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 Post subject: Hallicrafters S-38 Alignment Inquiry
PostPosted: Sep Mon 20, 2021 9:11 pm 
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Joined: Aug Tue 31, 2021 11:43 pm
Posts: 6
I have restored a Hallicrafters S-38 (the early one with the BFO tube and the BFO knob on the front panel) in a version which does not have the two RF trimmer capacitors adjustable from the top of the chassis (designated A10 and A13 in a SAMS Photofact document I downloaded). Someone had re-wired the power supply section probably to compensate for one or more sections of the filter cap having gone south so I replaced the filter cap and restored the wiring to match the schematic diagram. I also re-capped the signal paths.

The radio started out with nothing but a very LOUD 120 Hz hum from the speaker and now it actually sounds sort of like a radio.

After replacing so many caps and replacing the tubes I am performing IF and RF alignments. I have never worked on a radio like this so now when aligning it I have several questions:

1. When performing IF alignment I am injecting the signal to the stator plates of the rear of the tuning cap as described in the literature. Most RF signal generators (mine included) have a 50 ohm output impedance. I am wondering if this causes a problem by loading down the circuit node where the signal is injected? If so how do I avoid doing this?

2. Related question is what amplitude signal should I be injecting on the rear stator of the tuning cap? I do not want to overload the circuitry because that is likely to render the alignment operation meaningless. Should it be 10 microvolts, 100 microvolts, 10 millivolts, 100 millivolts etc. or what?

3. When performing RF alignment I am injecting the RF signal to the antenna terminal through a RMA dummy antenna as mentioned in a manual I read (the one described at ai4fr.com/main/page_ham_radio_electronic_repair_rma_schematic.html). Again, what amplitude signal should I be injecting in order to avoid overloading the front end?

Nothing I have read anywhere addresses this issue.

Also someone in response to a related question said that good results can be obtained by shorting the AVC to ground after the first resistor in the AVC line then measure the negative voltage from the detector. Can someone tell me: 1) which resistor is being identified? and 2) At what circuit node do I measure the negative voltage from the detector?

I once heard that a dumb question is the one that you failed to ask . . . .


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 Post subject: Re: Hallicrafters S-38 Alignment Inquiry
PostPosted: Sep Tue 21, 2021 12:04 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8157
Location: Near Brandon, Iowa
1) No, it won't. Don't worry about it- the sig gen could be a perfect "hard source" for all it matters.
(2-3) Use the lowest RF output level possible to get the audio tone from the sig gen out of the speaker. The absolute level is irrelevant and probably varies a bit from one radio to the next of the same model; just use the lowest level needed to barely get the output tone. Too much and you will engage the AVC and that will just complicate matters. Set the volume control to maximum to avoid masking the minimum RF input level at which the tone is present on the output.

An analog output meter (AC voltmeter) works best for output audio peak signal assessment. (It's tough to do this with moving numbers using a DMM.) And the human ear is not a very accurate output meter (although that's probably a controversial statement).

The Riders alignment procedure will work perfectly well; if you want to experiment with other techniques such as locking AVC to the gain stages and then monitoring the AVC source node voltage with an o'scope or VTVM, that's fine- but it won't necessarily produce superior end results. All you are trying to do in aligning this set is to achieve two things:
(1) Ensure that the IF filters peak at the correct IF frequency; and
(2) Adjust the front end trimmers and padders to make the dial indicator needle register accurately on each band (as accurately as the adjustment constraints of the design permit). Perfect linearity from one end of the dial to the other on all bands is probably unachievable.

This is a very basic radio, essentially an AA5 with additional bands. It is what it is and it can't be materially enhanced by "improving" the alignment procedure. I own several restored (by me) examples of this radio- all different sub-models- and, while I haven't aligned any of them for about ten years, I don't recall that any were problematic in this regard.


