Channel Master 6519-No Sound

First topic message reminder :

I'm a newbie to radio repair, and transistors in particular.  I have the CM 6519 radio that I got as a kid in the early 1960's and decided I'd see if it still worked.  I needed to replace the 4 D-cell batteries, and all the electrolytic caps were cracked so I replaced them.  Turned it on and no sound.  I've been working on it since.  Attached is the schematic.--Well can't figure out how to do that-I don't see an option.

In checking the voltages on the transistors I found the following issues with some of the voltages:

I replaced the X1 Mixer early on, with no success, but Voltage readings ok.

I also replaced X2 Oscillator recently but with no success. Voltages still ok.

I finally replaced the X5 AVC transistor, but with no success. However, Voltages on B and E are low and VU Meter stopped working.

I checked all the Voltages for all the transistors, and all the Collector Voltages on all the transistors check OK.

All the other Voltages for the B and E are also within spec, EXCEPT for the B and E on X1, X3 and X5!

I didn't replace X3.

The Voltages read as follows (Spec/Actual):
X1:
B. -0.2/-0.06
E. -0.1/-0.003

X3:
B. -0.3/-0.07
E. -0.1/-0.011

X5:
B. -0.1/-0.009
E. -0.1/-0.003

As you can see in the Schematic, these 3 interact with each other.


I've also noticed in this transistor radio (6V, Pos. ground) that when it is turned off, and I connect my voltmeter with the negative probe to chassis, and the positive probe to any ground point on a component, like the Negative pin on an electrolytic, or its outer case, or an IF transformer case, etc., that I get a DC Voltage reading of -6 volts, or a hair less. I also get this same reading when I check the voltage across the terminals of the speaker.

Then if I turn the power on, all of the voltages go dramatically down and into the mV range.

Does this indicate a short somewhere in the circuit? Should I get a voltage reading when the radio is turned off?If so, how is the best way to try to find the short in this type of radio?



So any help would be appreciated.  Thanks.

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.

Thanks.  Fingers-crossed for something to be heard!

You should hear the demodulated audio at that point, so, if your RF / Mixer, and IF sections are all working, and the X11 diode, the components in K5 are all working properly, that is your first stage of audio signal at point 23. Making yourself a probe cord coupled through a capacitor (something about 0.47uF or lower is fine) would do the job, just start out with that amp at a LOW volume and connect the cord shield to the + / Positive battery terminal.  Heck, with a few more components you could make yourself a RF demodulator probe too and check your IF stages, there are quite a few schematics with a diode, a few caps and resistors on the web. Trying to make low cost things you can use is one of the most fun parts of this hobby.

PoorMan-Yes I've got a guitar amp I could connect to the base of X5.  I'll try that this afternoon.  Should I just expect to hear "noise" or "oscillation", or something else?

The winding resistances don't sound far off at all. The Schematic actually shows 3.5 ohms between 2 and 5 ( 5 being close to a center tap) , 2.5 ohms between 3 and 5, and 6 ohms  (adding 2.5 and 3.5) for the full 2 to 3 winding, so, your primary winding measurements are about spot on. You could try desoldering one lead of X11 and see how your readings change. Be careful not to overheat it though, maybe keep a small alligator clip on the lead you are desoldering to sink some heat away from it. If you find it is defective , replacing it with a BAT41 silicon - schottky diode from a reputable supplier, as most IN60's sold on the web will just be rebranded versions of something else, or so aged even if NOS they may not be reliable. Do you have any type of audio amplifier you can hook up to the base of X5, using a capacitor to coupe it to the amp so the DC doesn't get passed on, and see if you get anything there?

On A1, if I put the Red probe on either of Pins 2, 5, or 3, and the Black probe on either pins 4 or 1, my DMM gives me an Ohm reading of 150K ohms!
If I reverse the probes Black for Red, then I get a reading of around 7K ohms.

Why is this?  Does it have something to do with the diode being attached to Pin 4?

I went ahead and checked A2 and A3, and both of them had readings of 6.6 on 2 & 3.  So is that because they're in circuit, or do both of them also have bad Primary windings?  

A2's Secondary read 0.5
A3's Secondary read 0.3

I tested the A1 in circuit, no power, and here are the results:

1 & 4 = 1.1 ohms   Schematic shows 0.8 for the secondary

3 & 5 = 3.1
2 & 5 = 3.5
2 & 3 = 6.3     Schematic shows 3.5

So is there a problem with the Primary side since it's reading is so much higher?

Looks like the Secondary side is OK?

