This FT817ND was regularly used to drive a 4 Meter transverter. Its owner reported that for no obvious reason it was now ‘dead’, refusing to power up from the internal battery, the charger or an external PSU.
This was indeed the case, the rig showing no signs of life. The inline fuse in the positive supply cable was intact. Close inspection internally, revealed a burned PCB track near to the power input jack. More worrying was the finding of a direct short to ground from the positive power rail- measured from the cathode of D1085.
My initial fear, given these rigs’ reputation for ‘eating’ PA transistors, was a short in the RF power amplifier. However this proved, thankfully, not to be the case. The PA board was exposed and the wire link supplying it with DC was unsoldered. The short to ground persisted.
Have a look at the circuit diagram fragment below taken from the Service Manual:
The burned PCB track is indicated. Note that it is in the negative ‘ground’ side of the supply. A little digging on the web unearthed several reports of exactly the same area of PCB track burning out, but no good explanation as to why it had happened. Also, I found no mention of a direct short to ground from the positive supply line.
Diodes D1085, D1084, and D1083, were all electrically intact. In order to determine where the short circuit might be, the common mode choke T1035 was unsoldered and removed from the PCB; see PCB removal below. Thankfully, after this, the short circuit was no longer present and continuity checks confirmed that the two windings on T1035 were shorted. T1035 is specified as a Tokin SH-322. I was unable to find a supplier of this component so I decided to rewind it. The original windings were unsoldered from the pins and the windings removed from the toroidal core. Sure enough, the two enamelled copper wires were fused together and electrically continuous.
The choke was rewound using two lengths of 28 SWG (0.38mm) enamelled copper wire, side by side (bifilar) with six turns passing through the toroid center as per the original. The ends were cut and soldered to the pins (taking care that the windings would connect positive to positive, negative to negative). The choke was then refitted to the PCB.
The burned PCB track was carefully cleaned with a soft glassfibre pencil and was then bridged with solder, reinforced with a short piece of 26 SWG tinned copper wire which was also tacked to the ends of the surface mount capacitors C1455 and C1456.
The radio would now power on and function when connected by the power jack but not when using the internal rechargeable battery, and neither would the battery charge. This was explained by the finding of an open-circuit fuse F1001, a 3.15A rated surface-mount 0805 device. This was replaced with a Vishay MFU0805FF03150P100 a 3.15 amp very quick-acting SM fuse. The tranceiver was now fully functional – Total cost (excluding time) less than £1!
- Remove the top cover- 7 crosshead screws.
- Disconnect the loudspeaker.
- Disconnect the smaller flat ribbon cable at the rear of the board by gently pulling upwards.
- Disconnect the larger flat ribbon cable at the front of the board by gently lifting the plastic locking mechanism with a fingernail at each end.
- Disconnect the two grey coaxial cables by gently pulling and twisting (the metal plugs, not the cables!)
- Remove 5 self tapping screws- one in each corner and one mid-board.
- Ease the board upwards off the 3 pin connector below.
Replacement is the reverse procedure.
How did it happen?
Clearly too much current was passed through the PCB track which burned. Protection diodes D1084 and D1085 would appear to protect against simple reverse polarity of an external power supply and even if not, surely the in-line fuse (in the positive supply cable) would have blown before the PCB track burned.
One very unlikely possibility is that the primary fault was the common mode choke becoming short circuit. This would explain the fuse in the battery supply (F1001) blowing , but doesn’t seem to explain the burned PCB track.
The only plausible explanation would appear to be:
- The chassis was connected to an external power supply negative via external equipment such an RF connection to a transverter.
- The power cable jack was inserted into the power socket on the radio and the negative lead (without an in-line fuse) was mistakenly connected to the PSU positive terminal.
- Current can now flow via the negative lead, unhindered by a fuse, through the track which ultimately burned and through one winding of the choke to chassis and back to PSU negative. A high current would flow causing the weakest link to fuse (the PCB track) and in this case causing the common mode choke windings to short.
- Once the choke is shorted to ground there is a short circuit across the internal battery, which causes fuse F1001 to blow.
This would appear to suggest that there should be an in-line fuse in both the positive and negative power supply leads and the owner of this radio has been advised to fit one before putting it back into service.