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FT 991 A deaf in IPO mode, receives normally in AMP modes in HF bands

Rodrigo Freire edited this page Jul 6, 2022 · 17 revisions


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

Attenuator Circuit Truth Table


The original R1005 resistor is an out-of-production Kamaya RMC1/10-151JTP, 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.

Buy here: 1712-CPF-A-0805B150RECT-ND

But... I can't solder SMD!

And how do you expect to remove the failed resistor? SMD component removal is at least 3 dB more complex than soldering a new one.

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 Pine64 Pinecil or a TS100 soldering iron.

If if you never did SMD soldering it's all about the soldering iron - It will surely enable you to perform this repair in less than 5 minutes - without burning or melting everything else around. And never leave your 991A nearby any transmitting HF station again.

Fallacies, Radio Butchering & Absurds

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.

I fixed with a standard resistor just like the Youtube video and it worked!

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 - this is not your FT-7B. Show some respect.

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.

"Oh but the resistor hack works, and the radio receives again"

Your radio does not deserve that. Show some respect to your money. Or get back to your Cobra 148 GTL DX. 10-4?

"What about DXpeditions, with several stations working nearby"

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.

"Hey we work in a Club station and several stations are nearby and active at the same time"

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 have a radio and it never had an issue"

I would invite you to check your radio sensitivity - and then compare with 991's sensitivity.

"zomg what an Yaesu design failure, how come"

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.

In other words: A protection device. Would you bother so much if was it a blown fuse? It was a blown resistor - which happens to be part of the attenuation circuitry too. Double win.

Or do you replace your fuses with other ones with higher amp rating at your home to avoid tripping? Does that sound smart to you?

Where is the FT-991 / FT-991A Protection?

Let's start with the antenna side - let's talk RX.

FT-991 and FT-991A Antenna Circuitry

  • The signal comes from the Antenna Connector (blue arrow) - and the signal is immediately greeted by a 300V Gas Discharge Tube (GDT) surge arrester (D4027), to divert any stronger static or voltage that might be present at this stage. Notice the high voltage, it has to withstand the TX voltages (since... antenna connector), so this is why such a high rating. One very important note: GDTs have a limited lifespan and they are not entirely happy to enter in operation - this component is rated 300 discharges in conservative scenario, and only 5 discharges in more aggressive discharges. When exhausting its life time, it will just quit diverting static to the ground.

  • Then, the signal moves to the TX/RX relay. This relay defaults to the RX stage (black triangles), following the green line. Right after the relay, a back-up 500V GDT (D1046) clamps more aggressive voltage discharges. Then, the signal enters a coaxial connector and moves from the HF/PA Unit to the Main Unit.

FT-991 and FT-991A Main Unit Protection Circuit

Signal comes from HF/PA Unit and enters the Main Unit (dark green line). The first component that the signal will see is a different kind of Surge Arrester - a 1608 SGX Solid-state 35V ESD Supressor Device (D1002). Differently from the hundreds of volts GDTs that you saw next to the antenna, this one is solid state because of the low clamping voltage (35V). Again, this component is also not entirely happy to enter operation, withstanding typically around 500 discharges during its service life span.

A side note - take into account that static charge builds up quickly during approaching thunderstorms - check this static charging from a approaching thunderstorm video - it gets over hundred of volts in matter of seconds. How fast to reach mere 35V and causing (D1002) to bleed the static (and decrease your arrester life time)? So, disconnect your antenna during adverse static scenarios.

Then- the signal gets in a tee. Two important things happens at this tee:

  1. The signal goes to the Attenuator relay (red line) which will either:

    a) Bypass the attenuator entirely (orange line);

    b) Add a 68 ohm ground load (R1012) to the circuit, attenuating the signal in tandem with the 150 ohm resistor (R1005) (light green)

  2. The signal is also monitored by (D1004) (pink line) - where:

    a) If the attenuator is off (AMP1 or AMP2 modes without ATT engaged)

    b) If the sensed voltage exceeds 2V (a very strong voltage, nonetheless)

    c) THEN it will trigger an emergency mode activation of the attenuator, by biasing the (Q1005) transistor (yellow line) and flipping the (RL1001) to ATT mode, regardless the absence of the ATT signal.

If the radio is powered off, the Attenuator circuit is commuted by default - the relay black triangle depicts the relay default state. More than just attenuating - it is also protecting the radio front-end.

"But... What about the surge protector rated 35V, where we are talking about 2V over the resistor?"

That's a surge protector rated 35V. 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 - which will (happily) cause to blow your attenuator resistor.

For the Icom fans...

Your IC7300 and IC7100 has no such protections. No GDT, no resistor, and no ESD discharge at all.

Final word

I bought the latest FT-991A Technical Supplement (Revision G), and I can see that there are three Main Unit revisions: The original (lot 1-3) and two subsequent revisions (lot 4-13 and 14-current).

There were small circuitry changes somewhere else in the board, but none changing the aforementioned resistor. If the resistor rating was a design failure/error, it surely would have been changed, but it was not- So it is there at the actual rating for a reason.

Hope that helps; 73 de PY2RAF.

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