Author Topic: Magnavox Micromatic Model: 2ST647 New Needle - No Sound  (Read 4338 times)

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #15 on: March 23, 2018, 08:59:09 PM »
If I'm reading this correct, you were feeding your console into external speakers that were also connected to another receiver?  If that's the case, the first thing I would do is to isolate them, (disconnect the wires on the back of the Magnavox) and set the speaker selector on the Magnavox console to Internal.  Eliminate any outside problems first.  If you still don't have any sound from the console there is a possibility that the amplifier might have been damaged.  I would check for fuses that may have blown.  When you disconnect the wires on the back of the console, make sure the wires don't touch, I'd hate for a second device to be damaged by shorted wires.

What happens is when two amplifiers are connected "back to back" the more powerful one can apply more power to the turned off device than it's rated for.  Even though the device is turned off it's still vulnerable.  If I'm not correct in how things were connected, or if you were just connecting Pioneer speakers my mistake in the interpretation, please disregard.
Steve

electra225

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #16 on: March 23, 2018, 09:42:28 PM »
You mentioned on time that you could faintly hear the radio at one point.  IF that transformer was fried, the radio would be dead.  My guess is the transformer is okay.  Bulging caps are a possiblilty.   My understanding is that replacing all the electrolytics in a solid state amp is an indicated procedure.  Like Chris said, don't borrow trouble.  Concentrate your efforts on something that will let you move forward.  And don't count on lots of encouragement on "that other forum."  Some of those guys have a dim view of console stereos.  That is why this forum was started.  We will be happy to help you any way we can.
I don't need Google.  My wife says she knows everything.

John B

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #17 on: March 23, 2018, 10:14:11 PM »
SeniorSteve,
When you disconnect the wires on the back of the console, make sure the wires don't touch, I'd hate for a second device to be damaged by shorted wires.


I've taped the speaker wires from the Pioneer.  If I have fuses I'll check them too.

electra225
Bulging caps are a possibility.   My understanding is that replacing all the electrolytics in a solid state amp is an indicated procedure.


I can do the caps if I can see 'em.  It's the little transistors and resistors that I'm afraid of.
I'm getting pumped up and ready to take it on.  I have to now, I just lost my little red nozzle when I sprayed air into the tuner section.  lol

John B

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #18 on: March 24, 2018, 09:30:38 AM »
Hello John B, I now understand what you were trying to accomplish.  Here's what I think happened, when you put the switch on the Magnavox in EXT, what that does is to connect the speaker connections on the back of the console to the amplifier inside.  There wouldn't be any connection to the speakers inside.  As I understand it, you wanted to play your Pioneer receiver on the console speakers, but instead what happened is the Pioneer amplifier and the Magnavox speaker outputs were tied "back to back".  Solid state amplifiers don't like that, and the more powerful one will win out. 

I'm not all that familiar on the Magnavox models, but can you tell me if there are two chassis on your set.  On the higher end models they had the tuner chassis, and the amplifier chassis.  The lower and mid power models just had one chassis that housed both sections.  I couldn't see from the photos which chassis you had.  I'm sure this isn't the first time something like this has happened.  Also I believe you model is from around 1966 which means it could possibly have germanium output transistors, which are now rather expensive.  They are also easily damaged by shorting the output or back feeding from another amplifier.

Lets get this figured out and your console working.
You have lots of help here.

Steve

electra225

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #19 on: March 24, 2018, 11:29:11 AM »
Referring to pictures of the chassis and the wiring diagram, it appears to me that there is only one chassis in this instrument.  The OP also stated in an earlier post that he could hear the radio faintly in the speakers.  I'd think that if a transistor was out he would not have any sound at all.  My solid state troubleshooting skills are rudimentary, at best, so I ask this in the interest of learning.  This last fact would also clear the power transformer of suspicion.  My suggestion to John would be to connect the cartridge to the Magnavox amplifier per the diagram posted earlier.  Turn the instrument on, set the function switch to "Stereo" or "Phono", set the speaker switch to "Int".  Then touch your finger to the terminals on the back of the cartridge.  You should get a loud buzz or hum in the speakers.  This one test will check the cartridge, amplifier and speakers in one shot.  If you do not get a hum, then start from the speakers and work backward to the cartridge.  A schematic is vital for successful troubleshooting.  Learning to use a multimeter is vital as well.  Guessing and throwing parts at something is likely to yield less than satisfactory results.  That paper diagram pasted in the cabinet is helpful for figuring out if the components are wired properly.  Performing directed tests in logical order is vital for successful troubleshooting.
I don't need Google.  My wife says she knows everything.

