Too busy at work and dont have the time to confirm or look at a schematic so take this with a teaspoon of precaution.
Also a link to the source. GF delete if not OK
Mark IV, IVA and Mark III HiFi mod/ direct inject.
Saw this posted else ware. I can not confirm this was successful nor have I ever performed this HiFi mod. It is just information by others that may or may not be useful:
Browning Mark 4 and 3 HiFi mod from Night Crawler:
To bypass the clipper refer to the schematic for the Mark 4 Transmitter.
1. Remove V204 the 12AU7 that tube is used for the clipper.
2. Find C220 and L601 on the schematic locate those parts in the transmitter.
3. Disconnect the end of C220 that's connected to L601.
4. Connect that end of capacitor C220 to pin 2 of of V204 the 12AU7 after the tube is removed.
5. Do not reinstall the 12AU7
The clipper is now bypassed also C213,214 and C215 can be changed from .001uf to .01uf for more low frequency response.
If you want to inject the audio directly to the modulator instead of connecting the one end of C220 to pin 2 of the 12AU7 connect it to your rack gear.
Question: "Would I need more output from my rack if I direct injected the audio? I see that would be bypassing the 12ax7 which is the pre amp tube if I'm not badly mistaken. Would I need more output from the rack gear to drive the 6bq5? Thanks again for our help. Oh yeah also clipping c220 before or after the point that it goes to c219? Would I need more output from my rack if I direct injected the audio I see that would be bypassing the 12ax7 which is the pre amp tube if I'm not badly mistaken."
Answer: “Yes you would.”
You can inject the audio from the mixer into the 12AX7 through the mic jack but the inter-stage coupling capacitors C213,C214 and C215 determine the frequency response of the microphone stage those can be changed to a .01uf or a .05uf to improve frequency response.
Try it both ways and see what works best for you, also R219 will be your mic gain control and R603 the clipping control will not longer function after the modification.
Clip C220 before C219.C218,C219 and L601 are used for the high frequency filter for the clipper so you don't want anything connected to that circuit.
Observation: "My friend Marc asked me if his Browning Mark 3 transmitter would be a candidate for high fidelity audio on AM. The Mark4 uses the same audio amplifier circuit. 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 and a Step response with a audio function generator. Looking 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, this is 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 audio signal and produces odd order harmonics. 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 and reducing the equivalent capacitance across the secondary of the modulation transformer. This will greatly improve fidelity and eliminate distortion of the transmitter. To my surprise the modulation transformer frequency response is quite good. The low end does suffer a bit because the Modulation Transformer lacks inductance but remember this was designed for communication audio. Output response was 60 to 15,000 Hertz (-3DB) 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. I do have wave file to demonstrate the results. The phone jack output from the receiver was feed directly into digital recording"
Read more: http://grumpy.proboards.com/thread/7590 ... z5CpfESOQe