FT 991 A deaf in IPO mode, receives normally in AMP modes in HF bands
There are few articles floating over the internet reporting FT-991 or FT-991A gone deaf after exposed to nearby transmitting stations - With the catch that the radio receives again if radio is either in AMP1 or AMP2 modes. The radio only works if the Attenuator Circuit State is Off, as per the below table:
| IPO Mode | ATT Mode | Attenuator Circuit state |
|---|---|---|
| IPO | OFF | ACTIVE |
| IPO | ON | ACTIVE |
| AMP1 | OFF | OFF |
| AMP1 | ON | ACTIVE |
| AMP2 | OFF | OFF |
| AMP2 | ON | ACTIVE |
Attenuator Circuit Truth Table
The original R1005 resistor is a out-of-production Kamaya, RMC Series, 150 Ohm, 0805-size, Thick Film, Automotive grade, +- 5% tolerance, 1/10 W. This component is not stocked at Digi-Key or Mouser.
However - there's a direct replacement that meet or exceed the criteria.
Digi-Key sells a TE Connectivity, CPF-A Series, 150 Ohms, 0805-size, Thin Film, Automotive grade, +- 0.01% tolerance, 1/10 W.
The bold items exceeds the original part specifications.
Thin Film resistors are high precision parts and also sports lower parasitic inductance and capacitance - both highly desirable characteristics in RF circuits.
And how do you expect to remove the failed resistor?
You can actually fix it, with the correct part: It's all about the soldering iron! I promise!
If you are in a rush, buy these $10 USB soldering irons at Amazon. It has the needed thin tip to replace and solder the new tiny resistor. Hook it to your computer's USB port or power bank and do the rework. Pro-Tip. Disconnect the radio from everything, leave it floating. Some of these USB soldering irons have 5V at the tip - and if the radio is grounded, it can cause a short - and we won't want it.
If you want to do stuff in style, I would suggest to treat yourself an nice TS100 soldering iron.
If if you never did SMD soldering it's all about the soldering iron - either will surely enable you to perform this repair in less than 5 minutes. And never leave your 991A nearby any transmitting HF station again.
Some websites and Youtube videos suggests (to my horror) replacing the 0805-sized 100 mW resistor by a common / stock through-hole resistor to overcome this issue. MY EYES, OH MY EYES /o\ Yes, sure it will resolve - if you solder a jumper wire over the failed resistor the radio will receive again too!
But- Please. Don't. Do. That. To. Your. Radio.
Keep reading.
Some were quick to point to "What about DXpeditions, with several stations working nearby" - or still "Hey we work in a Club station and several stations are nearby and active at the same time" - or; "I have a radio and it never had an issue", or "zomg what an Yaesu design failure, how come".
Or yet: "That resistor is undersized, let's use a resistor with higher wattage".
Let's talk about each ill-informed (and non-educated) guesses.
Congratulations - You are definitely receiving. Did cross your mind that jumping a wire across the pads would also make the radio receive again?
HOWEVER by substituting the chip resistor with the common through hole resistor you have botched all the effort that the designer put in your radio to keep RX chain silent, matched, efficient. You paid almost a grand for this radio, are you going to crap its reception with this hack? Do. Not. The film in common resistors has a wound pattern which causes it to act like... An Inductor. Do you know what an inductor does in Radio Frequency? It eats the signal. Oh, that legs, that long legs - you are also adding a lot of parasitic capacitance, detuning what the designer worked his arse to keep under reins. Detune = Changing impedance. Impedance mismatch = loss of signal and performance. You are friggin deafening your radio.
Your radio does not deserve that. Show some respect to your money.
Any serious DXpedition works with - surprise - band pass filters, which allows to filter out the neighboring strong signal at other band and not overload (or.. burn) the nearby station at other band. With good BPFs, it is even possible to share the same antenna at the same time with two transmitters! Check DXEngineering or Vibroplex for some Bandpass Filters.
Wrong. Don't. Not right, not fine, not a good idea. Your receiver is tuned to receive signals that are almost a whisper, in the order of nanovolts. While the rigs have bandpass filters prior to the receiver front-end, the spurious from the nearby transmitter, given the proximity, can be strong enough within the BPF frequency and zap some front-end component, like diodes, mixers, amps, desensitizing your front-end via AGC actuation, etc.
No. Don't. Be well informed. Be smart. If using simultaneous nearby stations, protect your station with a BPF that matches the frequency that you are working.
I would invite you to check your radio sensitivity - and then compare with 991's sensitivity.
While this was designed by humans - and humans are prone to failures - Only FT-991 and FT-991A uses this 1/10 W resistor, while FT-891, FT-DX1200, FT-DX3000, FT-2000 uses the same 150 R resistor - though in ¼ W variants. Other than this discrepancy, this initial signal treatment (let's call it front porch) borrows the same design across these six different radios - And all, but the FT-DX3000 uses even the same Surge Arrestor, the 1608SGX. So, why did Yaesu change the F..ancy resistor?
I don't really know - but this front porch circuit is a tried and tested (read: mature) design, across several other Yaesu equipments - and we all know that Japaneses are extremely conservative - Only FT-991 and FT-991A features this specific component change. Why change? It surely has a reason. My hunch is that it is designed to do exactly what happened: have the resistor damaged open if conditions are adverse instead of more expensive or extensive damage ahead in the receiving chain.
Take a look at the below circuit:
See the red line? If the radio has the attenuator off and a strong signal shows up, that signal follows the red line, and if strong enough, will flip the relay just like if the ATT line was active. Think self-defense.
The calculated required RF voltage to trip the relay is around two Volts (think again about the nanovolts). And there are some 'jeniuses' talking about the radio withstanding 5.7 V...
That's a surge protector. Notice the word surge. Read: Static discharge. That one that builds up in windy and dry weather in your antenna; as well with approaching thunderstorms. Rapid discharge. And not continuous signal like a neighboring transmitting station.
Hope that helps;
- RF.