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Comet 200 amp repair. (Previous failed project of mine.)

D

doffo

Well-Known Member
Oct 14, 2012
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476
73
Hello,

Its been awhile since I messed with this amp. I did my best to add SSB Biasing, and thats when it took a dive and burned up on me. (The two resistors next to eachother were to make up for the burned up resistor I didn't have), and it still cooks the resistor when you transmit. Basically it draws max amps and roasts that resistor.

I am assuming either both or one of the MRF455 is toast. How would I go about trying to see if the MRF455 is bad? Lets say only 1 is bad, is it possible to run the amp with just 1 of the MRF455 in place?

Winter is coming up so just getting some projects together to work on. Thanks in advance for any guidance. :)

Comet 200 Amp 20251102_190522.jpg
 
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Added bias? Where? The center tap of the input transformer has a RF choke on it, with the other end grounded. Won't put any bias voltage on the transistor base leads that way. The resistors that burned appear to be in line with the radio drive power. A failed transistor tends to short internally from collector to base. This burns up the half-Watt 10-ohm resistor attached to each base lead. They look okay in this pic. I would blame the burned resistors on excessive drive power. A pair of MRF455 will be overdriven by any radio that shows more than 4 Watts forward modulated power. And half or less that much carrier. A Cobra 2000 that busts a gut to swing 14 is about the perfect size to drive this amp.

What size radio did you drive it with?

73
 
nomadradio said:
Added bias? Where? The center tap of the input transformer has a RF choke on it, with the other end grounded. Won't put any bias voltage on the transistor base leads that way. The resistors that burned appear to be in line with the radio drive power. A failed transistor tends to short internally from collector to base. This burns up the half-Watt 10-ohm resistor attached to each base lead. They look okay in this pic. I would blame the burned resistors on excessive drive power. A pair of MRF455 will be overdriven by any radio that shows more than 4 Watts forward modulated power. And half or less that much carrier. A Cobra 2000 that busts a gut to swing 14 is about the perfect size to drive this amp.

What size radio did you drive it with?

73
Click to expand...
I removed the failed SSB Bias parts lol.... I put back stuff to the way it was, but it still burned up the resistors you see in the picture... The second it seen any power, it instantly cooks the resistor. It was on my trusty 146 GTL radio which does around 10 watts SSB max on peak modulation, and kept the AM carrier to just under 2 watts....

I think you helped me immensely on how to go about putting the stuff together, but in the end it just cooked.

Just at the point for this amp to restore it back to working order.
 
Any chance there's an excessively-tall solder bump on the bottom surface of the pc board? That could create a short to ground against the heat sink surface. There should be no current through those resistors beyond the radio's drive RF.

73
 
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nomadradio said:
Any chance there's an excessively-tall solder bump on the bottom surface of the pc board? That could create a short to ground against the heat sink surface. There should be no current through those resistors beyond the radio's drive RF.

73
Click to expand...
Hard to take a picture, but for sure no short against ground.
Comet 200 Amp 2 20251102_220802.jpg
 
Oops, Here is the picture of the other side.... Thanks for your input Nomad. Your advice is greatly appreciated all of these years. If this amp is a lost cause then no worries. Just was hoping to bring it back from the dead... :)

Comet 200 Resistor Side 20251102_222216.jpg
 
Here's a pic of a silver streak 200 linear. Exact same board as the comet 200. The resistor that was burned shows as a 47ohm-2 watt resistor. Probably connected to the hi/lo switch and pads the input drive down in the lo position. You can check this by putting the hi/lo switch in the hi position and connecting an ohmmeter across the com and lo terminals on the switch. Maybe your value and wattage rating are wrong. If replacing the resistor fixes the problem, I would replace the caps inside the yellow squares with silver mica types. ( 2nd Pic. ) Just good practice on that circuit.

J.J. 399

COMET AMP.png



SS1`50.png
 
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unit_399 said:
Here's a pic of a silver streak 200 linear. Exact same board as the comet 200. The resistor that was burned shows as a 47ohm-2 watt resistor. Probably connected to the hi/lo switch. Maybe your value and wattage rating are wrong. If replacing the resistor fixes the problem, I would replace the caps inside the yellow squares with silver mica types.

J.J. 399

View attachment 74728


View attachment 74727
Click to expand...
Those pictures help. Thanks. :)
 
Allright......New development...

Kept the amp on the low switch, and now its working?! But you can't go on the high side, as thats when things go south and the amperage draw sky rockets passed 30+ amps. Least this time the relay didnt fully stick on and cook something.


Even if I gotta keep the amp on the low side, ill be fine with that, as long as it works. Surely the amp doesnt draw more than 30 amps while flipped on Hi on a 2watt am carrier??

------

After using the amp for a little, that resistor I replaced does get HOT to the touch. There might still be something going on with it though it works on the low side.
 
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AudioShockwav said:
What are you driving the amp with, what radio?
Click to expand...
I have a couple, but currently an old 140 GTL radio and have the AM carrier set at just under 2 watts. The moment you even try it on the Hi setting, it will skyrocket the amperage 30+ when you key up instantly. Of course I let go so it turns back off.
 
Check out the hi/lo switch. top terminal should go to the 47 ohm resistor. Middle term should go to center pin on input SO239 when the amp is in xmit. Bottom pin should not be connected to anything. If it is, disconnect it and test the amp again. Are you certain your wattmeter is accurate. Maybe doing more then 2W DK. Was the hi/lo in HI when the resistors smoked ??
 
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unit_399 said:
Check out the hi/lo switch. top terminal should go to the 47 ohm resistor. Middle term should go to center pin on input SO239 when the amp is in xmit. Bottom pin should not be connected to anything. If it is, disconnect it and test the amp again. Are you certain your wattmeter is accurate. Maybe doing more then 2W DK. Was the hi/lo in HI when the resistors smoked ??
Click to expand...
The amp was on Hi when it let out the smoke, but this was when I tried to add the whole SSB Bias circuitry to it earlier this year. I removed that all out, but it still gets that 47 ohm resistor hot.
 
If the 47 ohm resistor gets hot when the switch is in hi, there is a bad connection in that circuit. Or a miswire. You may have to bite the bullet and remove the board fro from the heatsink. Can you post a picture showing the connections on the hi/lo switch.
 
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