SX-25 AC line voltage ????

[w8pu]

I have an SX-25 that I have been restoring for the past year. I replaced several tubes, all the old original paper capacitors, and some resistors that were way out of tolerance. One question I have is just what input line voltage should I use? I have been using a variac and applied 110 to 115 volts but at these voltages the filament is below 6.3 vac, closer to 5.9 vac. I also notice very low sensitivity on 10 meters. If I increase the line to 125 vac I am at 6.3 vac and 10 meters begins to improve. The other I have read that the SX-25 was designed to operate between 110 vac and 125 vac. Any ideas?

Gary W8PU

[KM1H]

Those tubes are designed for +/- 10% filament tolerance. Running them on the low side increases life with minimal signal degrdation.

The biggest problem with most Hallis is that they skimped on the transformer iron and they run hot at todays line. Some sets Ive measured are actually entering saturation at 123V. It works fine on 110-113V.

A very easy solution is to use a bucking transformer which is as simple as a 10-12V filament transformer. Only the secondary current is important and a 1A style has more than enough overhead for most receivers.

The SX-25 is a slug on 10-15M; the only help is some tube and component mods or an outboard preselector.

I restored, and aligned my SX-25 and after a quick listen around it went on a shelf where it will stay. It is a damn good looking radio; I also have the SX-24.

Carl
KM1H

[w8pu]

Thanks Carl for your response. I did hear that there modifications to the SX-25 but I havent been able to locate any. I owned a SX-25 when I was a young buck in Junior High School and spent hours listening to the Hams on AM. I only recently found the one I have now which is probably in a lot better shape then the old one which is long gone.

73, Gary

[WQ9E]

Gary,

I agree with Carl's assessment on the Hallicrafters transformers; they do run hot if the line voltage is elevated and they are none too rugged.  The SX-42 and 62 are the same.

Since yours does come to life a bit at higher voltage, any chance your B+ is sagging due to old electrolytic caps?  I see you mentioned the paper caps but did you also do the electrolytics?  Leaky filter caps and a leaky output stage cathode bypass cap will cause increased current draw not doing your transformer any favors.  I imagine you already got the coupling caps to the 6F6 control grids when you did the recap. Finally, if the type 80 rectifier is getting weak it will also reduce your normal B+.

The SX-25 I have is not great on 10 either and the RME DM-36 is needed to make either my Defiant or Super Defiant practical for use on 10 meters.  It also gives them a nice ole buzzard look.

[w8pu]

Roger,
 Thanks for the reply. Yes I have replaced the filter capacitor. The B+ voltages at the tubes are running a bit higher than what info I could find. The SX-25 manual nor the riders info give voltage info. I have plenty of signal strength and audio on most bands but band 4 is lacking, no background " hiss " at all even with the volume all the way up. I do hear stations around 18 mHz though but still nothing like the other bands. If anyone has any info on the proper voltages at the tubes I would appreciate having it passed along.

73, Gary W8PU

[WQ9E]

Gary,

A guide to voltage readings is on Bama:  http://bama.edebris.com/manuals/hallicra/sx25volts-dial/

Check the voltages closely on the converter stagea and I would also try a different 6K8 to see if that helps the top range.

[w8pu]

Roger,
 Thanks for the info .. that is the same voltage chart I have. My B Plus readings are higher even at 115 AC. I replaced the 6K8 with a NOS tube by hand picking one that gave me the most signal with 28 mHz from a signal generator. I havent checked lately but I have noticed when poking around in the past that RF from the local oscilator drops off quite a bit on band 4.

73, Gary

http://home.roadrunner.com/~w8pu

[WQ9E]

Gary,

I went through this with an NC-183D years ago, a very big performance drop off on the the range including 10 and 15 meters.  I finally traced it to the oscillator coil which had been damaged and repaired; apparently the repair reduced the coil Q causing a drop in output.  I pulled the coil and rewound it and the sensitivity came up where it belonged.

Take a good look at the coil for dirt/film buildup and also inspect the caps associated with this range for leakage or other problems.  I went through some major problems with an SX-62A that someone had gone crazy with TV tuner cleaner/lubricant and saturated the coils for the 2 VHF bands.  The top range wouldn't oscillate at all and the lower range dropped out about halfway through the band.  I used some electric motor commutator cleaner to cut through all of the old gunk and got that receiver going again.

