[Amps] Amp Supply LK500ZB - strange failure mode

k0zx Rich k0zx.co at gmail.com
Thu Oct 2 22:05:49 EDT 2014


Right you were Colin. After pulling and swapping the result was one of 
them had the symptom and the other didn't. Pretty clear cut. So I guess 
I'm on the hunt for another, first price i've seen was 189, hoping can 
do better than that. Thanx for the suggestion, I got focused in on 
possible semiconductor failure, replaced the zener in the cathode path.
Are u and owner of one of these amps?
rich


On 10/2/2014 9:07 AM, Colin Lamb wrote:
>
>
> Hi Rich:
>
> Pull one tube and try it - then the other.  I suspect one tube.
>
> Tuning will change and the input swr will increase - but you are 
> looking for an obviously bad tube.
>
> Assuming the filaments are not in series and the cooling is not solely 
> through the socket, it will fire up fine.  Watch out for hv bite.
>
> 73,  Colin  K7FM
>
>
> On 10/2/2014 7:59 AM, k0zx Rich wrote:
>> Wonder if anyone has experienced this failure mode with this or a 
>> similar amp:
>>
>> This amp is 80s vintage and since then has been a solid performer, 
>> grounded grid with two 3-500z. But it has a developed a problem where 
>> at power up (no rf drive, ant relay input not active for tx), the 
>> plate current rises to about 3x normal (about 300ma, 100 is normal), 
>> stays there for 5 to 10 seconds and then drops to zero.
>> If enabled to transmit with  antenna relay input on, plate idle 
>> current is normal, going from 0 to 100ma.
>> If apply rf in, it will output but at only about .6 or .7 its normal 
>> output.
>> After inspecting the schematic am not seeing an obvious source for 
>> the problem.
>>
>> Any suggestions appreciated
>> Rich
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>



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