Lou,
Yes good old Icom and the 781 overshoot problem.
The amps were Alpha 87A's (3CX800's) and the 781. The spike would send 200+
Watts on the first syllable to the amplifier and it would BLOW out the PIN
diode T/R switch.
ICOM continued to deny a problem and ETO ended up taking an 87A and 781 into
the shop and hooking a scope on to it and finding the ALC overshoot problem.
This find was because almost everybody who owned a 781 also owned an Alpha
87A and they were dying with warrantee repairs.
ICOM still continued to deny there was a problem.
Then it was found looking at the circuit that almost ALL 781's had a missing
cap and a couple switched around resistors in the ALC part of the circuit.
Almost ALL 781's suffer this affliction. Very late ones are OK.
ICOM still denys there was a problem.
I had a problem with occasionally BLOWNING UP the plate choke in my 77DD
amplifiers and Alpha DD conversions when running in the high voltage
position. This was purely SPIKE TRANSIENT INTOLERANCE.
The Signal One CX11A's also had this problem and would unload 300+W spikes
even though the power control was turned down.
Most modern transceivers have a problem.
I pioneered not using ALC between the amplifier and rig back in 1971,
however with these semi-modern SS HF rigs I've gone back to ALC use. It is
one more level of rig control and also allows me to turn up the drive and
get the old AVERAGE POWER back that has been lost to over active (but still
not perfect) ALC.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Gudguyham@aol.com
Sent: Sunday, March 18, 2007 10:37 AM
To: g8gsq@eltac.co.uk; amps@contesting.com
Subject: Re: [Amps] drive power
Some years ago a couple of contester friends of mine who were running the
Icom 781 (don't remember the # but it was the top of the line radio then)
They
both were running Aplha amps using tetrodes I believe. The amps only
required like 30 watts of drive, so they had to back the drive level back on
the
Icom radios. More often than not the Alpha amps would trip off on high
grid
current due to overshoot of the Icom radio. They both had the same problem
and
the both called Icom. Icom denied that the radios had any overshoot
problems. It was a night mare for these guys. All I remember was they
were really
angry. Don't know the final result, but that is what I remembered. Lou
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