low am modulation MK III

[dirtyjob]

had a guy give me a browning mk III transmitter, i hooked it up to my mkIII receiver andfound that the transmitter has very low, next to zero modulation on am, the rf carrier is great and healthy at 4 1/2 watts, when i tried ssb it operates fine, modulates very well there.

Short of packing this up and shipping it out for repair where or what should i look for as far as am modulation issues on this transmitter ?? My net research has shown the mode switch may be at fault but I would appreciate any insight from you guys on this.

[TheCBDoctor]

had a guy give me a browning mk III transmitter, i hooked it up to my mkIII receiver andfound that the transmitter has very low, next to zero modulation on am, the rf carrier is great and healthy at 4 1/2 watts, when i tried ssb it operates fine, modulates very well there.

Short of packing this up and shipping it out for repair where or what should i look for as far as am modulation issues on this transmitter ?? My net research has shown the mode switch may be at fault but I would appreciate any insight from you guys on this.

Hi Dirtyjob

You will need Sams Photo Facts CB-79 for that radio. I can look it up tonight to see what the board layout is. Most radios use an audio circuit to produce AM while SSB is produced at the balance modulator. If it is the same for a Browning Mark III I will let you know. If that is the case then you have a bad audio tube in the transmitter. The receiver has its own audio tube.

It has been a long time since I worked on a Mark III so my memory is a bit foggy on that radio. Once I look over the schematic I can give you a more precise response.

Here is a link to my website on that radio.

[Please login or register to view this link]

[Please login or register to view this link]

Respectfully,

---

Monday, 13 May 2013, 16:55 PM

Hi dirtyjob

Check out the AM modulation tube 68Q5. It may be bad or it may have a problem in the related audio circuitry with 68Q5.

Respectfully,

[bossman327]

I would check the 12ax7 and the 6bq5 also known as an el84 tube in the modulation circuit. There should be a pot next to the 12ax7 for your modulation to turn clockwise to turn the modulation up. I prefer the JJ brand of tubes which a 12ax7 is only 10 bucks and an el84(6bq5) is only about 15 bucks. I would start there as someone has already said. Bossman

[Island_0860]

The good thing is that the low modulation is constant...not intermittent.
Should be an easy fix and you have a backup xmitter to boot....some guys. ...lol i see your rig pictures...good luck DJ

[dirtyjob]

thnx guys, ill check that tonight, i have a tube tester and many extra tubes, i hope thats the case, its just a pain to bring the rig to my small bench. I will report back with my findings.

[KOA4705]

thnx guys, ill check that tonight, i have a tube tester and many extra tubes, i hope thats the case, its just a pain to bring the rig to my small bench. I will report back with my findings.

The Mark 111 is the best out of the bunch for AM broadcast audio if that's a goal with a few simple mods.The only real problem with the audio chain design is the voltage limiter circuit. The limiter generates distortion by clipping the peaks of the audio waveform due to odd order harmonics (fourier analysis).This circuit is followed by a 3 pole low pass filter to reconstitute the signal to remove the harmonics...... sounds good in Theory. Concerning the low level audio issue do the simple things first tube swapping look for obvious damaged components in the audio chain etc beyond that you will need some basic test equipment.
High Plains Drifter