TM-255 modifications

This webpage describes 3 modifications:
–  Enabling to use of the TM255 for microwave operation with DB6NT equipment.
–  Modification for power-up problems
–  Change of memory battery

1) Enabling to use of the TM255 for microwave operation with DB6NT equipment.
The modifications:
–  12 volt at the antenna plug in transmit mode (wire to 12 volt TX, insert 10 K resistor, insert 470 pF capacitor)
–  3 Watt output in low power mode (adjust VR 401)


2) Modification for power-up problems
My TM255 has had the problem that it not always starts-up.
After reading the emails in the TM255 group, I concluded that more amateurs have reported this problem in the forum.
After investigations and a better understanding of the schematics, it became clear that the processor on the LCD print did not reset correctly. If you like you can measure this at the base of Q3 (or pin 4 of IC5). When applying 12 volt to the set, a clean reset signal should become visible at the oscilloscoop. Instead of a clean reset signal a lot of spikes were noted here. To make a long story short, a better de-coupling of IC5 was the solution. The easiest solution I found was to change C6 (0.01k) to a tantalum C of 0.68uF (other values are also OK, but this one fitted very well). To solder an extra C on top of the existing one is ofcourse also OK.
Conclusion: if your TM-255 has problems at start-up: power-up your scoop and look to the reset signal at the LCD print.

Note: Bill, K0ZL reported that this modification worked quite well. He was so kind to make a few photos (Tnx Bill !)


3) Change of memory battery
Sooner or later the TM255 will lose memory when switched off …….. the memory battery is empty! Changing the battery is the solution. To avoid searching for such a special battery, you can replace the battery by a lithium CR2032 in a battery holder. Changing the battery in the future is now very easy.

The original battery is replaced by a battery holder

Insert a CR2032

Hans, DK1MHW found an alternative solution for placing the battery holder