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Author Topic: SX-24 Issue  (Read 1093 times)
W4IJ
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Posts: 54




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« on: January 16, 2019, 02:08:53 PM »

I've got an issue with my SX-24 that I haven't yet been able to resolve. 

The audio stages are fine - the rf amp stage is fine, but the signal is not making it through the 6K8.  The 6K8 is the first detector, the mixer, and the oscillator!  Complicated little metal tube - a triode section that is the oscillator and a hexode section that is the mixer and first detector.

The tube tests fine but I don't have a spare to replace it with -yet!  The tube specs show a grid resistor of 50K for the triode section, but the tube socket is situated underneath the tuning coils and is almost impossible to reach, thus I haven't checked it yet. 

Before I make a career out of getting to that resistor, has anyone had any experience with this receiver that might help me out in case I have overlooked something simple? 

Here's a link to the Bama Archive that has the schematic:

http://bama.edebris.com/download/hallicra/sx24/sx24riders.djvu
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G3RZP
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Posts: 1090




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« Reply #1 on: January 17, 2019, 02:56:07 AM »

Why don't you pull the tube and use a multimeter going into the tube socket from the top to measure the resistance between pin 5 and pin 8? That should tell you if the grid resistor is OK? While you are at it, you can measure the resistances between pins 3,4 and 6 and the B+ line. All with power off, of course.
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KE4OH
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Posts: 217




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« Reply #2 on: January 17, 2019, 07:15:24 AM »

I have an SX-24!

The 6K8 is sort of a touchy tube, so I would definitely suspect it. These were used a lot until pentagrid converter tubes came out, and then they were pretty much abandoned by radio manufacturers. They are bad to not oscillate if everything isn't just so.

But the resistor is also a good suspect. Those old ceramic tube resistors are not the best design and are now around 80 years old.
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73 de Steve KE4OH
W9GB
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Posts: 3351




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« Reply #3 on: January 17, 2019, 02:29:44 PM »

Start with a Good “6K8” and use VTVM or DVM to check resistor value.

RCA 6K8 Tube (January 1938) Datasheet
https://www.radiomuseum.org/tubes/tube_6k8.html

The 6K8 triode hexode valve was “purpose designed” as a radio frequency mixer.
http://www.r-type.org/exhib/aaa0058.htm
The valve has a common cathode and is unusual in that the triode and hexode control grids are internally connected.
The local oscillator would produce 7.5 Volts peak to peak signal from the triode section.
The input signal would be coupled to the frequency changer via the top cap connection to grid 3.

As a metal enveloped valve the top cap connection is interesting.
The connector is standard but is set in an insulating disk that is then sealed into a shaped end of the envelope.
This appears to have been quite difficult to manufacture.
==
New Old Stock (NOS) available for about $4.00
http://www.worldtubecompany.com/mm5/merchant.mvc?Screen=PROD&Store_Code=WTC&Product_Code=6K8N&Category_Code=1K-N
Suitable substitutes: 6K8GT, 38568K, CV1945, VT-167.
==
Practical Design of Mixer Converter Circuits
February 1941, QST Magazine
by Curtis R. Hammond (W9PKW)
http://www.rfcafe.com/references/qst/practical-design-of-mixer-converter-circuits-feb-1941-qst.htm
« Last Edit: January 17, 2019, 02:39:31 PM by W9GB » Logged
G3RZP
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Posts: 1090




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« Reply #4 on: January 17, 2019, 02:51:03 PM »

According to 'Radiotron Designer's Handbook' by Langford Smith, the 6K8 was designed to get over the problems of pentagrid mixers, which include negative input resistance above about 15 or 20 Mc/s because of transit time.

Try pulling the tube and measuring the resistances from the top side of the socket.
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KE4OH
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Posts: 217




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« Reply #5 on: January 18, 2019, 05:47:27 AM »

According to 'Radiotron Designer's Handbook' by Langford Smith, the 6K8 was designed to get over the problems of pentagrid mixers, which include negative input resistance above about 15 or 20 Mc/s because of transit time.

And yet 6K8 or a miniature equivalent (is there one?) is almost never seen in post WW-2 gear. Pentagrid converter tubes are the rule. I imagine that pre-war pentagrid tubes might not have been very good, hence the 6K8's popularity.

