Hallicrafters SX-99 Alignment procedure

Rod,
 Double check the original wiring for that band.
 I ran into an RCA console  AM w/ 3 SW bands that came in for rebuild.

Got all but 1 band to work just great. Beat my head on the wall for a couple of days. 
   Another tech took a look and found the FACTORY had MISWIRED it!!!.
 Moving a couple of wires and it literally worked better than new.

 That was one that got up & bit me hard. I had never thought to check for that. Boy did I feel stupid....

Because of Frank's latest post on Mica Capacitor replacement in critical circuitry, I got to thinking about the SX-99 and Band (4) that I never got to work. This is the set with the band switch that had the metal contact sections coated with some kind of awful insulating crud. Well, there are (2) micas that could be involved on band (4). (1) in the RF coil section and (1) in the Oscillator coil section. I may get busy and try replacing those two caps with some newer ones and see what happens. I don't have the cabinet anymore and basically set the chassis aside for parts (dial cords gone too) but will consider giving it another try.
73, Rod  WB6FBF

Here is a short follow up to my previous post of aligning the IF stages of a Hallicrafters SX-99 receiver. Previously, I had found the crystal frequency to be 452.6 kHz (Hallicrafters and other manufacturers used the same 455 kHz crystal since at least the early 1940s). Again, I used a TS-375 A/U VTVM to monitor the negative output voltage from the 6H6 second detector at the output of R14 (47K Ohms). I adjusted the signal generator to zero beat with the BFO in the receiver to get the generator on the right frequency. I checked and peaked the primary tuning of T1 first. It was right on.

This time I wanted to check the tuning of the secondary winding of T1 (crystal filter IF can). I had skipped this step last time. I found that the tuning of the secondary of T1 was right on frequency at 452.6 kHz. Pretty amazing after some 60 years. After the realignment, I temporarily shorted together the antenna and ground terminals and readjusted the S-meter to zero.

This is the second TS-375A/U I've had. They are good for doing adjustments on receivers and transmitters. They only go up to 300 Volts though. I have been thinking of buying another one just to get the MX-660/U DC probe. The current one I have is missing the DC probe and the handle. The holder for the DC probe is also slightly damaged in mine. It's hard to find one where everything is ok on it.

They are rather heavy and hard to pick up without the handle. I have a clip lead going up into the dc probe socket for now terminated in a 5 Meg resistor. The 5 Meg is needed to keep the meter in calibration.

Yes. You are right that there is a Raytheon sub-miniature rectifier tube in the larger AC probe. Sometimes the cords on these probes are bad.

Rod  WB6FBF

Hey I just Realized I too have a TS/375.
Is a shelf queen (Collector of a lot of DUST)
Never once did I test it. I probably Should though. (SIGH)

How good does it perform?
I understand there is a miniature tube in the probe (Pencil Tube most Techs. call them.)
After reading your post I guess I may add that one to the For Sale Posting.

Out of sight out of mind I guess.

I got busy yesterday and set up the SX-99 receiver to do the IF and BFO alignment. I wanted to get the BFO setup so I could key the Johnson Viking II transmitter to zero in the receiver at 3870 kHz for monitoring purposes. By the way, the BFO threaded shaft had been violently bent some time before so I had to straighten it as best as I could.

The alignment setup is makeshift. I have very little room here at present and no work bench as yet. Maybe I can get one installed next month.

I have the alignment procedure. I followed Hallicrafters suggestion to skip the first two steps that cover adjusting the secondary of the 1st IF transformer which is also the crystal filter transformer. Even though it has been 60+ years since the set was made and probably does need touching up. I used my new signal generator which I have called a clone of the B&K Precision model 2005A. I am not sure where these models originated. I found it to be quite smooth in tuning the range covering 455 kHz. It has a BNC high level output jack for using with a frequency counter.     

The first round of the alignment I made the adjustments by ear with the signal generator's modulation on somewhat contrary to the instructions. I could not find the crystal frequency easily with the modulation off and the receiver's BFO on as instructed. I did find what I thought was a good output at 453.6 kHz. The instructions said the crystal frequency could vary from 450 to 460 kHz so this was ok. I then zero beated the BFO to this frequency and reinstalled the BFO knob. Then I adjusted the remaining IF transformers to this frequency. These IF adjustments seemed rather broad tuning by ear but the later on-air tests were good. 

I decided later to redo the realignment this time using a TS-375 VTVM to monitor the output voltage of the diode detector at the output side of the 47K Ohm resistor R14. The TS-375 has a really high input impedance and also an additional 5 meg resistor in the DC probe. This worked out really well as it didn't matter whether the signal generator's modulation was on or off and there was no need to turn on the BFO. Having the signal generator's modulation on could help to speed things along though when you can actually hear the signal. Well, I was able to get a really strong output from the crystal filter this way at 452.6 kHz. The tuning was really sharp as is to be expected. Maybe only 200-300 Hz wide at this point. I readjusted the BFO setting again and then went on to realign the other IF transformer slugs to the new frequency. I was able to get a positive peak voltage reading on the VTVM for these other IF adjustments. These latter adjustments turned out to be very close to those I did by ear. Only one of the IF transformers was off by any significant amount after some 60 years. The 3rd IF can's primary side was the one that was somewhat off.  

Thinking this procedure over I wonder about the usefulness of finding and aligning the set to the crystal frequency. So far, I haven't  used the Broad Crystal or Sharp Crystal selectivity positions much on the SX-99. In the past, I have only found two receivers where the crystal filter worked well. One of these was an RCA AR-77. The xtal filter worked really well here and was useful on phone. The other was a National NC-173. The filter in this one also worked well. Many of the others I've tried left a lot be desired. The phasing control (part of the crystal filter circuit) has not been a useful one in my operation of such receivers. Seldom do I have the phasing control set for maximum selectivity (for copying CW) or to eliminate heterodynes as it was also intended. I would much rather have had the manufacturers leave out a user adjustable phasing control.

A story comes to mind about the ease of and usefulness of these filters to improve selectivity. I have read that National did a survey on the use of the xtal filter in the HRO by amateur radio operators and found that most of them did not use it even for copying CW. Most of these older receivers with xtal filters that I have tried are not good for phone reception. I'd say this is because at 455 kHz the Q of the circuit is too high and too much sideband cutting occurs. Plus the fact that the adjustment of the phasing control is problematic. Now I may get a lot of arguments on this from others but this is what I have found from my use of such receivers.

P.S. I tried to attach some pictures of the alignment setup but for some reason the "Max size per file" was stuck at "0 Mb" and wouldn't let upload any.