[Drake] L75; What went KERPOW?? **ANSWER!!**

[Neil Rosenberg]

Dave, that "resistor" is in fact used as a fuse, and is sometimes called 
the "Glitch Resistor".  It's meant to self-destruct if a surge in current 
would otherwise harm the amplifier.  You'd rather lose a 10 cent resistor, 
than something much more valuable.

As to what can cause "glitches", that's where the real mystery is, and what 
I'd be concerned with in the case of your L75.  If it happened once, it 
will happen twice.  Of course, you could always just buy a gross of 
resistors and get really good at replacing them.

Parasitic oscillation is one possible cause of sudden over-current 
events.  It could also have been a bug (yes a real bug) walking between the 
plates of your variable capacitor.  If this happened, the amp briefly acted 
like a bug zapper, which looks like a glitch.

Good luck with your search;
Neil, K1VY

P.S. In most amps I've worked on the glitch resistor is enclosed (but NOT 
encased) in some kind of protective material, like an oversized piece of 
CLEAR silicon tubing.  This will help prevent the shrapnel from flying 
around when it blows.




At 10:59 PM 9/7/2004, Dave Edwards wrote:
>First, thanks to all who took time to send emails with ideas. What a
>powerful tool the internet can be!!
>Anyway, I had some time to pop the lid this evening on the L75.
>As soon as I started to juggle it around on my lap to get the cabinet screws
>off, I could hear something sliding around inside. Open 'er up, pull out the
>chunks and what do we have? a resistor half. It reads 82 and a multiplier of
>divide by 10. Hmmmm...  .82 ohms...this sounds familiar!
>Sure enough, I looked at my notes for the L7, and the same thing happened a
>long time ago.
>What did I do then? How did I get another .82 ohm resistor? It doesn't sound
>very standard. Perhaps I made some notes in my L7 manual!  Pulling out the
>manual, which is in a plastic bag...out fell two 2 watt, .82 ohm resistors.
>Now I remember!!   Good 'ol Drake sent me three of them, many years ago..
>What was bizarre is that the .82 resistor acts as a fuse to protect the rest
>of the power supply components. Half the resistor blew away. The other half
>was still intact, and measured about 300 ohms. IT WAS STILL WORKING!!
>Perhaps what was left was glowing red hot, and would never even make it
>through a qso. I only sent a few V's to see that it was still working. I'm
>sure the rest would soon have blown in half as well.  So, I popped in a new
>resistor, put it all back together and sent a few CQ's . All seems fine, but
>I'll have to have a long QSO at 500 watts or so to be sure. As I recall, the
>L7 just blew for no reason as well.
>By the way... the beep noise I heard turned out to be my APC battery backup
>gizmo for the PC which must have been disturbed by the AC line fluctuation
>that the amp caused. Or perhaps it SENSED and AC line fluctuation which is
>also what blew the amp????
>We'll never know...just as long as the amp don't have another fit!!
>Thanks again guys!
>...Dave
>
>
>
>----- Original Message -----
>From: "Dave Edwards" <kd2e at comcast.net>
>To: <drake at mailman.qth.net>
>Sent: Monday, September 06, 2004 6:17 PM
>Subject: [Drake] L75; What went KERPOW??
>
>
>Have not heard much from the Drake list lately...Anyway, wonder if anyone
>can suggest a likely culprit in the innards of my L75 amp?  I was having a
>chat on 20 CW. The amp suddenly made a beep sound (like a warning
>buzzer...what the heck could that have been??) and then a "ker Crack!"
>sound.
>I hit the standby switch and finished the QSO without the amp.
>Later, I turned the amp back on and it seems to be working fine. But, there
>is that yummy smell of burnt bakelite in the air.
>So, when I get the chance I will pull it out of there and pop the lid to see
>what is going on. Can anyone suggest what could have gone south, and yet
>allow the amp to keep on working?
>Just now it keys down at 320 mils and 2500 volts. The output about 650
>watts.
>Any thoughts would be helpful.
>If you are interested, I will post my findings here after I "dive in" !
>Thanks all;
>...Dave
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