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 Post subject: Re: Hallicrafters S-38 Alignment Inquiry
PostPosted: Sep Tue 21, 2021 2:48 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 827
Location: Suburban Chicago
I agree with the above. If you want to align by measuring the AVC the circuit below will help. In this radio you would put a temporary jumper or cliplead around the "S5A" contacts shown below. This will disable the AVC but keep a small bias on the converter and IF amp tube grids. Now you can measure the AVC voltage at the end of "R5" indicated below. This schematic is the one found in the Hallicrafters service bulletin for the S-38, the parts may be numbered differently in other S-38 schematics. You want to keep your signal generator output in the linear range of the AVC detector if you align using AVC. Measure the no input voltage and keep the AVC voltage dips while you are tuning about 0.5 to 1 V lower than the no input voltage. You do this by gradually turning down the signal generator as you align. If you start with a radio that is already aligned and you are just touching up the alignment then whatever signal generator voltage you start with may be good enough for the whole alignment. If you start with a radio that is far off alignment you will have to ride the signal generator voltage more closely as you bring the successive stages into alignment.

Attachment:
AVC_Alignment2.jpg
AVC_Alignment2.jpg [ 554.59 KiB | Viewed 102 times ]


I don't know that the AVC method is any better than any other method. It could be convenient in radio factories because you could use dummy loads on the speaker outputs to avoid having ten tuning stations blaring test tones simultaneously and it might eliminate the need (ie, expense) for an oscilloscope at every tuning station.

Maybe you've already seen this thread and it sparked your question but if not here is more on the subject of AVC tuning:

https://antiqueradios.com/forums/viewtopic.php?f=6&t=345842

NOTE: In an earlier version of this response I said that you can pull the BFO tube and switch to CW mode to avoid adding the jumper or clip lead. If you ever work on a radio with a separate BFO tube and parallel wired filaments from a power transformer that trick would work. But in transformerless sets like this one the filaments are series wired and you cannot pull the BFO tube without disabling the whole radio! If you read that earlier version, sorry about the senior moment!


Last edited by khutch on Sep Tue 21, 2021 4:10 pm, edited 4 times in total.

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 Post subject: Re: Hallicrafters S-38 Alignment Inquiry
PostPosted: Sep Tue 21, 2021 3:45 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 827
Location: Suburban Chicago
Post deleted, error in message above corrected there instead.


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 Post subject: Re: Hallicrafters S-38 Alignment Inquiry
PostPosted: Sep Tue 21, 2021 4:46 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16897
Location: S. Dartmouth MA 02748-1225 USA
My observations begin with where did 120 hz tone in the speaker come from when it should be a 60hz.? Was this hum condition remedied?

For your safety and electrical device safety, use an isolation transformer when LIVE servicing this radio.

It is common practice to use an output meter on the speaker or at the plate of the output tube reference to chassis. That is an AC meter coupled with a .1 mf cap in series with the plate connected lead. Set meter 10 volts or lower. Beware if meter has a metal case connected to the negative lead.

The generator can be coupled with a link, meaning there is no electrical contact. A 10 turn hank of insulated hookup wire about 1-1/2" or a bit smaller in diameter connect to the shield and the live of the generator output lead. Use the insulated link to lightly couple the generator to the oscillator coil or in the vicinity of the stator of the oscillator tuning condenser. Using a link allows for light introduction of RF and fully isolates the generator from accidental contact with a live AC chassis... Follow all the steps for the alignment in the Riders/Hallicrafter data. Beware setting the BFO, should zero beat on the desired IF frequency, may involve a balance of mechanical settings as well as electrical, unless data says otherwise. When zeroing the BFO turn off the modulation on the generator, the whine is distracting. Note, some adjustments may interact, the technique of "rocking" may have to be applied to get proper tracking of the OSC vs ANT adjustments.

As long as the generator is kept at a very low level the AVC will not influence the settings. That includes any noise that may enter the radio, noise will effect the AVC.

For the RF create the "mock" antenna advised and couple to the generator using a twisted pair gimmick cap of about 5 to 10" of hookup wire. No chassis connection to the generator.

Same rules, keep the generator output as low as possible. Modulation at 30% 400hz. (or so). Beware that the oscillator is set to the correct peak in the HF bands and not an image or RF alignment will fail and the radio will be deaf on some parts of some bands.

Note when done, band conditions may not be favorable and radio may be "dead" to be sure, set the generator for HF and place an "antenna" on the generator some distance away, 10' or so, modulate, antenna of 5' should be adequate or more to determine all is O.K.

Be sure to use the correct tube type, the 12SA7 and the 12SK7 are metal tubes. If glass are used they must be shielded and may not align properly with the substitutes even if shielded.

GL

Chas

_________________
List' & I will Enchant Thine Ear


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