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #20 on: March 24, 2018, 11:55:43 AM »
I'm wondering what the condition of the Magnavox was before the external speakers were connected to it.  Was the radio functioning properly?  Does the radio work now?  These questions are assuming the speaker selector is in the "INT" position.  What I'm trying to zero in on is if something may have happened to the chassis.  We have to get the radio working first to verify its operation before we tackle the changer. (My opinion).  Once we know the radio is working, the same circuitry is used for the phono and we should be able to get it going, or figure out what to get next. 

1.  We are assuming the power transformer is good, do you have the dial lights?
2. Test the tuner, does the tuning meter work or the stereo fm indicator illuminate?

With those two items, we can figure out if the unit is getting power and B+ voltage to the tuner at a minimum. 
Next step is the amplifier, does the tuner have any volume, and is it distorted. 

As much as I don't want it to be in the chassis, I want to start there.  If the Pioneer receiver wasn't turned up very loud, there may not be any problems, but if it was turned up quite high, there is a much higher probability it may have created a problem in the Magnavox.  I don't know how much power the Pioneer has, but the higher the power output the higher likelihood of an "oops".

Sorry to ramble,
Steve


John B

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #21 on: March 24, 2018, 03:04:28 PM »
Steve and electra225,

I'm rolling!  The radio works.  The dial lights up, the tuner meter needle works, FM indicator illuminates.  Nothing from cartridge.  Only a faint sound from radio.  Possible model number found on chassis:  204-13-20.
I cannot see any bulging capacitors.  I cannot find the "magic bullet" fuse.
I have LPS NoFlash Electro Contact Cleaner to use on the tuner knobs or anything else.

You guys ramble all you want, that's why I'm here.  lol

John B

John B

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #22 on: March 24, 2018, 03:34:57 PM »
electra225  "A schematic is vital for successful troubleshooting."

I found a schematic.  In fact, three of them.  Sam's 841-6 has my model, 204-13-20.  I can attach it now and use the Download for the others?  I found them by chance by doing a search for Magnavox 204-13-20.

John B

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #23 on: March 24, 2018, 04:11:16 PM »
Are you saying the radio works with good volume, or the tuner functions (meter and stereo lights work), and you have a weak sound from the speakers.  I'm trying to identify if it's in the amplifier at this time.  I did download the schematic and indeed you do have the germanium transistors for outputs.  We have a good starting point. At this time what I need to know is if the radio has good volume out of the speakers to eliminate the amplifier.

Steve

John B

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #24 on: March 24, 2018, 04:23:35 PM »
Steve,

The radio seems to be working but the sound is very, very faint.  I almost can't hear it at all.  Should I spray the cleaner I have on the knobs?  Maybe sometimes a good cleaning would help.  I cant find a fuse and to me I don't see any bulging caps.  Where can I upload the other schematics?  They are 15MB.   

I appreciate the help,

John B

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #25 on: March 24, 2018, 04:30:33 PM »
Thanks John, you've answered my question, and I did download the schematic.  This will help a lot!  I will look into the amplifier as far as what to tell you to look for.  Do you have a meter?  It may be something to look into.  What I think may have happened is the output transistors may have shorted. 

Steve

John B

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #26 on: March 24, 2018, 05:01:14 PM »
Steve,

I have a couple of CEN-TECH meters and after a search in the garage I found my 150 page booklet on "Electrical Testing With Multitesters".  One of the meters is still in the package that I got at Harbor Freight.  Buy something and get something free.  Take your time.

John B

electra225

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #27 on: March 24, 2018, 05:19:23 PM »
Steve, what evidence is there to support your theory of shorted output transistors?   
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SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #28 on: March 24, 2018, 05:38:50 PM »
I don't have the evidence yet, but some voltage checks will tell me if they were damaged.  A couple of checks will tell.  The output transistors are the diamond shaped  metal devices on the black heatsink in the center of the chassis.  They are insulated from the chassis.  I know there is power because the radio works, (although you really can't hear it).  What you need to check is the voltage on the cases of the outputs, on two of them you will have the supply voltage (around 36 volts) and on the other two you should have half of that, (around 17 volts).  If the transistors have shorted, it normally opens the emitter resistors (R88 and R104 with the possibility of opening R91 and R107 in the process).  What I'm looking for to start this off is that on two of those output transistors, you should have about 17 volts on them.  The reason you hear the radio ever so lightly is you are hearing the signal that is coming from the driver transistor before the output stage.  The transformer that is between the driver and the output transistors "protected" the amplifier from more serious damage.  That's what I think. 

Steve

SeniorSteve

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Re: Magnavox Micromatic Model: 2ST647 New Needle - No Sound
« Reply #29 on: March 24, 2018, 05:45:10 PM »
I forgot to add if the voltages are incorrect, the chassis will have to be pulled.  All of the resistors associated with the output stage are under the chassis and located under the output transistors.  If you get to this point you will see the resistors will have a bulge in them.  I'm hoping I'm wrong.  Also this is a positive ground chassis in that any voltage measurements will be negative with respect to the chassis.

Steve