Sigma CD... HALP

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noschematic
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Sigma CD... HALP

Post by noschematic » Tue Aug 07, 2018 6:25 pm

Repeated crowbars happen until the Sigma Diamond drive finally goes down. Then it takes an act of God for it to come back up. It CB almost immediately when I beam. I removed the high watt resistors and it still fired. I removed the HV wire going to the CB assy and it did not fire, but failed to beam(with the HV going to GND that should be about right?). I have looked in the beam supply and the power supply before that.

I couldn't adjust the heater voltage to spec so i ordered a new rheostat, but i remain apprehensive.
My collector current is a bit higher than my good working cab(V2).
IOT is a D2100

I'm scared. :D

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NECRAT
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Re: Sigma CD... HALP

Post by NECRAT » Tue Aug 07, 2018 7:36 pm

noschematic wrote:
Tue Aug 07, 2018 6:25 pm
Repeated crowbars happen until the Sigma Diamond drive finally goes down. Then it takes an act of God for it to come back up. It CB almost immediately when I beam. I removed the high watt resistors and it still fired. I removed the HV wire going to the CB assy and it did not fire, but failed to beam(with the HV going to GND that should be about right?). I have looked in the beam supply and the power supply before that.

I couldn't adjust the heater voltage to spec so i ordered a new rheostat, but i remain apprehensive.
My collector current is a bit higher than my good working cab(V2).
IOT is a D2100

I'm scared. :D
When you said you can't adjust the heater voltage, do you mean on the Thyratron? Is the Thyratron at 6.3V?
Have you tried bringing the HV voltage down a tap or two to see if stable?

While not on a Sigma, on my old Comark I had this exact issue, and I had to run the tube into Beam without the drive, at a lower HV for a few days, and slowly bring the HV up, then slowly bring the drive up. CPI said it had to do with a buildup on the grid. I know in the Sigma (which I have now but not a ton of experience with), the only way to bring the tube up without drive is to kill the power to the IPAs.
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noschematic
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Re: Sigma CD... HALP

Post by noschematic » Wed Aug 08, 2018 5:17 am

Yes, the thyratron sorry. I am headed up the the site today with the new part. I'll let you know what I find out.
Thanks for the help!

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KPJL FM
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Re: Sigma CD... HALP

Post by KPJL FM » Wed Aug 08, 2018 6:58 am

Last few times I had that happen, the tube had arced inside the input cavity.
$$$$
Trim to fit, paint to match, tune for minimum smoke.

noschematic
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Re: Sigma CD... HALP(fixed!)

Post by noschematic » Wed Aug 08, 2018 4:15 pm

Well after installing my rheostat(twice), it came back on. However, not without a CB and a couple faults. I then opened it back up and noticed a dark spot with a crack on the HV wire running from the high watt fuses on the CB assy to the Isolation cab(?). I'm guessing I had a bad arc. I replaced it, fired it up, and it came on with no CB. Wish i could have saved the 3,4000 dollars on the rheostat, but I'm learning!
Thanks guys!

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Re: Sigma CD... HALP(fixed!)

Post by NECRAT » Wed Aug 08, 2018 7:31 pm

noschematic wrote:
Wed Aug 08, 2018 4:15 pm
Well after installing my rheostat(twice), it came back on. However, not without a CB and a couple faults. I then opened it back up and noticed a dark spot with a crack on the HV wire running from the high watt fuses on the CB assy to the Isolation cab(?). I'm guessing I had a bad arc. I replaced it, fired it up, and it came on with no CB. Wish i could have saved the 3,4000 dollars on the rheostat, but I'm learning!
Thanks guys!
So I will suggest if you had one HV cable fail, you're soon to have others. I would replace the rest of them as well.
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grich
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Re: Sigma CD... HALP

Post by grich » Thu Aug 09, 2018 9:14 am

KPJL FM wrote:
Wed Aug 08, 2018 6:58 am
Last few times I had that happen, the tube had arced inside the input cavity.
$$$$
On our old Sigma, we started with one tube, then added another PA section. Tube 1 lasted the life of the transmitter in our plant, which was until we fell back to our old VHF frequency and back into the cradle of solid-state bliss. The second PA tube became troublesome after a couple years, and had to be replaced.

I don't miss the KABOOM of the CB trips.

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Re: Sigma CD... HALP

Post by NECRAT » Thu Aug 09, 2018 10:41 pm

grich wrote:
Thu Aug 09, 2018 9:14 am
I don't miss the KABOOM of the CB trips.
Aww but they are so much fun.

* especially at 3:30am
* on an older style transmitter with the air-gap crowbar circuit.
* in a room that echos.

Adds life to your blood! (and loss of your hair, vision, sanity.......)

Having been in both, I still prefer tube transmitters. My old DCX3 was my baby. I kept that girl running smooth for years. I do miss her.
All will be replaced with post repack solid state rigs. The above DCX3 has about a month - month and half to live.
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