Tait T-800 II – modification for digital modes (D-Star, Fusion and DMR)
Hardware modifications on the T-835 RX (VHF) module only !
1) Open the module (take both side panels off)
2) Locate resistor R223, remove it. Do NOT add resistor R338 on the open pad (discriminator) !
3) Solder a 1.5k to 2.2k resistor between PCB hole P207 and PCB hole on the picture below: Leave the Op-amp stage intact (IC-390) to maintain a high enough audio level
This will bypass a lot of the audio circuit (Op-amps etc.) on RX that will lead to delay and phase shift/phase distortion in digital modes. Audio is outputted at 15-pin D-sub at rear panel of module on pin 6 (PL-100). This pin
goes to back-plane pin 24 (Unbalanced RX Audio Out, with no SQL) on back-plane 800-50-0001
The discriminator audio (IC350, NE614AN) on T-835 RX (VHF) is very low output, so the first Op-amp stage needs to be intact to have enough audio level. The first Op-amp is a TL 084 (IC390).
The balanced audio path on the module is still fully functional, so you can use this path for ordinary FM mode. In FM mode the SQL (RX Relay or RX Gate) circuit is used with CTCSS so the module will only open SQL
when a valid FM signal with correct CTCSS is detected.
In digital modes the SQL circuit is not used, so the digital path is unaffected by CTCSS etc.
The T-855 RX module (UHF) is different and requires a different modification !
1) Open the module (take both sidepanels off)
2) Locate resistor R223, remove it.
3) Install resistor R349 (100 Ω, SMD size 0805) on the open pad, see picture below:
This will bypass most of the audio circuit and allow discriminator audio to pass directly to Pin 6 on on the 15-pin D-sub (PL-100) on rear panel of the module. This will eliminate delay and audio phase shift/phase distortion etc. in digital modes.
The Audio Chip (MC3361CP, IC310) in the T-855 (UHF) is a lot more powerful than in the T-835 (VHF), so the first Op-amp stage is not necessary in the T-855 UHF module. IC310 has an output of 1 Vp-p for 60% system deviation. So it can drive the input of MMDVM Zum board directly.
The balanced audio path on the module is still fully functional, so you can use this path for ordinary FM mode with CTCSS. You MUST use CTCSS in FM, if not the digital audio will also open SQL and go directly from RX to TX via the balanced audio path. That will make a mess out of everything.
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MMDVM Settings
Turn the MMDVM RX pot (the one to the left) fully clockwise (minimum level) until it clicks. Turn the MMDVM TX pot (the one to the right) fully counter-clockwise until it clicks (max level).
For T-855 (UHF) Only:
The RSSI value is dependant upon the RSSI level POT inside T-855 RX regarding setting RSSI level to 2.0 V at test point with a signal input on RX of 3.0kHz deviation at level -110dBm.
