Low Output From My Mark III AM/SSB

[amastron]

Hi all,
I noticed the other night that my transmitter is only showing 2.5 watts output on multiple watt meters.
Friends have told me it seems to not be as loud, s-meter wise as it was so my question is do I have a tube going bad like the 7558 or is it worse?
And, is there anything I can check or do at my end to get her back to 4 watts? It's all stock and I never use a "box" on it, I don't even own one!

TIA
Tom

[Tombstone (R.I.P.)]

Did this power reduction happen overnight or did it get lower and lower over time? Yes, first check your 7558 tube. If it's okay try peaking for more wattage. I believe that 6GH8 serves as the oscillator and the final driver, you should check it. There's a transformer with a slug in it that controls the tube but it's very delicate and has a sharp point where it is peaked for full drive, not a slow increase or decrease in power as you turn it. If you're unsure about this leave it alone as you can throw the transmitter off frequency. When it's peaked that should also be the point where the frequency is correct. There are two points at which the oscillator shows peak but I forget if it's the setting closest to the chassis or farthest from the chassis. It obviously is at the correct position or the radio wouldn't have run correctly before. it might need a tweak. With the transmitter keyed up, SLIGHTLY turn the slug in each direction and stop when it peaks. You may have to slightly heat this slug with your soldering iron to get it to turn. Use an alignment tool that fits the slug, not a screwdriver, you stand a chance of cracking the slug and then then the slug will have to be replaced. I doubt that it's the problem anyway but at least check those two tubes. One or more of the power resistors might have changed to a higher value too over time. One may have cracked and not working also. These resistors are the long ceramic ones closest to the final output tube. I assume that your antenna match is within limits. I would bet that it's the 7558 final. These hints are at the fringe of my knowledge, 2600, fixr, or Sandbagger can help you more. Good luck!

Tombstone

[amastron]

Not sure when this started, I don't keep a watt meter on it.
I was told by a good CB friend one day it sounded low and then another said I wasn't giving him as much on his S-meter.
This was re-done by Barketts back on 1/15/08, sorry Fixr and total cost was $384.17 I would hope he replaced the tubes at least!
In place of the 6GH8 I have a 6GH8A, not sure about that I only know enough to not let all the smoke out at once.
Guess I'll buy a few new tubes, 7558 & 6GH8 and try that first.
Antenna match is good at 1.2, I did put a dummy load on and it made no change in output. Also dug out my manual and checked the BIAS and went through the output tunning check. Thanks BTW for the quick reply Tombstone, I appreciate that very much.

Tom

[Sandbagger]

Aug 2, 2010 20:33:49 GMT -5 amastron said:

Not sure when this started, I don't keep a watt meter on it.
I was told by a good CB friend one day it sounded low and then another said I wasn't giving him as much on his S-meter.
This was re-done by Barketts back on 1/15/08, sorry Fixr and total cost was $384.17 I would hope he replaced the tubes at least!
In place of the 6GH8 I have a 6GH8A, not sure about that I only know enough to not let all the smoke out at once.
Guess I'll buy a few new tubes, 7558 & 6GH8 and try that first.
Antenna match is good at 1.2, I did put a dummy load on and it made no change in output. Also dug out my manual and checked the BIAS and went through the output tunning check. Thanks BTW for the quick reply Tombstone, I appreciate that very much.

Tom

As a general rule, alignment doesn't drift unless something changes to alter the impedence of the affected stage. So your best bet is to first check the tubes. If it turns out that the tubes are not the problem, and an alignment doesn't bring it back, you should start looking at voltages. If the B+ voltage drops to the output and modulator tubes, it can cause the problems you're seeing.

[FIXR]

Tom,

What is the SSB output?

Fixr

[amastron]

On LSB and USB it's between 1to1.5 watts when I speak in the mic and the Mod Meter just does move a little, not much at all.
If I just key it on SSB it shows nothing on the watts, speaking normal it bounces up to 1.5 watts if I'm close to the mic, not like AM where I can set back a bit and see the Mod meter bounce.

Tom

[amastron]

UPDATE** fixr got me all stright with some new tubes! Thanks fixr.

amastron

[cbrown]

Glad Fixr was able to help sort it out.