I received the board today along with the Motorola engineering bulletin and
a list of construction hints and ran across my first problem. The bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear this up
for me. I'll post the results when I get a reply.

Hello DR. Death:

R4 and D2 make up the Bias voltage and current that is feed to the
base's of RF Amplifier Transistors Q3 and Q4.

The resistance value of R4 will be a dead nuts on requirement. But may
not really need to be a exact value, within a certain range. R4
resistance value was probably called out 47 Ohms as this will work.

I would use 4 each, 2 watt resistors wired in a parallel/series way to
make a 8 watt resistor, as a 6 watt resistor is needed.

My simple diagram shows 4 each, 47 ohm resistor wired in a series
parallel method. This allows the resistor to have the same value as a
single resistor but 4 times the power rating.


++++++ ++++++
++++++ +++++ ++++++++
++++++ ++++++



http://www.communication-concepts.co...63300sharp.pdf

Jay in the Mojave



DR. Death wrote:

I received the board today along with the Motorola engineering bulletin and
a list of construction hints and ran across my first problem. The bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear this up
for me. I'll post the results when I get a reply.

Correction:

" The resistance value of R4 will be a dead nuts on requirement."

This is incorrect.
The resistance value of R4 will NOT be a dead nuts on requirement.

Jay in the Mojave



Jay in the Mojave wrote:

Hello DR. Death:

R4 and D2 make up the Bias voltage and current that is feed to the
base's of RF Amplifier Transistors Q3 and Q4.

The resistance value of R4 will be a dead nuts on requirement. But may
not really need to be a exact value, within a certain range. R4
resistance value was probably called out 47 Ohms as this will work.

I would use 4 each, 2 watt resistors wired in a parallel/series way to
make a 8 watt resistor, as a 6 watt resistor is needed.

My simple diagram shows 4 each, 47 ohm resistor wired in a series
parallel method. This allows the resistor to have the same value as a
single resistor but 4 times the power rating.


++++++ ++++++
++++++ +++++ ++++++++
++++++ ++++++



http://www.communication-concepts.co...63300sharp.pdf

Jay in the Mojave



DR. Death wrote:

I received the board today along with the Motorola engineering
bulletin and
a list of construction hints and ran across my first problem. The
bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear
this up
for me. I'll post the results when I get a reply.

"Jay in the Mojave" wrote in message
...
Hello DR. Death:

R4 and D2 make up the Bias voltage and current that is feed to the
base's of RF Amplifier Transistors Q3 and Q4.

The resistance value of R4 will be a dead nuts on requirement. But may
not really need to be a exact value, within a certain range. R4
resistance value was probably called out 47 Ohms as this will work.

I would use 4 each, 2 watt resistors wired in a parallel/series way to
make a 8 watt resistor, as a 6 watt resistor is needed.

My simple diagram shows 4 each, 47 ohm resistor wired in a series
parallel method. This allows the resistor to have the same value as a
single resistor but 4 times the power rating.


++++++ ++++++
++++++ +++++ ++++++++
++++++ ++++++



http://www.communication-concepts.co...63300sharp.pdf

Jay in the Mojave



DR. Death wrote:

I received the board today along with the Motorola engineering bulletin
and
a list of construction hints and ran across my first problem. The
bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear this
up
for me. I'll post the results when I get a reply.


The bulletin uses a 1N4997 diode for D2 and a 33 ohm for R4. The
construction hints uses a MJE243 Transistor for D2 as they state the 1N4997
is no longer available and R4 as 82 ohms. The 63R resistor parts set has it
at 47 ohms. So the choice I think would be either the 47 or 82 ohm.
I was able to find a 1N4997 replacement NTE5800 and might try it with the 33
ohm as the MJE243 will be a bit harder to install as it goes between the PCB
and heat sink and has to have a spacer. Thanks for your input, if the 1N4997
doesn't work like expected I'll give the series parallel a try.

