The "Desert Ratt" is a clever design for a sensitive and well behaved
shortwave regenerative radio. It has been doing the rounds in various
forms since 1994, and quite a few people added new twists.

As I kept seeing roughly the same stuff popping up all over the place,
and found it somewhat hard to keep track of which version was an
improvement over another one, I did a bit of investigation to figure
out who did what, and what the direction of development was.

Read it all at:
http://webfools.net/filippo/desert-ratt.html

BR

Filippo/SpamHog
N1JPR/I2
http://filippo.ru.ru

Hi,

I have a question about the Desert Ratt 2... do you have any idea what
the dimensions of the form for the onboard coil are?

Thanks,

The Eternal Squire

SpamHog wrote:
The "Desert Ratt" is a clever design for a sensitive and well behaved
shortwave regenerative radio. It has been doing the rounds in various
forms since 1994, and quite a few people added new twists.

As I kept seeing roughly the same stuff popping up all over the place,
and found it somewhat hard to keep track of which version was an
improvement over another one, I did a bit of investigation to figure
out who did what, and what the direction of development was.

Read it all at:
http://webfools.net/filippo/desert-ratt.html

BR

Filippo/SpamHog
N1JPR/I2
http://filippo.ru.ru

SpamHog,

Here's another question:

I've been building the Desert Ratt 2 today, starting with the audio amp.
Problem is that I am getting attenuation rather than amplification.

I have questions about the biasing network surrounding Q4. R9 and R10
are specified as 1.2K (1200 ohm). The schematic states that the
emitter and collector voltages should be 3.0 Volts. However, my version
develops an collector voltage of 7.2 Volts and an emitter voltage of 1.7
Volts. My base voltage is 1.8 Volts (which is in the ballpark of what
the schematic states as should be 2.0, I am not nearly as worried about
that one.

I had used a 2N3904 as recommended. On the assumption that the
transistor could be bad, I subbed a 2N2222. Same result.

It seems to me there might be a misprint with respect to the values of
R9 and R10.

Any ideas?

The Eternal Squire

The Eternal Squire wrote:
Hi,

I have a question about the Desert Ratt 2... do you have any idea what
the dimensions of the form for the onboard coil are?

Thanks,

The Eternal Squire

SpamHog wrote:

The "Desert Ratt" is a clever design for a sensitive and well behaved
shortwave regenerative radio. It has been doing the rounds in various
forms since 1994, and quite a few people added new twists.

As I kept seeing roughly the same stuff popping up all over the place,
and found it somewhat hard to keep track of which version was an
improvement over another one, I did a bit of investigation to figure
out who did what, and what the direction of development was.

Read it all at:
http://webfools.net/filippo/desert-ratt.html

BR

Filippo/SpamHog
N1JPR/I2
http://filippo.ru.ru

I have questions about the biasing network surrounding Q4. R9 and R10
are specified as 1.2K (1200 ohm). The schematic states that the
emitter and collector voltages should be 3.0 Volts. However, my version
develops an collector voltage of 7.2 Volts and an emitter voltage of 1.7
Volts. My base voltage is 1.8 Volts (which is in the ballpark of what
the schematic states as should be 2.0, I am not nearly as worried about
that one.

The voltages on the schematic are wrong.
Whatever the base voltage is, the emitter
should be 0.6 to 0.7 volts lower. Then
you can calculate the emitter current as
Ve/Re (around 1 mA in this case). The
Collector voltage is approximately Ie/Rc
lower than the supply voltage, or something
around 7V here.

The fact that your emitter voltage is only
0.1V less than the base voltage says that
either the transistor is bad (which you seem
to have eliminated, you have a bad or wrong
value component, or a wiring mistake.

I'd suggest disconnecting C15, C16 and
C17 to isolate the stage, and repeating the
dc measurements.

73, John - K6QQ

John Moriarity wrote:
I have questions about the biasing network surrounding Q4. R9 and R10
are specified as 1.2K (1200 ohm). The schematic states that the
emitter and collector voltages should be 3.0 Volts. However, my version
develops an collector voltage of 7.2 Volts and an emitter voltage of 1.7
Volts. My base voltage is 1.8 Volts (which is in the ballpark of what
the schematic states as should be 2.0, I am not nearly as worried about
that one.


