Thanks for the replies to my queries on the LK550 -- I got some good info.
The replies generally support my theory that arcs in the gassy tube caused
plate choke damage (bunched turns), which in turn caused RF to leak into the
meter and ALO circuitry on at some operating frequencies. It's possible that
mistuning due to the bad meter readings caused the plate choke to burn up,
but more likely it was due to the previous damage from the arcs. Now I know
to take bunched up coils on a plate choke seriously.
A glitch resistor in the B+ line might have saved the plate choke, so that
mod is going in first. Billy Edwards, formerly of Amp Supply and now with
Omega, is building me a relacement plate choke with new parasitic networks.
He also has a nice supply of parts to fix cosmetic damage to the meter
covers, knobs, etc.
Billy is very friendly and knows a lot about the history and performance of
these amps. These items may be of use to other LK-series owners:
1. I didn't mention earlier that my amp is labelled "Amp International",
which puts it among the last amps manufactured by the company before it went
under (the QST ads stopped only one or two months after the name change.)
2. The step start board in mine is original equipment, not an add-on.
According to one report I got, earlier versions of the LK550 didn't have it,
so it must have been added later. Towards the end, they ran out of their own
boards and started using Billy's boards, which are labelled "Omega
Electronics". In fact, they used to send the amps to Omega to get the boards
installed, and perhaps some other finishing touches. The give-away is the
green paint on the board's screws, The factory slopped green paint on every
solder joint and many screw heads so they could detect when the amp had been
modified or repaired. This is also an indication of late manufacture. Billy
thought that they switched to red at the very end, so amps with that paint
might be the very last of all.
3. Another indication of late manufacture is a black wire running across the
back of the ALO board. This fixes a fairly serious design bug. The ALO
circuit will trip when it detects excessive plate or grid current.
Originally, if you set the meter function switch to "Volts" instead of
"Grid", the ALO would fail to trip on excessive grid current. The black wire
takes care of that.
4. There are at least two versions of the ALO board. One has green pots and
the other has black pots. The direction you turn to increase or decrease
sensitivity is exactly opposite between the two versions. With the black
pots, turning them counter-clockwise increases sensitivity (decreases
current required to trip), and turning them clockwise decreases sensitivity
(increases current required to trip.) With the green pots, it's the
opposite.
5. You don't have to remove the meters (difficult!) to replace the covers.
You can pry them off from the front. They were glued about 1/4 inch in from
each corner. Just slip a screwdriver between the cover and the faceplate at
that point and gently pry forward.
Hope this is of use to someone.
73, Dick, WC1M
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