Collins 30L-1 Output drops at higher bands

[Tim Freeze]

I just bought a 30L-1 at a local auction.

It puts out 500 watts on 3.5 MHz and 7.2 MHz

On 14.5 MHz it's 350 Watts.

On 21.2 and 28.25 MHz its 200 Watts.

Any thoughts on where to start trouble-shooting?

I'm thinking of checking the bias voltage and working from there.

I'm driving it with a Ten-Tec Omni -D with 100 watts output. The reflected power in the exciter is low, less than 1 watt.

I've ordered the power supply HV upgrade kit since the caps are old and this amp has sat for many years.

Thank you for your suggestions.

[Ralph Mowery]

Post by Tim Freeze
I just bought a 30L-1 at a local auction.
It puts out 500 watts on 3.5 MHz and 7.2 MHz
On 14.5 MHz it's 350 Watts.
On 21.2 and 28.25 MHz its 200 Watts.
Any thoughts on where to start trouble-shooting?
I'm thinking of checking the bias voltage and working from there.
I'm driving it with a Ten-Tec Omni -D with 100 watts output. The reflected power in the exciter is low, less than 1 watt.
I've ordered the power supply HV upgrade kit since the caps are old and this amp has sat for many years.
Thank you for your suggestions.

The tubes are probably soft. The power supply voltage could be low due
the bad capacitors, but that would not account for the drop off in power
as you went higher in frequency.

You did mention the swr was very low so hope you checked that on all the
bands.

[Tim Freeze]

Post by Tim Freeze
I just bought a 30L-1 at a local auction.
It puts out 500 watts on 3.5 MHz and 7.2 MHz
On 14.5 MHz it's 350 Watts.
On 21.2 and 28.25 MHz its 200 Watts.
Any thoughts on where to start trouble-shooting?
I'm thinking of checking the bias voltage and working from there.
I'm driving it with a Ten-Tec Omni -D with 100 watts output. The reflected power in the exciter is low, less than 1 watt.
I've ordered the power supply HV upgrade kit since the caps are old and this amp has sat for many years.
Thank you for your suggestions.

The tubes are probably soft. The power supply voltage could be low due
the bad capacitors, but that would not account for the drop off in power
as you went higher in frequency.
You did mention the swr was very low so hope you checked that on all the
bands.

Thank you for the advice. I did check the -170 bias source and it is -166.5 V DC with 5.5 volts ac ripple. So I'll be replacing the rectifier with a modern one.

Tim

[Scott Dorsey]

Post by Tim Freeze
Thank you for the advice. I did check the -170 bias source and it is -166.5 V DC with 5.5 volts ac ripple. So I'll be replacing the rectifier with a modern one.

You could put 1N4007 in there and it wouldn't hurt anything and might improve
long-term reliability. But I'd first replace that 10uF capacitor, which you
already know is bad and is most likely the source of that ripple.
--scott

[Scott Dorsey]

Post by Tim Freeze
I just bought a 30L-1 at a local auction.
It puts out 500 watts on 3.5 MHz and 7.2 MHz
On 14.5 MHz it's 350 Watts.
On 21.2 and 28.25 MHz its 200 Watts.
Any thoughts on where to start trouble-shooting?

There's not much inside there. I'd start by checking the finals to make
sure they are good, and then I'd start doing simple in-circuit checks on the
plate and screen resistors to make sure the thing isn't getting misbiased.
Those are common failure points.

Before doing anything else I would replace the power cord and wire it so the
hot lead is on the fuse and switch side (which was not the case for most
original installs) and if you are running it on 240V, add additional fusing.

I would check the 1800V lead with a proper high voltage probe. I would also
check the -170VDC supply and the filament supply to make sure they are within
a reasonable range; you can check the filament supply with it idling.

If you haven't replaced every damn electrolytic yet, now would be a good time.

Post by Tim Freeze
I'm thinking of checking the bias voltage and working from there.

Check ALL voltages. But check for baked resistors too.

Post by Tim Freeze
I've ordered the power supply HV upgrade kit since the caps are old and this amp has sat for many years.

I'd replace the .01uF RF trap capacitors on the AC input with modern X1 types
too, just for safety.
--scott