auto-select under control of the radio. Putting it in "manual" mode, band switching works properly - so this indicates the CPU and switching logic local to the tuner is in order. I tried two different interface cables and verified connections for each.

This leaves the isolation diodes between the remote connector and the CPU. I'll post a reply with results of testing/replacement when I dive back into the unit in case someone encounters the same problem.

73 - Fred, N8YX

Did some digging into the tuner this morning. First order of business was to verify correct voltage on each of the four Band Data lines as the various bands were selected by the '980. This checked out.

Next, that data was verified on D03-D06 at the cathodes first (nearest the radio - consult the Instruction Manual schematic) then at the anodes (nearest the uPD7507 CPU). All check except for the "B" line. This line makes up part of the control nibble for 3.5, 7, 18 and 21MHz. If the right voltage isn't present, the tuner will merely go to the nearest band which matches the nibble value (e.g., 14MHz in binary is 0101 and 21MHz is 0111 - MSD first.) A check of diode D04 showed it to be bad; it was replaced but the problem persists.

All pull-up resistors and other components in the area check out, though if you examine the "B" data line of the CPU at the CPU itself with an ohmmeter, it reads 110KOhms - while the other (properly working) data pins read 101K or thereabouts.

This leads me to believe the CPU is partially bad. Note that if you select Manual mode, ALL data lines are pulled low at the CPU and band switching is handled via a different means - the tuner will still work properly in this fashion so in Manual the defective pin isn't an issue. Unfortunately, the device is a factory-programmed part. I'm half tempted to redesign the tuner to use a BASIC Stamp or similar embedded micro that has a lot of I/O pins, barring me being able to find another tuner or CPU Board.

73 - Fred, N8YX?

Your logic makes sense: if the blocking diode was defective-shorted (vs open), it could expose the MCU pin to raw IO voltage (vs emulating a low-side driver). If the FC-757 is like the 980, Yaesu didn't seem to provide a huge amount of protection to the IO. The FILT pins on the ACC1 plug go right into CMOS logic gate inputs for example.

I haven't reverse engineered this aspect of the radio (as I only wish I owned the 757 tuner), so I'm going to have to wish you luck!
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-Mat Breton, N8TW