Hallicrafters SX-28 Alignment

WHILE THIS MESSAGE APPEARS, DO NOT USE THIS PROCEDURE. IT'S STILL BEING DEVELOPED
THE REASON ACCESS IS GIVEN IS SO SOME FRIENDS CAN HELP MAKE IT GOOD

This page describes all of the required equipment for this procedure.

This is handy for converting period and frequency.

Oscillator Alignment

First alignment task is to insure all of the bits in the oscillator module are adjusted properly.

The oscillator module has only B+ via R71 as an input. The only output is from V4 pin 6 (cathode) and C68.
V3 pin 5 (grid) receives the output of the oscillator module. Unfornatuly, you can't get to that pin.

Equipment Requirements for Oscillator Alignment

Scope
Optional: Frequency counter
Optional: Tube extender
Screw driver

Common Instructions For All Bands

It might be possible¹ to connect a scope probe to the mixer end of the mica capacitor going from V4 pin 6 to V3 pin 5.
It might be possible to connect a scope probe to V3 pin 5.
If so, position the radio upside down to get access to the oscillator adjustments.

If that doesn't work out, position the radio on its side, heavy power transformer down, and install the tube extender under V3 and select pin 5. Connect the scope to the test point on the tube extender.

Band One

Switch the band switch to band one.
Tune main dial to 1400 KHz.
Observe a sine wave on the scope with a period of 0.54 ㎲ or 1.855 MHz on the counter.
If corrections need to be made, go here.
Tune main dial to 600 kc.
Observe a sine wave on the scope with a period of 0.95 ㎲ or 1.055 MHz on the counter.
If corrections need to be made, go here.

Band Two

Switch the band switch to band two.
Tune main dial to 2.8 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.
Tune main dial to 1.6 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.

Band Three

Switch the band switch to band three.
Tune main dial to 5.6 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.
Tune main dial to 3.2 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.

Band Four

Switch the band switch to band four.
Tune main dial to 11 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.
Tune main dial to 6 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.

Band Five

Switch the band switch to band five.
Tune main dial to 20 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.
Tune main dial to 11 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.

Band Six

Switch the band switch to band six.
Tune main dial to 36 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.
Tune main dial to 22 MHz.
Observe a sine wave on the scope with a period of _____ ㎲ or ____ KHz on the counter.
If corrections need to be made, go here.

455 KHz IF Alignment

Second alignment task is to insure all of the bits in the IF chain are adjusted properly.

The IF chain, which consists of transformers T1, T2 & T3 and tubes V5 and V6, has four inputs:
The cathode return voltage via R16 and R21.
The screen voltage via R17 and R22.
B+ via V18, R23, and R30.
The output of the mixer V3 pin 3 via T1.
The only output is from V6 pin 8 (plate) and T3.
V7 pins 5 and 5 receive the output of the IF chain via T3. Unfornatuly, you can't get to that pin.

Sections in whatever this color is were copied from elsewhere and don't make any sense. Yet.

