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 Post subject: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 12, 2018 3:33 am 
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Location: West Point, PA
The Lafayette KT-135 has a 270pF fixed cap in series with a 5 - 80 pF mica trimmer to tune the antenna.
I tried to substitute a standard air dielectric trimmer in place of the mica trimmer and found that Band D (16 to 30 megacycle) would not oscillate.

I don't understand the purpose of the 270pF cap in series with the trimmer. It seems to me you can leave it out of the circuit.
Can anyone enlighten me?

In my experiments with the standard variable cap I tried various values in series with it, from no capacitor to 75pF to 750pF, but Band D would not work.

Does the mica trimmer have different electrical characteristics than an air trimmer?
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MicaTrimmer.jpg
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The manual can be found here:
http://www.wass.net/othermanuals/Lafayette%20KT-135.pdf


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 12, 2018 3:45 am 
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Location: Austin, Texas
I'm fairly sure the 270pF is there for safety. It gives a second level of protection from electrical shock if the variable cap should short. Any value of 270pF or more should work.

I don't think this cap is related to your band D problem.

Jay


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 12, 2018 4:02 am 
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It's not an item that usually wears out, what happened to the original C2?

You need C9 there at the specified value; the "new" C2 needs to cover the range of the original (5-80pf). Both are parts of the tuned circuit. If you isolate these two components (C2 and C9), they are two capacitors in series. When C2 is at the bottom range the combine value is the equivalent of two capacitors in series (270pf, 5pf), at high range it's the equivalent of (270pf, 80pf). Use the series capacitor equation.

https://www.allaboutcircuits.com/textbo ... apacitors/

If the set stops oscillating in band D, the cause of the problem is more likely to be somewhere else. If other bands are (more or less) ok but band D is completely dead, first place to check is the band switch.

Also, C3a is for bands A, B, C; C3b is only for band D. So check that too.


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 12, 2018 1:35 pm 
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Location: Tokyo
I agree with Jay, I think it was used to provide protection from the hot chassis. The Knight Space Spanner uses what appears to be the same compression cap, same range, but without that 270pF fixed value cap.

On my Space Spanner (which has nearly the identical circuit as your KT-135), I find even just 5pF is too much to couple the antenna directly to the tank on the high frequency end of the SW band. I replaced that compression cap with a switch with different fixed cap values. Something like 100pF, 47, 22, 10, 5, 2, and no cap (just the capacitance of the switch itself). Around 15MHz, I use the 2pF position; anything larger and the detector won't go into oscillation. And that's with an indoor 15 foot antenna! For a 50' long wire outdoor antenna, it would have to be even smaller. For the BC band, I generally use 22pF on the high end, 47pF on the low, but sometimes use smaller values to attenuate very strong signals.

Try this as an experiment: try the D band without any antenna; the detector should easily go into oscillation. Then connect your antenna to the receiver terminal post through a gimmick cap, a few twists of wire. Sometimes just a single twist is all it takes.

Have you considered adding an untuned RF amp with an attenuator control between the antenna and detector? Easy to do.


Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Fri 13, 2018 2:26 am 
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Thanks for the replies!

JnTX You've solved the mystery of the 270 pF cap for me.
AJJ, there is nothing wrong with C2.
My KT-135 has been altered to the point where the only original KT-135 parts left are the coils and the front panel.
However, a member here has one I consider museum quality (if it were to deemed worthy of a museum.) Very nicely soldered, all original parts and pristine front panel.

However, the antenna trim cap is giving him problems. Since mine has been altered so much I thought I'd just mail him my antenna trim cap. First, I had to make sure I wasn't going to disable the radio, so I started experimenting.
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KT135.jpg
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shinkuukan The radio oscillates on band D with NO antenna! It doesn't pick up anything but it pops and then hisses about 1/3 of a turn on the regen control.

Is my antenna too long? It's 100' feet of wire running up to the attic. The variable cap I used (and killed band D) was 10 - 60pF.
From what you've written, I need a SMALLER value.


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Fri 13, 2018 11:41 am 
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Is a 100' wire antenna too long? For band D SW it certainly is. For band A MW, it's probably fine.

Try about 6' for the antenna. You'll still probably have to adjust the antenna trimmer down to around the minimum capacitance, but the detector should go into oscillation, at least on the bottom end of band D (top end might still be a problem). If it doesn't go into oscillation, reduce the length to 3' and try again.

Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Sat 14, 2018 2:08 am 
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Location: Austin, Texas
Rob,

Thanks for all the insights. I was inspired to get my Space Spanner out of storage. I built it in 1957 and it had many modifications and parts replacements. It is now a pile of parts and will get reassembled with all new caps and other parts as needed along with a clean chassis. Now if I can just find the knob for the antenna trimmer. I know I have it somewhere.

