Help needed with SX-28 connections...

[a_strong]

The resurrection of the much hacked about SX-28 receiver that I have inherited is progressing well but I am stuck with tracing the connections to the RF coil switch box under the chassis.

The problem I have is that there are few clues about how they were originally arranged because of the butchery that it has been subjected to in a previous life. I have applied a 20VDC to the 280V B+ rail at the reservoir capacitor C49 and all the valves that should have voltage on their anode and screen pins have it, with the exception of the 1st RF amplifier, V1, which is hardly surprising considering that three wires are still unconnected under the chassis. I have established that there is a low resistance connection between pin 8 of V1 (anode) and wire 1. The resistance of this path varies with the setting of the band switch which would suggest that it should be connected to the 280V B+ rail. However, there is no path between wire 1 and pin 6 (G2) of V1 which suggests that maybe R4 is missing.

I would appreciate help in sorting out these wires and replacing and rewiring the broken tag strip in the bottom left hand corner of the attached image.

Many thanks - Alan G3WXI

[Station X]

Alan.

I have two SX28s. I'm having a photographic session tomorrow, so I'll take some under chassis pictures for you then.

[Station X]

I'd forgotten how heavy SX28s are. Having lifted one of mine on to the bench I feel an overwhelming desire to restore it and hear the push pull output stage. That won't happen though as I have about 5 other projects in progress.

I hope my sketch is of help to you. I have made no attempt to compare it with the circuit diagram. As I say the colour code on one resistor is indistinct. It may be shunted by design or by faulty components elsewhere.

The set will remain on the bench for a few days so let me know if you want anything checked. I took some pictures, but they are not very useful.

[Station X]

On checking I see that there are no 3.3K resistors in an SX28. My guess is that the resistor in my sketch is a replacement for R10 which is 3K. This leaves plenty of possibilities for the designation of the other resistor.

EDIT. I'll try looking at another set tomorrow.

[PEHamel]

Take some time to look at http://antiqueradio.org/halli12.htm
There are some pictures and a blow-by-blow what to do (and not to do).
Enjoy the feeling when it finally works. Mine had been hooked to a low-ohms speaker and had burned up the transformers and the heat killed the magnet on the s-meter.
But it finally worked.

[Station X]

No help at all from the other set. Some previous owner has stripped out the entire PSU and AF sections. He probably used them to build a guitar amp or something.

Returning to the original set I disconnected the unidentified resistor and it still measures 441 ohms. I swabbed some of the dirt off and the colour code "appears" to be brown, black, something where something could be red. That would make it 1K, so perhaps it's R5?

That being the case the two wires in the centre of my sketch would be the 280V HT rail.

The two wires which go into the RF compartment are terminated on the bottom of T20 in the 2nd RF Compartment and the bottom of S11 in the 1st RF Compartment. I think this positively identifies the resistors as R5 and R10.

[a_strong]

Hi Graham - many thanks for the sketch and the subsequent information. I came across the attached image on the Internet today which confirms your 3K and the 1K, with the proviso that the critical band on that resistor is obscured. I go along with the 280V and I will implement the wiring tomorrow and see if that restores the front end H.T. connection but only with 20 volts on the rail, just in case! Interesting about the 441 ohms of the 1k resistor. I have been rebuilding the AF section today and found every component way out of tolerance, both Rs and Cs.

I managed to pick up a 15W push-pull 6V6 output transformer from the USA (Ebay item number: 400118486973) for £12, including shipping which is the same as the cost of shipping alone from a UK dealer. It is 8K anode to anode load with 4, 8, 16 ohm secondary which is not ideal for an authentic rebuild but probably more convenient and I hope that the 6V6s won't be too concerned about the lower primary impedance. My set has been so chopped about that I am only aiming to get the it working and make it look tidy, with no ambition to make it look like it just came out of the packing case.

I have already discovered the antiqueradio.org website

Alan

[dave walsh]

Saw that image myself this afternoon Alan, researching the 28. I didn't know about this set but I can see why it has such a reputation. Good luck with your efforts. I suspect that the insides don't matter that much, if you can get the look and the performance back! Dave W

[PEHamel]

Alan,
I had an inexpensive "engraved name" badge made that I glued to the speaker terminal area of mine saying that the output transformer had been changed "to minimize shock hazard and match modern speakers".
The next owner is therefore warned.
The transformers in mine caught fire - there actually was foil on the fuse when I got mine.

[Station X]

To clarify my earlier posts.

The wire from R10 (3K replaced with 3.3k) goes into the second screened compartment from the front of the set. It termnates on T20 which is of course connected to T19 through to T24.

The wire from R5 (1K) goes into the third screened compartment from the front of the set. It passes between T13 and T14 and terminates on what is annotated as S11 in the manual. This is the coil next to T14.

[Station X]

Alan.

In a PM you asked me about the SX28's crystal. This is located under the bandwidth control switch wafers. Right in the middle of the attached picture.

If it's like the one in the HRO Receiver it should be possible to dismantle it for cleaning.

Correct operation of the Crystal Phasing Control depends on accurate alignment, with the IF stages aligned to the crystal's frequency which may vary from the nominal IF.

[a_strong]

Thank you for confirming that the mystery object that I suspected might be the crystal is indeed the crystal. I cannot remember if I told you that the set is now working (after a fashion) and my next task will be to replace the Rs and Cs in the RF box but I am more than a little apprehensive about taking that job on. I strongly suspect that the set requires realignment but I do not want to embark on that until I am sure that the RF and oscillator stages are not suffering from leaky or out of tolerance components. I have a reasonable signal generator and frequency measuring equipment so providing that there are no nasty surprises lurking inside the coils or IF cans I should be able to work my way through that.