It sounds like your 1N60 diode is functional, albeit a bit leaky reverse biased, but it still should give you audio. You should get a resistance reading of under 10 ohms going across pins 1 and 4, the secondary winding of the A1 transformer coil. If you don't, that would be where your signal stops. The primary side shows 3.5 ohm across the full coil between pins 2 and 3, with a lower resistance of 2.5 ohms between pin 3 and the pin 5 tap, so I would expect something similar to 2 and 3 on pins 1 and 4.

I feel like the problem is in the Secondary of the A1 transformer.  I've got almost -4.9 VDC going into the Primary, but -2.2 mV coming out the Secondary on Lug 4, and -1.6V coming out the Secondary on Lug 1 that goes to the diode and to GND.  Wouldn't the voltages on both lugs of the Secondary be the same? When it gets to the Base of X5, its -19.2 mV.  

On A2, the output on both lugs of the secondary are -.65 vDC, so substantially higher.  And also similarly on A3.

So is there a way to check that transformer?

0.25 V for forward bias--Diode setting on DMM, negative probe to diode GND and positive lead to other side of diode--POWER OFF
160k ohm for reverse bias--Ohm setting on DMM, Positive probe to diode GND and Negative lead to other side of diode-POWER OFF

So if I did this properly, the reverse bias is very low.  Trouble?

Poorman, could you tell me how to setup my DMM to do these tests so I'm sure to do them correctly?  Thanks.

The X11 - 1N60 diode and the K5 R-C network are what produce the audio frequency signal from the last 455Khz IF transformer to pass onto the X6 - 1st AF amp transistor. There's a chance the X11 is either open or shorted (more likely open), and it often seems the old germanium diodes don't age well. It should have a  forward bias voltage drop around 0.15~0.3V when measured with a diode check setting on a DMM, and also it will have some measurable leakage when reverse biased, but probably above 500K ohms if checked with the ohm meter setting. If it doesn't behave like that, it won't produce audio to pass onto X6. If that never happens, you will never see the base of X5 go higher in voltage as well, but X5 is designed to reduce a high level signal by turning on and then reducing the bias on X1 to keep a signal overload from happening.

I now realize that maybe the reason I don't get any signal at the Collector on X5 is it might be blocked by the diode.

PoorMan-I checked the voltages as you suggested on the Base of X5.  It's practically 0 V when in the NAV position and 0.015 mv when in the Normal position.  I had previously installed new electrolytic in the radio.

I'm not using a Signal Generator for this next step.  I assume that if my antenna is working and the X2 Oscillator and X1 Mixer are working, then I should be getting a 455 kHZ signal on the rest of the circuit that I can probe to find.  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.  Then when I probe the Collector on X5 I don't get any signal.

So I assume, if my probing was correct, that the problem lies between X4 and X5, and that's consistent with the low Base Voltage on X5.

So I"ll start going backwards from the X5 Collector, and see where I get proper voltage.

Does this sound correct?

The voltage on the base of X5, which is your AVC control transistor, is grounded if the M3 / NAV switch is in one position, and is connected to the negative pole of C5 the 30mF electrolytic cap nearby. If the voltage is low in both NAV switch positions, double check the polarity of the cap if you replaced it already, if not, pull it and check it for ESR and leakage. A reversed e-cap will create a situation where it behaves like a failed one and will pull that base to  the common + level like a short circuit. You can check your local oscillator in that radio by holding it near another AM radio, tuning the other radio to about 1200khz, and then tune the 6519 to about 750khz, you should hear a off pitch tone on the other radio, but if you don't, there's trouble with the oscillator in the 6519

Doing good so far!

It is easy to make a mistake, but you figured out what you did wrong on the signal injection.

Please keep us up on your progress. You are doing great so far.


    Very common problem:  if you are unable to turn down the volume, it is quite often a broken trace to ground from one end of the volume pot. I saw this a lot on car radios and home stereos where someone "punched" the volume control. Something to remember for future repairs.

I created a link to my google drive with the ful 300dpi jpg file for viewing, it's not blurred like the compressed image here

Channel Master 6519 Schematic

The next things to look at are, have you tested those replacement transistors for proper junction behavior before you installed them? Are they good - functional germanium transistors of the right type ( PNP or NPN, RF or audio fT, or transition frequency?) Sourcing proper replacement parts is very important, and sometimes what is advertised for parts and what you get are 2 different things if you buy from places like ebay.

[You must be registered and logged in to see this image.]
Posting a schematic wasn't bad, I spliced the 2 pages together, saved it at 300dpi, but it drops resolution here it seems.

