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[Amps] al80a

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Subject: [Amps] al80a
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Fri, 2 Aug 2002 09:08:54 +0100
carl seyersdahl wrote:
> In my FWIW dept. I looked over the entire amp and didn't find anything of
>conncern. I did find a burned resistor which turned out to be in the
>external 12vdc line. , but no burned bandswitch contacts, no evidence of
>anything unusual, but still I knew something was wrong.
> I tried something interesting , tho, I took the tube out and laid it on the
>bench and hooked up the filament with jumpers , connected the ohmeter
>between the fil and the grid and when I fired it up, there it was, about 30
>ohms between the two terminals !!! So, now I know the tube is no good. I
>should have a known good one tomorrow to check it out. In the meantime I
>have seen several things that need to be done. glitch diodes, glitch
>resistor, new suppressor unit, etc. Live and learn (or re-learn??)

Take care about the glitch diode. In an SB-1000 (same animal) that 
someone else had "pre-repaired", I had weird meter readings on 160m 
until the glitch diode was replaced with a 1N5408. It turned out that 
his no-name diode was rectifying at 1.8MHz!

Part of the problem was RF leaking back into the power supply 
compartment on the B+ line. No amount of bypassing at the base of the RF 
choke would cure it, because the RF is being induced onto the B+ lead. 
It needs some series impedance right there in that lead. With some 
difficulty I managed to squeeze in a glitch resistor (one of the 
long-bodied glass types) right at the base of the RF choke, with an 
extra .01 bypass at the cold end right by the hole in the shield.

While you have the cover off, it's also worthwhile to fit a step-start 
relay. Rich's simple circuit is hard to beat: the relay is a 12V DC 
type, powered from the existing +12V rail. At power-on, the step-start 
resistor in series with the mains, so it takes a little time for that 
rail to come up, and this generates the delay.

What else? Oh yes, a few things to speed up the TX/RX changeover, and 
save the antenna relay from arcing on semi-BK or VOX. With modern 
transceivers, when you hit the key in semi-BK you have maybe 10-15 
milliseconds between the PTT line going low and the RF appearing. It's 
simply a race against time, to switch the antenna relay in the amp 
before the RF arrives.

First, install a transistor switch in the SB-1K/AL-80A (powered from the 
+12V DC line) so that you can use the direct solid-state PTT option in 
your transceiver without blowing it up. That gets the PTT signal into 
the amp several milliseconds faster than going via the relay inside the 
transceiver. Second, install the K1KP/W6XX speed-up circuit for the 
antenna changeover relay in the amp itself.

I gave a complete circuit in my RadCom column a few months ago (sorry, 
it's not on the web) but the building blocks are on W6XX's site - look 
for the "IC706 PTT interface" and the "Relay speedup circuit".

These two mods make the antenna relay change over much faster than 
before, and greatly reduce arcing. It isn't a complete cure because 
there is still occasionally some contact bounce, and of course it isn't 
as fast as a vacuum relay (see Rich's site).

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)

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