I was handed an Alinco DR-135 + T3-135 to troubleshoot. The reported symptoms were that it “never repeated the owner's Foxtrak based mobile station”.
The T3-135 is an OT3 tracker/TNC built on a card to fit inside the Alinco DR-135 in the place provided for their own EJ-41U TNC. The rear DE9 port provides data access to the T3-135's A and B serial data channels.
First step was to confirm the reported symptoms.
The station did not repeat my own Foxtrak packets, and further examination showed that it did not decode them, but did decode other off air signals.
It was obvious during the confirmation tests that the transmitter audio was grossly overdriven, The owner had been advised it is factory set and not user adjustable.
The transmitter drive was adjusted using OTWINCFG and other parameters set for its intended uses as a fill-in digi. Configuration was straightforward and reliable (unlike the USB interface on the OT-USB).
Receiver sensitivity was checked ok.
A number of drive tests were done with various trackers using the T-135 as a UI digi. It would never decode the Foxtrak packets, but worked on all other sources available at the time.
The matter of Foxtrak / OT3 interoperatibility is discussed in more detail at:
- Interoperability of FoxTrak tracker and Argent Data's OT3 – Part 1; and
- Interoperability of FoxTrak tracker and Argent Data's OT3 – Part 2.
During testing to explore the interoperability issues reported above, various levels of deviation and pre-emphasis on and off were used to test the T3-135. Certainly the T3-135 transmitted without pre-emphasis which is a performance disadvantage in a network where most stations use pre/de-emphasis. It is likely that the receiver did not use de-emphasis to be consistent with the transmitter and provide optimal performance in a homogenous network.
The Alinco DR-135 + T3-135 appears to march to the Kenwood tune of not using pre/de-emphasis, a performance disadvantage in a network predominantly using pre/de-emphasis.
It is fair to say that the OT3s are not tolerant of the signal from the Foxtrak whereas as other modems do tolerate it. Depending on your position you might regard the Foxtrak or the OT3 as the problem.
I did have an OT3m on order to explore the issues further, but cancelled the order because of slow shipment.
As ham innovation dies out, ‘making' diminishes, and interest in APRS wanes, Kenwood grows in dominance of the network, and APRS digi operators seem to favour Kenwood's not using pre/de-emphasis (they are often Kenwood owners).
The local digi (VK2RHR-1) which used a T3-135 has just been replaced with a Microsat device. There is a stunning improvement in decodes by the digi which speaks volumes for the T3-135.