[Hallicrafters] SX-28 Dial Tracking Issue - Update

Thu Aug 18 21:24:21 EDT 2016

I couldn’t wait for the weekend, so I took to the radio this evening after
work and I’m pleased to report that I got it taken care of. A much closer
inspection of the main tuning capacitor revealed a slight misalignment of
the outboard plates on C2 (the oscillator portion of the main tuning
capacitor). It had apparently been slightly knifed at some point in its
life. I very carefully moved the plates ever so slightly at a time, then
checked the center frequency. It got closer to spec, but of course it
moved the end points. Repeated alignment of the end points combined with
very fine tuning of the capacitor plates got it as close as I’ve ever seen
a radio of that design get. I really hesitated to muck with the plates,
but when the padders didn’t do it for me I had nothing to lose. Tracking
is reasonably good on all bands, orders of magnitude better than when I
started. The plates were moved very little, and a casual view of the
capacitor might not reveal that they’d been adjusted.

Many thanx to you guys for your suggestions.

73,
Jim
WA2MER

On 8/18/16, 1:19 PM, "Hallicrafters on behalf of Richard Knoppow"
<hallicrafters-bounces at mailman.qth.net on behalf of
1oldlens1 at ix.netcom.com> wrote:

>     I did mean minimum capacitance. Because capacitance is proportional
>to the inverse of the spacing, that is it goes up as spacing becomes
>less, the capacitance of a symmetrical capacitor will be minimum when
>the center plate is exactly centered between the two outside plates. It
>is somehow counter intuitive but works that way. If the rotor appears to
>be centered it likely is but very small differences can cause some
>variation in dial calibration so its worth checking.
>     I had forgotten that the SX-28 has padder caps as well as trimmers.
>  These are more likely to be causing the problem.  Getting padders set
>right is a bit of a juggling act. Sometimes reversing the order and
>adjusting the low end with the padder and the center frequency with the
>coil slug is necessary. So, you must do the padder, the slug, and the
>trimmer back and forth to get it on frequency.  Once set its unlikely
>the padders or slugs will change much.
>
>On 8/9/2016 6:29 PM, Jim DiMauro wrote:
>> I appreciate all of the thoughtful responses.
>>
>> Richard: The rotor plates of the main tuning capacitor appear to be
>> centered with respect to the stator plates, and the spacing seems to be
>> consistent throughout the entire tuning range. However, a check of the
>> capacitance is certainly in order and I will do that when I get back
>> inside the radio. I believe that you meant to say “maximum" capacitance
>> when you referred to when the plates are fully meshed, but I got your
>> point. Regarding the fixed padders, I had thought of that as well, but I
>> left them alone because the error is consistent among all of the bands
>> with regard to the amount of physical movement of the main tuning
>> capacitor required to correct the error near the center of the bands.
>> While I’m not prepared to rule out the fixed padders, it seems unlikely
>> that whomever might have messed with them resulted in almost exactly the
>> same error across the bands. That, and the lack of an adjustment
>>procedure
>> made me shy away from the padders, at least for now. Finally, I have no
>> intention of knifing the plates, since they appear never to have been
>> tampered with. It’s my guess that they left the factory untouched, and
>>I’m
>> determined to find and correct the root problem and not resort to
>>messing
>> with the plates.
>>
>> Jacques: See my comments above regarding the fixed padders. Also, I’ve
>> already tried the three point alignment procedure without success. At
>>the
>> end of it all the end points realigned properly and the band center
>>error
>> returned as before.
>>
>> Charlie: I did indeed check the LO frequency, which provides high-side
>> injection. It’s proper on the five bands I checked, and just for yuks I
