Author Topic: Console.. no, wait, portable repair and restoration  (Read 1068 times)

MikeNinNH

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Console.. no, wait, portable repair and restoration
« on: March 04, 2017, 03:46:59 PM »
So last week I set off on my first repair and restoration of a vintage Magnavox console I got for a few dollars at a thrift store. Yesterday, thanks to advice I read here and from a friend who works on vintage telco gear, I replaced the power can capacitor and a couple of the Eire capacitors on the 1964 phono coffee table and got the amp working (I still can't believe it).  I was listening to my iPod with it, as the turntable needed work.

So today I'm out at another thrift store and unbelievably found a portable stereo Magnavox with the same turntable insert (the Micromatic we are all familiar with here). Its turntable works fine as it turns out, so I swapped it into the console and voila, thats done. Now I'm turning to the portable. Its model number is 4SC242R (or 46C242R, its a lil faded).  It has a 3-tube amp marked 8001-40 in ink. Not a sound coming from it, the tubes glow orange a bit, thats all. Is anyone familiar with these models? It looks like someone already replaced the can-style power capacitor with .. three? .. new ones.  The transistor amp on the console only had 2 caps in the can I replaced. I guess I need a schematic. I'll search the download area.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #1 on: March 04, 2017, 04:25:00 PM »
So, no dice on the schematics in the download area. I did learn that it's a Magnavox Holiday portable stereo and it looks exactly like this one:

http://www.ebay.com/itm/Vintage-Magnavox-Stereo-Portable-Record-Player-Turntable-/121156911341?hash=item1c3583c0ed%3Ag%3AzkoAAOxy7S5SBSB6

It's an SC-242 - specifically, 4SC242R.  1962, I believe.

UPDATE: Found the amp schematics online, posted them in the Downloads area. it uses the 80-01-00 amp, it appears. And yes, it has three common-negative caps in the power supply. So, with no sound at all, my next suspect is the tubes. They're the original Magnavox tubes. perhaps they've died. However, I'm happy to hear suggestions how to approach troubleshooting this.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #2 on: March 05, 2017, 08:13:38 PM »
So, I'm approaching two different issues. On the coffee table console with the transistor amp, its nice and quiet as long as the source of the music isn't plugged into power or as long as its not the turntable. otherwise, I get a 60mhz hum, especially in one channel. I'm still looking for the schematics of it, but its the same as in this video:
https://www.youtube.com/watch?v=Yz-vw7RRneY&t=592s

In the absence of schematics, I'm going to assume that I have more capacitors to replace. I'll look it over further.

In the meantime, that tube amp - I think the tubes are good as the silvering is, well, silver, and they glow fine. the .47 600V capacitors look bad tho, so I'll probably replace those.  They have brown spots on them, look toasted.

TC Chris

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Re: Console.. no, wait, portable repair and restoration
« Reply #3 on: March 05, 2017, 08:45:50 PM »
So your hum appears on external sources or the turntable--but not on the radio (AM or FM)?  That suggests grounding issues.  Sometimes turntables are happier with a separate ground wire from chassis to chassis.  Sometimes not.  Check the ground side of the tonearm cabling and the connections at the cartridge.  Soldered securely?  Or if push-on connectors, are they cleaned and snug fitting?  I assume that the other "external sources" are something like an iPod or phone?  Check the cleanliness of the input jacks on the amp.

Chris Campbell

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #4 on: March 06, 2017, 12:35:52 PM »
Hi Chris, thanks for responding - I will look into the things you suggested I verify. My console is basic - just a turntable and the amp, no radio, tape deck, etc.  I tested with the iPod by unplugging the phono jacks of the turntable from the amp and plugging in a phono-to-mini cable in its place.  There is a very slight hum, more pronounced when I plug my laptop into the amp while the laptop's plugged into its AC power supply.  When I plug in the laptop WITHOUT the power supply, there's a noticeable hum.  That's why I figured maybe some of the caps I haven't replaced are bad - perhaps they're caps that filter noise out from the phono or whatever I plug into the amp.  But the hum is VERY noticeable from the turntable, so I'll look into what you suggested.

Now if only I had a clue about the portable Maggie's tube amp. So far based on research, I think I'll probably replace the rectifier, the sand resistor, and see what happens next. Found a conversation about the same tube amp on another forum.

TC Chris

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Re: Console.. no, wait, portable repair and restoration
« Reply #5 on: March 06, 2017, 11:22:11 PM »
The hum:  maybe an impedance mismatch and an input overload?  Your digital devices are probably putting out more than the phono cartridge did.  And it's a high impedance device, isn't it, while the audio output of the digital devices is probably pretty low.  Maybe try a lowish resistor across the digital output and a higher resistance one in series?  That way the digital device output sees a fairly low impedance load and the Maggie input sees a high one?

