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[Amps] al80a

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Subject: [Amps] al80a
From: (2)
Date: Fri, 2 Aug 2002 07:40:16 -0700
>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. 

Ian -- How much RF potential did you measure across the 0.01uF HV-bypass 
without a glitch resistor?

>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.

Some radios R/T switch in 5mS. 

>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.
What is the measured make-speed of the stock relay with this circuit?

>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).

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,  

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