Help needed with SX-28 connections...

[PEHamel]

Alan,
I passed along the SX-28 manual with my receiver, so it is gone. It has been a while, so make sure that the IF crystal is 455 before you try to align to 455.
There is high voltage on the plate trimmers and the decoupling resistors can be changed in value by shorting the trimmer to the can when trying to align it.
Mine had the wiring so rotten that I needed to buy spaghetti tubing and rewire the coil decks one wire at a time.
You have access to epoxy, so epoxy a piece of bare stiff wire in the coil form where the terminal (I think you call them TAGs?) was broken off.
That will give you a safe place to solder the litz wire and not break it off at the coil.
Do you have the Hallicrafters or the military manual ?
There are some capacitors in the oscillator section that the books warn
"Do NOT TOUCH". There are trimmers and padders and feedback or linearity caps if I remember right. Sorry I don't remember more.
Pat

[a_strong]

Good morning Graham - there area around V9 has many new components in it, which could be the result of modifications but I suspect the problems that I am currently dealing with result from haphazard attempts at fault finding, probably without any documentation and a similar amount of experience. The quality of the soldering is quite appalling in a number of places but will be corrected during the re-capping process.

I plan to work through your earlier notes and try to restore things to back as they should be. Fortunately the band associated with these components is working, albeit with reduced sensitivity (R5 Live on 909kHz reads S6 on the meter with 50' of wire aerial attached) so the effect can be assessed after any change that I make.

My set has a switchable transformer and it have incapacitated the switch less it should become moved accidentally when I keep inverting the set. So far I have remembered to remove the rectifier valve so it has not been decapitated (yet)!

Might I ask you to check the dial reading with the tuning capacitor fully meshed and fully open? Clearly there is something wrong with the dial setting on my set.

Enjoy your holiday!

Alan

[a_strong]

Good Morning Pat - many thanks for your advice, especially regarding checking the crystal frequency.

I have the Hallicrafters manuals for both the SX-28 and the SX-28A and the military manual. My set is definitely an SX-28.

I was initially deterred from investing any time or money in the set when I saw it used fabric coated wire but it turned out to be in remarkably good condition, considering its age and the conditions under which it has been stored for the past ten years. The top of the cabinet and tuning capacitor box were both very rusty but inside the box and underneath the chassis were reasonably clean. The top of the chassis would benefit from having the transformer and capacitor box removed so that it could be properly cleaned but that is low on my list of priorities. I plan to renovate the gear-box some time in the future and the top of the chassis can be wire brushed and given a coat of grey enamel at the same time.

Beneath the chassis someone has made a very clumsy attempt to replace the Rs and Cs, and that will have to be put right in the course of time.

When I first got the set at least a dozen wires or components were disconnected but with a great deal of help from Graham the connections have been traced and remade. It is no wonder that he is in need of a holiday


Best wishes - Alan

[a_strong]

Hi Graham - just to put this to bed, at least for the time being, I have attached a drawing of T13 as it is at the moment. The blackened winding worries me a little and my eyesight is not good enough for me to form a reliable assessment as to whether the coil should be black, with the limited view that I can get of it.

Therefore I plan to leave any further work on this area until I remove the coil deck to replace the capacitors. However, I fear that I may be looking for another T13. An entire 1st RF coil deck would be the answer, but out of the question financially, unless I am very fortunate.

Alan

[Station X]

The positioning of the dials relative to the tuning capacitors is easy. With the tuning capacitors fully meshed and fully unmeshed the cursor should line up with the correct end of the scales. The bandspread scale has these points marked 0 and 100 on the logging scale. If you set that up first you'll be able to see how the main scale should be set.

T13 has three windings. None of them are black, they are wound with fabric covered wire which I would describe as a candle wick colour. It is reasonable to suppose that two of the three windngs are wired in series or parallel to form one electrical half of the transfromer, but I wouldn't like to say which windings.

TRANSFORMER T13.

WITH REFERENCE TO THE CIRCUIT DIAGRAM. ie Top means the connection nearest the top of the diagram.

PRIMARY WINDING.

Top connection goes to the wave change switch via a white wire with blue stripes. This is the NE tag nearest to the chassis when the set is inverted.

Bottom connection is commoned to T14, T15, T16, T17 and T18. This is the NW tag.

SECONDARY WINDING.

Top connection goes to trimmer T12. This is the NE tag nearest to you when the set is inverted. (The tag which has been ripped out of the former).

Bottom connection is commoned to T14, T15, T16, T17 and T18. This is the SE tag.

To correct an error in a previous post the litz wire on the NE tag nearest to you with the set inverted goes to the inside of the winding nearest to you with the set inverted. Whether it terminates on that winding or passes through it I cannot say.

[Station X]

Looking at your picture in post #12 all three windings appear to be intact. What you describe as a blackened winding is in fact a piece of wire.

[a_strong]

Hi Graham - as I said, poor eyesight! Many thanks for putting me right on that point.

Regarding the dial adjustment, the coupling at the capacitor end of the extension spindle was slipping on the capacitor shaft. It has been repaired at some time but when it was refitted the screws were partially hidden by the end of the screening box and there is not room to get a nut runner onto them. They appeared to be tight, but not tight enough apparently.

It looks as though the screening box is going to get an early bath because whilst I was removing the screws from the rear coupling to clean up the slot, one fell inside the box and is lodged somewhere between the front end of the box and the front end of the capacitor. I have tried upending the chassis and shaking the set, as much as a septuagenarian can shake a 78lb radio set, but it refuses to drop out.

The challenge will be how to remove the box securing screw that is directly under the extension shaft without removing the gear box. So maybe the gearbox will get an early overhaul as well!


Alan

[Station X]

Alan.

These days I can't repair a valve set without the aid of an illuminated magnifier. Transistor sets are very difficult for me to work on and anything involving ICs just about impossible. You're not alone.

I've come to the conclusion that every carbon resistor, wax paper and electrolytic capacitor in my SX28s will need replacing. It looks like this gets progressively more difficult as you work your way back from PSU and AF stages towards the aerial. I'm determined to get at least one set working though. Every time I hear someone on the air saying they're using an SX28 I go green with envy.

Anything I can check for you please ask.

[a_strong]

Good Morning Graham - well I should not be troubling you for a while. I will be retrieving the screw today, which means removing the tuning capacitor box. I have removed all the screws but one (the one under the main tuning capacitor drive shaft). I could maybe get at it with a small o/e spanner but since the other screws were very tight (the heads of two of then sheared off) I want to use a nut driver so as not to damage the head.

To gain access to that screw I will need to remove the shaft, and to remove the shaft I need to remove the front panel and gearbox. I plan to clean up the screening box and recondition the gearbox whilst they are off the chassis.

I have already replaced the Rs and Cs in the power supply and audio stages and following my investigations of the RF deck replacing the Rs and wax paper Cs in that does not look as intimidating as it did at first.

Adam from the coating company has been in touch and is sending me a sample of the finish for the cabinet so things are progressing well.

Alan

[FrankB]

I might have an OEM service & op manual in my storage for that unit.
If you need a dead tree copy, please let me know.
Frank