Admin edited :
Updated---Changed model from 6915 to 6519

The 9v battery is in the Elenco kit, and the CM6519 is 6v-4 D cells.  Sorry for the confusion in my post.

NOTE:   >>>>>>BTW Radio Museum and other posts show this set as 6 volt, not 9 volt <<<<<<

Using the wrong voltage battery can cause endless problems
.
Please be aware that some manufacturers used a   BATTERY HOLDER FOR 4 AA BATTERIES THAT USED A 9 VOLT BATTERY SNAP CONNECTOR TO THE HOLDER. Very easy to mistake this for a 9v set.

(Sometimes the radios used a positive ground and an earth ground.
 Seems strange but even some TV sets used this arrangement. Dual grounds are quite confusing at times.)


This radio has a known history of the grey caps all going bad.


 Sams published service data in Photofact Set:  623 Folder-6 and TSM 23 (Transistor Radio Series)

Cliff-I already read that and sent up a photo of the schematic using both of the methods, but I don't see it.  I'll attach it to this reply using the host method again, and maybe it will work.

Instructions are How to post Pictures is in our tab FAQ at the top

This radio is Positive Gnd., so all of the readings are going to be Negative numbers.  I connect the Negative probe lead from the DMM to the chassis for the GND, and then use the Positive probe to probe the components, etc. as described.  So I don't think what you suggest is the solution, but there may be some short or something going on.  As I said, if I probe the circuit with the Switch turned OFF, I read the Negative B+ voltage, ie. -6.0 vDC, and then if I turn the Switch ON, the readings read an appropriate Neg. value--other than these crazy values on the B and E of these 3 transistors.  The other components, ie. transistors, have Correct voltages when the Switch is turned ON--it's just these 3 transistors with the really low voltages. Something seems to be blocking the B+ from getting to them, and these 3 transistors ( the 2 IF power amps and the AVC transistor) are all linked. I didn't make myself clear.

I tried to post the Schematic, but I don't think I was successful.  Can you tell me what I need to do to post it?

Thanks for your help.

To me, it sounds like, you are putting the negative of your meter onto the negative battery terminal to do your measurements, which in this case is incorrect. The negative battery connection is basically your B+, and that side is not switched, so, when you are seeing voltage on the IF cans, you are reading the difference in potential between the battery + and -, but when you close the switch it shunts your meter, dropping it to zero. Connect the meter negative to the battery positive, turn the power switch on, and recheck your measurements, you will see they are now negative values closer to the marked schematic values.

I figured out how to post an image.  Here's the schematic of the Channel Master 6519 transistor battery radio.

I'm a newbie to radio repair, and transistors in particular.  I have the CM 6519 radio that I got as a kid in the early 1960's and decided I'd see if it still worked.  I needed to replace the 4 D-cell batteries, and all the electrolytic caps were cracked so I replaced them.  Turned it on and no sound.  I've been working on it since.  Attached is the schematic.--Well can't figure out how to do that-I don't see an option.

In checking the voltages on the transistors I found the following issues with some of the voltages:

I replaced the X1 Mixer early on, with no success, but Voltage readings ok.

I also replaced X2 Oscillator recently but with no success. Voltages still ok.

I finally replaced the X5 AVC transistor, but with no success. However, Voltages on B and E are low and VU Meter stopped working.

I checked all the Voltages for all the transistors, and all the Collector Voltages on all the transistors check OK.

All the other Voltages for the B and E are also within spec, EXCEPT for the B and E on X1, X3 and X5!

I didn't replace X3.

The Voltages read as follows (Spec/Actual):
X1:
B. -0.2/-0.06
E. -0.1/-0.003

X3:
B. -0.3/-0.07
E. -0.1/-0.011

X5:
B. -0.1/-0.009
E. -0.1/-0.003

As you can see in the Schematic, these 3 interact with each other.


I've also noticed in this transistor radio (6V, Pos. ground) that when it is turned off, and I connect my voltmeter with the negative probe to chassis, and the positive probe to any ground point on a component, like the Negative pin on an electrolytic, or its outer case, or an IF transformer case, etc., that I get a DC Voltage reading of -6 volts, or a hair less. I also get this same reading when I check the voltage across the terminals of the speaker.

Then if I turn the power on, all of the voltages go dramatically down and into the mV range.

Does this indicate a short somewhere in the circuit? Should I get a voltage reading when the radio is turned off?If so, how is the best way to try to find the short in this type of radio?



So any help would be appreciated.  Thanks.