[Hallicrafters] sr400a

Tue Nov 11 10:33:18 EST 2014

Good morning Rocco,

Yes the demod probe is the the old rf probe.  I have several that I build myself but they're clunky.  I use the probe frequently so bought one that is convenient to use.  Search on ebay for “RF detector probe green”.  They are about $30.00.  The capacity is 5pf and they use Schottky diodes with a turn on of about .25v.  You will need a detector probe to do sweep analysis in the 6.5Mc I.F. system.

The meter that I generally use is the Micronta 22-208A FET VOM.  They are extraordinarily inexpensive but superb meters --- as good as any.  For high energy RF measurements, (Antenna lead on RF amplifiers) I use the HP410B one of the best low capacity RF meters.

The Micronta measures P/P the HP410B RMS.

Using the Demod probe creates a very effective and inexpensive RF voltmeter.

Kindest regards Jim K9AXN  

-----Original Message----- 
From: lardiere at ix.netcom.com 
Sent: Monday, November 10, 2014 11:08 PM 
To: Jim Liles 
Subject: Re: [Hallicrafters] sr400a 

Jim,

What is a "demod" probe?  One of the old fashioned diode based RF probes?  I have an old Heathkit version of one of those but never thought to try it with a modern high impedance meter.  

I understand what you mean about 10X probes loading down circuits, but I have yet to find a good low capacitance RF voltmeter, at least at a semi reasonable price.  I did find a modern scope probe with 6 pF capacitance, and I may give one a try.

73,

Rocco Lardiere N6KN

-----Original Message-----
>From: Jim Liles <hallicrafterssr2000 at k9axn.com>
>Sent: Nov 10, 2014 12:10 PM
>To: Walt Cates <cateswa at msn.com>, sdw at dwdorset.co.uk, hallicraftersradios at yahoogroups.com, hallicrafters <hallicrafters at mailman.qth.net>
>Subject: Re: [Hallicrafters] sr400a
>
>Good afternoon Walt et al,
>
>The reason for the Heterodyne oscillator appearing distorted on the higher bands is the 20, 15, and 10 Metre bands use doubling crystals i.e 20 Meters is 10.25 Mc fundamental for 20.5 Mc second harmonic.  Every second wave is muted.   
>
>The output on 80 and 40 is significantly higher than 20,15,and 10 Metres.  Use the manual to verify the voltages and do not use a scope even with a X10 1Meg probe, the capacity of the probe will warp the measurement.  Use a vtvm with a low capacity probe i.e. HP410B or C.  If you don’t have either, use a Demod probe and VTVM, works very well.  The measurements as acquired in the manual are right. 
>
>BTW, Walt, I do not subscribe to the Yahoo Hallicrafters site but I do watch your contributions to the WZE site.  Great job!
>
>Kindest regards Jim K9AXN
>     
>
>-----Original Message----- 
>From: Walt Cates 
>Sent: Sunday, November 09, 2014 12:49 PM 
>To: sdw at dwdorset.co.uk ; hallicraftersradios at yahoogroups.com ; hallicrafters 
>Subject: [Hallicrafters] sr400a 
>
>
>
>
>Simon. See red below for a quick response. I'll be back Monday with more on the het osc.  Dear Walt
>
>
>further to earlier posts, I went back and started from beginning again.
>
>
>no problem with carrier osc.
>
>
>the heterodyne osc is ok on 80 and 40 showing over 4vpp at pin 8 V2.
>
>
>however 20 and above low and waveforms distorted. volts on V12 ok except pin 6. according to mauaul shold be 95v. also resistance shown as 40K. however 150v line has a resistance of 8k and pin 6 is fed through R78, 100R and R 77 10kR. so could not possible get to 40K. the R78 and R77 agreed with circuit diagram. however part lists shows R78 as 1K and R77 as 33K. The schematic is correct, the parts list is incorrect. In the original 400 R78 & R79 were 1K and 33K. The change in value was not updated in the 400A parts list.
>
>
>Which is correct?
>
>
>what ealse should I check to get Het Osc giving correct output on higher bands?
>
>
>With regard to incorrect voltage at tie point cr11 and l20 found that r18o on audio cathode had gone up to over 1K on replacing with 120R volts now 15 on rx but still a little high on tx at 2.5v. pulling v15 leaves volts at 2.5. Ok, this is good now. That should solve some of your problems. It is a good idea to replace both C146 (25uf) and C218 (5uf) if they are original. You may want to try a quick alignment at this point to verify what faults remain.
>
>
>in your earlier response you said that CR11 should be 7v in both RX and TX. Rx is 7 v but TX neg 1.8v. The anode should be 7v in receive and transmit modes. The cathode should be 14-16vdc in receive and less than 2vdc in transmit mode. To read the bias voltages on this switching network you need to pull V12. The load of a meter in the ckt, with rf present will turn the ckt into a rectifier and produce the negative voltage you are seeing. You can also measure the cathode voltage below L20 where there is no rf from the het osc.
>
>
>bye the way in your repair & Rest notes para 3-5-2 C you state that if the volts are correct the either CR11 or C47 are bad. I think this should be C107. You got me there, my error it should be C107 you may also add C106 or a shorted L20 to the possible faults.
>
>
>Simon G8OJ 
>
>
>Best Regards, Walt Cates, WD0GOF
>
>A generous person will prosper; whoever refreshes others will be refreshed. 
>
>       
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