>> tried to realign on the other side but couldn’t do it. I also checked to
>> ensure that I didn’t align to an image. BTW, you didn’t insult my
>> intelligence. You don’t know me or my capabilities, and so you didn’t
>>take
>> anything for granted. I appreciate all suggestions, no matter how basic.
>> Even the most experienced people can sometimes overlook things.
>>
>> All: I found a ceramic trimmer capacitor in parallel with C2, the
>>section
>> of the main tuning capacitor that tunes the oscillator and that is used
>>on
>> all bands. It’s located on the frontmost section of the tuning capacitor
>> and right next to V4. There’s no mention of it in the manual and it’s
>>not
>> shown on the schematic. It seems to require an unusual tool to adjust
>>it,
>> and perhaps that may provide the answer to my problem. I will give that
>>a
>> try before anything else.
>>
>> I’m not sure if I’ll have time to work on the radio over this coming
>> weekend, but whenever I’m able to get to it I’ll be sure to update the
>> group; might be a week or two. Again, thank you for your suggestions.
>>
>> 73,
>> Jim
>> WA2MER
>>
>>
>>
>>
>> On 8/9/16, 9:38 AM, "Hallicrafters on behalf of Jacques Fortin"
>> <hallicrafters-bounces at mailman.qth.net on behalf of
>> jacques.f at videotron.ca> wrote:
>>
>>> Hi all,
>>>
>>> I have found at least two sources that gives the same tracking
>>>adjustment
>>> sequence.
>>> The procedure is called three point tracking alignment and involves
>>> padder capacitors in the local oscillator tank circuits.
>>> This can be resumed as:
>>> 1.	Adjust the COIL INDUCTANCE to obtain the proper alignment at the
>>> CENTER of the dial.
>>> 2.	Adjust the TRIMMER CAPACITOR to obtain the proper alignment at the
>>> HIGH end of the dial.
>>> 3.	Adjust the PADDER CAPACITOR to obtain the proper alignment at the
>>>LOW
>>> end of the dial.
>>> 4.	Repeat the sequence until no improvement is possible.
>>>
>>> This should be the original procedure used at Hallicrafter's….
>>> Explained under Oscillator tracking at :
>>> http://users.tpg.com.au/users/ldbutler/Superhet.htm
>>> Once properly done, re-alignment can be performed using only LOW and
>>>HIGH
>>> adjustment points (two point tracking), as described in the SX-28(A)
>>> manual.
>>>
>>> 73, Jacques, VE2JFE
>>>
>>>
>>> -----Message d'origine-----
>>> De : Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] De la
>>> part de Richard Knoppow
>>> Envoyé : 9 août 2016 00:15
>>> À : hallicrafters at mailman.qth.net
>>> Objet : Re: [Hallicrafters] SX-28 Dial Tracking Issue
>>>
>>>    I forgot about the padders. I would certainly check those before
>>> doing anything mechanical to the tuning capacitor.  Sometimes you can
>>>get
>>> things on by setting the low end with the coil and the mid band with
>>>the
>>> padder. Do this a few times until both are on. Then do the high
>>>frequency
>>> trimmer and check the low and mid again.
>>>
>>> On 8/8/2016 8:29 PM, Jacques Fortin wrote:
>>>> Hi Jim,
>>>>
>>>> My best guess is that somebody have played with the local oscillator
>>>> padders
>>>> (FP1 to FP6 in the manual, or C7 to C12 in the schematic).
>>>> If someone, in the life of this set, have tempered with those, it can
>>>> well explain why you succeed to align the bands ends and be off at the
>>>> centers.
>>>> The effect of the padders is like changing the shape of the variable
>>>> capacitor rotor plates (like spreading the adjustment sections in
>>>> other radios).
>>>> Both the SX-28 and the SX-28A manuals are specific about the FP
>>>> adjustments:
>>>> DON'T TOUCH THOSE.
>>>> But you know, some never reads the manuals....
>>>> Just try the following to see if this can be related:
>>>> On any given band (say the highest one) just carefully note the actual
>>>> position of the related padder (C7) then purposely turn it a quarter
>>>> turn in either direction, then realign the ends using C103 and S14.
>>>> How is the center tracking, now ?
>>>> It can be better or worse, but at least you will have a starting
>>>>point !
>>>> The TM11-874 schematic gives the padder capacitor values for the
