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   Home   Help Search  
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Author Topic: Alpha 91B Weird Issue  (Read 1939 times)
N4ATS
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« Reply #30 on: June 25, 2019, 03:25:24 AM »

 I have an external hv power supply. I am going to pull out the soft start board and see if there are any blown traces on the back that may not short the 20 ohm resistors. Make sense 
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W1QJ
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« Reply #31 on: June 25, 2019, 04:09:59 AM »

Can’t you leave the HV unplugged put a voltmeter across the resistors to see if the resistors  get shorted out?  You wouldn't have to go through the hassle of removing the board.

  Have you tried exciting the HV AC terminals with an auxiliary transformer fed directly with no step start to see how the rectifier / capacitor board plays out?  This would tell you if the rectifiers and caps are making DC voltage with no shorts or anything.  Something is pulling a lot of current it seems.
« Last Edit: June 25, 2019, 04:21:20 AM by W1QJ » Logged
N4ATS
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« Reply #32 on: June 25, 2019, 02:58:43 PM »

Glenn, R2 is about 1250 ohms , I will change it. Both 10 Ohms are about 9.5 ohms each so they are OK. SS is shorting out BOTH 10 Ohm resistors , no issue there. Appears it may be the transformers that you stated in an email today with the HV short internal. I sent you an email about borrowing one for a day and I will pay both round trips...
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W1QJ
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« Reply #33 on: June 26, 2019, 03:26:26 AM »

Glenn, R2 is about 1250 ohms , I will change it. Both 10 Ohms are about 9.5 ohms each so they are OK. SS is shorting out BOTH 10 Ohm resistors , no issue there. Appears it may be the transformers that you stated in an email today with the HV short internal. I sent you an email about borrowing one for a day and I will pay both round trips...

R2 in the primary area of the SS circuit looks to be a resistor that is in series with the blower.  Doesn't seem like it would have anything to do with your problem.

You think you have 2 shorted transformers???
« Last Edit: June 26, 2019, 03:47:20 AM by W1QJ » Logged
N4ATS
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« Reply #34 on: June 26, 2019, 05:02:22 AM »

 yes I am pretty sure both Transformers are shorted I think I got Glenn talked into sending me one to test and then I'll send it right back and order a new one if I have to. I have two 91b  amplifiers that came into the shop for repair with the exact same problem
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W1QJ
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« Reply #35 on: June 26, 2019, 05:23:25 AM »

Bill, Don't you have any other HV transformer that you can hang on the two AC inputs of the PS circuit to excite the PS?  Isolate the B+ and put a HV meter on the output to see if there is a problem with the diode/cap board?  Any transformer should do just to check to see if the B+ is coming up without a load on the transformer.

The step start is clearly working as shown in your video.  As soon as you hook up the AC to the PS board, things go bad.  Sure it could be you have two shorted transformers, but it certainly isn't on the primaries.  I am very suspicious of your diode/cap board having some problem.  Have you put a HV AC voltage meter on the individual leads of the secondary and measured for any leakage to ground on either leg of the HV AC secondary leads?  I have run across this before. 

I know that subbing in a known good transformer will tell the story, however you should be able to discover a problem through some testing.  I recall where I isolated the transformer core from ground and all was well, as soon as the core was grounded problems.
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N4ATS
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« Reply #36 on: June 26, 2019, 08:21:53 AM »

Yes. As stated above I already excited the HV part of the amplifier with an external power supply. Glenn  sent me an email about the Transformer problem.... he's seen this before.
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W1QJ
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« Reply #37 on: June 26, 2019, 11:12:11 AM »

Oh wow.  Good to note that this is a problem spot with these amps.  The TenTec Titan 425 transformers eventually short out in the one 120v primarily winding.  I replace the transformers with the Ameritron AL800H transformer for $247.00 vs $800 for a PD replacement.  I’ve got several transformers that are NG.  Does anybody rewind them?
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N4ATS
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« Reply #38 on: June 27, 2019, 10:17:34 AM »

Yes there's a guy in Tampa that rewound Transformers for me in the past. I lost contact with him. I'm probably going to resurrect it if I can although Glenn is supposed to get back to me if I can borrow one from Alpha for a quick test.  it's quite ironic that I got two 91B's with the same exact problem.....
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N4ATS
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« Reply #39 on: July 08, 2019, 02:45:23 PM »

Oh ya....

https://youtu.be/guttCsuMb3s

Thank you Glenn at Alpha for letting me borrow a transformer, you rock
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W1QJ
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« Reply #40 on: July 09, 2019, 04:43:46 AM »

WOW, two bad transformers!!  Like the Titan 425 amps.  I have a collection of shorted Titan 425 transformers.  I suppose they could be rewound if someone does a good job of it.  Does Alpha still sell those transformers?  Cost?  Good show Bill. 
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VR2AX
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« Reply #41 on: July 09, 2019, 05:22:04 AM »

Impressive. An old ETO version as well. I had one of the first in 1996. It seemed indestructible back then. Cudos to you for the perseverance.
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N4ATS
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« Reply #42 on: July 09, 2019, 07:21:53 AM »

Yes, Peter Dahl is making them for Alpha and are  about $550 per Transformer.
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K6AER
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« Reply #43 on: July 11, 2019, 05:24:48 PM »

For the last twenty years Alpha has been buying their transformers from:

Tranex, Inc.
2350 Executive Cir
Colorado Springs, CO 80906
Contact:   Jerry Smith
Phone:   (719) 576-7994
Website:   www.tranexinc.com

If you need a new transformer I would contact them first.
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