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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 23:39:32 +0100
2 wrote:
>>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?
I didn't even try. If you think about it, such a measurement is 
well-nigh impossible in that location, which is very hard to get to and 
bathed in an intense RF field.

You've also mentioned several times that the bypass cap at the bottom of 
the RF choke in that amp is rather low in value. That may be so, but 
once I had fixed the glitch diode problem, the amp seemed to behave 
perfectly well on Top Band in its original configuration. With a 
wirewound glitch resistor in the B+ feed, any RF current at the bottom 
of the RFC has to go through the bypass cap, so the problem is over.

>>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.
Unfortunately, yes. In that case, either change to a fast vacuum relay 
or improve the radio... preferably both!

Some older radios do their internal R/T switching using a slow 
multi-pole relay that also switches the PTT line to an external amp. In 
a lot of those cases you can win back valuable milliseconds by taking 
the amp PTT signal from the line that *drives* that slow relay. 
Generally the relay is driven by a switching transistor, so simply add 
another transistor sharing the input switching signal, and wire that 
directly to the PTT output.

More modern radios have a built-in TX delay, but you can lose a lot of 
that time by using the optional relay output for the PTT. *Always* use 
the solid-state PTT output - and if it isn't capable of switching your 
amp. Modify the map so that it is... but not by using a relay! Remember 
that it's all about *time*.

>>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 didn't test that particular one, but I have done measurements with a 
selection of other relays, and the K1KP circuit will roughly halve the 
switching time. With a 24V line, 24V coax relays come down from 15-20ms 
to 7-10, and Kilovac HC-1s come down to about 1.5ms.

With an open-frame relay as used in the SB-1000 you might guess at 
20-30ms before speed-up and 10-15ms after. These open-frame relays are 
not precision mechanisms, so you can expect quite a lot of variation 
from one shot to the next.

Bottom line with the SB-1000 is that after adding the speed-up circuit, 
the relay only arcs occasionally on first pressing the key. Without the 
modification, the relay would arc every single time. That's with a radio 
that delays the RF by about 12ms (IC-746), and a solid-state PTT driver 
to get the PTT signal to the relay ASAP.

I haven't made detailed measurements on this amp, but am pretty sure 
that even with speed-up there is not much reserve of time between the 
relay switching and the RF arriving. The occasional arcing must be due 
to variations in switching speed (this is not a relay in good 
condition). So what we have in this particular case is not a complete 
cure but a considerable improvement.

With the VHF amps using coax relays, the speed-up circuit has made all 
the difference between regular arcing and absolutely none. It really is 
well worth trying!

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

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