Poor Grounding in FT 991A Tuner Unit
After using your Internal Tuner to tune in 7 MHz and lower bands (also happening in 20 and 15m to as per report), the radio may tune it - but when you transmit, you are presented the HI-SWR warning - with VSWR tending to infinite.
This happens because the FT-991/A (and potentially the non-A models) has a grounding issue in the Tuner Unit.
I have seen floating over the internet a few articles mentioning this issue - this is because the solder contact points at the bottom of the tuner unit are bathed with varnish/lacquer/solder mask.
Initially I was skeptical of it - this looked like something too stupid and hey, mine is a 2018 FT-991A! - until I the day I needed to do a test in 7.290, a tad higher in 40m band and with higher VSWR and used the radio tuner. As soon I hit the PTT the HI SWR started flashing - and I went to investigate.
To my shock, it was indeed the case - at the bottom side of the board, the "soldered" contacts that are supposed to provide a better contact with the chassis, are all covered by solder mask!
Continuity test 1 - No continuity
Continuity test 2 - Trying other contacts. No continuity too.
Removing the Tuner Unit is pretty straightforward.
- Remove the side strap
- Remove the bottom cover
- The Tuner Unit is covered by a metal cover. Remove the four screws holding the metal cover and open it.
- Carefully remove the control cable (yellow arrow). Remember the lid holding it (violet star)
- Carefully remove the two coax cables (blue arrows)
- Remove the screws that attaches the tuner unit (red arrows)
- Extra Carefully remove the tuner unit by tilting it - there are thermal pads that sticks. Exercise care.
- Remember the position of the three thermal pads - they are exactly under the three relay banks.
Tuner unit removal. Image credits: Waruna Illukpitiya, 4S6WLX
After removing, check the flip side:
FT-991A Tuner Unit - Bottom side. Arrow points the ground connections to the chassis
Do the continuity tests on all four ground connections. If yours has no solder mask in any of them, reassemble your equipment. Otherwise... Keep reading.
Solder mask is hard to remove - you won't be able to remove it using isopropyl alcohol - So I went to remove it using a sharp and delicate knife, and gently removing the lacquer from the contacts, on all four contact points.
Continuity test - test 1
Continuity test - test 2
After you got good connectivity on your contacts, clean the area with alcohol, remove any residue and reposition the thermal pads under the relay bank.
Take the opportunity to clear with alcohol the contact point in the chassis where you will screw the Tuner Unit.
Reassemble the tuner unit, don't forget to reconnect the Control Cable, the Lid and the two coaxial cables. Ensure to give a good (not excessive) torque on the screws.
Close the equipment and that's it.
- But what about the screw? It has a copper pad that makes contact with the screw!
Well - technically yes. The board is grounded to the chassis: However, it uses two different contacts: The board makes contact with the screw, and the screw with the chassis. And this is good enough for DC power. But remember, we are talking about Radio Frequency grounding. Keep reading.
Now another think exercise: The RF signal that comes from the down side of the board and needs to go to the ground: It comes from the lead, goes to the track - and the track does not make any direct contact to the chassis - so it have to surface to the other side of the board through a (small) via, get to the other side, enter the track, flow to the screw - which finally gets contact with the chassis.
Far from optimal.
Hope that helps; 73 de PY2RAF.