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Amp Supply lk500ZB Help.

Discussion in 'Amplifiers' started by cbkidd1, Jun 14, 2019.

  1. cbkidd1

    cbkidd1 Active Member

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    IMG_20180416_152150902.jpg I got my lk500 ZB Amp back up and running finally. It was not easy to figure out. The trouble I am having now is the fan does not come on when I switch the power on to the amp. I traced the two leads from the fan one goes to the junction board I guess that is what it is called might be wrong on that. From there over to two Dale 15 watt 500 Ohm Resistors where both wires are connected one on each side. From there to the front high and low switch. The resistor on top can be moved from side to side or up and down? I can see when moving it powder comes out of the one side. So I am guessing that resistor is the cause of the fan not working? My question is there a way to work around the resistors in order to get the fan working? I am not sure if that has anything to do with keeping the ready light and xmit stby switch from working if the fan is not working? I have two Dale 10 watt 100 Ohm wire wound resistors if I could use them in place of the other resistors? Or could I wire it back to stock?. Here also is a stock photo of the same amp before it was changed. This fathers day is my birthday and I was hoping to be able to talk on my radio after spending two months on fixing my amplifier. Any help would be greatly appreciated. 73, 64648185_406865689920119_8036506052792942592_n.jpg


     
    #1 cbkidd1, Jun 14, 2019
    Last edited: Jun 14, 2019

  2. BlowinSmoke

    BlowinSmoke Active Member

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    That amp should have one high power resistor in series with one leg of its wires going to it from the 120v source. The hi/lo switch simply shorts out the resistor in high speed. In low speed the resistor is in line with the one leg and it slows the fan down. The original design may have had two resistors in series and only shorted out one in high speed and left one in and called that high speed. If you intend on running AM with that amp you had better make the high speed a direct hook up to the 120v supply with no resistor in line. The fan on those amps in high speed sound like jet engines.
     
    Shadetree Mechanic likes this.
  3. cbkidd1

    cbkidd1 Active Member

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    Evening Blowinsmoke,
    Hope all is well. I just found out that's not the only issue I am having. When I go to key the amp it won't key? I checked the cord running from the relay that loosecannon was nice enough to install in the radio to key the amp going over to the amp key input. That seems to be in working order. Do you think I might have a bad Zener Diode and that is keeping the amp from keying? Thank you for your help. Think I fixed the fan trouble.
     
  4. BlowinSmoke

    BlowinSmoke Active Member

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    Nope. A bad zener diode would not keep it from switching into transmit. All the zener diode does is set the operating bias of the tube. If the zener was shorted the only issue would be higher idle current which is not a big deal it just makes the tubes draw more current when not driven. Make sure you have the keying circuit hooked up right. How are you keying the amp? Using the radio or a foot peda; or what?
     
  5. cbkidd1

    cbkidd1 Active Member

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    Loosecannon traded me, my President, Jackson that needed some work for a President AX144 that he had restored for his self. And put in a relay that was hooked up to the pa-jack. So I could use it to key the amp. All was needed was for me to run a wire from the pa-jack on the radio to the relay jack on the back of the amp. So when I keyed the microphone it would key the amp. Now after replacing a few parts inside the amp, everything appeared to be ok. That is until I went to key the Microphone on the radio and found the amp was not being grounded in order to put out a signal? Not sure what is wrong? Should I just try to use a switch and see if it keys the lk500 without the radio? That way I will know if it's the relay inside the radio or a part inside the lk500 that is bad? 73,
     
  6. cbkidd1

    cbkidd1 Active Member

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    Got it working again. Just one Question. I am seeing 220 on plate current with microphone keyed and amp tuned is that too high for the plate current?
     
  7. BJ radionut

    BJ radionut Supporting Member and 6m addict

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    Idle current...might be a smidgen high...but pretty close for pair of 3-500's
    Do yourself a favor and don't run that thing on AM!
    GL All the Best
    Gary
     
    wavrider likes this.
  8. loosecannon

    loosecannon Sr. Member

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    hey cbkidd1, congratulations on getting the amp working!
    im sure that was no small task.

    glad to hear that the keying issue was resolved.

    as for the blower, i would start with a thorough check of your AC line jumpers.
    LC
     
  9. Dr_DX

    Dr_DX Active Member

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    factory was R1 = 300 ohm 20W and Optional R2 = 1K ohm 10W, with R1 being switched out with Hi/Lo switch. BTW, the 2 resistors are reversed on the Amp Supply Power Jumper Board schematic.
     
  10. nomadradio

    nomadradio Analog Retentive

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    The zener diode in the first pic looks pretty beefy.

    Mostly they fail as a short in this sort of service. A check with the continuity, or "diode test" function of a typical digital meter will reveal if it's shorted.

    A shorted zener will drive the tubes' zero-drive idle current higher than normal.

    73
     
  11. cbkidd1

    cbkidd1 Active Member

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    Thank's Chris for your knowledge and help. The amp I had working great after replacing the Grid current resistor D3. R21 100k 3w resistor. Glitch resistor 0.82. And the 20w resistor that comes off of the plate choke. I have a few pictures included of the meter readings I was getting before the magic smoke happened. I noticed every once in a while when using the lower channels below channel 20 on 11 meters. The wattage would drop way down from 250 watts to 5 watts output for a few seconds and then return. I didn't think too much of it until yesterday when it dropped off and I started to smell the magic smoke. So I removed the cover turned the amp back on and traced it over by the tubes. I then saw a spark every time I would key the microphone. Guess what it was coming from one of the two I think they are ceramic disc caps? That comes off the top of the plate choke. You had talked about them earlier in this post. As you will see in the pictures I took the one is black the other one looks find not to say that it might also be bad? Would you happen to have a link you could post that would show me where I could buy a few? From what is printed on them they are MDC 1000M Z5R 12 KV I guess ceramic Disc capacitors? When it comes to reading the part numbers I really have no idea what I am ordering to be honest. As you will also see the Plate current looks a little high so I am thinking I will also need a new zener diode as well? 73,
     

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  12. cbkidd1

    cbkidd1 Active Member

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    Almost forgot I appreciate everyone trying to help me as well. Hey, Matt, I have not forgotten about sending pictures of the radio to show Handy Andy the great job you did on the radio. I am very happy with it. I have just been so tied up with my health issues and this amp. I took them just need now to send.
     
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  13. Dr_DX

    Dr_DX Active Member

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    They are 1000pF 12KV disc capacitors. Here is a link to pick up suitable replacements for ~$7

    https://www.ebay.com/itm/US-Stock-5...UhaAnjz:sc:ShippingMethodStandard!78006!US!-1
     
    Dmans likes this.
  14. nomadradio

    nomadradio Analog Retentive

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    That zener is an easy test for an internal short with the ohms scale on a meter.

    73
     
  15. cbkidd1

    cbkidd1 Active Member

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    Thanks for the link Dr_DX. I was told that most caps on eBay that are 10KV and higher are almost always not suitable for RF amplifier use and will do exactly what mine have done. And mine were Chinese junk caps, That's why they failed. And you can tell when you search if you find caps with crazy high voltage ratings. Also, Caps over 6 KV are not easy to find if good quality. Now my question is to everyone is how do you know what you are buying? And who is a good source to buy quality 5-6 KV caps from?
     

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