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Post by hellcat on Jun 2, 2019 22:29:08 GMT -5
I have a Mark lVA with brand new tubes in the transmitter and receiver. After a 2 minute warm up it will dead key 4 watts. Everything is fine for about 15 minutes, then when you dead key or talk you can watch the wattage drop from 4 watts to 1 watt in a matter of seconds. Any suggestions on what to check or replace while troubleshooting this?
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Post by 2600 on Jun 2, 2019 23:24:59 GMT -5
I would first check the final tube's plate current when it's putting out full power, and then check it again after it "falls down".
You select either USB or LSB, should not matter which. Select "Ma" on the meter switch and key the mike. If the bias control on the rear is set correctly, the needle should fall on the small square mark. Go back to AM and let the radio warm up. When the AM power falls down, go back to sideband, and compare the "Ma" meter reading. If it's lower, maybe the final tube has trouble.
Maybe.
A 'new' tube may be better than the 'old' tube.
Might not. This is where a good tube tester pays for itself. We won't install a new tube in a customer's radio without testing it first. Troubleshooting a fault caused by a bad tube always wastes time compared with how fast the tester would have warned you before trying that tube in the radio.
If you don't see any change in the first test above after the power drops off, I would try swapping each of the 3 tubes on the RF circuit board, the one with V301,302 and 303 on it. First swap V301, the 6CB6. If the fault is still there after the warmup, move on to the 12BY7 in the middle.
The 7558 final is probably not at fault if it passed the "Ma" meter test. But swapping with the old one won't waste much time if it fails to make a difference.
If the 40-plus year-old electrolytic capacitors have not *ALL* been changed yet, the power supply may be at fault. Electrolytic capacitors are meant to last ten or fifteen years, not 40-plus.
73
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Post by hellcat on Jun 14, 2019 22:27:31 GMT -5
2600 this transmitter also has a jumper wire with a switch across R 607 which more than doubles the output of this radio when first warmed up, swinging to around 18 watts. Is this a safe mod for this radio? If not could this have damaged something? I swapped the 3 tubes out mentioned above and still the same thing.
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Post by 2600 on Jun 14, 2019 22:53:28 GMT -5
That switch is what we call the "nitrous bottle".
It gooses the carrier power beyond the available audio power to modulated it, and reduces the on-air audio level.
Makes the wattmeter read higher.
For a while. Tends to hammer the final tube. Not healthy for the 6BQ5 modulator, but not as stressful as it is for the 7558 final.
Running it with the nitrous switch closed for any length of time could weaken the final tube.
If new, unused spare tubes perform the same, this suggests the first set of tubes didn't get clobbered, and the fault lies elsewhere.
You said "the three tubes". Did this include the 6CB6 mixer tube?
73
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