AirKing 4602

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Re: AirKing 4602

Post by frank1956 on Wed Feb 03, 2016 2:18 pm

WC,
Here are my DC+ Voltages reading for the following tubes/pin

12SA7 Pin# 3 63.1 V, Pin# 4 63.3 V

12SK7 IF Amplifier Pin# 6 63.3 V, Pin# 8 62.9 V
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Re: AirKing 4602

Post by Wildcat445 on Wed Feb 03, 2016 5:34 pm

You are doing great, Frank!  Those are the readings we need to clear the IF transformer primaries of suspicion.  That proves that both the IF primaries are good and that the problem you were having reading small resistance with your meter was due to "parallax error", inaccuracy caused by meter loading, calibration, settings and impedence.  If the primaries had been open, there would have been no B+ on the plates of the tubes involved.  

If you would care to check the secondaries of the IF transformers to clear them of suspicion, you could try a couple different methods.  You could use a signal generator set to the IF and see if you hear the modulated note in the speaker, or you could use the bit of a thin-blade screwdriver. This test needs to be done with the radio turned on, warmed up a bit.  Tapping the grid, pin 4 on the 12SK7 with the screwdriver should produce a click heard in the speaker if the secondaries are both good.  Appropriate safety procedures should be employed to prevent damage or injury.  My guess is the secondaries will be okay.

With the IF transformers cleared of suspicion, the next step will be to go to the next stage ahead of the IF section, the converter and the local oscillator.

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Re: AirKing 4602

Post by frank1956 on Wed Feb 03, 2016 9:46 pm

WC,
I checked the secondary IF transformer using the bit of a thin blade screwdriver and tapping on the grid of pin 4 tube 12SK7 which produced a clicking sound heard in the speaker.
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Re: AirKing 4602

Post by Wildcat445 on Wed Feb 03, 2016 10:19 pm

Great job!  You have just confirmed that both IF transformers are good.  Both by measuring voltage and by stage isolation and circuit shocking.  

Our next move is forward to the converter stage, consisting of the oscillator coil, tuning cap, and the 12SA7 tube.  If you have another 12SA7 tube, it would be good to sub that tube.  Converter tubes are known to be finicky at times.  Measure all the element voltages on the 12SA7 tube. There are two tests to confirm oscillator function.  The surest way is check for negative voltage on the grid of the converter tube, pin 5.  There should be from -5 to -15 volts on the grid.  It is not critical that you see exact voltage, so long as you see some negative voltage.  The voltage seen will vary as the tuning cap is changed from low to high.  The voltage may vary due to signal strength, meter accuracy, and AVC action.  Another way is to use another radio, such as a small transistor model, to see if you can hear the oscillator squealing in that other radio.  You will turn the AirKing on, employing appropriate safety precautions, and tune it to 600khz.  Then turn the other radio on and run the tuner from low to high until you hear the oscillator tone.  You should hear it at a frequency that would be a harmonic of 600 kHz plus the 455 kHz intermediate frequency (IF).  So you should hear the oscillator at 1055 kHz and 1510khz.  It is difficult to describe what the oscillator sounds like, but if you hear it, you'll know what it is.  You will want to set your meter to (-) DC volts, not more than 15 or 20 volt setting. You will place the negative (-) probe of your meter on common negative (one side of the power switch) and the red probe on the grid of the converter tube, noted above.

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Re: AirKing 4602

Post by frank1956 on Thu Feb 04, 2016 12:38 pm

WC,
Good news, there is a neg voltage of -5.7 on tube 12SA7 Pin 5, and a Oscillator tone was heard at 1055 KHz and 1510 KHz. I also substituted out the 12SA7 with another I had.
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Re: AirKing 4602

Post by Wildcat445 on Thu Feb 04, 2016 10:58 pm

You are doing really well here, Frank.  Messing with oscillators is not the easiest part of radio repair.  You have proven, by performing directed tests and not by guessing, that the oscillator is running.  Subbing the 12SA7 did nothing to help in this case, but I have seen plenty of times when that was the cure.  It is always well to sub that tube in any case.  

There is one more stage between the converter and the antenna.  The radio you are working on has a tuned RF amplifier stage.  This stage helps strengthen the output of the radio by strengthening the signal input to the radio.  You need to check element voltages on the 12SK7 tube (the tube between the 12SA7 and the antenna).  You might switch this tube and the 12SK7 IF amplifier tube if you do not have another you can use.  If this tube is good, we will need to introduce noise into the antenna somehow to see if we can get the noise to be heard in the speaker.  Turn the radio on, observe appropriate safety procedures, and let it warm up for a bit.  Take your screwdriver bit and tap the antenna connection that goes to the tuning condenser.  You should hear that click in the speaker.

