Discussion in 'Amplitude Modulation' started by N8FVJ, Apr 14, 2020.
Did your pictures post properly? My computer can't find them for some reason.
I tried to edit and I timed out, photos were lost
See if this one shows:
Did we come to a conclusion of the best of the five designs in attachment?
Doing more tests, and a few revisions with an external SS driver amplifier to the
existing Ranger primary. I think this is as good as the existing modulator iron can
handle. I can get a bit better modulation depth at the expense of more distortion.
Ranger is lightly loaded to 35 watts output to reduce modulator requirements at 100 Hz.
Highest modulation test was at 6-kHz. first photo is 6 kHz modulation before I ran out
of drive voltage!
Second photo is 100 Hz at 35 watts RF output.
Lowest test photo was at 1ooHz. I wanted better low end response to reduce inband
I was of the impression you were going to, or had picked one of the circuits you posted in #50.
I only increased the 500pF capacitor and removed two capacitors in the audio preamp section. I read removing the driver transformer current is a good idea. There are 5 designs and some have issues with phasing and the mod transformer feedback winding. I cannot make heads or tales of what works correctly. Should I proceed with one of the five designs or leave the driver circuit as is.
I would recommend making all of the speech amplifier modifications first and then get on the air and elicit audio reports.
When you get your oscilloscope and are able to make additional measurements, go to the more advanced mods if you are still not satisfied with the rigs performance.
One has to remember that while making upgrade mods are fun, any one transmitter is only capable of a certain level of performance because of its deign and the economics of the period.
IMHO, if you want performance over and above what you have, then home build a transmitter from one of the many designs on the Interweb. And many here on QRZ have experience doing just that.
Very good advice.
There is also the option to radically redesign and modify what you already have to make it perform like you want. Junkers make good platforms for this to keep the purists from gnashing their teeth and screaming "blasphemy!" when you start drilling, blasting, and poking bigger glass out of the top of cabinets.
I bought a working Heathkit HO-10 monitor scope. I will replace all the electrolytic capacitors and the two .15uF 1600 volt capacitors before it fails. I will be able to view the Ranger lower frequency modulation. If bad news, I can raise the D-104 lower response by changing the input resistance. I can tune the mic for 10dB down at 100Hz, yet have a close to a flat 200Hz response.
I think it's best to make mods one at a time, and check out the result of each before proceeding to the next. Too often, some proposed mod doesn't work out as planned; if you have made several different changes and the end result didn't come out as expected, it's a lot more difficult to troubleshoot a series of possibly unsuccessful mods than just a single one. Multiple deficiencies in performance caused by multiple faulty modifications are likely to interact with each other and add up, making troubleshooting all the more difficult.
Yep, one example is the limitations of Ranger audio due to the modulation transformer. A better transformer (inevitably heavier and larger) would improve performance, but there is just barely enough room inside for the stock transformer. One alternative would be an umbilical cord that plugs into the auxiliary socket in back, leading to an outboard transformer, but then you have a mess.
Just as with the series of modifications, when building an entire transmitter from scratch I would highly recommend building it one stage at a time and getting each to work satisfactorily before proceeding to the next. For example, get the IPA working before assembling the driver stage, and get the driver stage working before assembling the final. Then build the speech amp and get it working before proceeding to the modulator stage. Probably the first thing to start with would be the power supply(ies), then proceed from there.
I knew a fellow who completed an entire homebrew AM transmitter in a 6 ft rack before powering anything up. It took him several months to get all the bugs worked out before he could use it. One danger is finding some component that didn't work out in one of the earlier stages, and the only way to mount another one that does work is to shuffle around some of the components of a later stage that has also not been tested. Building a multi-stage transmitter is like a game of chess.
But I would recommend evaluating what you have that you want to modifiy, and unless it really is a junker, think through the modification before drilling the first hole. I hate to see something like a nice 75A- series or a pre-war HRO receiver with a jagged hole blasted in the front panel from some Hammy Hambone modification that didn't work out.