 If I get home from the office at a reasonable time tomorrow I will make some measurements with my HP signal generator and let you know how mine performs along with range to range comparison.

[w8pu]

Roger,
   I have replaced all of the paper capacitors with orange drop capacitors. The only ones that I haven't touched are C48 and C50 the " Ceramicon " capacitors.
   I took another look at the voltage chart from BAMA. Most of the readings are higher on my SX-25. With 117 vac line voltage I am getting 328 vac from the transformer vs. 300 vac per the chart ( To get 300 vac I have to lower the line voltage to 108 vac ). After the filter choke I am getting 312 vdc vs. 280 vdc per the chart. All of my B plus readings at the tubes are 30 to 40 volts high. For instance the 6F6's have 321 vdc on the plates vs. 280 vdc per the chart.
    With 117 vac I am getting 5.9 vac on the filaments and the 80 rectifier is seeing 5.2 volts. I would be curious as to what you have for B plus at 117 vac line voltage.
   I did run a few checks on band 4 with my signal generator but stupidly didnt right them down. I was doing this while selecting 6K8's. I will take another look tomorrow. 
   Thanks again ...

   73, Gary W8PU
   

[ke7trp]

I use variacs on recievers.  I turn down the voltage until filements are down around 6.0 to 6.3 and leave it.  90% of the time. This is right around 115 to 117 volts AC.  I think your right in the ballpark

C

[N3DRB The Derb]

there's just not much you can do with the SX24/25 above 22 mhz, even when operating perfectly. 6SK7's and the like just dont cut it up there.

the hot lick if you want it to work up there and yet remain buzzardly is one of the RME preselectors. The one with the rounded corners  even matches the style of the 24/25. Those things make a night and day difference above 20 meters.

[WQ9E]

Gary,

Those voltages are not unreasonable and many of the Halli receivers run fairly high B+ which doesn't help the heat buildup, drift, longevity, etc.  Double check the value you have for C-42 (the input filter cap) since any increase in value here will result in higher B+.  The original was 10 uf.

Since the output tube plate supply comes of prior to the filter choke it is going to be at the highest B+ value.

How is your cathode reading on the 6F6 tubes?  A significantly higher voltage here indicates higher current draw and much of the current load on the power transformer is due to the output tubes so you don't want to get into the danger range for an already marginal power transformer. 

You could consider changing the type 80 tube rectifier to SS diodes plus a dropping resistor and use the no longer needed 5 volt rectifier cathode winding to "buck" the primary voltage.  Properly done this will result in a cooler running power transformer.  My SX-42 and 62 receivers are set up this way along with an inrush current limiter.

Also make sure the primary is fused because Halli seemed hit or miss with fusing during this era.

Does it look like your power transformer might have been replaced at some point?  Also, if possible, verify your AC meter reading against another meter.  The AC function is typically the least accurate of the meter ranges.

[KM1H]

Those RF and oscillator coils suck up moisture which affects performance. The only fix is to run 24/7 and monitor the sensitivity and injection; it should improve a bit.

Replacing the 6SK7's with 6SG7's will help above 20M. Revising the converter to a 6SB7Y will help quite a bit more as it reduces the ENR 4X.

The SX-25 was my first commercial set as a Novice in 1955 and was fine on 80/40. But when I decided to get on 15 it was dead and unstable; the cure was a HQ-129X. I swore off the 25 until one in mint condition came along locally a few years ago strictly for its appearance.

The DB-22A and other RME preselectors are great noise and overload generators. I modified mine by bypassing the 2nd stage 6BA6 but leaving the coils in place with a coupling cap. Then changed the other 6BA6 to a 6GM6 with a few component changes. The added RF selectivity is a big help with the lower and middle level sets for image rejection above 20M as well as a lower noise figure with the 6GM6. An unmodified SX-16 and HQ-120X come alive with it and Ive used it on the FB-7 when on 30M.

