[Amps] Three questions (all related to Amp Supply LK-xxx series 3-500Z amplifiers)

[Curt Nixon]

Recently spent a good deal of time with a few lks.  One I found the
internal connections in the meters was loose.  Also, found very difficult
to see cracks in the solder eyelets on the alo connectors.  I used a
precision power supply and dvm to validate the main shunt resistor values
to be sure what they were showing was correct.  You can set the alo limits
that way as well.   There seem to be several versions of the alo.  Bad
ground connections on the rectifier board and alo connections cause all
kinds of strange things.

On Sat, Oct 21, 2023, 2:50 PM wb0gaz via Amps <amps at contesting.com> wrote:

>  Working on a LK550 for a friend - amp has various control/metering/low
> voltage circuit problems (ALO/Meter board, QSK board, etc.) including
> evidence of probably two generations of prior issues in this area.
>
> Three questions (arising from poor documentation) -
>
> 1. (possibly specific to LK550). The LK550's ALO/Meter board appears to
> accept unregulated 12V (I guesstimate around 13-14VDC half wave rectified
> and filtered) from the onboard low voltage/filament (two secondary winding)
> transformer (in the LK550, the HV xfmr is external and is disconnected.)
> The ALO/Meter board shows some signs of damage from a prior life (a few
> cooked traces), but not yet that far along. Presently trying to understand
> the use of D7, which is a 1N4742 12V zener diode which sits as shunt across
> the unregulated (somewhat over 12) input from the low voltage part of the
> power supply PCB. I've never seen this configuration, where a DC power
> supply is driving a shunt zener directly, without a dropping resistor. The
> only way I can see that the Zener's current is being limited is by reliance
> on the intrinsic resistance of the corresponding secondary winding on the
> onboard transformer. By the time I encountered the amp (my friend bought it
> in this condition), D7 had been re
>  moved.
>
> 2. (likely common across the LK450-500-550-800 range) The two meter
> movements (one for plate current, one for grid current or high voltage)
> appear that they may be voltmeters in the millivolt range. They are shunted
> by bidirectional silicon diodes and a small (20-ohm-ish) resistor in series
> entering the diodes (presumably to limit damaging current into the
> back-to-back diodes), however, it is not clear from the schematic (which in
> the LK550 manual was evidently copied from a version of the amplifier
> without the ALO/Meter board, so close-but-not-exact) what the meter
> characteristics are (full scale volts or microamps, either is need to
> know.) I can work this out experimentally, but if there's anyone with a
> known spec, that would be really helpful.
>
> 3. (Probably common to grounded grid direct heated cathode amplifiers in
> general.) I'm working with the amp with the HV transformer removed, so the
> only power in the RF deck is filament voltage and the 12/24V low voltage DC
> circuit. The tubes are still present. When powered on, a small residual
> cathode current is seen on the plate current meter (and a small standing
> voltage at the grid current resistor-to-ground connect point.) Is this
> possibly due to the tubes rectifying a potential between the cathodes (hot
> with AC) and the grounded grids? After puzzling over this today, I think on
> next go I'll proceed with the tubes removed, but as this wasn't expected
> behavior, comment would be appreciated.
>
> As the issues the amp has are limited to low voltage DC/switching type
> circuits, the HV transformer is fully unplugged to eliminate that source of
> live circuit risk.
>
> Thank you for comments on above or related.
>
> 73 Dave WB0GAZ wb0gaz at yahoo.com
>
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