---------- Forwarded message ----------
Date: Sat, 19 Jun 2004 22:08:39 -0700
From: Pete Juliano <jessystems@...>
To: Ashhar Farhan <farhan@...>
Subject: Re: Fw: First QSO with the BITX20 Transceiver
Hi Ashhar,
For the crystal filter I am using 9.0 MHz computer crystals. I built a test oscillator and measured them with a frequency counter. Out of a group of 20, I found 4 that were within 50 hertz of each other. Those are the four in the radio. When I jumpered in a Commercial Filter to make a comparison, I didn't even have to touch up the carrier oscillator crystal so the HB filter has a very close center frequency to the commercial unit. The HB Filter does a nice job and is cheap --20 crystals cost me about $7 USD and within that remainder is another batch that could be used for a second filter.
Attached are several photos of my version of the BITX20 including the Linear Amp, an overall view of the boards with digital display and a close up of the main circuit board. Hopefully next week I will have it all in a box. The VFO and stabilizer are as large as the mainboard. I used sockets for all of the semiconductors so I could do a little testing of various transistors and IC's
The linear amp output filter has the following constants. 220PF, L1 which is 12 Turns of # 26SWG on an FT37-2 powdered iron core form (red core ~ 0.55 uhy). Then followed by a 220PF in parallel with a 330 PF (= 550 PF) followed by L2 same as L1 and then a 220 PF. The caps are ceramic on glass and rated at 100 volts. Really small caps. I used a 78L05 regulator and the bias adjust --I used what I had --a 25K ohm pot in parallel with a22K ohm resistor. Works fine. The linear amp board is 2inches X 2 inches (5CM X 5CM).
You will note two relays --one at the linear amp just like the schematic --but I also had to resort to a relay at the main board as there was a RF feedback problem. The second relay solved that problem.
The microphone is a low impedance --A D104 (lollipop) fitted with a Heil HC4 cartridge. I have a 25K ohm pot soldered right at mic connector and use that to set the mic gain.
The last two evenings 20 meters has been hot and the RX is doing a FB job. Despite my initial comments about receiver sensitivity it is more than adequate.
If I get a chance I may try my hand at an analog agc circuit.
Again a very nice radio and despite its subtle simplicity --it packs a lot of peformance.
I will keep you posted on my progress.
73's
Pete W6JFR
