The following is a collection of my notes from integrating the Tait T800 series II for MMDVM (Specifically DMR). We acquired a very large cache (about 40 total units) from a retired state UHF linked radio system. The units consisted of:
–Tait T855 receivers
–Tait T857 exciters
–Tait 100watt PA’s (dual) for a total of 200watts per channel
–Tait T803 tone remote with 803 backplane
–Scott Zimmerman STM32 MMDVM Repeater Builder board (V3)
I really need to give credit to Roar Dehli LA4AMA and Tom Brown N4TAB for the success of our project. I spent nearly a year learning about how to modify the Tait equipment with countless hours of limited success. The end result, the modification documentation from Roar was exactly correct. What I had challenges with was ALIGNING the Tait to work with the MMDVM. In our case, the equipment was aligned to work in the 468MHz range and we moved it down to 440Mhz. The original system was a voice/analog mode only. This proved to be a challenge to pass low end audio that DMR uses. After retuning the VCO and front end filters, we hoped to get the units working. Unfortunately, the detailed alignment process was needed. This included getting the two point modulation correct for the exciter along with tuning the IF’s on the receiver to allow low end audio.
The following are a collection of notes that is everything you need to be successful in the MMDVM/Tait T800 process. This document is made to provide a strategic summary of the tuning process. You need to be familiar with oscilloscopes, spectrum analyzers, 12db SINAD and deviation testing. Our bench environment consisted of an Agilent 8935, Siglent SDG1032 ARB, Atten ADS1102 scope. Of course, we highly utilized the on board MMDVMCAL with Pistar.
The first order of business is to reprogram the radios to the desired frequency. Retune the VCO and ensure it is locked. If you have access to a disciplined GPSDO, check that the main oscillator is in tune at 12.8MHz on both TX and RX
Transmitter or exciter- You need to ensure the two point modulation and limiter are adjusted properly. This is completed by following the instruction EXACTLY in the service manual. A diagram is included on where to tap the backplane for the test points. Once the two point modulation is set, the radio will pass low end audio with no issues. No further adjustments are needed on the exciter/transmitter. Ensure flat audio is selected on internal jumper. CTCSS pin 8 is the entry point for unbalanced audio. We used the Bessel null method for deviation.
Receiver- It was anticipated this would be the easier of the two to configure. Removal of R223 and installing R349 (100ohm smd) is required. In our case, it took a significant amount of time to tune the IF’s to accept the low end audio. It is important to enter the exciter via the unbalanced audio pin on Drange1. We used the ARB to sweep the audio between 67hz and 3000hz. The trigger was sent via the ARB to the Oscope for a clear view on the sweep into the IF. The sweep output of the ARB was sent to the MOD input of the Agilent 8935 also. The Agilent 8935 was the SIG GEN that also supplied the sweep audio to the N antenna port. Audio was taken from the flat audio pin for the scope. I included a pic of what the IF sweep looked like. An additional pic is included of the scope with exact measurements that a successful DMR wave was present. In our case, all units so far have met the spec with 12db SINAD of -111db sensitivity along with having a 0.0 BER DMR wave. We start out with the receiver by getting it to spec via 12db SINAD. Last, we turned off the sweep and kept the signal genset on the receive freq. We would transmit a DMR TG9 TS1 ID1 via MMDVMCAL and get the wave to replicate the voltage and look as pictured in the scope pic. Every result was a near flawless DMR receive with units so far. We followed the instructions in the Tait service manual except for the lightly coupling the IF and adjust the beat notes. We did adjust MOST of the IF pots during the process. L390 being the main adjustment.
In our case, the learning curve was alignment. The modifications that Roar sent out some years back are exactly on target. Our challenge was the actual alignment. Once the alignment is complete, you will enjoy a hassle-free, bulletproof DMR repeater.
Hopefully the documents are helpful.