[Drake] Drake L75 knowledge??

[Pfizenmayer]

Dave - There is a chance the original 3-500 popped over or is popping over 
and blew the 0.82 ohm -- that resistor is used as a HV "fuse" . The fact you 
are seeing anything out of the resistor now indicates it is kaput - did you 
measure it ? Just because it looks intact is not a  test .

One of the modifications in L7 and L75 is to add a 20 to 25 ohm 25 watt 
wirewound in series with the B+ to limit surge current when a tube pops 
over or you get an arc somewhere . I would highly recommend that be done no 
matter what.

Your  antenna SWR is not a problem .

One missing piece of data is whether the 0.82 was damaged before or after 
you put in the other 3-500 - from what I read , I don't think we know that .

Hank K7HP


----- Original Message ----- 
From: "Dave Edwards" <kd2e at comcast.net>
To: <drake at mailman.qth.net>
Sent: Tuesday, June 18, 2013 11:26 AM
Subject: Re: [Drake] Drake L75 knowledge??


> Thanks guys for the tips.
>
> Since you sent me some info, here is what I did......
>
> Pop open the cabinet
> Put in a spare 3-500 that I had.
>
> I noted that the .82 ohm resistor I put in Sunday is half blown in 
> half...bakelite is missing from half of it, but guts are still intact!
>
> Close interlocks and power on.
>
> Dim the lights
>
> When I simply key the mic....no audio...I can see a momentary flash from 
> that .82 resistor.
>
> If I say "one" , "one"....I see essentially a modulation indicator from 
> that resistor. The inner guts flash on and off.
>
> Each modulation peak was still hitting 500 watts, but I'm sure if I hit CW 
> key down, that resistor would fry the rest of the way.
>
> It seems odd that something on the "other" side of that resistor could be 
> so drastically out of range to cause that .82 to pop...and yet,
>
> it still works with perfect output...when it is working.
>
> So, while the capacitor/diode bank probably needs updating, Am I to assume 
> that is not the culprit at hand ...because the .82 is on the output side
>
> of that entire assembly?
>
> Changed tube, perhaps my antenna has some SWR....but the 30L1 has no 
> issues...not even glowing plate...so it can't be that bad. Meter shows 
> 1.8:1 or thereabouts.
>
> I have to do this while the iron is hot, or the 30L1 may just stay in 
> place there for years!!!
>
> .....Dave
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