[Hallicrafters] SR-2000 Audio additional comments

[James Liles]

Good morning Bob:
We use the sensor wrapped around the 1st RF amp 12DK6 in the SR-2000 with 
the configuration in the note, measuring 70uv with a 3uv signal injected 
into the antenna connector on the 14Mc band.  One thing to be aware of is 
the 2pf feed into a the 50 ohm input of the HP 8447D divides the signal by 
about 4 on 14Mc meaning that the true indicated value for that RF amp is 
more like 280uv rather than 70uv on the 14Mc band.  This indicated value 
varies by band, for example, the indicated value will be close to even or 
parity on 28Mc and lower on the lower frequency bands.  You can calculate 
the result for each frequency that you wish to measure and confirm the 
result with your signal generator.  A companion technique is to add a 2pf 
mica cap with a mini grabber to a short length of 50 ohm coax attached to 
the 8447D and use it to measure the passive amplification of the 
pre-selector and other tuned circuits.  This configuration does however 
introduce a slight load because of added lead length and component.  A 
slight touchup of the pre-selector provides corrected measurements.  The 2pf 
load is very close to using an active (FET) probe.

Kindest regards Jim K9AXN




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From: "Robert Nickels" <ranickels at gmail.com>
Sent: Wednesday, November 24, 2010 8:28 AM
To: "James Liles" <james.liles at comcast.net>; <hallicrafters at mailman.qth.net>
Subject: Re: [Hallicrafters] SR-2000 Audio additional comments

> On 11/23/2010 9:38 PM, James Liles wrote:
>> These sensors provide an approximately 2pf NON-INTRUSIVE sensor into the 
>> radio.  Use them on any
>> tubed radio.
>>
> Jim, that's got to be one of the cleverest service tips I've read in a 
> long time!  Thanks so much for sharing your experiences, I'm going to be 
> making one asap.   I've often struggled with alignment of RF stages where 
> even the slightest amount of loading from a scope probe or such is too 
> much.   I've used impromptu one-turn loops but as they shift position the 
> variation in signal is more than the change I'm looking for - but your 
> approach fixes that.
>
> I've always wished for a preamp or a few more gain positions on my scope 
> vertical input - do you get enough signal from even the low level stages 
> with this probe?   I know my 141T spectrum analyzer should have sufficient 
> gain and can't wait to try this out.
>
> Thanks again and please keep the service suggestions like this coming!
>
> 73 and HTD!
> Bob W9RAN
>