One thing that concerns me is that there are signs that someone has meddled in the RF box in the vicinity of T13 (image attached) so I am not rushing into anything.

Alan.

[Station X]

Set upside down with the back of the chassis towards you. The wire from what is now the top coil of T13 is connected to a tag which is rivetted to the former. The hole for this tag is just visible in your picture behind the trimmer. From the tag a wire goes to the trimmer capacitor second from the left in the group of four. This is to the right of the trimmer in your picture. This triimer can easily be removed for access by removing the single screw noting the washer between bracket and chassis.

EDIT. I now see that both wires from that coil in your set appear to be directly connected to fabric covered wires. I'll have another look later.

[Station X]

Sorry, but I can't help much more on this one. According to the diagram the bottom end of T13 goes to chassis, but without major dismantling I can't tell you how it gets there. I guess that a simple check for continuity between the two leads will confirm that they both go to the same winding of T13.

I note also that you appear to have two wires going through the grommet on the left of the compartment, or perhaps this is an optical illusion? My set has only a single wire here which goes to R5. Where does your second wire go to?

I'll leave the set on the bench for a week or two.

[a_strong]

Hi Graham - I have removed the screw from the trimmer and moved it to one side to afford a better view. Three of the connections to this coil have been tampered with and possibly a connection to the trimmer. One tag has become detached from the top of the coil former and appears to be connected to the lower winding of the transformer.

There is only one wire passing through the grommet as far as I can see but the light in my work area is not too good so I will take a closer look in the daylight tomorrow.

I am quite intimidated by the prospect of having to work in such a confined space and the coil appears to be wound with Litz wire to further complicate matters.

Alan

[a_strong]

Good morning Graham - as promised, I have taken a close look at the coil in question. I think that I misunderstood the grommet to which you referred. Three wires pass through the grommet between the second and first compartments, The connections are depicted on the attached images.

I tried to remove the side shield to provide a better view. The screws came out quite easily but it is being restrained by a couple of screened leads passing close by so I can only move it a fraction.

I appreciate that the amount of support that you can offer is limited by distance but reconciling the physical connections with points on the circuit is not easy unless one is quite familiar with the set.

Regards - Alan

[Station X]

I'll have a look and try to make a sketch of my set's wiring.

[Station X]

Set upside down with the rear of the chassis facing towards you. Front panel furtherest away from you. Assume you're facing North.

One wire runs through the West wall of the compartment in which T13 is located.
One end of this wire is connected to the South West (SW) tag of the coil to the right of T14 (S11?)
The other end of this wire is connected to the South end of R5 (1K)

----------------------------

Three wires run through the S wall of the compartment in which T13 is located.

Wire 1 is dark in colour.
One end is connected to the SW tag of T14.
The other end is connected to the NW tag of T11.

Wire 2 is white with red bands.
One end is connected to the NW tag of T14.
The other end is connected to the wave change switch in the adjoining compartment.

Wire 3 is white with blue?? bands.
One end is connected to the NE tag (close to chassis) of T13.
The other end is connected to the wave change switch in the adjoining compartment.

------------------------

CONNECTIONS TO T13

Tags nearest to chassis.
NW tag connected to SW tag of T14.
NE tag (covered above)
SE tag connected to SE tag of T14.
SW tag has no connections.

Tags furtherest from chassis.
NE tag connects to second trimmer from left.

------------------------------

THE WIRE YOU DESIGNATE 'X'

One end of this wire is connected to the aerial socket.
The other end is connected to the wave change switch.
This is as shown in your picture.

This wire has NO connection to T13.

The wire close to T13 which you show as 'X' is white with a blue stripe. I suspect it should be connected to the NE tag (closest to chassis) of T13.

----------------------------------

Sorry, but I do not know which wires of which winding of T13 terminate on which tags of the former.

[a_strong]

Good evening Graham - now there is something to get my teeth into! I cannot imagine why anyone should want to brutalise a piece of equipment in the way that this example has been treated.

I have dug out my signal generator and qualitatively checked a few things. Paraphrasing Monty Python, Band 6 is not dead, it is just sleeping! It appears that all the local oscillator is working on all bands but the frequency calibration is way off. Whether it is the IF stages that have been twiddled I will not know until I have squirted a 455kHz signal into the mixer but my suspicion is that it is.

Or it could be possible that the tuning dial is not positioned correctly with respect to the tuning capacitor. With the capacitor fully meshed the tuning dial reads 1560KHz. I cannot recollect seeing any guidance notes on setting up the tuning drive. There is a single tuning mark beyond the end of the Band 1 tuning scale which might be a reference point for setting up the alignment.

But the next job is to sort out the wiring to these two transformers and your notes will be invaluable for that.

Many thanks - Alan

[Station X]

I've yet to come across an SX28 which hasn't been brutalised. I do mean brutalised not modified. I suspect that some sets had their 120V mains transformers destroyed by being connected to 240V mains and were of no further use. One of my sets has a switchable 120/240V transformer and the other no transformer at all.

The set will remain on the bench in case you have further queries, but I'm off on my hols for two weeks from 25 May and won't be contributing to the forums.

I can confirm that the wire from the outer end of the top (nearest to you) winding of T13 terminates on the tag which has been ripped out of T13's former.

I've attached a picture showing the wires on the wavechange switch. It looks like it's wired the same as yours. It's the other end of the wires near T13 and 14 where the trouble starts.