On Thu, 21 Oct 2004 17:37:04 -0500, "DR. Death"
wrote in
:

snip
The bulletin uses a 1N4997 diode for D2 and a 33 ohm for R4. The
construction hints uses a MJE243 Transistor for D2 as they state the 1N4997
is no longer available and R4 as 82 ohms. The 63R resistor parts set has it
at 47 ohms. So the choice I think would be either the 47 or 82 ohm.


It looks like the resistors were chosen based on the characteristics
of the respective device, so keeping the match would probably be the
best choice. IOW, use the 82 ohm resistor with the MJE243 and the 33
ohm resistor with the 1N4997.


I was able to find a 1N4997 replacement NTE5800 and might try it with the 33
ohm as the MJE243 will be a bit harder to install as it goes between the PCB
and heat sink and has to have a spacer.


Avoid generic replacements. Unless it's reverse-referenced as the
original component, it may not be a drop-in replacement and is not
likely to have the same specifications as the original. Besides, using
the MJE243 is, IMPO, a better choice than the 1N4997 for a couple
reasons: First, as a diode it will have forward voltage drop (bias
regulation) similar to the emitter-base characteristics of the power
transistors (mainly because it -is- a power transistor); and second,
because it will have a better thermal connection with the heat-sink
providing bias regulation with improved thermal tracking. That is, of
course, assuming you are able to mount it physically close to the
power transistors. And when you mount it, use just a very thin film of
heat-sink grease -- avoid the "more-is-better" mentality.


Thanks for your input, if the 1N4997
doesn't work like expected I'll give the series parallel a try.


Using multiple resistors is an excellent idea. If the 33 ohm resistor
is used the power dissipated by the resistor will be slightly greater
than the 5 watt rating specified -- it would be much better to have a
resistor (or resistors) rated for 10 watts or more. Also, mount that
resistor (as well as the base resistors R5 and R6) slightly above the
board (1/8" to 1/4") or the heat will cook the board and the foil will
fall off. The extra wire also helps dissipate heat.

There was also the suggestion to use a Dale, but that's really just a
waste of money in this case -- the input impedance of the transistors
is so low that feedback from stray RF isn't going to be a problem.

Another issue: While looking at the schematic I was suprised to see
that there isn't a choke between the bias supply and the input
transformer -- there -really- needs to be one there! At the very
least, add a few beads on the wire from the center-tap of the input
transformer. A nice torroid or hash choke would be even better.

Anyway, while you are building this thing, keep in mind that it is a
very basic circuit (i.e, primitive, uninproved, bare-bones), so don't
expect much. And yes, it -will- need a filter.






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"Frank Gilliland" wrote in message
...
On Thu, 21 Oct 2004 17:37:04 -0500, "DR. Death"
wrote in
:

snip
The bulletin uses a 1N4997 diode for D2 and a 33 ohm for R4. The
construction hints uses a MJE243 Transistor for D2 as they state the
1N4997
is no longer available and R4 as 82 ohms. The 63R resistor parts set has
it
at 47 ohms. So the choice I think would be either the 47 or 82 ohm.


It looks like the resistors were chosen based on the characteristics
of the respective device, so keeping the match would probably be the
best choice. IOW, use the 82 ohm resistor with the MJE243 and the 33
ohm resistor with the 1N4997.


I was able to find a 1N4997 replacement NTE5800 and might try it with the
33
ohm as the MJE243 will be a bit harder to install as it goes between the
PCB
and heat sink and has to have a spacer.


Avoid generic replacements. Unless it's reverse-referenced as the
original component, it may not be a drop-in replacement and is not
likely to have the same specifications as the original. Besides, using
the MJE243 is, IMPO, a better choice than the 1N4997 for a couple
reasons: First, as a diode it will have forward voltage drop (bias
regulation) similar to the emitter-base characteristics of the power
transistors (mainly because it -is- a power transistor); and second,
because it will have a better thermal connection with the heat-sink
providing bias regulation with improved thermal tracking. That is, of
course, assuming you are able to mount it physically close to the
power transistors. And when you mount it, use just a very thin film of
heat-sink grease -- avoid the "more-is-better" mentality.


