BITX20 Transceiver (fwd)

Ashhar Farhan <farhan@...>


pete joins us on the group. he has details about substituting the tv
baluns cores with FT-37 cores. he has also pointed out that the receiver
gain is low. anybody else too who has experienced this problem?

i did encounter it on two sets that i assembled (out of three). in one set
it turned out o be a cold solder on an emitter bypass capacitor, in the
other case, it turned out that i had swapped the collecot load and the
base bias resistors on the audio pre-amp.

- farhan

---------- Forwarded message ----------
Date: Fri, 18 Jun 2004 08:14:12 +0530
From: Ashhar Farhan <computercorp@...>
To: jessystems@...
Cc: farhan@...
Subject: RE: BITX20 Transceiver


my hotmail box is choked. my other email address is farhan@....
and the mirror is on i work at phonestack.

i will cross post this mail of yours back to the group. if you are getting
dx, then the receiver is probably sensitive enough. it is an unusually quite
receiver design. as a result, it does give the impression that it is not
sensitive enough. the acid test is to listen to the receiver without the
antenna and with the antenna attached. if the noise goes up substantially,
then it is sensitive enough.

- farhan

Ashhar Farhan
71, Huda Heights, MLA Colony,
Banjara Hills, Hyderabad.
INDIA 500034.
Phone - +91-40-23314077

From: "Pete Juliano" <jessystems@...>
To: "Ashhar Farhan" <computercorp@...>
Subject: BITX20 Transceiver
Date: Thu, 17 Jun 2004 12:08:13 -0700

Hi Ashhar,

Thank you for your trouble shooting hints. I will go through those.
Actually I have been copying signals from all over the world at the QTH
here near Seattle Washington. Last evening I heard IK2's, UA5's, LU7, KP2
and the usual mix of stateside stations. So it is not like it is deaf.

Thanks for the link to the user's group.

Oh. I tried sending you some photos of the rig but I guess hotmail does not
like jpg files. Is there some way to get some photos to you?

I don't know that there is a problem --but as compared to my other homebrew
gear it seems to be slightly less senstive. I am using a 2 element beam and
a fixed wire beam so it is not a case of a poor antenna. But I will follow
your trouble shooting hints. However as you say it is easy to get parts in
the wrong place and if you see my 2X6 inch board it is chock full --so it
would be easy to put the wrong part in the circuit. Must admit I have to
use a magnifying glass to read some of those really small parts.

I must open the audio full gain to get to the pain level in the headpohnes
--usually with the LM386 -1 it is about 1/2 gain so that is the basis of my
comments about gain.

Yes the bandpass filter peaks up quite nicely and I tested the transmit
function. I get about 150 mv pk to pk with just the pre-driver and the
audio sounds about right. I had to put a series inductor between the
crystal and the small 22 Pf variable to get USB. I used a about 10 turns on
a FT37-61 core. The balanced modulator works quite nicely for only two
diodes --I am used to using either a DBM like an SBL-1 or TUF-1 or an
active device like a MC1496. Two diodes and a coil are pretty cheap and
just as good. Today I will build the final amp and then give it a smoke

Here is some coil data for you. I used FT37-43 toroids with a 13 turn
trifilar winding for the coils into the product detector/balanced modulator
and for the VFO mixer. I used a 13 turn bifilar winding for the pre-driver
and for the coils in the bandpass filter I used 22 turns on a T-50-6 iron
powder toroid. For the IRF510 final amp I will use 8 bifilar turns on a
FT-37-61 ferrite (~3 uhy). For the parallel driver stage I will use 13
turns bifilar on a FT37-43.

There are some very strong local stations here and that is where an AGC
circuit would be nice. Have you been thinking about an AGC modificaton.
Something simple where the voltage to the collectors on the receive vfo
mixer and post filter amp could be reduced based on received signal
strength. That might also enable an S meter.

Thanks again for a very nice project. I will let you know what I find in
the trouble shooting and when I get the final amp working.

Pete W6JFR
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