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 ground solder contact points at the bottom of the tuner unit are bathed with varnish/lacquer.
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 radio chassis, are all covered by some kind of varnish/lacquer!
Continuity test 1 - No continuity
Continuity test 2 - Trying other contacts. No continuity too.
Estimated time: 30-45 minutes.
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. Disregard the RTL-SDR in the picture, heh.
Tuner Unit screws
- Carefully remove the control cable (orange arrow). Remember the lid holding it (violet star, upper left side)
- Carefully remove the two coax cables (blue arrows)
- Remove the screws that attaches the tuner unit (red arrows)
Tuner unit removal. Image credits: Waruna Illukpitiya, 4S6WLX
- Extra Carefully remove the tuner unit by tilting it - there are thermal pads between the board and the chassis - and they stick. Move the board to unstuck it and remove from the radio.
- Notice the position of the three thermal pads - they are exactly under the three relay banks.
After removing, check the board's 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 varnish/lacquer in any of them, reassemble your equipment. Probably someone else fixed it. Otherwise... Keep reading.
This lacquer is hard to remove - I did not have isopropyl alcohol, don't know if is it effective to dilute it - So I removed it using a sharp and delicate knife, and gently removing the lacquer from the contacts, on all four contact points.
After you got good connectivity on all four screw contacts, clean the area with alcohol, remove any residue and reposition the thermal pads under the relay bank.
Continuity test - test 1
Continuity test - test 2
Take the opportunity to clear with alcohol the chassis' contact points 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.
Ivo LZ7ST also found that his transceiver covers were coated with a fine layer of black paint (the finishing of the radio) near the grounding points, that also contributed to the issue. Check his post in FT-991A Groups.io mailing list.
- But what about the screw? It has a copper ground pad in the board upper side that makes contact with the screw and it is attached to the chassis - that should be enough to conduct the energy right?
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 a thinking exercise: Let a RF signal that comes from the bottom side of the board and needs to go to the ground: It comes from the component lead, enters the board track - and the track does not make any direct contact to the chassis - so the signal have to emerge to the upper side of the board through a (very small) via, enter the upper side board track, flow to the screw - which finally gets contact with the chassis and is drained. While with a clear ground contact, the aforementioned signal would just flow to the nearest ground pad and make direct contact with the aluminum chassis.
Far from optimal.
To my absolute surprise and astonishment, the brand new FT-DX101D / FT-DX101MP are also affected by this same issue.
Yvo Muniz from Alpha Telecom was repairing an FT-DX101D that had an PCB holed out due to electrical discharge. In 101D, it happens that the RF connectors goes out in the Tuner Unit board. So in order to repair the board he went to examine against the existence of varnish/lacquer in ground pads.
Well - it was true. See the pictures for yourself.
FT-DX101D Tuner Unit - Poor grounding in Tuner Unit - Ground pads covered by varnish/lacquer.
Image: Yvo Muniz, Alpha Telecom
FT-DX101D Tuner Unit - Poor grounding in Tuner Unit - Removing varnish/lacquer from grounding pads.
Image: Yvo Muniz, Alpha Telecom
So I would strongly recommend checking your brand new equipment, as well your 991/991A too - seems to be a standard manufacturing practice.
Hope that helps; 73 de PY2RAF.
