Hallicrafters Skyrider Defiant SX24 Receiver

[Station X]

The two recent threads regarding Hallicrafters SX20 receivers, reminded me that for the last 20 years or more a tatty/verging on basket case Hallicrafters SX24 receiver has been residing in my attic. I decided to get it down and take a look.

A visual inspection showed:-

Rust.
Bottom panel missing.
Various fixing screws missing.
S Meter missing.
80 Rectifier valve missing.
Chicken head knobs are wrong for this set.
The upper part of the chassis has been painted a hideous gold colour.
Output transformer is not original.
Viewing windows for tuning scales discoloured, unsure about scales themselves.
Several components have been replaced, but the replacements have been tacked in just anywhere and the soldering is poor.

Apart from that it was OK! The tuning drive cords were intact and turning the knobs resulted in smooth movement of the scales.

[Station X]

I have a printed copy of the manual, which is readily available online.

Yesterday I got the set on the bench and removed the upper casing to improve access.

My main concern was that the set might have a 120 VAC mains transformer and that it might have been destroyed by having had 240 VAC applied to it. That would certainly have rendered the set BER, However there was a voltage setting knob mounted on the mains transformer and it was firmly locked in the 250 VAC position.

I set about identifying the various windings on the mains transformer and annotated the circuit diagram with their tag numbers. This wasn't such an easy task as it sounds, as the cotton covered wires had all deteriorated to a uniform grey colour making it impossible to trace them visually through the wiring harness. I had to resort to continuity testing using a DMM on ohms range, but even that wasn't easy as the 80 year old solder was resistant to having its oxidised surface pierced by test probes or croc clips. I got there in the end though.

While I was at it I checked the windings for continuity and inter-winding shorts. Fortunately everything tested OK.

As is often the case, the mains switch, which is ganged with the volume control, remained stuck open circuit even when switched on. For once a squirt of Servisol 10 cured it and I didn't have to dismantle the switch.

There was an evil looking waxed paper capacitor connected from one side of the mains supply to chassis, I snipped it out before it had a chance to explode. Next I powered up the mains transformer through a Variac, slowly bringing up the voltage whilst looking and sniffing for signs of distress. Everything stayed cool and all secondary voltages were present and correct.

With a major hurdle crossed that was enough for one day.

[Station X]

Today I did a bit more investigatory work on the PSU and audio output sections.

The smoothing choke had continuity. I don't think the smoothing can is original. It's 32uF/32uF 450V, whereas the spec calls for 30uF/10uF 350V.

I connected my capacitor reformer to each section in turn and they seemed to reform. I say seemed to reform because this set uses a potential divider to derive the screen grid supply and this draws a steady 10mA from the reformer which has to be deducted from the final steady current. Also there are a number of capacitors in this set which, if leaky, can draw current from the HT supply without the need for the valves to pass current. Time will tell.

Turning to the audio output section which uses a 6F6 it was obvious that the output transformer was not original. It didn't span the mounting holes and was held in place with a single self tapping screw. Apart from that it had a single secondary winding connected to a 4 foot long pair of wires which went nowhere. The original would have had a 5000R secondary tapped at 500R and routed to terminals on the back panel.

Testing the output transformer by applying 50VAC to its primary winding showed it had a ratio of 44:1. Somebody had obviously converted the set to use a low impedance speaker, but I have my doubts about the power handling capacity of the physically small transformer. It'll do for testing though.

The tone control circuitry has also been modified, so I'll put it back to normal.

I didn't bother testing the 6F6, as I have some tested KT63's which I can replace it with if necessary.

That's it for the moment. I've put the set under the bench and will resume work on it next weekend when hopefully I'll have an 80 rectifier to replace the missing one.

This will be a very long job!

[LyntonP]

Originally Posted by Station X

I have a printed copy of the manual, which is readily available online.

Yesterday I got the set on the bench and removed the upper casing to improve access.

My main concern was that the set might have a 120 VAC mains transformer

I set about identifying the various windings on the mains transformer and annotated the circuit diagram with their tag numbers. This wasn't such an easy task as it sounds, as the cotton covered wires had all deteriorated to a uniform grey colour making it impossible to trace them visually through the wiring harness. I had to resort to continuity testing using a DMM on ohms range, but even that wasn't easy as the 80 year old solder was resistant to having its oxidised surface pierced by test probes or croc clips. I got there in the end though.

The mains transformer in your set looks identical to the one in my S20. In addition my set had been modified with the addition of a solid state rectifier supply. The colours of the wires all look the same due to their age, so it took me some while to work out what was what. I converted the set back to a valve rectifier (including having to make a new transformer terminal for the rectifier heater which was missing).
Sounds as though you are facing many of the difficulties I had, with rust, missing parts, etc, etc.
Good luck with the restoration, I am sure you will be successful.
Regards
Lynton

[mrrstrat]

I have a minty SX-24 that is a fantastic performing receiver in my shop right now. The only thing I had to do was replace a 0.05uF cap that was melted into a socket. But aside from some repairs over the many decades it is stock.

Let me know if you need pix or other info I can get on it. This one, the SX-25, and the SX-28A I have are among the favorites I have right now in the shop.