Your radio has a loop antenna.  One end of the antenna loop is connected to the AVC bus with the other end connected to the RF section of the tuning capacitor.  There should be only a few ohms of resistance across the antenna loop.  Measure this like you did the IF transformers.  You will probably get a similar result--a reading of zero ohms.  You do not want to see infinity.  If checking the resistance across the antenna coil does not yield any usable information, try this.  Take another piece of wire and make another antenna.  You can use almost any kind of wire you have on hand, the longer the better.  Make sure you do not ground this wire when the radio is running!  Just stretch this wire out on the floor, connect one end to the tuning cap where the antenna is connected.  Does the radio work now?

I need to point out that the radio should be turned off to check resistance on the antenna. And it has to be turned on to see if using another wire for an antenna makes the radio work. I forgot to clarify that.

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Re: AirKing 4602

Post by frank1956 on Fri Feb 05, 2016 4:29 pm

WC,
Thanks for the great information. I just want to clarify something before I move to the next step, I should have done this in my last post. The Oscillator tone that I heard in my last test should have been coming out of the working radio's speaker correct?
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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 5:31 pm

Correct.  You have it right, Frank.  The heterodyning of the local oscillator in the AirKing radio should be able to be heard, at intervals of the intermediate frequency (IF), tuning from the low end of the dial to the high end, in the speaker of the working radio.


Last edited by Wildcat445 on Fri Feb 05, 2016 5:36 pm; edited 1 time in total

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Re: AirKing 4602

Post by frank1956 on Fri Feb 05, 2016 5:32 pm

WC,
The 12SK7 and 12SA7 are good tubes. There was a few ohms of resistance across the antenna loop (barley enough to read) I tried to introduce noise into the speaker by using a screwdriver bit and tapping the antenna connection that goes to the tuning condenser, with no results (no noise produced). I also tried the wire and nothing.


When you say to check the element voltage are you talking DC voltage? If so 12SK7 Pin 4 (2 V), 6 (62.8 V) and 8 (24.8 V).
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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 5:47 pm

That's what we need to see, Frank. The tubes are okay. You have B+ to the 12SK7. Your noise introduction test and trying another antenna produced no positive results. The antenna coil checks okay.

Assuming you connected the test antenna to the proper place, let's try something different. If I remember correctly, you own a signal generator. Right? If so, we need to perform a directed test(s) to try and figure out why the AirKIng won't talk.

Observing appropriate safety procedures, such as having a .1uf at 600 volt (nominal) isolating capacitor in the "hot" lead of your signal generator, start up the generator and let it warm up and stabilize for 30 minutes or so. (You need the isolating cap to make sure no DC B+ gets to your signal generator thru the "hot" lead.) Now, referring to the alignment instructions in the schematic, we are going to basically do an alignment on the IF section of the radio as a first step. We know the oscillator is running, that the IF transformers are not open, that we have reasonable B+ to our tubes, that the audio section, from the volume control to speaker is working, so why is the thing so mum? My thinking is that the IF's are sufficiently mis-aligned that the signal is being interrupted in the IF stage. I am open to being wrong, but we need to prove it.

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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 5:50 pm

Let me see if this will work.......

http://illiweb.com/fa/pbucket.gif

Yep! Dandy! Why am I posting the schematic again, you ask? "Cause I'm too lazy to print it out so I can follow along. Every time I go back a page to view the schematic, I lose my post!

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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 6:19 pm

Okay, Frank.  Back to your project........

We are not going to connect the signal generator how it is recommended on the schematic.  What we want to do is merely find out where and why the signal is not getting to the speaker.  Why your introduction of noise was not heard in the speaker.  You will want to connect the hot lead of the signal generator to the grid of the 12SK7 IF amplifier tube.  Connect the "ground" lead of the generator to the metal radio chassis, anyplace out of the way so you can work.  You will set the signal generator for a 400 cycle audio note, modulated at the IF frequency of 455khz.  Turn on the 400 cycle modulation on your generator and set the dial of the generator to 455.  The radio dial can be any place for this test.  Turn the radio on, turn the volume control wide open and let it warm up for a bit.  You may want to turn the output level on the generator up pretty high until you hear the tone from the generator in the speaker of the radio.  As the circuits become more in alignment, you will turn down the output of the generator.  Ultimately, you will need the output of the generator at such a level until it can just barely be heard in the speaker.  For now, just hearing the tone is all we want.  

