I picked up an MTR-3B at the RSGB Convention in a very advantageous deal.
I used a non points scoring activation of Birks Fell G/NP-031 last month to give the MTR a test drive. I worked a decent number of stations but received signals seemed / were perceived / to be low. When I checked the RBN later I noticed that most of the hits were slightly off the frequency which I had set on the MTR.
My gut feeling turned out to be right - the RX band pass filters were seriously out of tune, in fact it was as if they were set in the worse possible position.
I calibrated my MTR-3B today, it involved removing the phono socket and removing the PCB. Once the PCB was free from the housing, I was able to get at the 7 small trimmers on the display side of the board. I let my frequency counter warm up for quite a while (I did a pile of clothes ironing whilst waiting!). I verified my frequency counter using a GPS locked oscillator.
The calibration routine itself isn’t too bad, it’s just a case of measuring a 10MHz test signal and adjusting for exactly 10MHz. I also tweaked the IF frequency and BFO for the best response.
Using my ancient 'scope and a home brew signal generator, I was able to loosely couple a test signal via a leaky dummy load into the MTR. It was then just a case of feeding in a 40m, 30m & 20m test signal and peaking the rx band pass trimmers for each band for a maximum response. It was clear to me at this stage that my MTR had never been through a BPF alignment procedure. To be noted is that if the test signal is too strongly coupled, then it’s difficult to find the actual peak. I find that laying the signal generator output lead next to my special leaky dummy load works really well. Using a dummy load means the rig is nicely terminated.
After the calibration procedure I then had to rebuild the rig putting the PCB back into the enclosure and soldering the connectors where needed. I found that the wire used for the connectors was very thin and was easily broken. I remade all of the connections just to try to prevent future failure.
Hopefully the rig is now in good health.
Dining room table rig hospital…