The MJE243 mounts close to the power transistors. The hints say to use a
mica washer and coumpound. I was certainly a little concerned with using
replacement 1N4997.

Thanks for your input, if the 1N4997
doesn't work like expected I'll give the series parallel a try.


Using multiple resistors is an excellent idea. If the 33 ohm resistor
is used the power dissipated by the resistor will be slightly greater
than the 5 watt rating specified -- it would be much better to have a
resistor (or resistors) rated for 10 watts or more. Also, mount that
resistor (as well as the base resistors R5 and R6) slightly above the
board (1/8" to 1/4") or the heat will cook the board and the foil will
fall off. The extra wire also helps dissipate heat.

Glad you mentioned that. I would have mounted them against the board.

There was also the suggestion to use a Dale, but that's really just a
waste of money in this case -- the input impedance of the transistors
is so low that feedback from stray RF isn't going to be a problem.

Another issue: While looking at the schematic I was suprised to see
that there isn't a choke between the bias supply and the input
transformer -- there -really- needs to be one there! At the very
least, add a few beads on the wire from the center-tap of the input
transformer. A nice torroid or hash choke would be even better.

When I bought the transformer set it came with the beads and wire and the
hints tell where to put the choke.

Anyway, while you are building this thing, keep in mind that it is a
very basic circuit (i.e, primitive, uninproved, bare-bones), so don't
expect much. And yes, it -will- need a filter.

LOL, no doubt. I have a pretty good low pass in line filter that I will use.
Thanks for the info Frank.






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DR. Death wrote:

I received the board today along with the Motorola engineering bulletin and
a list of construction hints and ran across my first problem. The bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear this up
for me. I'll post the results when I get a reply.


Hello DR. Death:

One other thing that could be done to get a feel for the Value of R4 is
look at the other Motorola Engineering Bulletins, and see what they use
for the resistor. 33 and 47 ohms looks like a good value.

One problem I see is that because this resistor needs to dissipate 5 to
6 watts, a lot of these resistors used are too small in there power
rating. Thetas why the 8 watt series/parallel circuit works so well. And
the 2 watt carbon Resistors can't be too expensive. See:

http://www.rfparts.com

You could have a 33 ohm and a 47 ohm resistors, and switch between them
to see what what works best.

And of course you could always measure the current of the two
transistors at .6 to .7 Volts applied at the Base and Emitter of the Q3
and Q4 Transistors. This with ohms law would tell the range of resistors
used for the bias supply for a pair MRF242 Transistors.

Jay in the Mojave

"Jay in the Mojave" wrote in message
...
DR. Death wrote:

I received the board today along with the Motorola engineering bulletin
and
a list of construction hints and ran across my first problem. The
bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The web
site resistor package lists it as 47 ohms. Hopefully they can clear this
up
for me. I'll post the results when I get a reply.


Hello DR. Death:

One other thing that could be done to get a feel for the Value of R4 is
look at the other Motorola Engineering Bulletins, and see what they use
for the resistor. 33 and 47 ohms looks like a good value.

One problem I see is that because this resistor needs to dissipate 5 to 6
watts, a lot of these resistors used are too small in there power rating.
Thetas why the 8 watt series/parallel circuit works so well. And the 2
watt carbon Resistors can't be too expensive. See:

http://www.rfparts.com

You could have a 33 ohm and a 47 ohm resistors, and switch between them to
see what what works best.

And of course you could always measure the current of the two transistors
at .6 to .7 Volts applied at the Base and Emitter of the Q3 and Q4
Transistors. This with ohms law would tell the range of resistors used for
the bias supply for a pair MRF242 Transistors.

