New 300 Maco Problem (idjjoker1)

[paws264]

I took the liberty of starting a new thread for a post at the bottom of a long archived message.

I have an orange face later model maco 300 using the 5- 2057's as soon as i turn it on the tubes randomly turn blue and then after a minute or so start glowing orange. i have checked all the tubes on the tester and show no shorts . i have even only put 2 of the 4 finals in and get the same results . also depending on where i turn the air-variable caps at a certain point the transmit light comes on , but obviously the relay is not engaging and there is no exciter. my first thought was a short to ground on a cap at a socket so i have removed each one , one at a time and checked for short but none. Could it be a bias problem? Also if i try the amp out quickly without leaving it on too long it does work and good numbers too , but when i get it tuned it is at the point where when i un-key it has no receive and the transmit light is on. Any help would be great , especially since i can't find the schematic on the orange face version . Thanks

The Tricky CB Site is down so, access to documentation is limited.

Your amplifier is going into "self excited oscillations" (boiling).

Pull the driver tube(s) out of the first stage and, see if the final tubes still start to turn on by themselves.

Yes, it is a bias problem; the amplifier appears to use a self-bias scheme on the first stage that could turn it into an oscillator.

Using this diagram:

If, you look at the driver tube circuit, input tuning lash up between the cathode and the control grid is an accident waiting to happen; try adjusting the input tuning to see if that changes the symptoms that you are seeing.

If and when CB Tricks is back up, I will look for the Orange-Faced model schematic to see what is different between the two units other than tubes.

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