Jay


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Sun 15, 2018 2:44 am 
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Jay, this may be of interest, concerning the Space Spanner SW band:

The SS tunes 6.5 to 18MHz, oddly just missing out on the important 49 meter BC band. This is because the bandswitch puts a fixed cap in series the main tuning VC, reducing the maximum capacitance. Only on the SW band. Remove that cap and the SS tunes down to about 5.2MHz. However, the detector often won't oscillate down there, there isn't enough feedback, it requires another turn or two on the tickler coil.

I removed that cap and also removed one turn from the SW tank coil. I didn't touch the tickler coil, but since the tank coil now has fewer turns, the relative amount of feedback from the tickler increased enough to easily let the detector oscillate. The low end is now about 5.8Mhz, nicely including the 49 meter BC band. Top end now goes to almost 20MHz (when the bandspread is at minimum C), so I added a small cap (about 4pF, I'll have to check) across the tank coil. That lowered the high end to about 18.5Mhz.

The catch is that the SW dial calibration is lost, not that it was so precise and accurate to begin with.

Why did Allied restrict the tuning range of the SS? I suspect the prototype versions worked fine down to 5.2MHz, but the actual production models caused problems. Depending on the individual 12AT7 selected and perhaps the wiring too, some SS would oscillate, some wouldn't. Since the SW coils were already made, the easiest solution was to raise the minimum tuning frequency to 6.5MHz by adding that cap.

For both the Space Spanner and the Explor-Air, I think adding a grounded grid RF stage is worthwhile. The main benefit is you can easily and smoothly adjust the RF signal level, and that's so important with a regen.

Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Sun 15, 2018 3:30 am 
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Rob, your suggestions worked perfectly. I used a trimmer cap that had one stator plate and one rotor plate, something like 0 to 4pF. Band D oscillated very nicely even using the long antenna. I tried a 12 foot piece of wire and didn't notice much difference as to when the regen control would kick in.

Unfortunately, for some reason the whole band is dead at the moment. The only thing I could pick up was an LED light bulb. When I turned out the light the buzzing stopped. There were four configurations I was using. Original antenna tune mica cap, long and short antenna, two plate air variable cap, long antenna and short antenna.

The other bands worked nicely with the air variable, but the broadcast band didn't have much signal with a short antenna. 40 meters was OK with a short antenna. There is some flame thrower Spanish station on 40 meters now. I prefer Radio Moscow.

Thanks for all your insight.

Now, this capacitor needs to be isolated from the chassis and I may have to move a coil to get it to fit, and NOT come into contact with the chassis. I wish there was a NOS replacement, but so far I can't find one.

The KT-135 got us to the moon in the 1960s.
http://analogdial.com/KT135/KT1359.html

Mike


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Sun 15, 2018 1:43 pm 
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>Unfortunately, for some reason the whole band is dead at the moment. <

Yes, the sad world of SWL, 2018. I never thought I'd miss Radio Moscow, but here we are. And I suppose someday I'll miss CRI. I seriously miss Radio Australia, especially the domestic SW broadcasts. During winter here, the 31 meter and higher bands are dead by 7pm. It's sometimes so eerily quiet that I'm convinced something must be wrong with the receiver. Outside of contest time, ham bands are also pretty quiet. There used to be a lot of ship to shore maritime communication in the 2 to 3MHz range. Well, I've still got WWVH, for the nonce.

Again, you don't have to use a VC between the antenna and tank coil. A switch with a variety of fixed value caps works well. Emphasize a choice of small values rather than large. And you don't have to worry about insulating the switch from the chassis.

Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Mon 16, 2018 3:06 am 
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I like the idea of the switch. I have a 12 position rotary switch in front of me.
However, the variable antenna tune cap is helpful in resolving SSB, which is kind of hard to tune, so I don't want to lose that functionality.

Mike


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Tue 17, 2018 12:57 am 
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Mike, how about this: mount a fine tuning control in place of the antenna trimmer. This can be an actual VC or a pot controlling a reverse biased 1N4007. Put the antenna trimmer (VC or switch) in a separate metal box, connect it to the receiver with a short length of mini-coax. Ground the coax shield to the metal box, not the KT-135 chassis.

Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Tue 17, 2018 2:16 am 
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Here's an idea for KT-135 and Space Spanner owners. Find yourself a 19EZ8. This is 9-pin, triple triode identical to the 12AT7 triodes. They did this by connecting two of the cathodes and one of the heater leads together (that heater would normally be grounded to the chassis). The heater is 19V/150mA, so it can inserted into the heater string. Here's the data:

http://www.mif.pg.gda.pl/homepages/fran ... 6/6EZ8.pdf


This tube gives you some interesting possibilities. You could use the independent triode as a grounded grid RF amp, and the two remaining triodes as the detector and the first audio. The first audio would have to be slightly modified to use contact bias with the grounded cathode. A grounded grid RF amp would eliminate the antenna trimmer cap problems. You replace that cap with a pot to control signal levels. While the GG amp doesn't really improve SW sensitivity, it does help on the BC MW band when using a shorter wire antenna (I use just 15' indoors). Also, regeneration control is usually less finicky with a GG amp.

Or you could add an additional stage of audio, perhaps with some NFB from the 50C5 stage.

Or you could connect two triodes in parallel and use that for the detector. That would double the transconductance, and higher transconductance seems to make for a smoother, more reliable oscillator.

Finally, the higher heater voltage helps to offset the higher line in voltage of today.

Rob


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Tue 17, 2018 2:44 am 
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shinkuukan wrote:
Mike, how about this: mount a fine tuning control in place of the antenna trimmer. This can be an actual VC or a pot controlling a reverse biased 1N4007. Put the antenna trimmer (VC or switch) in a separate metal box, connect it to the receiver with a short length of mini-coax. Ground the coax shield to the metal box, not the KT-135 chassis.

Rob


Rob, The KT-135 already has a band spread (fine tune) control. I even put a vernier on it. (Killed the looks of the radio.)

Instead of all these modifications I just need a comparable part. It looks like this.
Attachment:
MicaCap.jpg
MicaCap.jpg [ 43.3 KiB | Viewed 257 times ]


I found this online, but it is the wrong value. Maybe I can find the correct value, then remove the shaft and bracket from one of these and make my own.


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Tue 17, 2018 2:50 am 
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shinkuukan wrote:

This tube gives you some interesting possibilities. You could use the independent triode as a grounded grid RF amp, and the two remaining triodes as the detector and the first audio.
Rob


It might be easier to add a single triode because the KT-135 already uses a 12AT7 as the detector and audio amp.
Unfortunately, at the moment this is beyond by technical abilities. I'd need a schematic of the RF AMP.

I like the idea of the pot, then I could just do away with the capacitor, and I have plenty of room on my modified radio for another tube.

Mike


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Wed 18, 2018 12:58 am 
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Mike, the fine tuning is in addition to the bandspread. The bandspread by itself is too coarse. The fine tuning control has a range of just 2~3pF. I use either a very tiny VC (hard to find) or the old reversed biased 1N4007 diode with a pot trick. I prefer the latter because with a quality pot the tuning is so smooth. Yes, you can use a reduction drive, but as you noted, that really changes the front panel appearance.

For reference, here's a grounded grid RF amp I made some years ago for the Space Spanner. The 6AB4 is the same as a 12AT7 triode. The 500uH and 2.5mH chokes I later found were unnecessary: today I use a 2.2K cathode resistor and a 22K plate resistor, but these values are not at all critical. The 1pF coupling cap is simply a gimmick of twisted wires. The signal level pot can be anything from about 1K to 10K. This amp produces very little gain: there's no tank circuit to begin with, and a lot of what gain is created is lost through the shunting capacitance of the tube socket, the wiring, etc. What it does do is isolate the detector from the antenna and allow control of the signal level without changing the tuning or regeneration.

I understand you may prefer to keep the radio exactly as it was originally designed. This is just food for thought.


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GG AMP 6AB$.png
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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Wed 18, 2018 3:31 am 
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What is the chassis connection verses the "G" connection?


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Wed 18, 2018 3:39 am 
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G is the B- bus that is isolated from the chassis. The cathodes (pins 3 and 8 ) of the 12AT7 are connected to the B- bus.

Jay


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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 19, 2018 12:53 am 
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Jay, 'G' is not exactly a B- bus but a single point. Each stage has a B- collection point (like the star system in audio amps) and that in turn is connected to G. G is the negative terminal of the first filter cap and the end of the heater string. While all this did reduce 50Hz hum, I still got too much noise and tunable hum from the direct connection to the AC line, so I eventually went to a transformer PS.


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SS PS.png
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 Post subject: Re: Lafayette KT-135 antenna tune capacitor
PostPosted: Jul Thu 19, 2018 3:13 am 
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Rob,
Yes, a good plan to have a single point return. I did take all of the AC currents to a single point in my SS but didn't isolate the low current DC returns.

I don't see a surge limiting resistance for the 47uF cap in your diagram. Do you depend on an isolation transformer to limit surge current?

Jay


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