Hallicrafters SX24

[VK7ZL]

I have just acquired an SX24 Skyrider Defiant for A$150 (about US$130). It is in good condition and in working order but will require a bit of work replacing all the old caps, probably a few resistors as well and new perspex windows. Radio's like this are very hard to come by here in VK.
It has had a couple of mods done to it. The power transformer has been changed to a much larger one which would have been done to suit the 240 volt power supply voltage here in Australia. It has been fitted with a xtal calibrator unit, a BFO control (or is that standard?) and the 6SK7 (RF amp) has been replaced with a 6AG5. (see attached photo's)

I have a couple of questions - Is there any thing in particular I should look out for while I am in the process of replacing the capacitors and what type of caps are best to use for replacing the old wax/paper caps?

[WQ9E]

I imagine not that many Defiants found their way down under so congratulations on finding one.  I have one and also its SX-25 Super Defiant brother and they do a nice job on 40 meters and lower, stability is not so great on the higher frequencies.

I get my caps from Mouser and use plastic film and ceramic caps except where mica is required.  With small modern caps replacement is simple and generally allows you to put the bypass cap and ground exactly where it is needed.

Hank Van Cleef posts on the old boatanchors mailing list and you have what he would call a "blue collar" receiver-it has some scars and personality from its long life.  It has a few modifications but as long as they were properly done I would consider keeping them.  Does the BFO control provide variable injection?  This was used on some earlier Halli models (and quite a few from RME).  My SX-25 came modified with a 3 position crystal controlled BFO.  I did remove this and saved the crystals for use in other receivers requiring single 455 Khz. filters.

Don't be surprised if your SX-24 doesn't perfectly match the manual or schematic since that is a common "feature" of Hallicrafters receivers.  You might want to take some good close up shots below chassis before you start in case you run into questions later.

[KM1H]

That appears to be the universal power transformer option from Hallicrafters, I have an SX-25 with the same appearance but a little taller.

The stock BFO is front panel adjustable.

Any modern caps can be used including disc ceramic for bypassing and audio.

The 6AG5 is a poor choice for an AGC controlled stage. A 6BZ6, 6DC6 were the common choices in later radios.

Have fun.

Carl

[VK7ZL]

The stock BFO is front panel adjustable.

The 6AG5 is a poor choice for an AGC controlled stage. A 6BZ6, 6DC6 were the common choices in later radios.

Have fun.

Carl

I have traced the wiring back on the right hand knob and it is the S meter zero pot. Next I will check how the 6AG5 is wired in, it shouldn't be to hard to change it to one of the tubes you mentioned.

[The Slab Bacon]

Bob,
      The 6ag5 was probably chosen as a lower noise tube. (or down there that was what the guy could get) (or possibly to get better sensitivity on the higher bands)If the original octal socket is still intact, you might want to try swapping the 6SK7 for a 6AB7 or AC7. Sometimes this does wonders for the noise figure on some of the "older" radios.

My SX-28 has a 6AB7 in the first (or maybe second) RF amp stage and has surprisingly good sensitvity all the way up to 10M. Most of those radios from that era were pretty deaf on the higher bands.

I also had an SX-62 that someone over the years changed the IF amp tubes to miniatures (6AG5s) and it worked very well so I left it that way. I believe this was a popular practice some years back.

                                                               The Slab Bacon

[KM1H]

The 6AG5 was available surplus and was an easy choice at first before overload and AGC were important by the huge postwar ham growth and KW rigs became common with all the surplus tubes.

The 6AC7 is a sharp cutoff and a poor choice as a RF amp for reasons mentioned. The 6AB7 is a nice tube and Halli even suggested it in the 2nd RF stage after WW2. One of my SX-28's is setup that way plus I used a very low loss antenna coil form for the highest band. Its almost as hot as my slightly souped up HRO-60 on 10M.

Carl

[The Slab Bacon]

The 6AG5 was available surplus and was an easy choice at first before overload and AGC were important by the huge postwar ham growth and KW rigs became common with all the surplus tubes.

I've often pondered making up adapto-sockets with old octal bases and trying out 6AK5s. I have a ton of them N.I.B...................

[KM1H]

Ive done that with better tubes, 6GM6, 6EH7, 6ES8, 7360, etc.

The only place I use 6AG5's is in the SX-42 and both SX-62A's as thats what they were built with and its not worth the bother to change......its almost impossible to work in the RF sections.