Carl

[w8pu]

Thanks Carl and Roger,
   Yes I did replace the filter capacitor with larger values. I used a nos twist-loc that is a 4 section 60 ufd.
    I have noticed that the output of the 6K8 drops off drastically on Band 4, its about 1/4 of what it is on Band 3. I also noticed that I can drastically improve reception below 21 mHz by peaking RF1, RF2, and antenna trimers for that portion of the band instead of per the manual up at 30 mhz. If push comes to shove I may leave it at that. I will try the 6SG7 idea. The tube socket for the 6K8 is tough to get to so I may not op for changing anything there. Over all the old gal works pretty good otherwise and has great audio. I just want to make sure its working as good as it should.
    Thanks to everyone for the help, any further ideas are always welcome.

73, Gary

[w8pu]

   By the way the plates of both 6F6's are at 318 vdc, grid at 309, and the cathodes are at 21 vdc, 4 volts higher than the chart. The transformer is original. I did however replace the audio output transformer so I could drive an eight ohm speaker. The original was missing.
   I will continue to run with the variac at 117 vac.

73, Gary
   

[WQ9E]

Gary,

I would replace the filter input with a 10 uf unit; you can just disconnect that section from your twistlock cap and wire the replacement in beneath the chassis where it won't show.  Using 60uf as the input section to the filter greatly increases the heating of your power transformer in addition to creating excessive B+.  For some transformers this is fine but I don't suggest it for the older Halli units or you are likely to need Gary's rewinding service soon.  

60 uf on the output side won't hurt anything and will help the filtering but for a choke type filter system you normally do not want to increase the value of the input capacitor.

[Jim, W5JO]

Any voltage reading within 10% is acceptable, and this includes your line input voltage.  So if your book says 110 volts then +/- 11 should work unless you have something wrong in your unit.  Change that input filter cap back to 10 or even 8uf  to help the B+.

[KM1H]

I would replace the filter input with a 10 uf unit; you can just disconnect that section from your twistlock cap and wire the replacement in beneath the chassis where it won't show.  Using 60uf as the input section to the filter greatly increases the heating of your power transformer in addition to creating excessive B+.  For some transformers this is fine but I don't suggest it for the older Halli units or you are likely to need Gary's rewinding service soon. 

The 80 rectifier is rated to only 10uF for the input cap and is prone to damage way up at 60 uF.

Carl

[w8pu]

Sounds good, I will remove the 60 ufd sections. My schematic shows 30 ufd before the choke, and 10 ufd after.

Thanks guys

73, Gary W8PU

[WQ9E]

Gary,

It should be 10 uf before (the input side of the filter which is the part that also connects to the audio output transformer primary) and 30 uf after.  There is no problem at all using your current 60 uf section on the output side (the side furthest from the type 80 rectifier) but the input side definitely should be kept to spec.

Once you do this, your B+ voltage should fall very close to that shown in the voltage chart.

[KM1H]

C-42 and C-43 are reversed, aka bass akwards.

Carl

[w8pu]

So they are reversed on the schematic ... ok, I will make the correction. Thanks ...

73, Gary

[w8pu]

Rodger and Carl,
  I installed a 10 ufd electrolytic on the input to the choke and reconnected a 60 ufd section of the Twist-Loc capacitor after the choke. The voltages did lower some but the curious thing is now my filament voltage is more near normal. With 117 vac line input I get 6.2 to 6.3 vac on the tubes. At 115 vac line input I still can get 6 vac on the tubes and the dc voltages, though still higher than the chart, are closer.

Thanks for your help ...

73, Gary W8PU

[WQ9E]

Gary,

This sounds good.  Your filament voltage was lower earlier because of the heavy load placed upon the transformer by the high peak charging current of the input filter capacitor.  Your transformer and rectifier will both be much happier now!

Some of the tube DC voltages are fed through a fairly high impedance source so if you are using a modern DVM with a 10 meg input resistance the readings will be higher than those on the chart which were recorded with a 20K per ohm VOM that loads the circuits more heavily on the lower voltage ranges.  But as Jim posted earlier, if you are within +/- 10% of the stated then everything is fine anyway.

[w8pu]

I have my trusty Simson 260 I bought new in the late 70's as well as a Fluke DVM ...

73, Gary W8PU