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MTR-5B Sidetone Volume

Hi all,
I am making progress with my MTR-5B based GoBox project, and in the course of trying a few QSOs, I noticed that the MTR-5Bs sidetone volume is insanely loud, as compared to the MTR-3B.

Fortunately, I think I found a solution: Heinz, HB9BCB has found that resistor R26 seems to be the devil and must be replaced by a 100-120k variant. I have not yet tried this but will shortly.

Here is Heinz’ original post:


73 de Martin, DK3IT

Because the sidetone volume was also very different on the MTRs from different production batches, the resistor in question was replaced by a 500k miniature trimmer, what allows a comfortable setting.
Caution: The traces on the PCB are very narrow/fragile …

My home soldered MTR has been absolutely fine. I wonder if there was a production error or parts substitution.

Anyway, seems like the fix is quite trivial.

My back up battery is also very well behaved, I did have an initial problem with the provided battery, but I replaced it with a good quality branded battery and it’s worked flawlessly ever since.

73, Colin

I implemented Heinz’ fix a few days ago. 100 k worked okay in my case (MTR-5B from LNR precision, new 2-line LCD batch). I won’t redo it, but 120 - 180k might have been even more appropriate, so Heinz’ suggestion to use a 500k trimpot makes a lot of sense.

73 de Martin, DK3IT