Note that this page is updated frequently but the actual printed and posted notes, schematics, and layouts are always a bit out of date!

• I have just completed 4 receivers (7/20/08), all of which work.  I used 5 Volt powered DVM's (the only ones available when I ordered), which required some minor circuit changes.  See the Notes on the 2008 DQ Receiver Boards for the changes and also for info on using CR123A lithium cells for power.
  
• The parts list has been updated (4/14/08) after I actually placed an order!!
  
• These notes apply to boards with dates prior to 2008, as well as to the DQ receiver portion of the new 2008 boards.  They do not apply to the new 2008 Class-E beacon.
• The new 2008 boards have only one change.  U12 (and C42) have been added to provide 9VDC for DVMs in the complete receiver that cannot work on 12VDC.  U12 is not shown on the schematic or the front panel layout.
  
• Note that on the RF Amp board R20 (2M) has become R20A (1M) plus R20B (1M) in series. This is shown in the parts list and color layout, but not on the schematic or on the actual circuit board.
• A good test of proper receiver operation is to disconnect the loop, null the audio tone with RF gain at minimum, then increase RF gain in 32Hz mode. You should hear random noise from the RF amp as the RF gain is increased.
• A simple way to test for proper shielding of the RF amplifier is to first do the above test, then change to 1 Hz mode.  With the receiver carefully nulled at minimum RF gain, slowly increase the RF gain while listening.  The receiver should remain exactly nulled (check by rotating the null control) up to the point where random noise masks it.  For the complete receiver, check with the gain switch in Hi and Lo positions. My receivers pass this test with the cover installed (and grounded), but fail with the cover removed.
• Even in a properly shielded receiver, there is a small amount of magnetic field radiation that can be detected by a nearby receive loop. To see this effect, audio null the receiver without the loop connected, then reconnect the loop, set the receiver to high RF gain, and move the receiver near the loop. Note that maximim coupling occurs when the receiver is near the flat side of the loop, ie near its axis.  This coupling makes little difference when searching, but could cause a small "offset" in Ground Zero location.  When doing the final, precise, locating, it is best to squat down while holding the vertical receive loop at arms length ahead of you, with the plane of the loop passing thru both you and the receiver.
• Another source for a varactor diode (with leads) that should work is Dan's Small Parts and kits. The part no. is MV-209.  It is listed as 32pF.  Its spec sheet says 30pF at 3V reverse.
• I am told that Electronics Plus has a couple of leaded NTE-614 varactors.  Dan's Small Parts has several leaded varactors that I haven't tried.
  
• NEW! If a 3 MHz crystal is used for operation in countries with 50 Hz power grids (to operate between harmonics), the operating freq will be 2929.69 Hz and the audio output will be 366 Hz instead of 437 Hz.  You must change the freq of the two audio bandpass filters to 366 Hz by changing R15 and R45 (on the newer boards with two bandpass filters) from 1.87k Ohms to 2.87k Ohms 1%.
  

• Once the loop is tuned for minimum DC current, the MOSFET will not overheat without the added heat sink. Use very short ON periods (<5 seconds) when doing initial tuning if the aluminum heatsink is not added.

• On the schematic of the Complete Receiver, R19 is the 5.6k dropping resistor for the Power-on LED.

• C31 and C32 (0.1uF B+ bypasses) were left off the Simplified Receiver schematic by mistake. They are shown correctly on the layout diagram.
• In the Simplified Receiver the Vcc power to the RF amp can be connected directly to the on/off switch.
• The Simplified Receiver uses one 9V alkaline or can use a 12V lead-acid without modification.
• Many items can be deleted from the "complete receiver" parts list. Delete on detector board: the DVM, U2, U12, C42, V1, U11 & SOCKET, R47, R48, R49, R50, R53, R54, U8 & SOCKET, R27, R28, R29, R30, R31, R32, R33, R37, C21 (A & B), C22, C38, R37, R39, R40, R41, R42, R43, D5, D7, S4, S6, PIEZO ALARM, R9 can be a 1-turn linear pot.  Delete on RF amp board: R55, R21, S3, R22 can be a 1-turn audio taper pot.  Also delete one 9 Volt battery socket.  This list may not be complete.