June 29, 2012

HMY2K8 Homebrewing for 20M Band

As I had just finished Homebrewing the JOTA 40M band receiver. I did not had any other projects in mind. while sifting through my shack I stumbled upon a HMY2K8 Kit which I had collected 2 years ago. Although I had unsuccessfully completed this complicated board on 40M which is presently with VU2HMY(Dr. Rajashekar) himself for alignment, like a die hard homebrewer and with a renewed interest I wanted to build another one this time for 20M Band.

This blog gives you a story of my 2nd HMY2K8 kit building experiences, difficulties that I have encountered while homebrewing. My plan is to only build the main board and use external DDS VFO and external 50W PA. The VFO and PA boards which came  along with the kit was kept aside. Also I will be giving you a detail of my understand of each section which might be of some use to other HAMS who are planning to build HMY2K8.

The main board consist for the following sections in the Receiver chain.
  1. Broad band RF amplifier
  2. Bandpass Filter
  3. RX/TX Mixer (TA7358)
  4. SSB Filter
  5. Switching circuit (74HC4053)
  6. Product Detector (TA7358)
  7. Broad band pre driver amplifier
  8. Broad band driver amplifier
  9. Audio amplifier
  10. MIC amp
As I had mentioned; HMY2K8 board is a complicated. Well atleast its complicated for me, You might ask what makes it so complicated. First of all it is a very compact PCB, all sections for RX and TX are on a single board (unlike JOTA where you have different boards for RX and TX). The soldering pads on the board is very very(Stress) close to each other which invariably touch where it should not touch when you solder. Some of the soldering pads do not have enough solderable area to solder, so you have to scrape the track bend the leads and solder, at the same time keep checking for shorts under a magnifying lens.

Building Audio Amplifier Section

As Dr. R. RAJASEKHAR, VU2HMY had instructed, the first and easiest section that has to be built is the Audio amplifier section. This section consist of below mentioned components. 
  1. IC4 - LM380 (14 Pin version)
  2. Q3 - BC549
  3. C60 -
  4. C61 -
  5. C62 -
  6. C63 -
  7. C64 -
  8. C65 -
  9. C66 -
  10. C76 -
  11. C68 -
  12. C69 -
  13. C70 -
  14. R30 - W
  15. R31 - W
  16. R32 - W
  17. VR3 - W
  18. LS1 - 8 OHMS
LM380 IC
  • The LM380 Audio amplifier chip is a general purpose amplifier with a set gain and a set bandwidth
  • The device is suitable for driving a small 8 ohm speaker
  • It operates from a single supply rail making it convenient in battery powered devices, the quiescent output voltage is half the supply voltage.
  • The chip is supplied in a standard 14 pin DIL plastic package
  • Take care that the chip has suficient space as it may become warm in use!!
  • The center pins (3-5, 10-12) on the chip are used as a heatsink and so should be connected to a suitable area of PCB.
  • Use IC Base and plug in the IC only after soldring all compents in the section.


Specifications

Supply Voltage10 to 22 volts
Input impedance150k
Voltage gain50
Output power2 W
Bandwidth100kHz
Peak Current1.3A


BC549 Pin configuration


  • BC549 is general purpose Transistor used in Low noise stages in Audio circuits.
Broad band RF Amp
Before we dwell deep into the circuit construction first lets understand what is the function of a RF Amplifier. As per  defenation" RF Amplifier amplifies the weak incomming signal coming from the antenna and pass it to the next stage"
Parts list for Broad band RF Amp Emitter =
Q1 -

Testing Broad Band RF Amplifier
After constructing the RF Amp section I first checked the resistance from BF494 to ground : Collector = ?, Emitter = 180 W, Base = 1K.

If resistance checks are OK, proceed to voltage checks.
Apply 12DC to RF Amp circuit and checked the voltages accross the pins of Q1 the voltages That I got is given  below. 

Collector = , Base = , Emitter =.

If above mentioned voltages varied more that  plus/minus 20% recheck the circuit for missed or wrong values.

Important Voltage readings


Important Aligenment and tuning Information

Testing Product Detector and Carrier oscillator CO/PD
  1. First apply +12V DC for they crystal which you are using (LSB or USB) or Short the rimmer to the ground
  2. Apply +12V DC to Product Detector section, Check if you are getting 5V at pin no 9.
  3. Bring a radio or rig which is tuned to 10MHZ near the board or connect frequency counter to pin no 7 or Inject signal vial oscolloscope and see the wave form.
  4. Connect a long wire antenna to pin no 1, you should be able to hear some station. This is because signal from pin 1 will be mixed with 10Mhz (from crystal) which will pass throu O/P and sould will be heard via SPK.
  5. Trim the trimmer and observe if there is any variance in pitch, Then we know if the SSB stage is working or Inject 10Mhz signal from Function generator or GDO or any crystal oscillator and you should be able to hear a whistle note.
Note:First test for Receiving part, automatically Transmitt side works.

Testing Side Band Filter.
  1. Touch the antenna wire to the output of the crystal filter to hear some Radio station.
  2. Then connect the antenna wire to the Input of the crystal fiflter, All other signals will be filtered and audio will be very low.
  3. This confirms that the Side Band stage is working.
Aligning  RX/TX Mixer
  1. Tune a VFO (2.9 ~ 3.0Mhz), Inject it to pin no 8 or RX/TX Mixer.
  2. Then connect the antenna to pin no 1 of IC and tune the VFO to receive 7Mhz.
  3. Use an alignment screw driver and align LSB/USB (depending on band) crystal trimmers to get exact beat note. This complets the alignment of carrier oscillator.
Aligning Band Pass Filters
  1. Inject signal from Long wire antenna to the first of the Band pass filter IFT (L6) and tune it until you hear maximum audio, Beware that excessive force will break internal core.
  2. Repeat the above step for the secong IFT.
Testing RF Amplifier  for Recrive mode Q1
  1. Connect a long wire to the collector of BF494 (Q1), you should be able to hear 7Mhz or 14Mhz signal.
  2. Now shift the long wire to base of Q1 and observe the increase on signal as the gain increases.

    (To be continued)

1 comment:

  1. Anonymous10:12 PM

    interesting, vu3inj

    ReplyDelete