[Amps] TL-922A - Loud Pop, Blown Fuses

[R.Measures]

On Dec 5, 2004, at 2:14 PM, Clay Curtiss W7CE wrote:

> Thanks for the responses.  I didn't think about an internal tube short 
> as a
> possibility.

Hello, Clay -- An internal tube short makes very little sound.  A loud 
pop is in the atmosphere, not a vacuum

> I removed the plate caps from the 3-500's and the HV  measures
> normal (2100V CW, 3100V SSB). Is it logical to assume that one (or 
> possibly
> both) of the 3-500's has developed an internal problem

If the anode-grid leakage is <10uA @ 8kV, the vacuum is good.  If the 
grid-filament breakdown potential is >5kV, the tube should not short 
when it is hot.

> or should I be
> looking for other causes as well, such as a bias supply problem.  I 
> don't
> have access to a hy-pot tester, but a simple resistance check from 
> filament
> to grid on both tubes doesn't reveal a short.

Tubes with only a few kV of withstanding potential will grid-fil short 
when hot, and this problem can not be detected with an ohm-meter.
>
> One thing I didn't mention earlier (because I don't believe it's 
> relevant)
> is that the small filament choke (L20) that connects to the filament
> transformer center tap, has overheated in the past.
> Since one of the tube
> filaments came unsoldered from the pin (a simple soldering job fixed 
> it),  I
> figure all of the extra current flowing through the small choke was the
> cause of that.  Other than some discoloration, it appears ok.
>
> Any and all suggestions are welcome.  I don't want to risk my power 
> supply
> by blowing more fuses trying to determine which of the tubes might be 
> at
> fault.

Does the 922 blow fuses with the tubes removed?
>
> 73,
> Clay  W7CE
>
>
> ----- Original Message -----
> From: "K7RDX" <k7rdx at earthlink.net>
> To: "Clay Curtiss W7CE" <w7ce at curtiss.net>
> Sent: Saturday, December 04, 2004 5:42 PM
> Subject: Re: [Amps] TL-922A - Loud Pop, Blown Fuses
>
>
>> Hi Clay, pull the tubes and start the amp again. If plate voltage is
> normal
>> and the fuse doesn`t blow again, shut everything down and inspect the 
>> tube
>> sockets for burnt or damaged rf chokes from the three grid pins to 
>> ground.
>> If one of the tubes flashed over, some of  these chokes will probably 
>> be
>> destroyed or at least damaged. 3-500z tubes sometimes develop a short 
>> from
>> grid to filament after a period of use and will flash over on the next
>> start-up. If you have access to a hypot tester, a shorted tube is 
>> easy to
>> spot...Hope this helps you find the problem...A flashover might 
>> damage the
>> bias as well as the high voltage supply. 73,Jim.
>> ----- Original Message -----
>> From: "Clay Curtiss W7CE" <w7ce at curtiss.net>
>> To: <amps at contesting.com>
>> Sent: Saturday, December 04, 2004 12:05 PM
>> Subject: [Amps] TL-922A - Loud Pop, Blown Fuses
>>
>>
>>> Yesterday morning when I turned on my TL-922A, there was a very loud 
>>> pop
>>> followed by blown fuses.  I replaced the fuses, changed the setting 
>>> to
> CW
>> to
>>> reduce the B+ voltage and tried again.  Similar result with several
>> smaller
>>> pops and then blown fuses.  I use a footswitch for T/R control and it
>> wasn't
>>> depressed nor was I transmitting with the exciter when I powered up. 
>>>  I
>>> should also mention that I've heard a pop on power up two times
>> previously,
>>> but neither time caused a blown fuse.  I opened it up today for a 
>>> visual
>>> inspection and see no signs of arcing or anything unusual.
>>>
>>> Before I dig into this thing, has anyone else experienced this 
>>> problem?
>> Any
>>> hints or suggestions to reduce the troubleshooting time will be
>> appreciated.
>>>
>>> Thanks and 73,
>>> Clay  W7CE
>>>
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org