a swing mod (more involved)

Uncle Ronnie 336 said:

if you can post those, i would love that. im gonna try to get ahold of one of my friends who has the 66xl. I would like to see the horrors of my work 10-12 years ago

Click to expand...

I saved 11 pages and I don’t know how to post them here . Lazarus posted a few different radio adaptations ie Galaxy, President Jackson and the modulation transformer type radios. Nomad and LC were active in that thread. Here’s a snippet from page 11 from Laz:

I'm gonna give a condensed description of the circuit in the OP, in an effort to prevent having to repeat it over and over again in private messages,

OK, here goes, How It Works,

The Darlington pair Q1 and Q2 are simply acting as a pass transistor,

R1, R2, and R4, make up a variable voltage divider which sets the idle bias of the Darlington pair, thus controlling the unmodulated carrier level,

R1 sets the maximum level at which R2 can adjust the idle bias level of the Darlington pair, therefore lowering the value of R1 will increase the maximum idle bias level, R4 sets the minimum level at which R2 can adjust the idle bias level of the Darlington pair, therefore lowering the value of R4 will decrease the minimum idle bias level, D1. D2, C1, and C2, make up a circuit known as an envelope detector,

C2 allows a small sample of the high level audio signal to pass through it to D1 and D2, where it is rectified, C1 then filters this rectified audio signal into a smooth DC voltage which proportionally follows the audio signals amplitude or envelope,

This smoothed out DC representation of the audio envelope is then fed to the base of the Darlington pair where it upsets the idle bias level set by R2, thus causing the carrier to rise in proportion to the audio envelope, therefore the louder you speak into the microphone the higher the carrier will rise as a result, and the value of C1 will determine how fast the carrier decays after modulation stops, for a slower decay time, increase the value of C1, the function of R3 and R5 is simply to allow each of the controlling circuits (the voltage divider and the envelope detector) to work independently without effecting each other, in other words, the envelope detector needs to be free to raise the bias on the Darlington pair without being loaded down by the setting of R2, without the use of R3 and RS that wouldn't be possible,

With a little effort tinkering with component values one can easily have this thing achieving a 100% modulation envelope at all times,

ideally the average DC output voltage of the Darlington pair into the modulator winding of the audio transformer, should be half that of the peak to peak voltage measured across the modulator winding at any given time during modulation, in which case the modulation envelope will be maintained at or near 100% at all audio levels,

OK, i think that pretty much covers everything

Thanks for that 711!

I might have that thread saved somewhere but i'll never find it.

I was a bit surprised by Nomad's response to Ronnie's post because i remember him being in the original thread.

Did anyone ever try this with a radio that also had an RFX unit on the back?
that could be interesting!
LC

From Lazarus pg 8

for those of you who may be trying to implement a 29-LTD or similar chassis with this circuit, i thought i would share a little problem i ran into when i did mine, and the solution to it,

because of the way the the 29 and similar type chassis are modulated through the primary winding of T1 instead of the secondary like real radios are, i had to place a 25v 470μf electrolytic cap across the emitter & collector of the Darlington pair, in order to restore the receive audio that was lost, due to the loss of inductive coupling through T1 when it was isolated from the B+ supply,

the only other solution to this problem would be to rewind T1 with 3 separate windings, one for the primary, one for the modulator, and one for the speaker, which is how it should have been done in the first place, i don't like the way it's wound at all, the wire is too small and theres not enough turns, both of which are largely responsible for the less than desirable low end response that this chassis exhibits,

here's a pic to help illustrate how to apply this modification to this type chassis, i have edited everything out thats not related to this modification and stretched a few lines here & there for clarity:

Also for a Galaxy

loosecannon said:

Thanks for that 711!

I might have that thread saved somewhere but i'll never find it.

I was a bit surprised by Nomad's response to Ronnie's post because i remember him being in the original thread.

Did anyone ever try this with a radio that also had an RFX unit on the back?
that could be interesting!
LC

Click to expand...

Froggy was active, so was Bow, Gadget, Sonoma, 2x4 and Billy to name a few. I never got around to trying it.

yeah neither did i.

i did have the note about the 470uf cap, so that must've come from you.

and thanks for all the diagrams!
LC

Uncle Ronnie 336 said:

corrected the schematic too.

Click to expand...

I guess I gotta make a board to try it now. My perf boards all look like poop lol.