Mark 3 meter pegs

[Van Lifeson]

So, I have read a lot about the meter pegging with modulation on a Mark three. What causes this? I have read that it could be the mode selector switch. I have inspected mine and I do not see any signs of traces, such as the carbon deposits people talk about. Doesn’t mean they are not there, but I don’t believe this radio was used very much and the switch looks to be in very good condition. But, my needle does peg violently when I have the switch set to modulation. Why does this do this?
Thanks

 

[Dmans]

First thought is that the transmitter is improperly aligned.(I.e.-Too much mike gain)

If you mean it stays stuck in the extreme RH corner even during receive, some voltage measurements are in order.

73
David

 

[Van Lifeson]

It does NOT stay pegged in receive.

 

[Unit 75]

Usually it's the mode switch that starts to break down that causes your problem. Do NOT use any kind of cleaner on it. I have also seen the 7558 tube not working correctly with the modulator circuit that pegs the needle hard. If you say your mode switch is ok, then try another final tube and see if it rectifies the problem. Just re-adjust your bias in ssb mode and peak your radio for max rf out on your wattmeter. Good luck
-Trainman

 

[DoctorAudio]

Check R53 and make sure it's still 10K ohms

 

[nomadradio]

If it only pegs when you talk, that's normal. Apparently Browning wanted you to turn down the internal mike gain to the 100% level.

If it pegs while receiving, that's bad juju. Can burn out the coil in the meter. When the first circuits in the selector are breaking down you'll generally find that the resistor pointed out below looks overheated or just burned.

There are two kinds of Mark 3 SSB transmitters: Ones that have had a breakdown in the mode selector, and the rest of them that will.

Just have to run it long enough. A radio with 500 original miles will take longer for the selector to break down than a high-mileage unit. But it's not a question of if, it's a question of when.

Gotta get organized and post the 'cheap' solution, hot-wiring the radio for AM only, taking the failed selector out of the circuits.

73

 

[Van Lifeson]

That would be great. An easy solution to make it AM only would be awesome.
Take the worry out of it blowing up LOL.

 

[Van Lifeson]

If it only pegs when you talk, that's normal. Apparently Browning wanted you to turn down the internal mike gain to the 100% level.

If it pegs while receiving, that's bad juju. Can burn out the coil in the meter. When the first circuits in the selector are breaking down you'll generally find that the resistor pointed out below looks overheated or just burned.

There are two kinds of Mark 3 SSB transmitters: Ones that have had a breakdown in the mode selector, and the rest of them that will.

Just have to run it long enough. A radio with 500 original miles will take longer for the selector to break down than a high-mileage unit. But it's not a question of if, it's a question of when.

Gotta get organized and post the 'cheap' solution, hot-wiring the radio for AM only, taking the failed selector out of the circuits.

73

Any idea why someone would put another 250 ohm in parallel with the one you show?
That’s what I have. What would this do? Should I remove it?

 

[nomadradio]

The original single resistor is there for filtering the rectifier ripple. It also causes some drop in the B+ voltage to the audio and RF power stages. Adding a resistor in parallel reduces the resistance in the power supply circuit, and raises the voltage supplied to the tubes.

Makes the wattmeter read higher. Does reduce supply filtering a bit, but probably not enough to tell. When a customer just wants to see more wattage from this model, we double the capacitance of the main filter capacitors. This allows us to use a resistor in that spot with a lower resistance, and get the same effect as simply soldering a second parallel resistor to the original one.

You'll see the carrier power fall a bit if you remove the extra resistor, and a bit less peak power on modulation.

73

 

[Van Lifeson]

The original single resistor is there for filtering the rectifier ripple. It also causes some drop in the B+ voltage to the audio and RF power stages. Adding a resistor in parallel reduces the resistance in the power supply circuit, and raises the voltage supplied to the tubes.

Makes the wattmeter read higher. Does reduce supply filtering a bit, but probably not enough to tell. When a customer just wants to see more wattage from this model, we double the capacitance of the main filter capacitors. This allows us to use a resistor in that spot with a lower resistance, and get the same effect as simply soldering a second parallel resistor to the original one.

You'll see the carrier power fall a bit if you remove the extra resistor, and a bit less peak power on modulation.

73

Besides removing the resistor in parallel,
What else can be done to lower the dead key
On a mark 3?