Chris Campbell

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #6 on: March 07, 2017, 08:37:09 PM »
It's possible, I keep the volume down low when I plug in a digital device.  I mistyped my last comments - I get the hum on the laptop when it IS plugged into its AC power supply, but not when it's NOT plugged into the AC - which suggests either a ground fault or a bad filter cap somewhere.  Or both. I had a REALLY loud hum even when no sources were plugged in, before I replaced the can capacitor, which has a 500uf/1000uf capacitor pair, positive common inside it. As 500 wasn't available, I had to use a 470uf capacitor in place of the 500, with a 25V rating (the can was 20V).  That cleared up most of the hum.  but there are still several other electrolytic caps on the board, and at least one of the ceramic disk caps looks a bit ugly. So I have some things to look into. Part of the fun restoring these old devices! :)

I've ordered some parts to replace on the old tube amp, in the meantime.  Looking forward to working towards getting that one working again too. :)

TC Chris

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Re: Console.. no, wait, portable repair and restoration
« Reply #7 on: March 07, 2017, 11:43:02 PM »
Check the schematic to see what the ceramic capacitor does.  If it's in the audio path, try to replace it with a suitable film capacitor.  They apparently sound better than ceramics.

If your laptop has a three-prong plug on the power supply, try using a three-to-two adapter to eliminate the ground connection and see what that does.  Could be a ground loop.

Chris Campbell

AstroSonic100

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Re: Console.. no, wait, portable repair and restoration
« Reply #8 on: March 08, 2017, 11:14:22 PM »
[Long but important]

Your tube amp does not have a power transformer to isolate it from the AC line.

If you look at the schematic for the amp you will notice that one side of the AC line goes to the changers on/off switch which then connects to a 10000mmf 1000 volt cap, probably a ceramic disc.  This is used to isolate one side of the AC line from the chassis.

If you look at the schematic, cap C13 (.047 600 volts) goes from B- to the chassis, the symbols are shown for each type of ground (B- and chassis).  The other side of the AC line goes to the changer power plug for the motor and a 22 ohm resistor in series with the rectifer which puts out 140 volts B+. 

The center pin of the turntable power socket goes to the B- line and also the 10000mmf cap connected to one side of the on/off switch..

You are getting hum while running your laptop on AC because the ground (shield) side of the audio cable is going to the input jack which is also grounded to the chassis through C2 a .047 600 volt cap.  When the laptop is not used on AC it is shielded from the AC line, therefore no hum.   

Both C2 and C13 are isolating one side of the AC line from the metal chassis.  This reduces but does not eliminate a potential shock hazard.  You might want to invest in a line isolation transformer to use while using the amp outside the cabinet.  These are available from Antique Electronic Supply.  The isolation transformer is 110 volts in and 110 volts out, which isolates the amp chassis from the AC line.  The isolation transformer works the same as a power transformer does in larger tube amps.

In the days of tube radios and phonos, manufacturers eliminated power transformers in their sets to cut down the cost of making them, thus the low price.

Once the chassis is back in the cabinet, it doesn't become a shock hazard, unless the owner takes the back off the set, touches the chassis and a cold water pipe at the same time.  A portentially lethal combination.

As a child, I was "bitten" by a phonograph without a power transformer.  The metal tone arm was mounted directly to the metal chassis under it.  I was standing barefoot on the concrete of the patio and touched the tone arm.  I felt a very mild tingling sensation and dropped the tone arm.



Ray

AstroSonic100

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Re: Console.. no, wait, portable repair and restoration
« Reply #9 on: March 09, 2017, 01:11:19 PM »
This is some information on why a transformerless chassis such as the one Mike has presents problems for electrical safety.

In a wall outlet, the longer slot on the left is connected to the "neutral" side of the 110 v line.  This goes to earth ground, not to be confused with the ground pin on a three prong cord which also goes to earth ground.  The shorter slot goes to the "hot" side of the 110 v line. 

Today's polarized two prong plugs have a wide and narrow blade.  The wide blade connects to the neutral side of the 110 v line.  The narrow blade to the hot side of the 110 v line. 

Most of the vintage consoles and radios we collect have no wide and narrow blades.  There is a 50/50 chance that the plug will be inserted in the correct position.  Older outlets had the same size slots on both sides of the outlet. 

If the plug is connected correctly, the hot side of the line connects to the diode in the Magnavox amp.  The neutral side connects to the on/off switch on the changer.  If the plug is reversed, the hot side is connected to the chassis via the on/off switch and caps isolating the floating ground system from the metal chassis.  The neutral side goes to the diode.  In this scenario, if you touch the metal chassis and then touch a ground such as a water pipe, 110 v on the hot side will pass through you to ground.

You could replace the line cord with a new one with the polarized plug.  The wide blade would be connected to the bottom  pin in the phono power socket with the on/off switch.  The narrow blade (hot side) would be connected to the lead going to the resistor and diode, tube filament string, and the top pin on the phono power socket which goes to the phono motor. 

NEVER CONNECT THE METAL CHASSIS DIRECTLY TO A GROUND TERMINAL ON THE AC OUTLET.  ONLY USE A TWO WIRE, TWO PRONG CORD WITH THIS CHASSIS.

Here is a link to an article on Improving Safety and Reliability of AA5 (All American Five) table radios.  These sets are also transformerless and have the same safety concerns as the Magnavox amp 80-01-00.

http://antiqueradio.org/safety.htm
Ray

AstroSonic100

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Re: Console.. no, wait, portable repair and restoration
« Reply #10 on: March 09, 2017, 01:15:31 PM »
Mike, it occurred to me that you mentioned you swapped changers from the portable to your console.  The console is AC powered and as I mentioned above the amp in the portable is AC/DC.  BTW, AC/DC is used to describe transformerless sets.

You also mentioned that you were getting hum when using the turntable.  There is a difference in the wiring of the switch and isolation cap between a changer used in a transformer (AC) powered amp and one that is used in a transformerless (AC/DC) set.

I found this out when I installed a Micromatic changer from an AC/DC portable to an AC console with a power transformer.  I had hum.  I made the connection as shown in the attached PDF for a console, the hum went away.

I have attached a PDF file that shows the wiring difference.


Ray

TC Chris

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Re: Console.. no, wait, portable repair and restoration
« Reply #11 on: March 09, 2017, 08:01:45 PM »
And just to follow up on "AC/DC," this is a faint echo of the big dispute between Edison and Tesla over the relative safety of AC and DC power.  AC has lots of advantages for distribution--you can run the voltage up and down with transformers, allowing long-distance transmission from the generating station to point of use.  Tesla favored it.  Edison thought DC was safer of you were shocked. His early stations were urban and in particular in New York, and as I understand it, there were some portions of the city served by DC into the 1950s.  But we know which guy's theory won, and which one became famous. 

Chris Campbell

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #12 on: March 14, 2017, 01:54:13 AM »
It's been a busy last few days so I haven't been able to work on the fun stuff, or catch up here til now - thanks for all the thoughts and advice and suggestions!  Ive been reading up on the tube amp and the dangers posed by the lack of isolation, and will be following the precautions suggested. Hobbies are fun, electrocution isn't! I should have realized there might be a difference in the changers between the two models - thanks for the attachment!  I'm going to work on the console first, then circle back to the portable and it's tube amp.  Hopefully sometime later this week I'll be able to take the time to do that.

 I have to say I'm pretty impressed with the sound from the console, even with the hum.  It sits in the same room as our home theater and it's modern surround system.  Yes, the modern system with the benefit of around 50 years advances sounds better - but not so much better to my wife and I that it overrides the simplicity of simply popping a record on the old console and listening to that instead. Since it sits where a coffee table should, in the middle of the room, and has four speakers (two facing out from each long side of the console) it does a good job filling the room with sound.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #13 on: March 14, 2017, 02:59:12 PM »
Mike, it occurred to me that you mentioned you swapped changers from the portable to your console.  The console is AC powered and as I mentioned above the amp in the portable is AC/DC.  BTW, AC/DC is used to describe transformerless sets.

You also mentioned that you were getting hum when using the turntable.  There is a difference in the wiring of the switch and isolation cap between a changer used in a transformer (AC) powered amp and one that is used in a transformerless (AC/DC) set.

I found this out when I installed a Micromatic changer from an AC/DC portable to an AC console with a power transformer.  I had hum.  I made the connection as shown in the attached PDF for a console, the hum went away.

I have attached a PDF file that shows the wiring difference.

You hit the fastener on the blunt end with this one, AstroSonic100! The portable's filter cap was indeed connected AC/DC style, and the old turntable from the console - which I will still be cleaning and getting operational eventually - was wired AC. I snipped the cap's lead to the lug for the other ground wire on the former portable (now console) turntable and soldered it to the black, like the original turntable, and the hum is greatly reduced. I still have a lot of the original electrolytics and even a pair of paper-type caps on the transistor amp of the console, so I'll be looking to replace those (particularly a single one of the same model EIRE as two I've replaced already) to see how that helps.  but for now it sounds pretty darn good.  still need to find the schematic for this amp Found a close enough version here: http://www.radiomuseum.org/r/magnavox_ch_a505_01_00.html  From SAMS Set 683, Folder 9 - Will see how to get it downloaded from there.

MikeNinNH

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Re: Console.. no, wait, portable repair and restoration
« Reply #14 on: March 14, 2017, 03:55:49 PM »
Woot! Found the schematic for the console's transistor amp, finally, and put it in the downloads section. A505 series.