SX-42 Alignment

[Chris P.]

I posted this over on the Antique Radio Forum, but I haven't got a response so I figured I'd post it here.

Can anyone better explain steps 2 through 4 in the Sams alignment instructions? I have both the Hallicrafters service bulletin and Sams photofact and I can't quite understand what I am supposed to do.

What is the purpose of setting the BFO to a 1000 cycle tone in step 2? After that step should I turn off the BFO and turn the modulation on the signal generator back on?

On step 3 it says to slowly adjust A4 while rocking the signal generator. I am supposed to adjust it to a peak?

Thanks
Chris

[WQ9E]

Chris,

Those steps are the preliminary setup of the single crystal filter in the 455 Khz. IF. 

In steps 2 through 5 you do leave the signal generation modulation turned off and the BFO is used to provide a beat note so that you have an audio signal for the output meter.  Having the 1000 hz tone is not critical.  Setting up the crystal filter/IF alignment this way provides the most accurate alignment of the sharp filter position for CW (which is where it will be used). 

An additional note:  I have yet to restore an SX-42 or SX-62 receiver that did not have a leaky mica cap in the FM discriminator transformer (at this point I have gone through about a dozen of them).  So the odds are very high that your receiver will also have this problem.  When aligning the discriminator you should note a clean and sharp null on an AM signal as it is adjusted and if the capacitor is leaky the null will be broad and shallow.  Poor performance/high distortion on FM will be the result.  I have run into a couple of bad mica caps in the other IF cans but very infrequently.  The symptom you will find is the lack of a distinct sharp peak since the resistive loading of the leaky mica will kill the "Q" of the transformer.

[w3jn]

I had to replace EVERY mica in EVERY IF transformer in the last one I did.

Rodger, as a fellow veteran of about a dozen of these things myself, I've never seen one with a good audio output transformer - have you?  That tone cap in the plate of one of the 6V6s ALWAYS shorts and takes half the output transformer primary with it, so the 6V6 is running with no plate voltage and full screen voltage, promoting a meltdown...

[WQ9E]

JN,

This is one area where I have been very lucky I guess, only 2 with bad audio transformers so far and both were probably because of the tone control cap.  My first SX-42 was a real winner however because both the power and output transformers were defective.  I have half a dozen caps left to replace (the easy ones since the front end is complete) in the one I am doing for my daughter and then I will see whether the output transformer is good.   

I bought the first SX-42 for $20 at a hamfest and the seller told me it had quit working.  He said it would work for awhile with kind of a funny smell and then it would blow the fuse but if you replaced the fuse it would work for awhile again.  He kept replacing the fuse until it finally died from the leaky coupling cap putting positive DC on one of the output tubes.   Some people could damage an iron anvil with a rubber mallet and this guy was one of them.

[W7SOE]

I had to replace EVERY mica in EVERY IF transformer in the last one I did.

Rodger, as a fellow veteran of about a dozen of these things myself, I've never seen one with a good audio output transformer - have you?  That tone cap in the plate of one of the 6V6s ALWAYS shorts and takes half the output transformer primary with it, so the 6V6 is running with no plate voltage and full screen voltage, promoting a meltdown...

yup, replaced mine also.  I eventually had to get a parts rig for mine but I DO enjoy that FM audio in the shack!

My volume control is very "scratchy", could that e the tone control cap you guys are talking about?

Rich

[WQ9E]

My volume control is very "scratchy", could that e the tone control cap you guys are talking about?
Rich

The tone control caps are C-108 and C-118 and should be replaced for the health of your output transformer. 

If C-106 is leaky then you will have DC going through the volume control which will make it very noisy in a hurry. 

If you haven't done a complete wax/paper recap I would definitely replace the three of these along with C-109 and C-112 (leakage here will put a positive bias on the output tube grids resulting in collateral damage to power supply and output transformer) and C-111 (electrolytic cap which is the output stage cathode bypass, has the same basic failure effect as leaky C-109 or 112 since leakage/short here will reduce/kill output stage cathode bias resulting in greatly increased B+ draw.

