Re: {Collins} Question re 30S-1 to 32S( ) connection

Hi Bill and Lou

As an experienced RF engineer,  but relative newcomer to Collins collecting, I'd certainly echo Bill's comments regarding the value of the CCA archives; they are of immense value in bringing the motivated new-comer up to speed and are always the first port of call when queries arise. In the case of the 30L-1, there are several great articles, including at least one on the cause and treatment of medium and high frequency instabilities in some amplifiers.

As one might expect given the vintage, the 30L-1 is not a plug-and-play device and it behoves operators to familiarize themselves with some of the foibles of the amplifier.  This is of course a foreign to many new hams but I've also spoken with a number of older owners who do little or no testing of their equipment.  One of the things that's surprised me since returning to amateur radio is that people now do less monitoring than ever, despite the abundance of better and cheaper test equipment.  In addition to my bench equipment, I use a monitor scope (a nice old Kenwood SM220)  permanently in my transmit line and, based on my experience , 30L-1 instabilities are clearly visible to the practiced eye.  Naturally, more sensitive equipment can tell you more but I had no problem picking both high frequency and medium frequency instabilities in my 30L-1, a RE model probably manufactured in 1968-69.  These were actually at a fairly minor level and I was getting good signal reports when I first put the linear on the air.  The message is that if you don't look, you might never know.

With the help of the CCA archives the mf oscillation was pretty easily dealt with but my amplifier had a stubborn, low-level signal-frequency (or near signal-frequency) instability on 10m when connected to anything other than a dummy load.  Out of interest, I replaced my 3' drive cable and inserted an ~20' length of RG58 to the exciter and, interestingly, the 10m instability vanished convincingly on a spectrum analyser.  The exact length was not critical; any similar length worked.

The case for the long 30S-1 cable is clear and well written up, most especially in The Signal article referenced by Bill.  But the 30L-1 is treated as a bit of an afterthought and, to date, I have found no single convincing analysis.  I suspect that in fact there is none:  the 30L-1 is conditionally stable and a variety of causes, including the somewhat motley collection of tubes many of us have to use these days, various source and load configurations, etc.,  could all contribute to a given amplifier benefitting from something that changes the magnitude and/or phase of a feedback signal.

So, my take would be to start with a convenient length of exciter cable but be an active, responsible ham and check thoroughly for both high frequency and medium frequency oscillation.  If you find residual instability on the higher bands, it'd be at least worth trying the longer exciter cable, along with any other required (and more conventional) fixes.

Incidentally, regarding the more insidious medium frequency oscillation (~1 MHz), I undertook the CCA-recommended treatment of L3 and its swamping resistor, although I had earlier found that excessive external impedance or poor de-coupling in the keying line triggered the original problem and, when corrected, the 30L-1 was actually fine with its original components.  As a further aside, there is a Polish eBay supplier currently selling cheaply stocks of the replacement Epcos 56 uH choke having a high self-resonance frequency (70 MHz).

Best 73,

From: Collins [] On Behalf Of Bill Carns via Collins
Sent: Sunday, 15 October 2017 9:39 AM
To: 'Lou Fiore' <ltfiore@xxxxxxx>
Cc: COLLINS@xxxxxxxxxxxxx
Subject: Re: {Collins} Question re 30S-1 to 32S( ) connection

There need not be another thread on this. It has been discussed and
researched ad nauseum and the results are in the reflector archives and also
in the RX for your Collins on our website. You should always check there
for answers when having any issue.

RX for Your Collins:<>

The short answer is Yes, you can shorten it all you want!

Here is the link to the RX article,<>

and scroll down to page 16 of the Signal issue in which it appeared.

Hope this helps, and to others, let'
Use the archives and not redo this issue..

Thanks, Bill

Bill Carns, N7OTQ (Trustee K0CXX)
Past President, Collins Collectors Association
Founding Board, Collins Radio Heritage Group
Editor, Signal Magazine (Retired)
Wimberley, TX
512 618 2762 (Cell)
512 847 7010 (Home)

-----Original Message-----
From: Collins [] On
Behalf Of Lou Fiore via Collins
Sent: Saturday, October 14, 2017 4:12 PM
To: collins@xxxxxxxxxxxxx<mailto:collins@xxxxxxxxxxxxx>
Subject: {Collins} Question re 30S-1 to 32S( ) connection


A question for the group:

Figure 1-2 Note 5 of the 30S-1 manual warns against shortening the
originally supplied 20.5 foot cable connecting the RF output of a 32S( )
transmitter to the RF input of a 30S-1. Does anyone have any thoughts on

Thank you for your input.

73, Lou W2LTF

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