I have recently built a Sandford Wattmeter at the G-QRP buildathon in Telford, UK. I am in the shack this morning, and thought it would be good to test the small radios I have here:
A Kanga Rooster kit I build last year
An MTR-5B I bought second hand a couple of years ago
The Rooster delivers just over 2W on 40m, as expected. The MTR-5B (on a 9v battery) delivers around 2.5W on all frequencies except 20m, where the output is around 0.5W. This leads me to ask a couple of questions:
Have any of you seen such behaviour with the MTR-5B?
Would it be worth taking it apart to see if I can see any reason why this should be the case? I have downloaded and printed the construction instructions, and will browse through them to see if there are any hints in there.
Update - I have pulled it apart, and there is nothing obviously wrong inside. According to the instructions, the windings on L21 affect the power output. These were closely spaced, so I spread them out (which should increase the power), but with no change to output. I have sent an email to LNR Precision to see if they recommend anything else. I do recognise this is an obsolete radio which is no longer produced, and also that they are a small, very busy company so I will fully understand if I hear nothing back!
I think it would be worth opening the case.
If, after opening it, glued-together toroidal cores of the low-pass filters were to be found, it would probably be the same developer’s joy (…) as I saw with a colleague’s MTR5B.
After carefully removing the adhesive, I desoldered the input-side LPF inductors, rewound them, and then spread the windings to achieve maximum output power.
Unfortunately, the recording of the pleasing output power values ​​after the modification is no longer available due to PC changes.
The TX chain is common up until the switches and transmit filter. If only one band is down, it’s either a poor connection on the transmit filter or switch. I have known switches to become a bit iffy. The most likely scenario would be a bad connection or broken wire on one of the two 20m toroids.
The switches are exposed to the elements and water ingress, followed by corrosion, is a possible cause of failure.
Thanks Colin - I will have a closer look at the switches and the connections before pulling the toroids off the board to rewind them. I am also missing the holder for the battery - I am not sure if this was just not fitted, or whether some were delivered without it.
The battery was found to drain quite quickly and then contributed to MCU lock ups. The advice was to leave out the coin cell. The only real benefit to the coin cell was to preserve the real time clock setting, but it’s easy to set the clock at the beginning of an operating session if you’d like to see time on the LCD. If the RTC hasn’t been set, then nothing shows on the display where the clock would be, so it’s not an issue either way.
The MCU locking up can be a real PITA as the only way to get things working again is often to remove the coin cell, which means undoing the case. In my case, I used hex socket screws (my MTR-5B is a kit version supplied without an enclosure, not LNR Precision) for the case and I couldn’t remove the case cover on the hill when my rig locked up! I now keep a hex key with my rig, although I don’t normally fit the back up battery.
I am having a look at this again, and I have noticed that the backlight dims when transmitting on 20m - something which doesn’t happen on the other bands. Sure enough, putting a meter on the incoming supply (a rechargeable PP3 battery), the voltage drops to about 8.5v on all bands except 20m. On 20m, it drops to about 4.5v.
So I guess I am looking for an issue somewhere from the S9/S10 switches through the 20m output low pass filter to the S7a/ S8a switches. I’m not seeing anything with the hand held microscope, but I will keep looking…!
L21 rewound, and 20m power output is now in line with the others, except 30m - that had dropped. I rewound L19, and this brought 30m back to the same level.
So - now I’m a happy bunny, and have another field radio I can exercise once I have mastered enough CW!
One question for @M1BUU - Colin - should I hot glue the toroids again?