Why are we doing an alignment without really doing an alignment, you ask?  We have reasonably determined that the components in the radio are in sufficient condition that the radio should work.  We now need to determine what adjustment will be required to actually get the radio to work.  In this step, we are using the signal generator as a "troubleshooting" tool, not an actual "repair" tool as is typically the case.  You will want to connect an AC voltmeter, set to the 25 volt scale, to monitor the output of the generator.  You can use ordinary test leads to make the connection of the meter across the voice coil of the speaker.  Be sure you connect the meter to the terminals on the speaker, not to B+ or you might damage your meter.  

After you have made the appropriate connections, the radio and the signal generator have been allowed to warm up and stabilize, you will adjust the second IF transformer first.  You will make the adjustment to the coil closest to the speaker first, in itty bitty increments until you hear the tone clearly, and have peaked it on the output meter.  (An analog meter makes this procedure easier to monitor if you have one).  Then you will adjust the other coil on the transformer similarly.  Again, you are looking for maximum output on the meter.  Turn down the generator output after you hear the tone so that you can keep AVC action to a minimum.  Perfect alignment is not our goal just yet.  All we want is to hear the tone from the generator, reasonably well peaked.

Let me know if you have any questions.  We are only working with half the IF section at a time, so the radio may or may not work after the successful completion of this step.

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Re: AirKing 4602

Post by frank1956 on Fri Feb 05, 2016 9:39 pm

WC,
I guess I have good news and bad news. The good news is the second IF transformer peaked on the meter like a charm, the bad news is I could not get the first IF transformer to peak (meaning there was no change in the tone noise nor any movement of the needle on the analog meter at all). I tap on the IF transformer with a screwdriver and still no results.
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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 9:52 pm

Okay, let's back up for a moment.  My concern here was the second IF transformer.  We have not gotten around to the first IF at this point.  

If I understand correctly, you could get the second IF to peak.  Did you turn the generator down continuously, until very little signal could get the If to peak?  Did you hear the generator tone in the speaker?  

If you did not move the connection of the signal generator, the first IF was technically not even in the circuit.  After we have verified a couple details, we can go to the first IF.

You will need to move the connection of the signal generator to peak the first IF transformer.  It will need to be connected to the grid of the 12SA7 (pin 5?).  Then follow the same procedure for that transformer as the other one.  If you can peak the first If like you did the second, the radio should work somewhat.  You should hear a station coming thru.  Perhaps weakly.  

So, to review.  We need to get the first IF to peak like the second one did.  You will need to move the connection of the signal generator to the grid of the 12SA7.  You will need to keep generator output low as you can, since the AVC will change the output.  If you need to have more generator output to be able to hear the signal at the start, go ahead.  Just remember that too much signal will complicate getting the transformer to peak.  IF you hear the radio working, then we will do a complete, thorough alignment.

I may have not been clear about only working with one transformer at a time. Just work with the first IF now like I suggested and we can get it back on track. Did the second IF take a lot of adjustment to get it to peak, or was it pretty close to start with.

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Re: AirKing 4602

Post by Wildcat445 on Fri Feb 05, 2016 10:09 pm

I apologize if I messed you up. I can "see" what you are doing in my mind's eye. The problem is getting my thoughts to print where you can see them. That is a talent I have not yet mastered. I appreciate your patience.

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Re: AirKing 4602

Post by frank1956 on Sat Feb 06, 2016 2:45 pm

WC,
No problem. I am on track thanks to you. I got both IF transformers to peak to the best of my ability.
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Re: AirKing 4602

Post by Wildcat445 on Sat Feb 06, 2016 3:16 pm

Does the radio work?  Did the transformers require lots of adjustment to get them to peak?  Exact is not critical at this point.  I just need to know the answer to these questions so I'll have an idea of what to do next.

Most important.......did you hear the tone from the signal generator in the speaker after you peaked the transformers?

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Re: AirKing 4602

Post by frank1956 on Sat Feb 06, 2016 6:26 pm

WC,
No the radio does not work. The transformer required some adjustment, but not a whole lot.. The tone was heard in the radio speaker.
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Re: AirKing 4602

Post by Wildcat445 on Sat Feb 06, 2016 7:46 pm

Okay, Frank, thanks.

This is the third time I've typed this. The system keep kicking me off for some reason! Rolling Eyes

We need to go one step further back toward the antenna. Warm up your signal generator again. Set it to inject a 400 cycle note, modulated at 455 kHz. You will connect the hot lead of the generator at the point where the antenna connects to the tuning capacitor. With the radio on and warmed up a bit, tune slowly from one end of the dial to the other. You want to hear the note from the generator all across the dial in the speaker of the radio. You can use as much signal from the generator as you need, and can set the volume control wherever it is comfortable for you. If the note stops at any point, there may be a short in the tuning cap somewhere. If the note comes and goes at fairly regular intervals, the IF transformers may be tuned to a harmonic of the IF, something that happens occasionally. If you do not hear the note in the speaker at all, there may be an interruption of the signal between the antenna and the converter section. This test needs to be done carefully so that we can get some idea of where to look.

