WB0SMX

  MMR40 S-Meter / Power Meter Circuit Filed in homebrew on Jan.15, 2010 There is a simple S-meter/Power-meter circuit in the SPRAT Newsletter issue # 57 by Bill Bartlett, G4KIH. It derives its S-meter component from the audio stages, and gets its Power indication from a simple RF probe. To adapt this circuit to the MMR-40 took a little engineering, but it wasn’t too bad. MMR-40 S/RF Meter The original circuit is in Black above, with my modifications in Green. The modifications allow the use of a voltage regulator chip, rather than a 10 volt zener, which I didn’t have handy.  They also fix a problem with the power indication, described below. The first step was to figure out where to connect the circuit to the MMR-40.  The MMR-40 kit has a trace available off of U8 pin 1, labeled Audio Line Out on the board.  This is a good spot to pick up the audio for the S-meter, as it is before the volume control, so will not be affected by the setting of the volume...

  More Volts from the LM7805 Regulator Filed in homebrew on Jan.10, 2010 I began development of an S-meter/Power meter interface for the MMR40 today. The plan calls for a 10 volt supply, using a 10 volt zener diode for regulation.  Since I didn’t have a 10-volt zener on hand, I decided to use an LM7805 regulator chip, but expand the voltage output to 10 volts from its normal 5 volts. To do that I used the method set out in an article by Sam Ulbing, N4UAU in the January, 1999 QST, page 45. The basic circuit looks like this: S-meter Voltage Regulator In the article, Sam gives the general guidance of setting the current through R1 at about 3 times the normal quiescent current for the 7805 regulator.  He goes on to give the formula for the Output voltage from the circuit: Vout = 5 volts + (5v/R1 + Iq)*R2 Vout = 5 + (5/470 + .003)*390 Vout = 5 + (0.0106 + 0.003)*390 Vout = 5 + (0.0136)*390 Vout = 5 + 5.304 Vout = 10.304 volts The device he was using had a quie...

  Another MMR40 fix Filed in Kits on Jan.04, 2010 When I initially constructed the MMR40, I was going to use the original case.  So I put the band range switch on the bottom of the board to accommodate that.  Well with switching to the new case, I decided that I needed to move the switch to the front panel.  This had an unfortunate side effect.  It seems that the stray capacitance from the few inches of wire used to connect the new switch is enough to shift the VFO frequency down about17 kc.   Original Band Switch I had spent a good amount of time adjusting the coils of the VFO to make it cover from 7025 to about 7150 kc on CW, which includes the most popular QRP frequencies of 7040 and 7030 kc.  With the 17 khz offset, the new range was shifted to 7042 to 7170 kc. To rectify this problem, I pulled 1 turn off of the L2 toroid of the VFO circuit. L2 Toroid This shifted the VFO frequency up, and caused the rig’s final frequency to shi...

  Some MMR40 Fixes Filed in Kits on Jan.04, 2010 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 wer...

  MMR40 Chassis Work (Con’t) Filed in Kits on Dec.20, 2009 I was able to get a little more chassis work done on the MMR-40. Here is what the faceplate looks like now.  I’m still debating on whether to put in a jack for headphones on the front.  This will end up with a laminated photo overlay for a nice looking finish. MMR-40 Faceplate Ace Hardware had some nylon screws an nuts that I used to mount the PC board.  This prevents any shorts that might occur due to some traces being really close to the screw holes. Mounting Hardware So the board is bolted to the perfboard, with the controls sticking out the front panel just enough to tighten the nut on the microphone jack to add some stability. MMR40 Mounted in the chassis Here is a wide view of the board and external wiring.  All connections to the board are on headers, so it may easily be unplugged to work on it out of the chassis.  I haven’t tied up all of the wires yet, as it’s still a wo...

  MMR-40 Front Panel Filed in Kits on Dec.14, 2009 Here are just a couple more pictures I had in the camera of the MMR-40 Box Up… The plan is to put a piece of red acetate over the display, then the front panel image over the whole thing.  That should cover up any rough edges in the hole I cut out for the display. I replaced the incandescent light in the meter with a white LED that draws about 20 ma.  It gives it a nice color, and should never need replacing. I plan on putting both the digital display and the meter backlighting on an electronic switch that will light for about 1 minute when I momentarily press a button.  That will save batteries when I don’t need to know what’s going on at all times. Front Panel with displays powered up Here is the back of the front panel.  The KD1JV Digital Dial is on the left, and the S-meter/Power Meter is on the right. The Digital dial is held in place by two countersunk flathead screws through the front pan...

  The Haul – (Superstition ARC Swapfest 2009) Filed in homebrew on Dec.11, 2009 I got up at my normal work wake up time on Saturday morning to attend the Superstition Amateur Radio Club’s 2009 Swapfest at Mesa Community College. I was expecting more of the same as I had seen in the last few years, with dwindling attendance, mostly computer stuff, and bad weather. I’m happy to say I was wrong on all counts. Progressive Receiver Start This year, I found more good parts and part radios than I had seen in several years.  I was mostly hoping to find some variable capacitors at reasonable prices, as they cannot hardly be found new anymore.  They are necessary for a lot of the older projects I have been interested in building. I found a box of variable caps, with a tube socket and a microphone thrown in.  I offered $20 for the whole box, and was happy to have it accepted. Variable Capacitors There are a couple of transmitting caps in there and lots of rec...