[Amps] Alpha 87a Low receive

[Jerry]

Hi Bob,

I didn't say the pin diodes were indestructible only that in a well-designed
amp like the 87A, the failure rate is low.  It isn't the average power that
blows them but also the thermal impedance and pulse time to a spike.  I
cannot speak about the Alpha 86 but the 87A fault protections seem to work
quite well in protecting the amp and diodes from dumb mistakes. I have
transmitted inadvertently into an open line with full reflected power and
the 87A tripped without failure... you cannot do this to the pin diodes
repeatedly without breakdown but then what idiot does?  I do agree with John
W0UN that static spikes on an antenna not DC grounded are the most likely
culprits for sudden pin diode failure.  Also, I have replaced the TX --
UM4006D pins on a "non-working" estate sale alpha 87 and it was a fairly
easy and cheap fix ($45 plus 1 hour).  

PS --  I have a beautiful old Harris pin diode TX/RX switch in great shape
if anyone is interested.

73  Jerry NY2KW

-----Original Message-----
From: Robert Arenella [mailto:rja12 at comcast.net] 
Sent: Tuesday, June 14, 2016 1:03 AM
To: Jerry; amps at contesting.com
Subject: Re: [Amps] Alpha 87a Low receive

On 6/13/2016 6:03 PM, Jerry wrote:
> The TX and RX pin diodes have been remarkably reliable with infrequent 
> failure.

   I'll stick with my statement that pin diodes were / are the Achilles Heel
of all the Alpha Amps that used them, especially the 86. If they were so
reliable, why did Alpha design & build a replacement T/R board for the 86
that used vacuum relay switching?  Why did 86 owners buy EVERY ONE that
Alpha made?  They were not cheap, about $450 each.

   The pair of those Tx pins could handle about 20 watts of reflected power.
My 86 trips at about 2:1 SWR.  Do the math.  At the legal limit, that's WAY
more than 20 watts of reflected power.  P-O-O-F!


73,
Bob
N2OAM