Comet 200 amp. Wishing to add SSB AB Bias. Help!

[doffo]

Hello Fellow CBers!

After all these years, I wish I took the time to actually hear my audio on the other end to be sure I was even happy with it. As of recent, I learned that my SSB audio just sounds overall bad on the RM Italy 203P. Best described as crunchy on the peaks. Which set me to try and do something about it.

I struggle to fully understand schematics and electronics, but I do decent on soldering stuff together. After looking at the pinned thread, I want to be sure I am going about it correctly.

This is the Comet 200 amp picture from the inside.

If I am understanding, that green resistor near the bottom-left will have to be removed and put in place a 103 (.01uh) 500v ceramic cap?

Also, I am only finding TIP42C transistors, but are those ok to use in place of the TIP42G its calling for?

I will have to Microsoft paint over the picture to be sure I am soldering stuff right. I just wish to finally sound ok on SSB.

Any help on this would be greatly appreciated. I did get this picture from the pinned thread which looks to be based off the same design.

 

[doffo]

I did find some Radio Shack 1N5404 diodes just now in my parts bin. Safe to use those too?

 

[doffo]

Going to stop here for tonight. I managed to at least get the parts installed and Microsoft Painted in the missing parts I need to put in. But is this the correct way? The picture is kinda blurry from the thread, but I am assuming the center pin of the TIP42C is right on the 12v, the right pin going to the 40 ohm 10 watt wire wound, and the left leg going to the relay being a 1k ohm 1 watt resistor? Or should it be a 2 watt part?

 

[nomadradio]

Almost. The center pin (collector) of the TIP42 needs to be feeding the 40 ohm 10 Watt. The emitter should connect to the fat red wire. The 1k resistor will have about 12 Volts across it. The power it will dissipate is the voltage squared divided by the resistance. 144 divided by 1000 is just more than 1/8 Watt. Safe enough for a 1/4 Watt part. A half-Watt resistor would barely get warm.

73

 

[doffo]

Almost. The center pin (collector) of the TIP42 needs to be feeding the 40 ohm 10 Watt. The emitter should connect to the fat red wire. The 1k resistor will have about 12 Volts across it. The power it will dissipate is the voltage squared divided by the resistance. 144 divided by 1000 is just more than 1/8 Watt. Safe enough for a 1/4 Watt part. A half-Watt resistor would barely get warm.

73

Thank you so much! I will correct the transistor. I believe I may have 1/2 watt 1000 ohm somewhere in my parts. My brother has some misc. parts drawers, and I swore I seen some RF Chokes in there! Feels like a large step going towards the right direction.

Only question for now is, I thought The Commet amp was "auto ssb" in a sense that on either mode, it at least keeps the relays on long enough to keep the relays from chattering. Thats why I removed the 470uf 50V cap while I was underneath removing the green resistor. If that is the cap where I can increase the value to the 2200uf, I can just do so from the top side. (I hope I dont have to get back under the board again!)

I included the picture of the bottom of the board to maybe help if that is what the 470UF capacitor did on the topside.

I just found it strange on the bias thread, that he had to include the 2200uf, but the picture didn't show the stock 470uf cap that shoulda been there.

 

[doffo]

Bah... No luck on sourcing an RF Choke and a 10w 40 ohm wirewound resistor.

I did at least find some .01 50v and .01 100v ceramic capacitors. Will either work or do they need to be closer to 500v rating?

 

[nomadradio]

Those caps should do just fine. They won't see voltages anywhere near 50 Volts DC. There's nothing sacred about the 40-ohm value of the power resistor. I would put a 5k ohm trimpot in series with the 1k resistor in line with the transistor's base lead. You need to have a way to set the actual current draw of the RF transistors with no drive. Using a trimpot should give you some leeway with the resistance value of the 10-Watt resistor.

73

 

[doffo]

Those caps should do just fine. They won't see voltages anywhere near 50 Volts DC. There's nothing sacred about the 40-ohm value of the power resistor. I would put a 5k ohm trimpot in series with the 1k resistor in line with the transistor's base lead. You need to have a way to set the actual current draw of the RF transistors with no drive. Using a trimpot should give you some leeway with the resistance value of the 10-Watt resistor.

73

Is this where the .6-.7 volt needs to be set? Putting the positive lead from a DMM on the Cathode end or the Annode to set the voltage?

 

[nomadradio]

You're not setting voltage, really. Yes, it will be in the ballpark of 6/10th of a Volt. The exact voltage is affected by temperature, power-supply voltage and the manufacturing variation of one transistor to the next.

Your target is the zero-signal DC current through the RF transistors with NO DRIVE.

This usually involves keying the relay with a gator-clip lead, dummy load attached. A DC Ammeter in line with the amplifier's power lead will show the current drawn by the relay coil. front-panel LEDs and the bias base-current source all added together. With the bias pot set BELOW the setting where the RF transistors start to draw current, note the meter reading. This is your reference or "virtual" zero current. Advance the bias trimmer until the current reading begins to rise. Pretty sure that type transistor should have either 80 mA or 100 mA zero-signal idle current. Setting the trimpot for the original reference reading PLUS 150 to 200 mA should clean up the sound of sideband transmit. Setting it higher just gets the RF transistors hotter for no good reason. And you already know what setting it too low will sound like on the air. It's not an exact adjustment, just needs to be in the Goldilocks zone between too hot and too cold.

Once this is done, feel free to measure the exact base-bias voltage that was needed for this set of transistors. That exact voltage will be different from one batch of transistors to the next. That's the reason to make it adjustable. It's not the voltage that's important. What matters is the RF transistors' collector current.

73

 

[doffo]

Appreciate the information. Since the amp has a Pre-Amp, should I go ahead and turn it on when I take the mA draw prior? I hardly ever use it or will it not make too much of a difference?

I still struggle to understand how the relays are working or what gets them to click on over, but what two pins will get the relay to click on?

 

[doffo]

Allright, Think I am getting somewhere...

With just the bare amp and none of the added transistor and circuits, it was reading 60 mA with the relay clicked on. This is with the Pre-Amp off

Pre-Amp on, it was seeing 135 mA draw with the relay on.


With the relay off and no pre-amp, it was at 25 mA
With the relay off and pre-amp on, it was at 101 mA

is that about right?

 

[nomadradio]

is that about right?

Probably. No need to turn on the preamp. It's a receive-only feature, not part of the transmit side at all.

Once the bias is hooked up, subtract that 60 mA from the current draw to see what the RF transistors' idle current is.

73

 

[doffo]

Probably. No need to turn on the preamp. It's a receive-only feature, not part of the transmit side at all.

Once the bias is hooked up, subtract that 60 mA from the current draw to see what the RF transistors' idle current is.

73

Thank ya!! I will wait once the parts roll in and see what I can get in numbers.