Author Topic: Magnavox 1P3700  (Read 90 times)

David_ATX

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Magnavox 1P3700
« on: February 11, 2019, 06:55:57 PM »
I am helping my daughter fix this HIFI. One channel of the amplifier is not working. The Motorola transistor number (38PIC  F  D  8C) on the transistor case is no help. I was thinking that someone has had to get these replaced and might know a current model equivalent.

electra225

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Re: Magnavox 1P3700
« Reply #1 on: February 11, 2019, 10:27:26 PM »
We need more information in order to help you.  The model number of the chassis, typically printed in dark ink, will be needed to access service information.  You should only start one thread about your stereo in order to prevent confusion.  Welcome aboard!
I don't need Google.  My wife says she knows everything.

David_ATX

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Re: Magnavox 1P3700
« Reply #2 on: February 13, 2019, 10:36:46 PM »
The chassis that holds all the electronic components has two different number codes on it. One of them is 743222 and the other is R 214 04-00.

electra225

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Re: Magnavox 1P3700
« Reply #3 on: February 13, 2019, 10:53:51 PM »
The "R214-" number is the model number of the chassis, the one we need.  The other one is possibly a code of some kind, the "222" being possibly a date code.  I'm not up on my solid state date codes, so maybe someone else can help you.  Have you referred to the Downloads section at the top of this page?  Click on Magnificent Magnavox, select amps and/or tuners, then scroll down and see if your model, the R214 model, is listed.  You need a schematic to show you how the thing is wired and perhaps a parts list.  Good luck.
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: Magnavox 1P3700
« Reply #4 on: February 13, 2019, 11:08:32 PM »
I just went there and I find a schematic for the R 214 chassis, but I can't open t.  It's labelled as a pdf but when I try to open it, it's a "php" file--????

Chris Campbell

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Re: Magnavox 1P3700
« Reply #5 on: February 14, 2019, 10:35:56 AM »
I just downloaded the file, and changed the extension from php to pdf.  The file opens up fine.

Steve

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Re: Magnavox 1P3700
« Reply #6 on: February 14, 2019, 10:51:02 AM »
I just looked at the schematic and it does use the more expensive germanium transistors for the outputs.  I have found that most of the time they are good and it's something else.  Is there any noise or hum coming out of the bad side, (you may have to put your ear up to the speaker to check).  If it has a speaker selector switch, run it through the positions several times, the contacts get oxidized and scratchy like volume controls.  The setting it should be one at this time should be "Int" or "Internal".  Once that is checked we will have to find out how comfortable you are taking voltage measurements inside.  The thing that breaks these sets is when you connect up external speakers and the terminals get shorted together.

If we know your experience level we can help you better.

Steve

David_ATX

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Re: Magnavox 1P3700
« Reply #7 on: Today at 01:08:00 AM »
Thanks to all. I downloaded the manual that matched the Chassis number and it appears to have everything that one would need to troubleshoot this problem. I have already run through the easy stuff. such as corroded controls etc. The right side speakers do not work when connected to the terminals for the right channel. I temporarily connected them to the left channel terminals and the speakers worked just fine. This is what lead me to be suspicious of the amplifier output transistors. I have bit of experience with this sort of thing as I built a Heathkit receiver in the late 70's. I will start measuring and checking to see what results that gives me. There are a lot of things that could cause a channel to go out. This will be a huge help

SeniorSteve

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Re: Magnavox 1P3700
« Reply #8 on: Today at 10:24:19 AM »
One thing you have going for you is there are two channels and you can compare them.  First thing I would check is to see if the voltage is correct on the output stage.  The case (collector ) of the transistors are a good start, one should be the supply voltage and the other transistor in the channel should be at half the voltage (36 and 18 volts respectively).  This isn't a guarantee that it's working, but tells you that the DC parameters of the output stage is normal.  If those voltages are within reason, the next step would be to see if there is an AC signal at the output stage.  Turn the volume up to a normal listening level and put your meter on AC volts then measure the ac voltage on the collector terminal of the output transistor with the lower DC voltage.  You might run into several volts here, but the main takeaway is you should have similar voltages on both transistors with the lower collector voltage.  I've run into the output coupling capacitors open with the result of a "dead" channel.

Something else you can try to eliminate some of the circuitry is to feed a CD player into the "Tape output or monitor"  jacks.  I'm not talking about the "Tape Input" jacks on the back of the console.  This is essentially connecting the player to the volume control, bypassing the selector switch and first two transistors in the amp.  What you are looking for is consistency between the channels.  They may be very low volume or not sounding that good, but they should be similar.

Let us know what you find out.

Steve