The voltages on the schematic are wrong.

Yep. I had used LTSPICE to figure that one out. Collector and Emitter
DC voltages measured are same as on simulation, NOT on schematic!
AC voltages on nodes 3 and 8 from simulation agree with same on
schematic. I should measure the AC voltages.

-------------cut here-------------
Version 4
SHEET 1 1052 772
WIRE -16 224 32 224
WIRE 144 384 208 384
WIRE 272 384 320 384
WIRE 304 96 304 128
WIRE 304 16 144 16
WIRE 144 16 144 96
WIRE -80 224 -160 224
WIRE -160 384 -160 416
WIRE 32 224 32 240
WIRE 32 320 32 560
WIRE 320 464 144 464
WIRE 144 464 144 496
WIRE 144 272 144 384
WIRE 576 352 576 400
WIRE 368 176 144 176
WIRE 432 176 576 176
WIRE 576 176 576 272
WIRE 576 176 592 176
WIRE 32 224 80 224
WIRE 32 640 32 720
FLAG 144 496 0
FLAG 304 128 0
FLAG -160 416 0
FLAG 32 720 0
FLAG 576 400 0
SYMBOL npn 80 176 R0
SYMATTR InstName Q1
SYMBOL res 128 80 R0
WINDOW 3 -83 76 Left 0
SYMATTR Value 1200
SYMATTR InstName R1
SYMBOL res 128 368 R0
WINDOW 0 -44 67 Left 0
WINDOW 3 -82 105 Left 0
SYMATTR InstName R2
SYMATTR Value 1200
SYMBOL cap 272 368 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value .1µ
SYMBOL cap -16 208 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 36 34 VTop 0
SYMATTR InstName C3
SYMATTR Value .1µ
SYMBOL res 304 368 R0
WINDOW 3 48 65 Left 0
SYMATTR Value 100000000
SYMATTR InstName R3
SYMBOL res 560 256 R0
SYMATTR InstName R4
SYMATTR Value 100000000
SYMBOL Misc\\battery 304 0 R0
SYMATTR InstName V1
SYMATTR Value 9
SYMBOL voltage -160 208 R0
WINDOW 3 -85 -33 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value SINE(0 .1 400)
SYMATTR InstName V2
SYMBOL res -176 288 R0
SYMATTR InstName R5
SYMATTR Value .01
SYMBOL Misc\\battery 32 544 R0
SYMATTR InstName V3
SYMATTR Value 2.2
SYMBOL res 16 224 R0
SYMATTR InstName R6
SYMATTR Value 10000
SYMBOL cap 432 160 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 36 34 VTop 0
SYMATTR InstName C4
SYMATTR Value .1µ
TEXT -246 506 Left 0 !.tran 0 .01 0 .001
--------------------cut here-----------------

Whatever the base voltage is, the emitter
should be 0.6 to 0.7 volts lower. Then
you can calculate the emitter current as
Ve/Re (around 1 mA in this case). The
Collector voltage is approximately Ie/Rc
lower than the supply voltage, or something
around 7V here.

The fact that your emitter voltage is only
0.1V less than the base voltage says that
either the transistor is bad (which you seem
to have eliminated, you have a bad or wrong
value component, or a wiring mistake.

I'd suggest disconnecting C15, C16 and
C17 to isolate the stage, and repeating the
dc measurements.

73, John - K6QQ



Or I should report the emitter voltage more
exactly. I'll verify this. Tell me something,
how terrible an idea is it to use disk ceramic
for C15, C16, and C17? I have heard of
disk ceramics being capable of conducting DC
if made badly.

Thanks,

The Eternal Squire.

PS: LTSPICE is the ham's best friend! I
think it would be worthwhile to post the
entire SPICE file for the Desert Ratt 2,
wouldn't THAT be something!

Or I should report the emitter voltage more
exactly. I'll verify this. Tell me something,
how terrible an idea is it to use disk ceramic
for C15, C16, and C17? I have heard of
disk ceramics being capable of conducting DC
if made badly.

The disc ceramics should be ok, as long as they're
good.

I've never built this circuit, I'm just looking at it
as a paper exercise. DC measurements will usually
locate the problem. Spice may be overkill ;-)

GL & 73,

John - K6QQ