[Chris P.]

Thanks for the replies.

This radio was originally owned by my grandfather who had it recapped and aligned about 10 years ago. The performance has slowly been decreasing, s-meter wouldn't zero, and finally the bandspread string broke so I hauled it onto the bench. I found a few out of spec resistors including one on the 1st rf amp cathode that had doubled in value which caused the s-meter problem. I also found a couple of caps in the RF secion that had been missed. The dial calibrations were off and the s-meter wasn't very responsive so I figured it needed an alignment.

Rodger your post was very helpful but I am still confused about step 3. It says to slowly adjust A4 while rocking the signal generator. I can rock the signal generator and find the stronger and weaker signal but why do I need to adjust A4 and what am I supposed to look for when I do this?

[WQ9E]

Chris,

In this step you are adjusting the crystal filter to set it up for the sharpest null and the A4 setting should be adjusted so that its tuning is centered on the null.  The Sam's photofact wording is confusing and I would suggest pulling the SX-62 run2 manual from edebris:  http://bama.edebris.com/manuals/hallicra/sx62run2/ and look at adjusting the crystal filter on page 19 of this manual.

The SX-62 circuitry is very similar to the earlier SX-42 and I think the Halli manual provides the best description of what should happen during the crystal filter adjustment.  There are some major differences between the SX-42 and 62 so you are better off with the Sam's photofact of the 42 for most of the alignment but not for the section on adjusting the crystal filter.

As a bit of trivia, towards the end of the SX-42 run it was selling for only $5 more than the newer SX-62 according to the old Hallicrafters advertising in the handbook.  That extra $5 bought a much nicer receiver (for amateur usage anyway).

[Chris P.]

Thanks Rodger.

I think I got it now. I did the IF alignment and it seems to be working OK. This procedure for the SX-28 also helped. http://pcbunn.cithep.caltech.edu/jjb/Hallicrafters/SX28/SX-28_IF_ALIGNMENT_PROCEDURE.pdf

Now when doing the RF alignment I can't get a peak on A23 or A32. I guess I have more work ahead of me.

The S-meter still doesn't respond well on any of the bands. The strongest SW stations read an S5 at most, and there is no noticeable response on FM although FM works very well. Where should I start looking?

[WQ9E]

Chris,

What all have you done to the receiver thus far (cap replacement, resistor checking, tube checking)?  On a strong local AM broadcast station with a fairly long wire antenna connected does the receiver overload (distort)?  If reception is weak, you could have a weak tube or out-of-spec resistors as high likelihood causes of the trouble.  If it distorts with a low S meter reading then one or more AGC bypass caps could be leaky pulling down the high impedance AGC line.

If you have replaced the wax paper caps, then I would take tube voltage readings to see if any are far out of spec.  I have replaced a lot resistors in old Hallicrafters gear, in particular the 1 to 5 watt units. 

For the two trimmers that don't have a peak, try using alcohol to clean the trimmer cap body.  There is often a conductive path that develops from dirt and this will greatly reduce the coil Q.  Depending upon the trimmer style it may also have cracked and shorted or the rotor may have come loose from the adjusting screw.

[Chris P.]

It was recapped by someone else about 10 years ago. I have gone over his work and he did a very nice job, but I found a couple capacitors in the RF section he missed. I replaced them, but there was no improvement. I have checked and replaced quite a few resistors, and the tubes all test excellent. All of the voltage readings are in spec.

I found why trimmer A23 wouldn't peak. The primary of the broadcast band antenna coil was open. The break was right at the terminal so it was easily fixed. I still haven't figured out why A32 won't peak.

I've found the 330 and 390 pf micas in the 2nd IF can (can #207 on sams) are leaky. Could this be pulling down the AGC line? The radio does not distort on a strong station.

I'm wondering if I should go ahead and replace all of the micas in the IF cans even though the cans peaked well during alignment.

[w3jn]

If you got a nice SHARP peak (+/- 1/2 turn of the slug) during alignment, leave well enough alone.   If it was a broad peak (say +/- a turn or more) the caps are bad.