Browning Mark3 broadcast audio?

[KOA4705]

Hello All,
My friend Marc asked me if his Browning Mark 3 transmitter would be a candidate for high fidelity audio for AM. After reviewing the schematic it looked encouraging, for one thing the transmitter is plate modulated. The first thing in order was to sweep the audio chain with a signal generator and look at the audio response at the output of the transmitter with a scope. The stock response was 400 to 2800 Hertz with phase and amplitude distortion, which is less than telephone quality. The first two audio stages use a 12AX7 which are straight forward. The 3rd and 4th audio stage is a voltage clamper circuit. This limiter circuit clips the input signal which distorts the signal and produces harmonics (Fourier analysis). Which is then put through a low pass filter to remove the harmonics and have a so called reconstituted signal. The final drive stage uses a 6BQ5. By making a few simple changes to the 1st, 2nd and driver stage and eliminating the clipper circuit. To my surprise because the modulation transformer being the limiting factor, output response to date is 60 to 15,000 Hertz with low distortion. I might still incorporate a loop of negative of negative feed back from the secondary of the modulation transformer to a earlier audio stage to see if any further improvements. I also have provided a line level input so you drive the transmitter directly from a audio chain, for example ......microphone, mic preamp, EQ, and a compressor/ limiter which its output goes into the transmitter line level input
Joe

[Night Crawler]

If you haven't done it already the low end frequency response can be improved by the changing the interstage coupling caps.

C45 from .001 to .01 C46 from .005 to .05 and since the clipper and filter were remove couple the output of the plate of the 12AX7 speech amp to the grid of the 6BQ5 modulator with a .05

[KOA4705]

If you haven't done it already the low end frequency response can be improved by the changing the interstage coupling caps.

C45 from .001 to .01 C46 from .005 to .05 and since the clipper and filter were remove couple the output of the plate of the 12AX7 speech amp to the grid of the 6BQ5 modulator with a .05

Yes, of course all coupling capacitors have been fatten up to move the pole to below 10HZ, also the cathode bypass capacitor for the 6BQ5 needs the same treatment . Their is plenty of audio gain to apply negative feedback, the cost will be gain and stablity (nyquist criteria) for improved response and linearity.
Joe

[KOA4705]

In the near future I will post a audio clip of the Browning Mark 3 transmitter output quaility. As received on a Kenwood R1000 receiver in the AM wide mode. Also I will provide all the technical details for modifications which are simple and non invasive and is completely reversible.
Regards,
Joe

[KOA4705]

Audio byte after mods

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[209 first class]

wow, that has the feathery high end (treble) of a nice studio mic. you said the reciever was bandwidth modified, i find the problem 11m radios is the reciever is audio bandwidth limited, and the speaker usually does 2-300hz up to about 2k. some go to 3k. from 3-5k is where the silky treble is, its tough to convey over these 3 inch speakers. it sounds fantastic by the way. 2zero9

[KOA4705]

wow, that has the feathery high end (treble) of a nice studio mic. you said the reciever was bandwidth modified, i find the problem 11m radios is the reciever is audio bandwidth limited, and the speaker usually does 2-300hz up to about 2k. some go to 3k. from 3-5k is where the silky treble is, its tough to convey over these 3 inch speakers. it sounds fantastic by the way. 2zero9

Hello 209,
The microphone that I used is a Behringer B1. I used a R390A receiver which is a general coverage HF receiver for the posted recording. The R390a IF bandpass is selectable from the front panel, .1,1,2,4,8,16Khz .........I used the 16Khz position for the recording. Good point about the limited 11 Meter receiver bandpass, is this really the Normal 300-3k? Are there exceptions? A reciever IF amplifier tuned circuit band pass can be increased by lowering the Q (parallel resistor) or possible stagger tunning.If mechanical and ceramic filters are used you are pretty much stuck unless you replace the filter with a wider one. Yes 3" speakers don't do it LOL
Regards,
Joe

[209 first class]

i end up having to add like 12-16 db of 4k to even notice its there on some radios, the bandwidth filter is so tight. i know the scooped mids tone is not hifi, but with all the other radios peaking around 1k, its a good way to stand out. do you fix brownings? im looking for a light repair, and that mod sounds great. only having the bandwidth limiting on the reciever will require less eq and probably increase talk power from the transmitter having to reproduce less bass and treble to begin with. 2zero9

[Dirty Diaper]

Hi, I just found this area. I have several Browning Mark111's. I would like to do some of the mods you have talked about- changing caps etc. Just to increase the quality and volume of the transmit audio. I tried to view the clip, but it was closed. With-out ripping out the original circuits, what mods can I do to notice an improvement. I am trying to learn all I can about this radio.My friend-ilike2race has Brownings also. Thanks-J.T. Also several members have already helped me out with other issues.