At 05:59 PM 10/11/2005, Dick Green WC1M wrote:
Danny--
Many thanks to Dick, WC1M, for quoting the "old man" himself (I can
say that, Dick Ehrhorn is traveling on vacation and not reading this
reflector.
My somewhat flip comment about "read the manual" this
morning was due to two reasons -- 1) I was late for a dental
appointment and 2) I didn't have a 781 in front of me and I
was not sure of the exact names for the critical knobs.
The problem is setting gain/drive up too high and letting the
ALC loop control everything. This HAS to result in overshoot
since there is a time constant in any feedback loop. Proper
selection of component time constants can reduce this but
not eliminate the problem.
While there may be assembly differences (translation = errors)
in some 781s I can take ANY particular 781 and make it have almost
no overshoot to where it looks like there is an extreme overshoot
-- just by adjusting (or mis-adjusting) the controls.
Here is what the 781 manual says (in paraphrase)
-------------------------------------------------------------
BASIC SSB OPERATION
Monitor output power with the meter, key the rig and speak into the
microphone.
Adjust the RF PWR to the desired output.
THEN push ALC and adjust MIC GAIN which should be held in
the ALC zone.
For more talk power, push COMP.
------------------------------------------------------------
SPEECH COMPRESSOR OPERATION
Preset Mic Gain to center of range
Preset RF POWER to max COUNTERCLOCKWISE
Preset COMP IN
Push Po on meter switch
Select Transmit and speak into the microphone
Turn the RF PWR control clockwise to desired output
Push COMP on meter switch
Adjust the MIC GAIN control for an S-meter COMP scale reading between
10dB and 20dB
Push ALC on meter switch
Adjust DRIVE for a meter reading within the ALC zone
--------------------------------------------------------
CW OPERATION
Push Po on meter switch
Select Transmit
Key rig
Adjust RF PWR to the desired output power
Push ALC and adjust DRIVE for a meter reading within the ALC zone
---------------------------------------------------------
The sequence of these steps is a bit counter-intuitive and I have found that
if I tweak up the rig without referring to this sequence I can get all of the
meter readings to read "correctly" but with a leading edge
spike. If I follow
the above procedures then the meter readings are still correct but there is no
more spike.
I can't speak for other rigs, but I assume most of the high-end Icoms behave
the same way.
As to why you are seeing a Fault 1, Danny, I don't know. I was never a great
tech on the Alpha 87As. All of mine have performed very well and have required
virtually no maintenance. And as Pres. I was not on the bench all
that much! ;-)
But Glenn, AE0Q, can probably tell you about the Fault 1 -- although I know he
suggested sending in the amp. I would try the above tune-up
procedure first and
see what happens.
Fault 1 is related to back bias voltage on the PIN diode TR
switch. If it is a soft
fault it is due to the back voltage not switching to minimum level
with key down
and NO RF. It is a hard fault with the key down and RF present.
Since you admitted to being an appliance operator, Fault 1 diagnosis
is probably
something that should be done at the factory. It involves a lot of
disconnecting or
disabling components and checking voltages under various conditions. On older
amps the most likely scenario is that D15 is leaky. Later amps changed the
regulation of the RX bias voltage with a zener chain and D15 is no
longer needed.
Hope this helps and remember, I am not an 87A tech nor do I play one on TV!
73 es gl John W0UN
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