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Re: AirKing 4602

Post by frank1956 on Sun Feb 07, 2016 10:02 am

WC,
Preformed the about test several times just to make sure I was satisfied with the results. The tone from the signal generator was heard across the dial scale. It was steady and never stopped, it never came and went. I did notice the tone would get lower at certain point on the dial scale, not sure if that makes any differences or not. just thought I would mention it. Other then that everything seem to go well.
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Re: AirKing 4602

Post by Wildcat445 on Sun Feb 07, 2016 11:20 am

Okay, that's good. The tone getting quieter may be due to the alignment not being absolutely perfect. That should not make a difference at this point.

To review. We can get a modulated tone at IF completely thru the radio, from the antenna connection of the tuning cap thru the speaker. That shows us that the oscillator is running, the IF's are okay, and that most of the circuits in the radio are doing what they should. Our concern now is ahead of the tuning cap. The antenna.

Frank, there is a capacitor shown on the schematic as part of the antenna. Did you change that cap? Is there anything on the antenna that appears to be adjustable? I can't read the schematic well enough to determine that. We should hold off on tinkering with adjustments on the tuning cap until we figure out where we are losing the RF.

Your radio should be working. At this point, I believe we should try subbing the antenna one more time. This should cause some reaction in the radio. If nothing else, the noise level should rise. Is there an external antenna connection on the antenna somewhere? You could connect the extra antenna there and see what happens. We need some type of reaction from the extra antenna in the radio. Technically, it should make the radio play. There is no place else for us to go, really. There may have been an error made somewhere, but there is no evidence of that yet. Connect the extra antenna where you connected the signal generator for the last test, on the tuning capacitor. Let's see what happens.

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Re: AirKing 4602

Post by frank1956 on Sun Feb 07, 2016 3:05 pm

WC,
Yes I replaced that cap it is a .005 mfd @630v. I do not see anything on the antenna that is adjustable, nor is there a a place to connect an external antenna. I connected a make shift antenna to the tuning capacitor with no results!!
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Re: AirKing 4602

Post by Wildcat445 on Sun Feb 07, 2016 3:55 pm

Boy, Frank, I'm about out of suggestions!....... Shocked

Look on the schematic at how the antenna is drawn. On the extreme LH upper corner of the diagram. You will notice that the antenna is drawn as a coil of wire. One end of the coil goes to the tuning capacitor, the other end goes to the AVC bus. AVC, for review stands for Automatic Volume Control. Its purpose is to keep the input signal to the amplifier basically at the same volume for strong and weaker signals. If you follow the black line signifying the AVC bus, you will notice on the diagram, shown basically under the 12SK7 tube, a resistor. I can't read what value that resistor should be, but you need to identify and measure the resistance of that resistor. There are two resistors shown on the diagram, drawn between the 12Sk7 and the 12SA7. One is connected to pin 8 of the first 12SK7 tube, going up to a capacitor, that capacitor is then connected to another resistor going down to the AVC bus. You need to identify and measure these two resistors.

I am surprised that you are not getting any reaction out of subbing the antenna. That makes no sense. I suppose you could try totally disconnecting the antenna from the tuning cap and trying the external antenna idea again. If the antenna was shorted out to ground for some reason it might kill the external as well. That's pretty strange thinking..... Rolling Eyes

You could try these suggestions and see if we can find anything that way. You have not checked the resistors that I recall, so that won't waste your time. I hope someone else sees something that we are overlooking.

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Re: AirKing 4602

Post by Wildcat445 on Sun Feb 07, 2016 4:04 pm

This schematic is not the best I have ever worked with.  There are no resistance to ground reading for the various test spots in the radio listed on the diagram.  I wonder if the tuning cap or antenna is shorted to ground.  One would think that such a short would have killed the signal from the signal generator.  We got IF thru the set to the speaker.  We should be able to get RF.  Try this.  Make sure the radio is off for this test. Check resistance from where the antenna connects to the tuning cap to chassis.  Do it both with the antenna connected and with the antenna disconnected. See if there is a difference. Then check it to B- wit the radio turned OFF.  Something may be shorting out the RF.

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Re: AirKing 4602

Post by frank1956 on Sun Feb 07, 2016 6:02 pm

WC,
The resistor on 12SK7 connected to pin's 6 & 8 is a 4.3 K. The resistor connected to 12SA7 pin 8 reads 109 K on my digital meter, it is to dis-color for me to identify. I like you can not read the schematic.
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Re: AirKing 4602

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