[Hallicrafters] SX-28 Dial Tracking Issue - Update

Thu Aug 18 21:46:52 EDT 2016

     Congratulations:-)  I am very reluctant to bend plates. Mostly its 
not necessary. I suspect someone didn't know how to adjust the padders 
or had some other problem and just got in there and bent them.

On 8/18/2016 6:24 PM, Jim DiMauro wrote:
> 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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>>>>> ______________________________________________________________
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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
>> ______________________________________________________________
>> Hallicrafters mailing list
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>>
>>
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>
>
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL