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 Post subject: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Wed 19, 2018 9:17 pm 
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Location: South Jersey East of Philly
I'm working on a '54 Maggie console radio/phono (model 252MX) that came in with no FM....AM is fine. I checked for obvious problems (broken wires, antenna, burned resistors, bad caps), nothing. Eventually I got the FM coming in, now pretty good but for a couple odd things----The antenna is super sensitive to direction on stations, and if I stand in a certain spot near the tuner chassis, the signal either gets better or worse...almost like the chassis is part of the antenna at times...and moving the antenna may get that station back in but I lose others badly. If I tap on one of the IF cans, I can lose the signal...yet no connections are bad. It tunes the stations in the right spot on the dial....but you just can't get any one spot for the antenna decent. Could SMD cause this? I know this model has been on his forum with the disease, and there was some evidence of it under the IF can around the mica. I have no scopes...just a DVM and patience to work with....Here's the schematic if anyone has some thoughts on this....
https://app.box.com/s/h4yg9v5mmneu9nrx77lx6te4o2mlb5px


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Wed 19, 2018 9:32 pm 
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Location: Albuquerque, NM 87123
Sounds like there is a bad connection somewhere; either that or a capacitance or inductance is changing because of your tapping. I would also look at the front end, because your reception is changing so drastically.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Wed 19, 2018 9:49 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
QUESTION---Has anyone here had personal experience with SMD (silver mica disease) and what symptoms it exhibits? Are they similar to what I am having? The quirky unpredictable things that are happening with the tuner are really hard to properly describe...but if this sounds like SMD, I'd be interested to know. I did see some blackening around the mica under the IF cans.
Johnnysan---I'll check out those things.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Wed 19, 2018 11:22 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Colorado Springs, Colorado
Going to share an experience I had with a Magnavox console of late 50's vintage that had flaky FM reception. Just sitting there it would drift and jump around. I finally traced it to the tuning capacitor. One of the brass clips that ground the tuning shaft between the different FM capacitor sections was making intermittent contact. I burnished the contact surfaces with a piece of paper and also used a small amount of Deoxit. Fixed the problem.

May not be the source of your problem but worth checking.

Bob


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 1:11 am 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Thanks Bob...I'll check it out.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 1:40 am 
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Location: Mountains of Mourne. Ireland.
Tbirdkid wrote: If I tap on one of the IF cans, I can lose the signal...yet no connections are bad.

Open it and take a look... :)

Replacing mica capacitors in an AA5 I.F. transformer.
Watch and listen. viewtopic.php?p=2948956#p2948956

------------

Cleaning Vacuum Tubes Pins.
http://www.youtube.com/watch?v=UHhNCr2fYoU&t=2m12s

For suspect dry joints, touch them up (reflow) with your soldering iron.


Greg.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 2:00 am 
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You haven't said that there has been an FM alignment done. There are ways to do this without specialized equipment, but I don't have that method in front of me. It sounds like the FM signal is weak, and often an alignment is a cure for that.

SMD is not usually the cause for the symptoms you're describing.

We are assuming you have good reception there with other FM radios. Are you using a good FM antenna tuned to the middle of the FM band? Like a TV antenna, the FM antenna works best with the loop turned broadside to the station you're tuning to.

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 2:25 am 
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It is good practice to mark the cans, the base and the chassis with a sharpie - as not to install them backwards.

Image
https://i.ebayimg.com/images/g/-CIAAOSw ... -l1600.jpg

Image
https://i.ebayimg.com/images/g/KWUAAOSw ... -l1600.jpg

I see that you already have the service information.
viewtopic.php?p=2954834#p2954834

Greg.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 6:45 am 
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SMD is pretty common in this series of tuners, in the FM section. Symptoms are no FM, low volume, and/or FM comes and goes. I found SMD in the FM IF's and maybe in the discriminator; i can't remember for sure. I ended up replacing the caps in the first and second IFs and put in a NOS discriminator I found on eBay. FM now works ok, but not great.

Here's a link to the thread, if you haven't already found it:

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=208803

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 7:33 am 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Thanks for the link to your Maggie 'ordeal' Ed. You really dug in and went thru the wringer to attack the problem. Now I'm 90% sure this is SMD on mine. My FM IF cans look like yours on the inside top, where the 'black corrosion' is. Mine is the same tuner, and your thread is a very helpful guide. Actually..today I pulled the guts out again. (I thought I had it working...but soon as it was in the cabinet, it got cranky again, and got intermittent. Felt like stomping on that damn tuner...LOL) So I again put every thing on the bench, and it's playing perfectly on FM for now without doing anything! I'll see how it goes...but I feel confident the SMD is behind all this.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 2:07 pm 
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Ordeal is a good description. I would never have figured out the problem without all the help from the ARF members who pitched in. I hope you get yours resolved. I'm not convinced the tuners were all that great to begin with. Mine is used for playing LP's 99% of the time, and it is excellent for that. I put the four Magnavox branded 6V6's away for safekeeping and bought a matched set of Electro-Harmonic 6V6's from AES. I am very pleased with the performance.

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 2:22 pm 
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A word of caution--after months of playing great, my wife moved the set from one wall to another. The next time I went to play a record, the audio was awful. I tore everything out and set it up on the bench, and it played perfect for hours. I put everything back in the cabinet and crap again. I moved the cabinet back where it was before, and problem solved. Apparently it was too close to the flat screen TV which degraded the audio, even playing records.

So be aware that this model seems to be very susceptible to RFI. It could be those Russian 6V6's I put in it. :roll:

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 4:36 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Thanks for the extra tips Ed....I swear these Maggies are 'possessed'...Who would think the audio portion would be affected by modern TV RF? Actually the FM on this one seems to be pretty decent when it's working...I fired it up this morning and all is well so far on the bench.
Who know if it will like being put back in the cabinet?... :lol:
One thing odd is when I got the set there was a 6CS6 in place of the 6BE6 at V2 position. It actually works good, and seems to be a good replacement if the 6BE6 isn't cutting it.
TIP: I put a 25mfd25v cathode bypass cap on across the 3.3k ohm cathode resistor on the 6AV6...Really kicks up the gain and sound for me...Try it and see if you like it.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 7:37 pm 
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I did that too, but used a 47uF cap recommended by some of the guys on ARF. The weak bass response is common to some of these Magnavox amps. You can experiment with different values. The audio with the 47uF is very good, especially the highs; they are really brilliant. Bass is not going to sound like some of the contemporary stereo hi-fi amps, but I think they sound somewhat unnatural anyway. Here is a good thread on the topic:

http://www.antiqueradios.com/forums/viewtopic.php?f=13&t=185729&start=20

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 8:15 pm 
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More good stuff there to check out ED...thanks!
Not sure if you caught that the cathode bypass cap I added was to the '6AV6'...as opposed to the 6V6's. I see you mentioned a bypass cap increase on the 6V6's, and I will try that.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 8:39 pm 
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I did, but I thought 6AV6 was a typo. My 132AA amp just has four 6V6's, a 12AX7, and two rectifiers, I think. I think there are at least four different threads on the 252MX on ARF.

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 10:35 pm 
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Location: South Jersey East of Philly
Ed... The 6AV6 is in the TUNER...the last tube out (audio) to the tuner chassis. When you bypass that 3.3k resistor on it's cathode, the set really jumps alive..may be almost too much gain, but the radio really sounds good now. Try this if you can sometime...easy to get at, and reverse if you don't like it. Anyway, the alignment is touchy on these sets, but I found a pretty good compromise spot on all the IF cans. So far no sign yet of the SMD acting up for now. Now, if it all holds good when I do the transfer to the cabinet... :lol:


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Thu 20, 2018 10:46 pm 
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I got my fingers crossed for you! I will try that bypass cap.

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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Sat 22, 2018 2:06 am 
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Location: South Jersey East of Philly
Put the Maggie back together today in the cabinet. The good news is now I at least get FM all time. The 'bad' news is it is still very touchy with antenna direction. I get fairly good FM reception from 92-106MHz...but at either end of the dial, stations are very dependent on antenna direction. If I tune the alignment to pick up those end of dial stations better, I lose other ones in the middle. I tried a standard FM dipole antenna, and an antenna of just 2 lengths of wire...no significant help on either types. I have it at a compromise tuning now...ended up with one antenna wire hooked up to one FM terminal...When I hook up both wires up on either the dipole or wires, I get a lot of signal loss or cancellation at times....and standing in front of the set will throw off the signal too..It seems like the chassis is affecting antenna signal too at times if I touch it. When I do get a decent station, the sound is very good. Also, when tuning a station, I get kind of like two slightly separate signals to tune to on some FM stations---one very strong and overloading signal..and the other clear but weaker. I'm completely lost on this now...But am ready to throw in the towel and accept it as is...as is the owner.


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 Post subject: Re: Stumped On Magnavox 252MX With FM Flakey FM Reception
PostPosted: Dec Sat 22, 2018 5:12 am 
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This is not an area I understand very well either, but you may have aligned on the wrong peak. Someone who really understands alignment will hopefully jump in and explain how to align to the right IF peak.

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