Collins used the 6AK5 in the 75A1 and almost got laughed out of business. The early A2 wasnt much better with a 6CB6 but they soon went with a 6BA6. Even the vaunted A4 was an overload dog without serious rework. They were clueless in building a competition grade ham receiver until the 75S3B; then they went way downhill after that.

[VK7ZL]

Bob,
      The 6ag5 was probably chosen as a lower noise tube. (or down there that was what the guy could get) (or possibly to get better sensitivity on the higher bands)If the original octal socket is still intact, you might want to try swapping the 6SK7 for a 6AB7 or AC7. Sometimes this does wonders for the noise figure on some of the "older" radios.

My SX-28 has a 6AB7 in the first (or maybe second) RF amp stage and has surprisingly good sensitvity all the way up to 10M. Most of those radios from that era were pretty deaf on the higher bands.

I also had an SX-62 that someone over the years changed the IF amp tubes to miniatures (6AG5s) and it worked very well so I left it that way. I believe this was a popular practice some years back.

                                                               The Slab Bacon

The original socket has been replaced with a miniature type and due to the difficulty of getting at it under the tuning section I think I will leave it alone at this stage. The radio appears to be working well on the lower bands and WWV coming in well on 10Mhz. The S meter appears to function correctly.

There is one minor problem I have yet to come to grips with. The BFO works as it should on an SSB signal but as soon as it is switched on the S meter rises to S9 +10 and when the BFO control is advanced fully clockwise the meter rises to +40. It is the same result as turning the RF gain down. Any suggestions?

I came across an earlier post by Carl regarding the value of the filter caps. They had already been replaced but with a dual 50mfd can type so I disconnected the input side and replaced that with an 8mfd. The B+ voltage is now down to around 260V about 20V lower than it should be but the rectifier tube should be a lot happier with less C hanging on it. The plate resistor on the 6SQ7 had gone from 250K to 340K so that was replaced at the same time.

On a different subject, I was recently given a Spot Frequency Alignment Oscillator (see pictures) They were manufactured in Australia by R W Steane & Co probably around the 1940's. Did any of them make it to the USA?

[WQ9E]

There is one minor problem I have yet to come to grips with. The BFO works as it should on an SSB signal but as soon as it is switched on the S meter rises to S9 +10 and when the BFO control is advanced fully clockwise the meter rises to +40. It is the same result as turning the RF gain down. Any suggestions?

On a different subject, I was recently given a Spot Frequency Alignment Oscillator (see pictures) They were manufactured in Australia by R W Steane & Co probably around the 1940's. Did any of them make it to the USA?

The S meter going upscale is normal behavior for this style of receiver.  Normally, you want to turn the AGC off for CW/SSB reception and control the gain manually with the volume most of the way advanced while using the RF gain to set the desired audio level.   With the detector and BFO injection circuit of this receiver the BFO signal will cause the AGC to reduce the gain. 

If you haven't done so already, be sure and replace the coupling cap between the first audio plate and output tube grid as leakage here will cause a number of problems resulting in damage to parts.  Also replace the output stage cathode bypass cap for the same reason.

That is the first time I have seen that particular spot alignment device but it looks like a handy device.  I have a somewhat similar device that provides several common IF frequencies but it is line powered (tube type) and not as handy as the one you own.

[VK7ZL]

[/quote]

The S meter going upscale is normal behavior for this style of receiver.  Normally, you want to turn the AGC off for CW/SSB reception and control the gain manually with the volume most of the way advanced while using the RF gain to set the desired audio level.   With the detector and BFO injection circuit of this receiver the BFO signal will cause the AGC to reduce the gain.  

If you haven't done so already, be sure and replace the coupling cap between the first audio plate and output tube grid as leakage here will cause a number of problems resulting in damage to parts.  Also replace the output stage cathode bypass cap for the same reason.

That is the first time I have seen that particular spot alignment device but it looks like a handy device.  I have a somewhat similar device that provides several common IF frequencies but it is line powered (tube type) and not as handy as the one you own.


[/quote]
Thanks Rodger
The coupling cap had been replaced before with a .02uF which still measured OK but I just replaced it with a .047uF to be on the safe side. The cathode bypass is a very new looking cap so that should be fine.

I got the xtal calibrator going last night. It's output is 7Mhz. As the red lines on the main dial are almost invisible the calibrator is very handy.
The weather has warmed up today so out to the main workshop and fit a mains fuse to it.