[ETO_Alpha] Pin Diodes

[Jerry Cudmore]

Hi all. I originated the post regarding the pin diodes and would like to 
follow up with more information. There is a voltage divider from the 2800 v 
source comprised of 4 sets of 470k film resistors. Two resistors in 
parallel equal about 240k. One set of these were open and the other was 
about 600k. When I first got the amp a couple of months ago I looked inside 
to confirm the voltage taps on the transformer. At that time since this was 
a recent purchase, I did a careful visual inspection. I did notice one of 
those film resistors had a burn mark. But the amp was working so I did not 
"fix it" as sometimes we tend to do. Last Saturday the same diodes that 
were replaced before had failed. When I called Alpha they sent me the parts 
no charge under warranty. I was pleased with their assistance and believe 
customer satisfaction is a prime concern for them. The Alpha technicians 
admitted more 89 amps come in with this same pin diode problem that 87A's. 
The 89 and 87a use the exact same PA/RF/Power supply. So why do more 89's 
fail?.

I would tend to think operator error. In my case I am very religious about 
tuning and always have used my tuner with just the exciter. The amp never 
saw more that 1.5 to 1 swr any time. I also never push it past the 1500 
watts. So I still have no explanation as to why the diodes fail. I now have 
extras in case there is a next time. I also have never seen a fault 
condition on this amp because I never created a condition to cause it to fault.

Technical tip

If the diodes open you will have no receive rf. If they short you will not 
be able to get any power out since the soft fault will kick in because 
excessive current is drawn in relation to rf produced. Unfortunately there 
is no theory manual. I have learned quite a bit about the protection 
circuits, it is comprised of many op amps as comparators "watching" 
voltages currents, temperature. When it faults you never know what caused 
it to fail.

Maybe by thinking about what precipated a pin diode failure in these amps 
we can come to logical conclusions.

I would be interested in hearing from any users of 86,89, 87a owners who 
have had this type of failure.

Thanks, 73's

Jerry







At 08:03 AM 3/6/02 -0500, you wrote:
 >Hello to all Alpha users,
 >    I seem to remember Alpha had a problem on the '86 with low bias voltage
 >on the pin diodes.  I would suggest, if you have problems on an '87A, to
 >check and make sure the bias voltage is high enough.
 >    As someone else said, the service fellow in the southeast should be able
 >to help you if you don't want to go to the new Alpha in Colorado
 >    Good luck & 73,
 >George        K8GG
 >
 >----- Original Message -----
 >From: "Jerry" <k2jrc@pobox.com>
 >To: <eto_alpha@mailman.qth.net>
 >Sent: Sunday, March 03, 2002 1:14 PM
 >Subject: [ETO_Alpha] Pin Diodes
 >
 >
 >> Anyone know of a better part to replace the receive pin diode in the 87A,
 >> 89 series amps. The part is a KS1001, the amp uses 4 of these. My 89 has
 >> blown these twice this year. I never push it past legal.
 >>
 >> Thanks.
 >>
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