Equipment Requirements for IF Alignment

Scope
Tube extender
Signal generator
Screw driver
Dummy antenna

Procedure

Position the radio right side up. Power off.
Test each adjustment screw for free movement. Don't forget C29 through 31. They frequently bind. It would be most annoying to have to apply penetrating oil or whatever to free up something up half way through the procedure.
Attach dummy antenna to A1 and A2. Connect A2 to ground terminal.
Configure the front panel controls as described here.
Connect a 5,000 ohm 10 watt resistor across the 5,000 ohm speaker terminals. One of the terminals goes to ground. Connect the scope to other² one.
Remove V2. This effectively removes all of noise from the RF section.
Adjust signal generator to 1.4 MHz sine wave with 0.02 volts peak to peak³.
Connect signal generator ground to chassis.
Connect signal generator output to main tuning capacitor C1-1.
This is V3 pin 8 while on band one (AM). This is the 3rd grid of the tub eand ususlly carries the amplified antenna signal from the RF stages.
Power on.
Observe a 455 KHz sine wave on the scope. The 455 KHz period is 2.2 microseconds. If this frequency is off, it's because the signal generator is set wrong or the oscillator alighnment (above) needs revisiting. There aren't any other factors.
Increase signal generator output voltage a little. See if movement is seen in the output amplitude. This doesn't do anything useful but it's a nice checkpoint to see if you're on the right track.
Roughly adjust the aligning screws of T1. The lower screw is accessible through hole in right mounting bracket. Adjust for maximum output on the scope.
Adjust lower screw on T2 for maximum output on the scope. Do not adjust upper screw. (Why?)
Adjust C31 for maximum output. C31 is located here:
Adjust the air trimmer condensers at the top of T3 for maximum gain.
Switch to Crystal Broad Position.
Turn on BFO and adjust WHAT?? to a tone of about 1 KHz.
Vary the frequency of the signal generator while adjusting the top screw on T2 until the output goes through a maximum, dips down and starts going up again.
Adjust the phasing control for maximum selectivity and then back off the top screw on T2 until the output reaches a minimum value between the two maximum values first noted. The frequency of the signal generator should be varied over a small range while adjusting the top screw of T2.
A swishing note, in contrast to the usual sharp crystal tone will be apparent when the correct adjustment has been reached.
Switch to "Xtal Sharp" and adjust C10 for maximum output while varying signal generator frequency.
Two points of maximum output will be noted corresponding to two adjustments of C30.
Either one of these points may be used at which to leave C-30. a sharply peaked tone will result at the correct adjustment.
Switch to "Xtal Medium" and adjust C29 till the output is midway between the outputs reached while aligning the "Xtal Sharp" and "Xtal Broad" positions. The apparent sharpness of tone should be midway between the "Sharp" and "Broad" positions. Switch again to "Xtal Sharp" and set the signal generator to exact crystal frequency.
Set BFO front panel control to a tone of approximately 1 KHz.
Switch again to "Sharp IF" and carefully realign the IF transformers as earlier described in the first paragraph of these instructions.

BFO Adjustment

Set front panel control to zero
BFO switch ON
Signal Generator tuned to crystal frequency --selectivity switch in IF Sharp position-now, adjust screw on top of T4, after loosening lock nut, to zero best. (See Fig. 8)

Noise Limiter and AVC Amplifier Adjustment

Have the controls set as before except that the AVC switch is now in the ON position. Connect a high resistance type voltmeter across R49 which is connected between terminal #5 of the 6L7 tube and chassis. Connect a 50,000 ohm resistor across primary of TS (Red and Blue leads). Set generator at 455 kc as for IF alignment. Connect generator to grid of 6AB7 tube (pin #4). Rotate ANL control all the way to the right, or position #9. Adjust screws on top of T5 for maximum indication on DC meter connected across R47. Reconnect generator, as for IF alignment, to mixer grid of 6SA7 tube. Remove 50,000 ohm resistor which was inserted across primary of T5 during alignment. Remove grid clip off top of 6L7 tube. With generator set at 455 kc and ANL control at extreme right adjust wave trap trimmer C55 for minimum signal as indicated on output meter. (See Fig. 8 and Fig. 12 for location of adjustments). (4) With generator connected to 6SA7 mixer grid as above, replace 6L7 grid and turn ANL control to extreme left until switch clicks. Connect high resistance DC meter across 6B8 diode filter condenser C64. Adjust screw on top of T6 for maximum indication on DC meter across C.64. For RF and oscillator adjustment location and align- ment procedure see Fig. 12.

FIGURE THIS OUT - "Antenna trimmer adjusted for Maximum gain at each RF alignment point on all bands."

Credits:

Bill Feldman N6PY for SX-28 IF ALIGNMENT PROCEDURE (Rev A)

Footnotes:

On the two radios I had available, one connection was made to the mixer end of the cap (the insulation sleeve was a little short) and on the other, connection was made to V3.
I think I've seen the speaker outputs wired up differently, particularly if an improper output transformer is installed.
I suggest setting up the generator before connecting to the radio as my cheap generator turns on with the output set to 5 volts peak to peak.

Page: /Alignment/Old_radio/Old_radio_align.shtml

Last modified: Sunday, 31 May 2026