[vosperd]

I cannot see from the picture but are there horizontal slots in the side for some louvres?
The reason I ask is that the only parts I think I may still have of my sx24 is a pair of those louvres.
Don m5aky

[Station X]

mrrstrat.

Thanks. Detailed under chassis views showing the component placement would be useful. PSU and audio sections would be most useful right now.

Don.

The louvres are all present and correct, but thanks for the offer.

[mrrstrat]

I need to pull the sets out for pix (as they are to be listed this week for sale). I will start with the SX-24.

Originally Posted by Station X

mrrstrat.

Thanks. Detailed under chassis views showing the component placement would be useful. PSU and audio sections would be most useful right now.

Don.

The louvres are all present and correct, but thanks for the offer.

[Station X]

Thanks.

While you're at it could you check the value of R27 (tone corrector) please. Parts list says 5k, but my set has 500R fitted and it looks original.

[mrrstrat]

I am getting ready to pull it off the shelf now. The set is mostly original but looks like someone(s) got their hands in to do occasional repairs. The set fully functions and is quite impressive in how well it works.

[mrrstrat]

Inside Pix - set 1.

[mrrstrat]

Inside pix - set 2. The 0.05uF is my doing: the one that was there was actually melted into the socket and would have been an eventual surprise for a future owner at some point.

I have a source of these caps in surprising NOS bulk.

[mrrstrat]

Inside pix - set 3.

[mrrstrat]

Inside pix - set 4.

[mrrstrat]

Details on R27 from switch to where I found it. I can concur its about 5K.

I have the John Riders books: there is no more info on this set than what can be found on it (such as from Antique Electronics schematics). There is only three pages on this set.

I'll have it on the bench for a bit (and the bottom plate off of it) - getting pix and some short videos of its amazing performance and sound through a PM-23 (5000 ohm). Let me know if you need more detail - this one is not bad to move and get the bottom plate off of.

[Station X]

Brilliant. Just what I need. Many thanks.

Your set is in a really nice condition.

[mrrstrat]

No problem

Only glad to help - this forum is rather helpful and patient with newbie radio folks lol.

I wish this was my set, but the SX-28A here is in even better condition and is one to die for. This set is certainly a consolation prize (the guy only has $400 USD on it) - I am listening to it now. It is actually one of my favorites. I have the S-20R in the same condition that was from the same collection the SX-24 came out of (and it was in my price range). That set also looks and works just as good.

Originally Posted by Station X

Brilliant. Just what I need. Many thanks.

Your set is in a really nice condition.

[Station X]

I did some more work on this set at the weekend.

In order to improve access to the underside of the chassis I removed the cabinet completely, which meant removing the knobs and switch/potentiometer retaining nuts. Once the chassis was free I removed the fragile tuning scales and put them in a safe place.

In order to provide support for the inverted chassis I made a couple of temporary end panels from MDF and clamped them to the chassis using small G Cramps. It occurred to me afterwards that if I'd made the temporary end panels wider from front to back I'd also have been able to work on the chassis with it resting on its front or back without potentially damaging the switches and pots. Something to remember next time!

[Station X]

I then proceeded to replace the dodgy two core mains flex with a three core one having a proper clamp rather than a knot in the flex. The existing flex conductors had been "blobbed" on to the mains transformer's tags and once I'd unsoldered them I had to unsolder and unwind the remains of two former mains flexes. At the same time I replaced the waxie between mains and chassis with a Class Y cap. The waxie showed resistance on a DMM and a full short on a Megger. A 3 amp fuse was fitted in the plug.

Having Meggered the mains transformer's primary winding to chassis I felt confident enough to apply full mains to the transformer. The OFF/ON switch on the AF gain control worked and all secondary voltages were present and correct.

A NOS 80 rectifier valve was fitted and it lit up as expected. With the shorting plug plugged into the back panel and the TX/RX switch set to RX, about 350 VDC was present at the smoothing cap.

I turned my attention to the potential divider chain which supplies the screen grid voltages to some of the valves. As drawn it consists of a chain of three resistors and a common decoupling capacitor. I had to replace one out of spec resistor along with the decoupling cap. I discovered an undocumented 500R resistor in series with the feed to the screens. Studying mrrstrat's pictures showed that his set had the same resistor. Could be a documentation error or an undocumented factory mod. The screen grid voltage was OK at around 150 VDC.

[Station X]

I had intended to repair this receiver by following my usual procedure ie PSU first, AF stages, IF stages, then mixer and RF stages. However a lot of mods have been done to the output stage apart from the replacement output transformer. I gave up trying to reverse engineer these mods. I'll rebuild the output stage close to original spec later when I've sourced a suitable output transformer.

Hence I commenced the task of replacing all waxies along with out of spec resistors. This set has been heavily got at though. There are wires which go nowhere, tape bombs, evidence that components have been snipped out and components blobbed on just anywhere. Before replacing any component I'm checking its designation, function, value and connections. That takes time now, but should save a lot of time later. I expect to find missing components and extra components which will need sorting.

Hopefully work will resume next weekend.