[Boatanchors] LK-500ZC Amplifier Help

[Bill Cotter]

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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