[Boatanchors] LK-500ZC Amplifier Help

[Mark K3MSB]

Gents

Thanks for your comments.

My paper copy of the manual lacked the addendum in the electronic copy
provided by Steve and I've now printed it out.  Thanks Steve.

Bill, the 20K Resistor is across two of the Molex Connector pins.  I put a
temporary 1K resistor on the board (just for bench testing) and the results
are now what one would expect:   With the relay deenegergized I get 1K
across the pins.   With the relay energized, I get a short.

The "open" in my original post was because the 22K relay was blown apart.

The addendum states "Filament Current Inrush".   Is this what is shown in
Figure B-5 of the schematic coming off the filament transformer choke
center tap?  I find no C1 with a value of 20000 uF and R6 with a value of
110 (or 100 ) ohms in any of the parts list.

I just checked with the fellow that was tuning up the amp when it blew.  He
indicated the amp shut down completely -- which I assume means lights out
etc.   That would indicate this is a standard inrush board and NOT what I
wrote in the above paragraph on "filament inrush" unless it took out the
fuse.  I need to check that.

73 Mark K3MSB

On Sun, May 8, 2022 at 8:28 AM Bill Cotter <n4lg at qx.net> wrote:

> Mark,
>
> The board you mention sounds like an aftermarket step-start board,
> or a late addition to the Amp Supply design. If the resistor (20ohm
> 22w) is in series with the AC line, then a step start circuit is
> the case. However, one thing in your statement doesn't make sense.
>
> "When the relay is closed,  the 20 ohm 22W resistor is placed
> across two of
> the molex pins.   When the relay is open, there is nothing across
> the pins."
>
> Every step-start circuit I have ever seen places the resistor
> between the "C" common of the relay and the "NO" normally open
> contact. The resistor is in series with one leg of the AC line to
> the power transformer, when the power first is turned on. When the
> HV AC voltage on the secondary reaches full voltage and the
> capacitors are fully charged, there is enough voltage on the LV
> secondary to pull the relay C to NO, and short out the resistor.
> This happens in usually less than 1sec.
>
> I would dig much deeper into the problem than just replacing the
> resistor. If there was one or more shorted electrolytics and/or
> diodes in the HV PS, the HV would not rise to enough to pull the
> step-start relay, and the resistor would be severely overloaded.
>
> Likewise, if there was a tube flashover and short in the HV PS, the
> Step-start would likely drop out and fry the resistor before the AC
> line fuse would blow.
>
> My approach would be to pull the tubes, and power the AC primary
> winding of the transformer with a variac, watching the primary
> current for a graceful rise as the HV caps charge. If a sudden
> rise, you know you have a more serious problem.
>
> Good news: Harbach has a nice PS board replacement if you need to
> overhaul the HV PS.
>
> Best wishes,
>
> 73 Bill N4LG
>
>
>
>
> At 04:04 PM 5/7/2022, Mark K3MSB wrote:
> >Greetings.
> >
> >The gang up the radio club managed to blow the amp several weeks
> >ago while
> >tuning up. They pulled out a circuit board for me to fix that has
> >a 20 ohm
> >22W resistor that is blown apart.
> >
> >I figured the manual would help me but the online manual, as well
> >as the
> >paper one I have,are not too helpful (and I'm being polite).
> >
> >Yes, I can replace the resistor, but I'd like to know what it was
> >connected
> >to (and I'm not up the club where the amp is).   Normally, the
> >manual is
> >helpful, but not this time.
> >
> >The circuit board is 3.5 inches square and has a 12V DPDT relay, the
> >aforementioned 20 ohm 22W power resistor, as well as a 110 ohm
> >resistor and
> >2200 uF cap along with a 4 pin molex plug.
> >
> >The 110 ohm resistor is a dropping resistor for the 12V coil and
> >the cap is
> >associated with that.  No problem.
> >
> >When the relay is closed,  the 20 ohm 22W resistor is placed
> >across two of
> >the molex pins.   When the relay is open, there is nothing across
> >the pins.
> >
> >Looking at figure B-5 of the schematic,  these could be parts C1,
> >R6, and
> >K1,  but I can't find anything in the various parts list that matches.
> >
> >Any help is appreciated.
> >
> >73 Mark K3MSB
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