[Drake] Drake L75 knowledge??

[Dave Edwards]

Thought I would give everyone an update on the L75 that I've been 
working on.
First.....On many hams suggestions, I was told to not even turn it on 
with the old .82 ohm resistor that cracked in half.
It was working with it, but I could see flashing emanating from it. When 
I removed it, you guys were right!!  It was no longer .82 ohm....it was now
about 1K!!
I bought a bunch of replacment resistors and installed one.
As I mentioned, I had two replacement 3-500Z tubes.
I got them both at a hamfest...probably 15 years ago.
One would not even light up, so I installed the other one.
Problem <seems> to be solved!!
I had a 20 minute or so ragchew with a ham on 20 at about the 500 watt 
level.
With the original tube, the resistor would not have lasted that long.
Now...the tube that is in there. Seems to be working fine, but its been 
sitting on my shelf for 15 years...and even then, might have been 
someone's worn out pull??
I'll probably get a new one, and use my spare as a spare again.

So...thank you all for giving me all your tips and suggestions.
I'm going to give it another try tomorrow, a few QSO's, and I'll 
consider the problem solved!

.....Dave




On 6/18/2013 2:26 PM, Dave Edwards wrote:
> 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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