Channel Master 6519-No Sound

Your attachment doesn't show. You will most likely have to  buy another parts radio with a good speaker ,or, if that doesn't pan out, take some CAREFUL AND ACCURATE measurements for dimensions, note the impedance, and sit down with your favorite beverage nearby, and search the many possibilities available on the internet.  Amazon, Ebay, Parts express, Aliexpress,  are some of the places you might find something.

I'm putting this together and have discovered that the replacement speaker I bought, although it works fine, has a magnet that is too large to fit into the chassis housing.  I'm attaching a photo of the speaker.  It's the one on the right.  Any idea where I could find one, or what type of magnet setup its called?  The metal housing around the magnet is stumping me.  

Hopefully the jpg file I just uploaded is visible to you and others.

Thanks

Congrats on your successful repair!

We are glad to have been able to help you out on this project.

Complete Success!!  FULL VOLUME!!

I put in the 4700 ohm cap to replace the bad R9, and moved the jumper that was going from A1 to the Bottom/X5 Base side of the new R9 4700 ohm resistor to the Top Side of that resistor.

Got great volume, so much that it gets distorted when turned all the way up!!

Looks messy with the jumper, etc., but I can clean that up.  

I consider this a complete success.

Thanks to everyone for their help and patience!

Happy Holidays!!

The R9 is reading 6.29K ohms vs. 5.6K ohms.  I had hooked the jumper up to the back side of the R9 and that's how voltage was getting to the B of X5, but I really should have hooked it up to the front side of R9, but when I tried that I didn't get any Radio reception anymore--presumably because of the high resistance of R9.  The closest thing I've got is a 4.6K 1/4 watt.

Suggestions?

Hi Frank

He did previously state that he can inject audio at the volume control and get sound, not sure what level he put in, and there are a few coupling and bypass electrolytic's to check, but a C-E leak on the X5 transistor will pull down the gain on the X1 mixer transistor no matter what position the NAV/AVC switch is in.

On those old transistors- Germanium I suspect, soldering & unsoldering them can sometimes "heal" a bad junction. Nightmare to diagnose unless you have a replacement to swap out.
 If there is a 'lytic coupling cap to the volume control of from it, try replacing it. I remember some some radios they would open or be intermittent.

Is there any difference in level with the NAV switch turned ON? ( ON shunts the X5 base to common/positive and shuts off the AVC) X5 itself could have a C-E leak, which would also energize the AVC part of the circuit and attenuate the signal and audio. You could lift a lead on R10, and make sure it's a good 22 ohms, and or check your emitter voltage on X5, if it's more than -0.2 with no station tuned in, it's a leaker.

That's good news! You must have jostled something into working order playing around with the components near the X2 transistor. Now, I'm betting it needs an IF / RF alignment, and then, you will notice that some stations may overload  and distort once you get it aligned because your AVC circuit, which is designed to lower the signal on strong stations,  won't work until you either repair that K5 module or replace it with a 1K resistor and 2 - 0.01uF caps, which can be low voltage ceramics to keep them small. There may be a chance you can dig into that  K5 component a bit with a xacto knife and tack solder a new lead onto it and try it again, but building a new little network with a 1/8 or 1/4W resistor and 2 low voltage ceramics will probably fit also.

Now I"m actually getting a station!!!!

Low volume, but its RADIO!!!

A Christmas Miracle!!

You can go to the NTE website and look up the number for a cross ref part.
Also you can post the OEM & cross numbers here and one of us old timers might have one.

I think I found the problem with X5!  After I took the K5 combined component off the board I set it aside after I did the measurements I identified below.  Its bugged me trying to figure out how the voltage gets from A1 to X5 if the 2nd pin on K5 is going to GND.  I looked at the schematic again and realized this K5 has 3 pins, not 2 like all the others.  So this morning I looked at the component and I see that the 3rd leg is broken off at the base of the component.  That's what's missing and that's how the voltage would be getting to X5, but no longer is!

So I don't see how I could repair this component, and I don't think they're available anymore, so I guess I'll have to build my own.

Any easy way to do this, or other suggestion?

Don't worry about X5 yet, it does nothing until it gets a demodulated DC voltage fed to it from the RF signal through the X1 - X2 part of the circuit ,and then, when it does work, it's job is to reduce the audio signal, which are are not getting. If you set the NAV switch to NAV it shuts off the AVC and bypasses it anyways. Keep on working on the local oscillator transistor part of the circuitry.

No luck.  The B on X5 is still reading the same low voltage.

Anyway, I checked the voltage going into K1 from Pin 4 on the A1 transformer and it is 0.750 V, and coming out of the K1 its 0.3 nf when its connected in the circuit.

So I decided to take the K5 Combo out of the circuit and check it's readings.

I hooked up the DMM to each end of the 2 wires.  I set it to Ohms first, and it read Open O.L!  It should have read 1000 ohms!  Then I left the connection the same and switched the DMM to CAP and it read 18.2 nf.  There are two .01uf connecting on either side of the 1000 ohm resistor that then go to GND.

I don't know what this indicates.  I've never seen these Combo components before.  But it seems like something is rotten in Denmark!

