|
|
Kenwood TS-570D
FSK input failure due to common supply grounds.
A cautionary tale 8-)
They do say the do's and don'ts in the
instruction manual are there for a reason. I definitely found that one proved
recently.
The Kenwood TS-570D instructions recommend you should not use a common
power supply ground for the TNC and transceiver when using the RTTY FSK input on
the ACC2 connector. This I can assure you also goes for any other type of FSK
interface too, and failure to heed this warning can actually damage your
transceiver !
During some development work I connected the FSK RTTY Key input to an interface
that had a common 12V supply ground with the transceiver. The interface used a
simple 2N2222A transistor (in open-collector configuration) connected between
the RTTY Key input and 12V supply ground. The TS-570D's internal 5V pull-up
circuitry would allow the open-collector transistor to switch this logic line
between 5V (transistor off) and 0V (transistor on). The interface was powered
from the same 12V supply as the transceiver was using. Sounds simple, what could
possibly go wrong ?............
Lots ! This must have set up an
accidental RF ground current loop or caused a differential ground voltage that eventually damaged the transceiver !
All seemed to work OK at first, until I increased the TX power, then all hell
broke loose. The first symptom noticed was the transceiver transmitting a few
hundred hertz off frequency in FSK mode with an incorrect FSK shift frequency
too. It was also found that in CW and SSB modes the TX frequency was incorrect
and could be now be shifted by changing voltage level at the FSK input. The RTTY
FSK input should be disabled by the transceivers logic in these modes !
Obviously some serious damage had been caused to the TS-570D's circuitry. If any
transceivers TX frequency is uncertain you cannot put on the air until the fault
is corrected.
The TS-570D was placed on the bench for repair with a dummy load on the antenna
connector (as I would be selecting TX mode during the tests). The RTTY FSK input
(RTK on the circuit diagram) goes from the ACC2 connector via analogue switch IC508 and
diode D509 to the SLAB input (pin 1) of one of the DDS synthesiser modules
(IC501). This input can also be clamped by the output from IC510, an "AND" gate
IC, via diode D508. Its the analogue switch that should disable the FSK input in
CW / SSB modes, the clamp circuit (D508, IC510) operates in all receive modes.
This analogue switch is a tiny BU4S66 SOT23 surface mount device near one edge
of the TX-RX / PLL PCB. It's really a single element of what is normally inside
the well known CMOS 4066 quad analogue switch IC. A check with the oscilloscope
showed that this was where the problem was ! The control input to the analogue
switch was changing when switching between FSK and other modes, but the analogue
switch was stuck "on". Worse still, it was leaky to the 5V supply rail causing a
heavy current of around 30mA to flow when the FSK input was grounded. Cold
checks on diodes D509 and D508 showed them to be OK, and a scope check of IC510
showed that to be OK too. I breathed a sigh of relief as this meant the damage
hadn't reached as far as the DDS synthesiser itself ! This would have been very
bad news !
A new BU4S66 device was purchased and carefully fitted using surface mount rework tools, and the transceiver was tested again. All was now 100% OK.
The Kenwood TS-570D is a fairly nice transceiver to work on, but as most of it's components are surface mount types you definitely need to be skilled in reworking them and have the appropriate tools in the workshop. You also need good oscilloscope and meter probes with fine tips (and steady hand) when taking measurements and tracing faults !
The interface I was experimenting with was "fixed" from causing problems again by using an opto-isolator in it's FSK RTTY output to isolate the ground and prevent any ground loop currents from flowing. The problem has never re-occurred since.
The moral of this tale......................OBEY THE DO's AND DON'Ts IN THE MANUAL !!! Don't use common power supplies for TNC's and interfaces or use an opto-isolator on all digital inputs.
Unless of course you enjoy doing surface mount component rework 8-)