Whatever the design goals may have been, it can still be touchy to get them to oscillate unless everything is just right.
« Last Edit: January 18, 2019, 05:50:18 AM by KE4OH » Logged

73 de Steve KE4OH
KAPT4560
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Posts: 549




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« Reply #6 on: January 18, 2019, 11:39:55 AM »

 Substituting another 6K8 may give a difference in performance. These tubes may test fine on a tester, but run out of steam (especially at the higher frequencies) in application.
 You do want the signal and oscillator grids to be measured as a negative voltage compared to chassis.
 Replace all electrolytic and wax caps. Many of these carbon composition resistors will drift very high over the decades. The wirewounds seem to hold their value. Make sure that B+ is within spec.
 I have the vaguely similar SX-25.
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W4IJ
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Posts: 54




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« Reply #7 on: January 18, 2019, 05:04:22 PM »

S*U*C*C*E*S*S*    Grin Grin Grin Grin Grin

Many thanks to all who replied to my post:  G3RZP, W9GB, KE4OH, KAPT4560!

As a result of checking the resistances and voltages from the top of the socket, the results pointed directly to the B+ supply.  A bad connection had cut off the supply to the 6K8 but not the other tubes. 

The 6K8 is oscillating nicely, and the SKYRIDER DEFIANT is happily producing beautiful audio!

I really like giving new life to old things - hopefully someone will do the same for me someday!

Thanks again folks - you've really helped me a great deal with understanding the 6K8, and adding to my troubleshooting techniques!

73 and hope to see you on the radio.
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G3RZP
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« Reply #8 on: January 19, 2019, 12:26:07 AM »

Well done. Glad to have helped.

vy 73

Peter G3RZP
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W9GB
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Posts: 3351




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« Reply #9 on: January 19, 2019, 09:36:52 AM »

Curtis Hammond’s 1941 QST article mentions the 6K8.
http://www.rfcafe.com/references/qst/practical-design-of-mixer-converter-circuits-feb-1941-qst.htm
The 6K8 has an effect known as space­charge coupling which is experienced at high frequencies.
This effect is as follows: The oscillator voltage on the No.1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.c. voltage across the signal grid circuit.
This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 μμfd - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio.
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AC5UP
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Posts: 4546




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« Reply #10 on: January 19, 2019, 07:49:16 PM »

On a related note, space charge tubes were considered state of the art in the late 50's for car radio applications and by my experience worked well.  http://www.junkbox.com/electronics/lowvoltagetubes.shtml

At the time a typical car radio was AM only so it didn't take much and I still have a couple of late 50's Delco's stashed against the day I want to hear oldies in Vintage Tone.  10 to 12-ish volts on the plates (assume worst case = parked with a slightly degraded battery) and a single doorknob style germanium transistor for audio output. 



The radio was good for maybe 5 watts of audio and the permeability tuned RF / IF performed exceptionally well across the band.  On a typical road trip late at night the stations were elbow to elbow with enough selectivity for armchair copy, but Hi-Fi it wasn't...
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KA4UPC
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« Reply #11 on: February 11, 2019, 07:03:38 PM »

I just fixed an sx-25 with a problem with the 6k8. I saw 300 volts on the cathode pin because the 400 ohm resistor to ground had opened. I patched it from above b/c the socket was too hard to reach below. Here's a video of what came on.
https://www.youtube.com/watch?v=4G41ZH_0vNA
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KA4UPC
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« Reply #12 on: February 11, 2019, 07:18:08 PM »

I should have let the announcer finish. https://www.youtube.com/watch?v=B9EuzKdqktY
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KD0REQ
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Posts: 2354




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« Reply #13 on: February 12, 2019, 01:36:57 PM »

those Delco radios ran the TO-36 and driver stage in class-A. them there buggers would take a wet cell battery down in no time so you couldn't start the car.  they wre still being used in 1976 production... they got me the wrong radio when I ordered my 76 Skylark. due to a rubber strike, wasn't going to fight, but switch. the old radio went into the folks' camping trailer, after I pulled the class-A stage and put in a LM280 amplifier stage instead. in idle the original output pulled 5 amps.
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