Jay in the Mojave

If you are really concerned you will want to use 5% or better resistors,
which, can be hard to find in large wattage for cheap. If you want to burn
a little money look into the dale power resistors which have an aluminum
outer heatsink covering. Since this covering is sinked to ground it will
also help with stray RF and may prevent some internal oscillations thus
keeping harmonics down a tad.

http://www.vishay.com/resistors-disc...wer-more-five/

Chad

"Chad Wahls" wrote in message
...

"Jay in the Mojave" wrote in message
...
DR. Death wrote:

I received the board today along with the Motorola engineering bulletin
and
a list of construction hints and ran across my first problem. The
bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The
web
site resistor package lists it as 47 ohms. Hopefully they can clear
this
up
for me. I'll post the results when I get a reply.


Hello DR. Death:

One other thing that could be done to get a feel for the Value of R4 is
look at the other Motorola Engineering Bulletins, and see what they use
for the resistor. 33 and 47 ohms looks like a good value.

One problem I see is that because this resistor needs to dissipate 5 to
6
watts, a lot of these resistors used are too small in there power
rating.
Thetas why the 8 watt series/parallel circuit works so well. And the 2
watt carbon Resistors can't be too expensive. See:

http://www.rfparts.com

You could have a 33 ohm and a 47 ohm resistors, and switch between them
to
see what what works best.

And of course you could always measure the current of the two
transistors
at .6 to .7 Volts applied at the Base and Emitter of the Q3 and Q4
Transistors. This with ohms law would tell the range of resistors used
for
the bias supply for a pair MRF242 Transistors.

Jay in the Mojave

If you are really concerned you will want to use 5% or better resistors,
which, can be hard to find in large wattage for cheap. If you want to
burn
a little money look into the dale power resistors which have an aluminum
outer heatsink covering. Since this covering is sinked to ground it will
also help with stray RF and may prevent some internal oscillations thus
keeping harmonics down a tad.

http://www.vishay.com/resistors-disc...wer-more-five/

Chad


That's an excellent suggestion A little more money sure won't break the
budget on this project. Thanks.

"DR. Death" wrote in message
...
"Chad Wahls" wrote in message
...

"Jay in the Mojave" wrote in message
...
DR. Death wrote:

I received the board today along with the Motorola engineering
bulletin
and
a list of construction hints and ran across my first problem. The
bulletin
lists R4 as 33 ohms. The construction hints lists it as 82 ohms. The
web
site resistor package lists it as 47 ohms. Hopefully they can clear
this
up
for me. I'll post the results when I get a reply.


Hello DR. Death:

One other thing that could be done to get a feel for the Value of R4 is
look at the other Motorola Engineering Bulletins, and see what they use
for the resistor. 33 and 47 ohms looks like a good value.

One problem I see is that because this resistor needs to dissipate 5 to
6
watts, a lot of these resistors used are too small in there power
rating.
Thetas why the 8 watt series/parallel circuit works so well. And the 2
watt carbon Resistors can't be too expensive. See:

http://www.rfparts.com

You could have a 33 ohm and a 47 ohm resistors, and switch between them
to
see what what works best.

And of course you could always measure the current of the two
transistors
at .6 to .7 Volts applied at the Base and Emitter of the Q3 and Q4
Transistors. This with ohms law would tell the range of resistors used
for
the bias supply for a pair MRF242 Transistors.

Jay in the Mojave

If you are really concerned you will want to use 5% or better resistors,
which, can be hard to find in large wattage for cheap. If you want to
burn
a little money look into the dale power resistors which have an aluminum
outer heatsink covering. Since this covering is sinked to ground it will
also help with stray RF and may prevent some internal oscillations thus
keeping harmonics down a tad.

http://www.vishay.com/resistors-disc...wer-more-five/

Chad


That's an excellent suggestion A little more money sure won't break the
budget on this project. Thanks.



Frank said it would be a moot point to have RF rejection in this area, do
trust him more than me as he has WAY more RF knowledge than I do. I'm an
audio guy!

BTW what's the budget on the project? Just curious.

Chad