Floyd,
ALC and Ig is monitored by a common LM324 device from what appears to be the
same sampling line. U1A drives Ig LED monitoring while U1C drives the high
side of the ALC pot (via connector J9-6). In fact, U1A out bifurcates and
connects to U1C through R21.
I would check to ensure continuity between the high-side pot, connector J9-6.
and one end of R25. The output of U1C directly drives D11, a 1N4148 diode.
Cathode is to LM324 out. The diode's anode connects to the other side of the
R25.
Since you do have Ig readings, there's not much between the U1 sections and the
ALC pot.
P:aul, W9AC
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Floyd Sense
Sent: Saturday, February 16, 2019 2:05 PM
To: amps@contesting.com
Subject: [Amps] Alpha 89 ALC question
Please - this is a simple technical question about the Alpha 89. No lectures
about the evils of using ALC, etc.
I recently acquired an Alpha 89 amp from the original owner and am checking it
out thoroughly. The only problem I see is that no ALC voltage appears at the
ALC jack on the rear panel, regardless of the setting of the rear panel ALC
pot. It seems to me that the ALC voltage is developed by U1C on the power
supply board and that it's based on the grid current which must be measured
between the -HV rail and ground. (Haven't been able to discern exactly where
that is sourced in the schematic). At any rate, the grid current monitoring
obviously works as the Grid Current LED meter reads properly. But, no ALC
voltage at all with grid current showing up to around 100 ma on the meter. I
would think that I should see a gradually increasing ALC voltage (in the
negative direction) as the grid current increases. Can anyone familiar with
the circuit comment? Thanks for your help.
73, Floyd - K8AC
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