>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,
www.vcnet.com/measures.
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