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Some MMR40 Fixes


Some MMR40 Fixes

While I was working on assembling the MMR40 into its chassis, I ran across a couple of problems.

After I hooked up the antenna connector jumper from the SO-239 on the back of the chassis to the BNC connector on the board, I did a quick check of power out.  What’s this??? No Power?? How can that be???

Q9/Q10 circuit

So I started thinking about what had changed…

  • Mechanical assembly – no chance for metal to metal contact
  • Added several header pins for connecting Counter, RF Probe, etc – check for shorts
  • Removed band range switch and added header pins for outboard switch – check connections
  • Added power switch and wiring – check connections
  • Added Digital Dial Counter – check header connections – check BFO setting

After checking all of this, I still had no power out.  So next, I dug out the scope and meter.

  • The DC voltages to the TX/RX switching sections were as specified in the manual.
  • The DC voltages on Q10 and Q9 were normal.
  • Q9 was producing what appeared to be a reasonable waveform on its collector, but I could not see it on Q10 Gate.
  • The current in Transmit went up to about 90 ma, which seemed reasonable with no drive, but low with drive.

So it looked like I was missing drive from Q9 to Q10.

On the base of Q9, I had about 0.3 volts of RF signal:

Q9 Base signal ~ 0.3 vpp

Q9 Emitter had a similar waveform:

Q9 Emitter ~ 0.3vpp

Q9 collector had a strong signal into the transformer (about 40 vpp):

Q9 Collector ~40 vpp

The input to C48 had about 15 vpp (seems reasonable):

C48 Input from Q9 Circuit ~ 15vpp

When I checked the output of C48, I saw a very small signal (< 1 vpp).  At this point it appeared that I may have had a bad solder joint, so I took the board out of its case, and resoldered the connections in that part of the circuit.  Upon reassembly, I had the full 5 watts out.

Here is the Gate of Q10 when working correctly:

Q10 Gate ~ 10vpp

And the Q10 collector output into the transformer:

Q10 Drain ~ 40 vpp

And finally, the output from the antenna connector to a 50 ohm load:

Antenna Output ~ 45 vpp

So 45 vpp / 2 = 22.5 volts peak
22.5 * 0.707 = 15.9075
(15.9075^2)/50 ohms = 5.06 watts.


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