If it's a silver banded one, that would be  a bit more than 10%  out of tolerance, but, you it's not as far off as the other 2, it may or may not make a difference much besides marginally slowing down how fast the 0.01uF cap charges to energize the X2 base. I'd leave it for now unless the oscillator still won't run, but seeing they designed it with that much tolerance I'm betting it's not a deal breaker.

I disconnected one end of the R3, 10K resistor from the circuit and it reads 11.88K ohms.  So it's reading high by more than 10%.  I should replace that too don't you think?

The 2.2K ohm resistor measuring @ 1.98K puts it at over 10% out of tolerance, surprised it dropped in value, most carbon comp's go up in value with age. The capacitor is measuring way low @ 0.18nF , that puts it well out of tolerance, at about 180 picofarads, where the schematic value of 0.01uF is equal to 10,000 picofarads. While you are in the neighborhood, check the value of R3, the 10K resistor nearby, as that controls the current going into that K1 R-C network. They all act like a tuned spring to energize the base of X2 at a rate that starts an oscillation, then, that frequency is further tuned by the values of the L-C components selected by the M2 rotary switch for each band.

I pulled the K1 combo from the board and it reads 1.988k ohms in resistance mode on the DMM, and 0.18n farads in the Ohm position on the DMM.

So it looks like the resistor in the K1 may be okay, but the cap is not?

The 2SA92, has a higher "fT" or "Transition Frequency", which is the max frequency it will still provide gain on, of 45Mhz, while the 2SA92 has a fT of 30Mhz. In general, the 2SA92 should have a higher gain (130 for the 92 Vs 100 for the 93) as the only difference between the 2, I'd imagine a 93 would work in place of a 92, but not as well due to the change. If you can, double check that 0.01uF capacitor near X2, in that "K1" section. That cap helps to start the oscillator, if it's out of tolerance that will kill it.

The X2 Oscillator transistor is a 2SA92.
The X1 Mixer transistor is a 2SA93.

In looking for replacements, I found that these 2 transistors are considered equivalents.

I couldn't find a reasonably priced 2SA92, but I found a very good price on the 2SA93 from a reputable supplier that I've bought from in the past several times.

So when I replaced the 2SA92, I used the new 2SA93.

Could that be an issue?  The specs are the same.

But is there something different about these transistors such that they are not interchangeable?

Where does the figure of -0.18VDC for the Negative Bias come from?  Is it on the schematic somewhere or did you do a calculation or something to come up with that figure?

PoorMan--Thanks so much for that explanation.  It helps a newbie like me understand better what's going on.  So I'll go to work on those suggestions and see what I come up with.

Sorry to be such a PITA and posting so much.  But I do appreciate everyone's help.

X5, which is the AFC "gate" , will not turn on and not reduce gain on the X1 ( at test point 7) transistor until X5 "turns on" by having a DC bias on it larger than -0.18VDC  You could pull X5, and see if it makes a difference, but odds are it won't, and you may just have a X1 transistor that isn't good for a RF oscillator, even though it's voltages are correct according to the schematic, or, you have a broken trace, connection or component / coil before that. You need to have the local oscillator working, if it's not, you won't have the RF broadcast signal to modulate the IF signal that you do have, so, you will have silence.

I'm sorry, but yes I did try the oscillator test using a working radio and got nothing.  I've replaced both X1 and X2, the mixer and the oscillator transistors, and I've got good voltages at each of their pins.  The voltages on the B, C, and E's on all transistors until X5 are close to spec.  And the C and E on X5 is correct.  It's the B on X5 that's essentially reading practically no voltage.

Is there a way I can see if the AFC circuit is causing this voltage loss?  What if I disconnect it either after the C on X5 or on the Input of the AFC trim pot?

In your previous post - "So, I hooked up my little oscilloscope to it using a RF probe that I put together, and just for kicks I probed X3 and X4 at their Collectors.  Both are showing around 455 kHZ with a rough jumpy signal on X3 the 1st IF Amp, but a beautiful sine wave on X4 the 2nd IF Amp" 


 You stated you get a good 455Khz signal, but, it sounds like your mixer transistor isn't taking the RF signal and mixing it with the 455Khz. You never said if you tried hearing the local oscillator on another radio from the test I mentioned earlier, so, it sounds like the local oscillator isn't going.  If you are probing the collector of X1 and get nothing, you definitely have an issue back at that point, like you aren't getting a broadcast RF frequency to mix with the local oscillator, or, your LO isn't working. 

I should say I could just hear the amp's hum or crackling when I was touching something on the circuit in the radio, but that's it.

No luck.  Nothing came through the amp or the radio.

I used a breadboard and built this Audio/RF Signal Tracer from YouTube and used it for this test: https://www.youtube.com/watch?v=QUmPBBnzkRE

I used it in both the Audio and RF modes.  I connected it to my guitar amp, and probed the B, C and E's at X3, X4, and X5, and at the Volume Control and nothing.

Its funny, I could hear a radio station coming through on the amp but I wasn't connected to the Radio at any point in the circuit, and I could hear the station even if the Radio was turned off.

I should be able to connect for the test at the input of the Volume control also, which is before the 5600 ohm cap going to the Base of X5.

Given the voltages I've gotten at the Base of X5 though, I'm not expecting anything.