20 meter High Performance Direct Conversion Receiver (3). Oscillator troubleshooting.

Link to schematic

Did some more diagnostic work on the this unit.  

Notes: 

1. The QST article says that the cathode of D2 should be at 9.1V.  Actual voltage is 8.9 V.  Good.
2. The QST article says that the C21 should be 2 V rms.  Actual is 0.87 V rms.  So the oscillator voltage is down significantly from what it should be.  I checked all the resistor values in the oscillator circuit and they are as expected. 
3. Removed C22 to see if the mixer was loading the oscillator somehow.  No evidence of that.  C22 was nominal 10 pF, and measured 9.6 pF.

There are other components to check in the oscillator circuit, but I suspect the transconductance, gm, of the Q4 is down and may be the source of the problem.  

I will order some MPF102 JFETs to see if replacing the transistor helps.  Perhaps the transistor was toasted while soldering. 

I will pull capacitor C19, C20, and C23 to check, and will pull L6 to check as well.

I am planning to do some simulations to check the dependence of the oscillator output on JFET gm. 

20-Meter High Performance Direct Conversion Receiver (2)

Partially checked out the RX ckt on the bench today. I powered the board off a 12V supply.   No external components were attached, including the VFO tuning cap and the peaking  cap.  The antenna coil is still missing. 

The voltage at  oscillator transistor Q4 drain is 8.9 volts, so voltage regulation of the oscillator power seems to be working. 

The oscillator, without the front panel tuning cap, operates at 14.365 MHz.    Amplitude is 638 mV pp, so 0.23 V rms.  

Next step will be to install a new antenna coil and repair the peaking variable capacitor.  I went to Four States QRP to order the toroids, but weren't in stock.  I ordered some T30-6 rather than T37-6, so I'll try to use those. 

20-Meter High Performance Direct Conversion Receiver (1)

Figure 1.  Photo 20m RX PCB

Today I'm starting work on the 20 Meter receiver that Mike bought back in 2018.  The receiver is based on a April 1978 QST article. (Page 11). 

• The circuit is fairly simple and shouldn't be too hard to fix.
• I found a 12 volt 2 amp "wall-wart" power supply to power it with.  I don't have a jack to go with it.  I'll have to kludge something.
• The toroid that comprises L1/L2/L3 is missing.  I may have to order a T37-6 core to replace it.   

 

Layout of the reciever board taken from the magazine article. 

The view is from the copper side.  The component side is a mirror image. 

Parks on the Air Activation

Well, the day finally came for a true test run of my "QRP Go Pack."  The YL (Linda Bob) and I went up to Keystone Lake (KFF-2788) to test it out.  Finding a nice spot was a challenge but I built up the courage to crash a family Christmas get together at the Community Center high above the lake and set up my little QRP ukits hb1b with a random wire attached to a squid pole on the highest point I could find. I tuned up on 14.063ish but all I heard was crickets. I started to worry if my signal was getting out. I decided to go down to the WES (Straight Key) contest and see. I got a 589 from W4AFB (Keith in FL).
He told me he nearly drowned in Lake Keystone in 1987. Wow that was when Steve and I was operating field day on the same spot I was sending from today.

Putting up the antenna was quite a chore and managed to mangle the loop provided at the apex of the pole.  Then after making the lone contact I found that the top piece of the pole became stuck and had to take it home that way.  I had to saw off the top piece close to the joint and put a round of electric tape on the end to make a friction fit once again.

QRP Portable in Germany Video

Here is a nice video from German amateur radio for portable QRP.  Its in German but you can recognize much of the technical stuff.

https://www.youtube.com/watch?v=AuWYu57qbSM

4sqrp tuner kit

Just finished the 4sqrp tuner kit and tried it out in my back yard with a random wire and counterpoise attached to a 35 foot squid pole.  All I could tune was the 40 meter band.  I managed to contact Mark Lund WA0RAO in Iowa but only got a 520 rpt.  Today I shortened it to 10meters and removed the counterpoise and was able to tune to 3 bands and made a contact on ea with 2ea 579 and one 449 so I was quite pleased considering the bands of late have been pretty bad.  By the way, I was qrp on all the contacts and managed to dig a 1W qrpp out of the noise. 

This weekend I plan to activate the Keystone Lake Parks on the Air (POTA) with my new qrp portable outfit. 

It is a T-match for random wire or hwef (1/2 wave end fed) antennas.

20 Meter High Performance Direct Conversion Receiver & VXO Controlled 6W Transmitter

Well, I haven't been posting for a while.  My QRO rig (Yaesu FT-101ZD) has developed a problem with the VFO.  The frequency jumps around.  I have no idea what could be wrong.  My only guess is that there is a loose connection.  I suppose if it were a capacitor it would drift instead of jump.  I plan on at least pulling the board and have a look-see.  I wonder if I could get the external VFO and just bypass the internal one.

I have bought a You Kits HB-1B qrp rig from Ten Tec and am quite pleased with it.  I have made QSO all around the world with its spankin' 5W.  Last night I made contact with a station in New Zealand and received a good report.  I took it camping and had a ball operating from a park bench next to the lake.  The ticks and chiggers were there to great me....never going back there again.

My other QRP rig is the Rockmite 20 and it is a little harder to make good QSOs but I have worked coast to coast with it and yesterday morning I held a 1 hour rag chew with a fellow in Canada camping out in the boonies.

20 Meter High Performance Direct Conversion Receiver & VXO Controlled 6W Transmitter

Look in QST Apr & Dec 1978 for the article on how to build this transceiver.

I purchased this homebrew QRP rig off of ebay from its builder K1NYK.  The workmanship is very nice and I hope to have it up and running soon. Taken from the 1978 QST article it uses a VXO on the 20M band and has a direct conversion receiver (the lower PCB).  It should put out about 6-7W.  I tried the receiver but all I get is static.  This will be my first attempt at repairing a radio.  I first try to find the problem with a voltmeter.  I don't have a signal generator or rf detector and I guess I'll have to make something from plans off the internet.

I finally have accumulated all the parts for building a replica of my brother's QRP rig he designed that are posted elsewhere on this blog.  The NE602s where not that hard to find.  I've already started the project with a proto board.  I better get her done before I lose some of the parts. :)

UPDATE: July 25, 2014  A fellow ham has suggested that I clean the VFO capacitor to try and resolve the freq stability problem.  I've built and audio oscillator using a 555 timer to test the receiver and the circuit from the mixer to the phones is good.  Also I tested the transmitter and it is also good.  So I suspect the circuit leading up and including the mixer and BFO.  I'm trying to find out how to test those circuits.  I wish I had a signal generator that would do AF and RF.