[Collins] Collins 62S-1 Transverter Downconversion Gain??

Tue Sep 21 10:32:34 EDT 2010

In 1965 I modified a 75A4 in the shop at National to a 6GM6 RF and 7360 
mixer but havent found the test data yet. The improvement however was 
substantial on 10-15M and so was the signal handling. My primary operating 
was DX and contests on CW and SSB. Before that I had a Drake 2B which would 
fold up on 40M almost nightly.

In 1980 I used a 5722 NF device to tweak the A4 and wound up with a 8.6dB 
system NF on 10M.

In 2002 I bought a surplused HP 4970A NF meter from work and attacked the A4 
again resulting in a 6.8dB NF on 10M. Swapping the front end tubes with NOS 
didnt improve it by more than .2dB so I left the oldies in place. Id always 
wondered about the 5722 accuracy.
The A4 is still in use but primarily for 160/80M CW DX as its had numerous 
other changes dating back to 65 and is a better in the noise CW receiver 
than a loaded TS-940.

For 6M these days I use a vintage HA-6 transverter with a SS RX path with a 
measured 1.25dB NF from antenna to 28 MHz IF ports. Devices are selected and 
biased for signal handling and gain made up in the post mixer IF amp. An old 
Ameco nuvistor converter measures 2.3dB NF but the signal handling is poor. 
The HA-6 feeds a slightly modified TS-830, the combination has negligible 
phase noise and hears well way down into the band noise.

Getting back on track to the 62S1, it was a luxury item for the well heeled 
casual operator or when the bands were wide open.

Carl
KM1H

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: "Pete" <zilassoc at la.twcbc.com>
Cc: <collins at mailman.qth.net>
Sent: Tuesday, September 21, 2010 1:25 AM
Subject: Re: [Collins] Collins 62S-1 Transverter Downconversion Gain??

> I've not measured my converters that way. Way back in the 60s when I
> moved to Texas for a couple years with my new 75S-3B and a home made
> nuvistor converter and put together a 2m transmitter, I was working
> locals around Dallas and bragging about the line noise they were
> complaining about. One night one of them drove up to Allen from Garland
> and dropped off his 5722 noise generator. I soon found that I had a NF
> round about 15 dB. I worked over the converter getting it down to 3 dB
> or better (1 ma at 50 ohms), then I could hear power line noise from two
> or three different power systems some in phase with my local line and
> some 30 degrees away (no doubt the feed to my substation which probably
> had a Y primary, delta secondary transformer to give that 30 degree
> shift). And with the scope I could see noise pulses for each peak of
> both 3 phase systems.
>
> One of those hams, W5WXV ran considerable power and good antennas (held
> a nightly sked with a station in Qunicy Illinois about 900 miles
> successful most nights) and his signal was all over the bottom 200 kHz
> of the band (no CW subband those days, he was crystal controlled on
> 144.085). So with my noise generator (I found an AIL noise head,
> probably at Collins Surplus) I started adding attenuation between the
> converter and the IF but backed off when I started to affect the noise
> figure of the system. I also spent much time in the receiver and found
> its signal handling limit was the second mixer so I connected the second
> mixer grid to a tap on the IF coil and changed the mixer tube from a
> 6EA8 to a 6688 or 7788 (I forget which, need a new one probably now),
> from a tube with a gm of 5500 to a tube with a gm of 50,000 which
> increased the mixer gain and reduced the mixer noise significantly.
>
> Before the two mods, Al's signal was 20 or 30 places in 200 kHz, after
> just three, one pegging the S-meter on .085 and the other two at the
> noise level.
>
> Some one in the area had a 62S-1 and was playing with PLL for the LO for
> better precision but it wasn't working well, he drifted around more than
> my crystals did. I don't remember him being on for band openings. I
> heard decently, better than I was heard with my 10 watts but Al heard
> much better partly because he could copy way down in the noise by ear.
>
> According to a standard chart of receiver sensitivty vs NF and IF
> bandwidth, 4 dB and 2.1 kHz corresponds to a signal of about -138 dBm.
> Half a microvolt at 50 ohms is -113 dBm. 1.2 uv is -105 dBm. So if those
> give 10 dB S/N roughly 10 dB less would be threshold, or sensitivity, so
> -115 dBm and 6 kHz (AM selectivity) is 20 dB NF, -123 dBm is about 13 dB
> NF. Seems bad, but calaculations with low S/N get messy. I don't think a
> 75S-3 for HF should have that good a NF, its not needed and not of
> benefit. I know mine does poorer after I cut the signal to the second
> mixer so much.
>
> On 9/20/2010 6:25 PM, Pete wrote:
>> Jerry,
>> The pertent 62S-1 manual info is:
>> 1. RX Signal path uses a 6ER5 Triode RF Amp followed by another 6ER5
>> triode as a Mixer.
>> 2. Noise Figure approximately 4 dB
>> 3. System performance is such that: 1.2uV at 2M or 6M yields 10 dB
>> S+N/N on AM Signals through a 75S Receiver
>> (NOTE: the 75S3 receiver manual spec is 0.5uV yields 10 dB S+N/N)
>
> Since SSB is the mode the S-line was made for shouldn't that be in a 2.1
> kHz passband for SSB, not AM.
>
> If the NF of the 62S-1 was made with an automated HP NF meter that meter
> takes out the factor of the IF NF and the RF/mixer gain and shows only
> the NF of the RF stage along. Its not the system NF. And I'm of the
> opinion that the 75S will have a NF more like 8 or 10 dB (still quieter
> than most HF antennas) and the front end gain in the 62S-1 is not enough
> to overcome that. Though my 6CW4 nuvistor convertor with one RF stage
> (grid driven shunt neutralized) and another 6CW4 as mixer had enough
> gain to let it set the system NF nicely with room for 10 or 12 dB
> attenuation at the IF (IIRC) with my unmodified 75S-3B.
>>
>> This information tells me the NF of the cascaded 62S-1/75S3 system is 
>> worse
>> than the noise figure of either the 62S-1 and the 75S.
>> There has to be a net translation loss from VHF to HF in order to match 
>> this
>> specification.  I'm calculating there should be -8dB conversion loss 
>> based
>> on these specifications in the manuals.  Interestingly I'm measuring -12 
>> dB
>> conversion loss.  But, I'd like some confirmation on what it should be.
>> What do you think?
>> Pete
>> K5PZ
>>
> Not so much conversion loss as not enough conversion gain for the 2m RF
> section NF to make up for the generally poorer NF of the 75S-3.
>
> NF of a cascade amplifier is NF of the RF stage PLUS the NF of the
> second stage divided by the gain of the RF stage. That is the Noise
> Factor (not figure) of the system is the Noise Factor of the RF stage
> plus precisely the Noise factor of the second stage - 1 divided by the
> gain of the RF stage. So the system NF is never as good as the RF stage
> unless the second stage NF is the same or better than the RF stage, or
> the RF stage has infinite gain. But if the second stage NF is as good as
> that of the RF stage, then the third stage NF is significant unless that
> third stage NF is as good or the gain of the second stage is infinite.
>>
>>
>>
> 73, Jerry, K0CQ, Technical Adviser for the Collins Radio Association.
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