Browning MarkIII Transmitter - Meter Problem

[Space Cadet]

I've got a Mark III Transmitter, everything works as it should except when the meter switch is in the "MOD" position. When the switch is set to "MOD", the meter deflects full scale, and the radio will only transmit a carrier, no audio. This only happens when on AM, if the radio is set to usb/lsb, the meter behaves normally even while in the "MOD" position. Also, if the meter switch is in any other position, the radio transmits (and modulates) perfectly.

I have checked the diode (sr4), the cap (c55) and the resistor (r53), all of which test good. With the diode disconnected, (no connection from meter to modulation transformer), the meter still pegs, and I am reading about 270V across the leads that feed the meter. There are no *obvious* shorts that I can tell. According to the schematic, with the diode disconnected, the meter circuitry should be dead (when in the mod position), so I cannot figure out why there is HV on the circuit. Also checked all the electrolytics, which are ok as well... Any ideas?

 

[fixer]

MKIII meter

Check your personal messages.

 

[Space Cadet]

thanks!

 

[nomadradio]

Your mode (AM/USB/LSB) switch has failed. High Voltage is leaking across burned brown plastic on which the contact points are mounted. This permits juice to jump from a 300-Volt circuit into the meter circuit.

Very, very bad for the expensive meter if you do it too long.

Odds are that you will find that the resistor shown below is burned, if you look for it underneath, connected to one lug of the big 3-section filter capacitor. It should read red-red-orange-gold. Odds are that one or more of the color bands is burned black, or that the part is scorched and splitting open.

There are "patch" fixes for this that will disable the SSB modes. Don't think that info is posted on the web anywhere. A new switch is expensive to buy and also to install.

Oops.

Actually, this is an increasingly common problem. Seems that 30 years is an "awkward" age for this control. No good way to "fix" that switch that we have found.

73

 

[Space Cadet]

Thanks for the info guys. It was in fact the mode switch. I was able to scrape and "dig" away at the switch deck enough to disable the path (between the contacts) that had developed. The resistor was ok, and the meter appears to be working normally now, so hopefully she'll last... time will tell...

73 ~Vince

 

[nomadradio]

Right on Vince, congratulations.

Sounds like you caught it early enough. When R51 gets roached, stuff downsteam from it does, too. Glad to hear it still looked okay.

Makes me sorta wish for the brick-red high-voltage enamel paint. Seems to me it was called "GLPT" or "Glyptal" or some such. The idea was to smear it onto cracks in the insulation of H.V. parts in televisions, and stop the blue "fizz" corona discharge. Serves well at lower voltages, but getting a drop of the stuff ONLY into the gap you made sounds really tricky.

I recommend taking a magnifier and a bright light to the front deck of the mode switch. Look between other pairs of rivets for signs of darkening in the brown bakelite. You may find another spot where preventive medicine will prevent the next episode of this. Usually it gets dark around one rivet, before breaking down like that one did.

I gotta ask. Does SSB transmit still work?

So long as there is enough of the unburned plastic left, maybe you can rescue this switch, and get some mileage out of it.

73

 

[Space Cadet]

I like the idea of the HV enamel, seems like that could be useful in situations like this. I was carefull to take a good look at the rest of the switch, the only other thing I could find was a carbon / corrosion buildup on the rotating contacts. I was able to clean those up pretty nice with some alcohol and cotton swabs. SSB transmit is working great; seems I got lucky here...

73 ~Vince

 

[Eastside]

Your mode (AM/USB/LSB) switch has failed. High Voltage is leaking across burned brown plastic on which the contact points are mounted. This permits juice to jump from a 300-Volt circuit into the meter circuit.

Very, very bad for the expensive meter if you do it too long.

Odds are that you will find that the resistor shown below is burned, if you look for it underneath, connected to one lug of the big 3-section filter capacitor. It should read red-red-orange-gold. Odds are that one or more of the color bands is burned black, or that the part is scorched and splitting open.

There are "patch" fixes for this that will disable the SSB modes. Don't think that info is posted on the web anywhere. A new switch is expensive to buy and also to install.

Oops.

Actually, this is an increasingly common problem. Seems that 30 years is an "awkward" age for this control. No good way to "fix" that switch that we have found.

73

My mark III mode switch is shorting out and what you describe has happened.....I need to know how to do this patch.....don't care about the ssb.