>>>> SX-28A, but I do not find any alignment procedure for those...
>>>> Maybe other people will know better (I hope).
>>>> I am not "in" my own SX-28A right now, but I will be during the next
>>>> year for sure.
>>>> Please make us aware of your findings !
>>>>
>>>> 73, Jacques, VE2JFE
>>>>
>>>> -----Message d'origine-----
>>>> De : Hallicrafters [mailto:hallicrafters-bounces at mailman.qth.net] De
>>>> la part de Jim DiMauro via Hallicrafters Envoyé : 8 août 2016 21:39 À
>>>> : Hallicrafters <hallicrafters at mailman.qth.net
>>>> <mailto:hallicrafters at mailman.qth.net> > Objet :
>>>> [Hallicrafters] SX-28 Dial Tracking Issue
>>>>
>>>> Hi All:
>>>>
>>>> I recently finished restoring a 1941 vintage SX-28. The rebuild
>>>> included all paper caps, a few micas that were out of spec and all out
>>>> of spec resistors.
>>>> I proceeded with the IF alignment, which seemed to respond well to the
>>>> procedure in the manual, and then I did the RF alignment. That¹s where
>>>> things didn¹t seem quite right.  I set the low frequency and high
>>>> frequency alignment points on each band, and the main tuning dial
>>>> lines up perfectly at each of the low and high end alignment points.
>>>> The problem is that the frequency accuracy of the dial degrades as I
>>>> move from the end points to the center of the band, with the error
>>>> being greatest at the center. I don¹t expect a radio of this type to
>>>> track like a Collins PTO would, but the tracking error is excessive
>>>> when compared to other radios that I restored, like my NC-200 and
>>>> HQ-129-X, both of which are nearly spot on from one end of each band
>>>>to
>>>> the other.
>>>>
>>>>
>>>> I took detailed measurements that show the magnitude of the error on
>>>> the SX-28. Here are sample measurements from each band: Band 1 aligns
>>>> perfectly at the alignment end points of 600 kc and 1400 kc, but the
>>>> approximate band center shows a 20 kc error (i.e. an 800 kc signal
>>>> reads 780 kc on the dial); Band 2 shows a 50 kc error at 2000 kc; Band
>>>> 3 has an 80 kc error at 4000 kc; Band 4 is off by 200 kc at 7700 kc;
>>>> Band 5 is off by 300 kc at 14000 kc; and Band 6 is off by 500 kc at
>>>> 28000 kc. Again, in all these cases the dial reading agrees with the
>>>> signal frequency at the high and low end alignment points, and in all
>>>> cases the dial reading is lower than the actual frequency.
>>>>
>>>>
>>>> Interestingly, the error in terms of the physical movement of the main
>>>> tuning capacitor seems to be consistent among all of the bands at the
>>>> band centers. To clarify, the difference in the main tuning dial
>>>> logging scale (the metal disk on the main tuning knob) between the
>>>> actual and dial frequency at the center is about 30 points on all
>>>> bands. If it was due to a problem with the gear train I would not
>>>> expect the end points to remain accurate after repeated tuning up and
>>>> down the band. I took care to ensure that I didn¹t align on image
>>>> frequencies. The band spread dial is installed correctly and I did the
>>>> alignment with the band spread capacitor set for minimum capacitance
>>>> (100 on the logging scale). There are no obvious physical issues with
>>>> the main of bandspread tuning caps, and there is no evidence that the
>>>> capacitor plates had ever been ³knifed.² Any ideas would be
>>>> appreciated. The radio looks and sounds great, but the magnitude of
>>>>the
>>>> dial error is driving an OCD person like me nuts.
>>>>
>>>> 73,
>>>> Jim
>>>> WA2MER
>>>>
>>>>
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>>>
>>> --
>>> Richard Knoppow
>>> 1oldlens1 at ix.netcom.com <mailto:1oldlens1 at ix.netcom.com>
>>> WB6KBL
>>> ______________________________________________________________
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>>
>>
>>
>
>-- 
>Richard Knoppow
>1oldlens1 at ix.netcom.com
>WB6KBL
>______________________________________________________________
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>
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