Amplifier help

[lerch256]

Hello I have a gray 200 with mrf455 that worked fine and I turned it on and when I key up with power on to the box it'll dead key the one watt from the radio and then fall to nothing can't get any power through it with the box powered up but if I turn it off I can talk threw it no problem any ideas thanks in advance

 

[Robb]

Do you hear the relay inside the amp 'clik' when you key the mic?

Amp attached to a 30 amp power supply with good connections and using 10ga wire?

 

[lerch256]

Yes itll key the relay

 

[Big Kahuna]

Do you have the radio jumper going to the correct input? I know on older amps if the TX RX stickers are worn off sometimes people plug into the wrong side and nothing happens

 

[lerch256]

Yes it's a base amp but I've I thought it might be the supply so I hooked it up to a meanwell 35 amp supply at 14.5 volts just to see no change I've replaced the 455's with 1446's and no change also replaced so 239 the relay and keying circuit and variable but it still won't talk just bleeds the power coming from my radio

 

[lerch256]

Yeah I got them correctly attached it's just strange it'll allow me to talk through it but nothing comes out when powered on

 

[Robb]

Well, if you aren't getting output, the next logical question would be: are the pills getting voltage?
Are the relays getting 14v out of it to the board?
Then; If the pills are seeing 14v; then the radio's drive signal is not getting thru.

 

[lerch256]

Well i have 16 v at the pills on the power strip but there is nothing on the input side of the pills maybe that makes sense.

When the amp isn't powered off does it just bypass all the coax in the amp and just jump from one side to the other on the relay

 

[lerch256]

Thank yall for the help BTW

 

[Robb]

Well i have 16 v at the pills on the power strip but there is nothing on the input side of the pills maybe that makes sense.

When the amp isn't powered off does it just bypass all the coax in the amp and just jump from one side to the other on the relay

Test with 14v. Really, you shouldn't run any more than that if you want to keep those pills you put in alive.

Yes, when the amp is off, then the relay is not powered and allows the signal to pass from the input to the output.

 

[lerch256]

Ok thanks

 

[lerch256]

So what you reckon would be keeping it from allowing the amp to send the incoming signal through the pills and out the output

If I remember correctly it's 16 volts at idle but when the pills are on the voltage reads 13.8 at the pills it has a 10000uf cap on the power buss

 

[lerch256]

I guess some pictures would help I'll try and post some when I get home

 

[nomadradio]

Umm, this is the small base amplifier with a toroid (doughnut) transformer in the rear.

Puts about 22 Volts on the RF transistors on standby. The tiny transformer drops it to 14 or 15 Volts under load.

Tends to blow out the 12-Volt MRF-455 parts.

Found that putting a 30-Amp automotive-type relay between the power supply and the RF transistors' collector circuit would protect the transistors from the destructively-high standby voltage. Keying it along with the antenna relay would connect the power only when the transistors are drawing current, and the voltage pulls down to a safe level. Beefing up the keying transistor to handle the current for two relay coils is a good idea, too.

Odds are 99 percent or more that the RF transistors are toast. That's what this model is famous for, along with exploding power-supply filter caps.

The other solution is to use more-expensive transistors rated for a 28-Volt power supply. The relay is cheaper.

73

 

[Robb]

One would think that a bridge rectifier and a smoothing cap should be there - at least.
If they are there; I didn't see them.
A regulator and a couple of pass transistors wouldn't hurt either.