Sunday, September 12, 2010

Amp Supply Company LK-500ZC Rebuild

This project started when I lost a blocking cap on the B+ and it smoked the plate choke.  I have owned the amp for a few years now and knew it had some issues.  The ALO circuit had problems and was bypassed, the fan speed control circuit was not working, and the plate tuning cap had been arced badly and ground down.

With the ALO bypassed I only had plate voltage metering and no current metering at all.  I think the problem with the ALO is a burned resistor that is used as a shunt for the sensing circuit.  This makes tune up a questionable process, but with the forgiving nature of 3-500Z Eimacs I used the amp anyway.  The fan speed was controlled by a light dimmer I installed on the rear to vary the speed of the fan.  I am uncertain of why the original fan speed circuit was bypassed and the switch for high/low was missing.

During dis-assembly I noticed the band switch had the 15m possition burned.  I guess it's a good thing I never used the amp for 15m!

I have removed all components from the chassis and cleaned it. 

I have ordered new blocking caps and installed them along with the new plate choke.

And the new E.F. Johnson Plate tuning cap.
The stem is longer than needed for this amp and I will have to cut it down.

More to come as the project progresses....................................


  1. 11/11/11:

    RICH, K0ZX 303-901-5100

  2. Hi David, hope you get this. I just acquired an LK500-ZC. In going through it I notice it has a fuse block in series with the B+ just before the plate choke, mounted on tube socket board just in front of fan. I'm new to amps, but noticed the fuse after it blew. Puzzled about a 2A 250V fuse on a 3kV line. Your rebuild looks beautiful, so I thought you might have some insight on this. Thanks, Bill Kulze W2NVD

  3. Hi David,

    I have problems with ALO as well. How do you bypassed it?
    Thanks for the help.

    73 de Julio EA4KR

  4. Julio, I don't really remember how it was done. It was in that condition when I acquired the amp.

    I did manage to get the rebuild almost finished when I purchased another LK-500ZA. The ZA looked nicer to me so I ended up robbing parts from the ZC to put the ZA chassis in top shape. I sold the ZC chassis with the rebuilt bandswitch and air variable to a local ham who is in the process of converting it with the GI7B Russian tube.

  5. In the 1st pick, those red caps rated at 12KV, can you tell me the value on the caps? I need to replace some and the value is "smoked" off.

    1. Gary, I do not know right off, but check back and I will try and get the value and post in the comments.

      Just to be sure, you are talking about the group of paralleled caps near the band switch in the tank circuit. If that is so, they are used as a lumped value in parallel that is switched in as additional padding for the air variable caps. These are used for the 3.5 MHz position if I remember correctly, and another lumped value is switched in for the 1.8MHz position as well and those I think are located under the ceramic coil form on the divider in my amps.

      I'll try to get a value for you soon...

  6. Yes David those are the ones. The ones I smoked look like they are under the inductor but I was in the 80 meter slot on the band switch when it happened. I bought this amp new in 1987 so I guess it's about time to change them all since some of those let go.
    Gary - N5KDA

  7. Well, sorry for the long delay. I have sold the ZC chassis because I acquired a ZA that looks way better than the newer ZC chassis. I robbed parts from the ZC to put the ZA in taop shape. Another ham purchased the ZC chassis to use as a project to convert to the GI7B Russian tubes. The tank circuit and PS are in place for him, so all he needs to do is get the tubes to fit. As for the capacitor values, I tried to read them in my amp, but can't see the markings and I can't get a photo with the camera that I can enlarge and see them.