DX-50 works as speaker

[Big Don Ho]

DX-50 runs fine at 25 kW. At 50 kW output drops to about 3 kW with lousy audio and no error lights. Bizarre thing is station audio comes out of the back of the TX at an almost painful level. This is the third vacuum variable in the output network as other have failed over time. Large output coil get very hot where it connects to vacuum variable. I know Harris did a lot of engineering by accountants, is it possible the cap is under rated?

[RGORJANCE]

It has been many years since I was inside a DX-25/50 so it would help to know which of the 4 variable caps, by circuit designation, in the output network, that you are referring to. It's repeated failure might be caused by something else in the output network/load .

And the "accountant scenario" you refer to was called the "value analysis program" that Gates Radio did back in the early 1960's, as i recall.

Fossil

[Deep Thought]

"station audio comes out of the back of the TX"

That is definitely something arcing under modulation conditions.

It is possible to tune one of these into a load which will cause all sorts of issues at full power while showing 0 reflected power. Your first step should be to verify that the transmitter is looking into 50 j0 at the output terminals.

[COMMENG]

I would shut the transmitter down and check two things:
1) the internal arc gap for dust, dirt, gap, etc.
2) the power combiner mother board(s) for dark spots and cracked toroids along the secondary rail rod.

COMMENG

[dwalltech]

Anything arching even slightly at 50K would emit audio. I found that the flex strap from the output buss to the main output coil gets overheated and the connections go bad - usually melting the solder joints and/or the strap connector. This also creates heat and leads to further failures. I would power off the xmitter and then re-work ALL the connections from the output buss where it comes into the final cabinet all the way out to the antenna connection at the top of the of the cabinet. You also need to check the 3rd harmonic caps in the final area. Just one of the 7 caps can cause havoc!!!
Don't know if this forum allows for other info - but I have a couple of those caps not needed anymore (new).

[RGORJANCE]

Overheated/hot connections can very easily be found using a laser pistol grip thermometer. Been using them for at least 10 years. Run the xmttr for a day, put the backup on, open it up and start pointing. You will find the hot ones really fast.

However, be aware that the cheap laser temp devices really don't work well, or not at all in a strong RF field.

Fossil

[NECRAT]

The OP hasn't returned to the forum since asking the question. I will drop him an email next week for a follow up, as it will be a month since he posted.

[Big Don Ho]

OK, guys, here's the skinny:

The output of the variable cap connects to the tank coil (why do they call it a "tank circuit"?) via bolted tubing. At full power this point would arc with modulation. I noticed the bolt was literally red hot at 10 kW when I walked int the transmitter room before turning the lights on. The cheap thermal measuring device goes bonkers with the transmitter running at anything above 10 kW.

I removed the melted bolt and Scotch-brited the hell out of the connection point, flattened the tubing to increase the contact area and replaced the bolt. Problem solved.

It would have been more easily solved had I not been working alone. One can't see the back of the transmitter when they are around front working the controls. Alas, one more product of reducing engineering staff.

[dwalltech]

If you notice, on the 19th, I suggested the connection to the final coil. Had that go back several times myself - until I took it apart, cleaned it up, and soldered the heck out of with silver solder. Need another final coil, I have a spare one now.

[COMMENG]

OK, guys, here's the skinny:

The output of the variable cap connects to the tank coil (why do they call it a "tank circuit"?) via bolted tubing. At full power this point would arc with modulation. I noticed the bolt was literally red hot at 10 kW when I walked int the transmitter room before turning the lights on. The cheap thermal measuring device goes bonkers with the transmitter running at anything above 10 kW.

I removed the melted bolt and Scotch-brited the hell out of the connection point, flattened the tubing to increase the contact area and replaced the bolt. Problem solved.

It would have been more easily solved had I not been working alone. One can't see the back of the transmitter when they are around front working the controls. Alas, one more product of reducing engineering staff.

Glad you found the problem.

The tank term is a historical term emanating from the fact that an LC circuit "stores" electrical energy; on one half of the cycle in the capacitor, on the other half in the inductor (coil), much as a water tank stores flowing water before it is transferred to another tank. Water flow is analogous to the flow of charge (current) in the exchange - it's an old analogy from the early days of electrical theory.

http://www.rfcafe.com/references/electr ... 1-1-20.htm


COMMENG