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Old 28th September 2011, 11:03 PM   #1
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Smile Gramp's Zenith MLT 15

First post - this is a great forum.

My 97 year old grandfather (who I am very close with) recently moved into a senior apt, and wanted me to have his old cabinet stereo - it's a Zenith MLT-15. Came with a manual, no schematics. Looks like he bought it in '64.

I have some experience with electronics and speaker building, but this is my first foray into both tubes and phonographs (I have never owned a record player before!). Would love some guidance.

The unit hasn't been used much in the last 10 years or so. The tuner works great - no apparent issues other than a few speakers that need replacing, and the volume/balance potentiometer needs to be cleaned/replaced. The turntable appears to be in order mechanically (I recently changed the cartridge and tested all the working parts), but the only sound I get through the speakers is crunchy static. From a basic inspection, it appears there is a separate (and free-standing) pre-amp between the TT and the tuner. There is also an ungodly jumble of wires from TT to pre to tuner - really need to get in and sort those out.

Here are my newbie ?'s:

1. Other than the stickies in this forum, what are some good internet reads for getting my feet wet with tubes and vintage electronics?
2. Any general suggestions for cleaning this badboy up and starting to troubleshoot the phono issues? I've read that replacing the caps might be a good idea?
3. What are some good sources for vintage audio parts and equipment? I swear by Parts Express for most of my electronics needs, but this isn't really their forte...
4. Where can I get a set of schematics?

Even if this isn't a top-quality unit, I would love to get it in fine working order for nostalgia and to finally have a working record player.

Thanks all! This thing just made it to the top of my projects list...
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Old 29th September 2011, 12:42 AM   #2
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1) Pete Millett's book page has lots of good reading to download for free. Most of it is pretty technical, but there's some more general stuff near the bottom.
Technical books online...)

2) Since the tuner sounds good through the speakers, most of it is working ok including the power amp section. I'd guess that you have a bad cartridge or preamp that I think is solid state. So first verify which is defective and go from there. The cartridge may be an old ceramic crystal type. (perhaps an Astatic) If so, a modern moving magnet type won't work well at all since it's output is much lower.

3) Search engins are your friends here, but Antique Electric may have some stuff.
Antique Electronic Supply

4) The schematic is in Sam's Photofacts packet #685. Folder 14 is the actual Zenith console. You can buy these on line at places like eBay, or download (for a fee) at other places. Google search for them.
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Old 29th September 2011, 02:01 PM   #3
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Thanks for the help.

So the issue is in the phonograph itself - the pre works, although it doesn't sound terrific. It's an Olson AM-221, looks to be tubed. Could this have been original to the unit?

I would describe the sound from the unit as being "boomy", especially through that pre. This is in part because a few of the HF speakers need to be replaced.

Most of the pots are touchy as well. Is there any way to get to these in units like this without dropping the whole tuner/amp? I expect to have to do this sooner or later, but I'm really worried that It's never going to go back together correctly....
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Old 29th September 2011, 02:31 PM   #4
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Quote:
Originally Posted by rave0035 View Post
Thanks for the help.

So the issue is in the phonograph itself - the pre works, although it doesn't sound terrific. It's an Olson AM-221, looks to be tubed. Could this have been original to the unit?

I would describe the sound from the unit as being "boomy", especially through that pre. This is in part because a few of the HF speakers need to be replaced.

Most of the pots are touchy as well. Is there any way to get to these in units like this without dropping the whole tuner/amp? I expect to have to do this sooner or later, but I'm really worried that It's never going to go back together correctly....
I believe that the '64 Zeniths had the Micro-Touch tonearm. That was about the time that they stopped selling the Cobra-Matic changers.
They originally had ceramic cartridges. The preamp is not original.
Feeding a magnetic preamp into an input which was designed for a ceramic cartridge would probably explain the excessive bass.
You'll need to pull the chassis before you can clean the pots. I use DeoxIT to clean and lubricate pots and switches. It's not cheap but it works very well.
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Old 29th September 2011, 02:54 PM   #5
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I am glad to hear you are close with your Grandfather and that he thought to give it to you. Mine was 93 when he passed and I treasure the cathedral radio he gave me. Same story, he and my Grandmother bought it new with my mother was but a child.

If the pots are touchy this thing could just need a good bath. The selector switches as well as the pots are highly suspect considering the age of the unit. Try a little spritz with a good quality tuner cleaner. If they are really bad you may need to use a tuner degreaser first and then the tuner cleaner. There were times when I had to go back in and put a drop of light weight machine oil on the contacts to get rid of the noise.

As for the speakers, don't replace them just yet. It is possible to have the original speakers re-coned. It isn't too bad to replace the paper and spiders. The voice coils can be a bit tricky but it is possible to replace them too.

I have a question about the restoration job in reference to the caps. How will replacing the caps affect the value of the radio? Is it OK to just replace the caps or should the old caps be replaced with the same old style cans? Does it make the radio worth more if the old cans are dis-assembled and the new (smaller) lytics placed inside the old shell? I have several other radios I would like to restore and don't want to mess them up.

Good luck and have fun.

Mark
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Old 29th September 2011, 02:58 PM   #6
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Having trouble finding any info about the pre. Here is a very flashy photo. Note the filth - this is covering EVERYTHING on the chassis. The tubes are Telefunkin ECC 83/12, which means nothing to me yet. They were made in (Western) Germany.

Click the image to open in full size.
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Old 29th September 2011, 03:55 PM   #7
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Telefunken smooth plate ECC83 are quite cherished. Look for the little diamond on the bottom and be careful, the silkscreen lable is usually quite fragile.

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Old 29th September 2011, 05:01 PM   #8
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This thing is a blast already!

Looks like they are ECC83-12ax87's with the diamond on the bottom. One of the tubes looks a little discolored around the top - almost coppery. Is this a sign that it's no good anymore?

Click the image to open in full size.
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Old 29th September 2011, 07:27 PM   #9
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Quote:
Originally Posted by rave0035 View Post
This thing is a blast already!

Looks like they are ECC83-12ax87's with the diamond on the bottom. One of the tubes looks a little discolored around the top - almost coppery. Is this a sign that it's no good anymore?
It may mean nothing more than it has been used to some extent and may also be the chemistry of the getter used. It is probably OK, IMLE these tubes function well even after a lot of use - haven't seen many that were so bad as to be unusable. Be careful with them, the pair are probably worth more than the rest of the stereo..

It is also not necessarily the case that the tweeters in this thing are bad, it could well be their cross-over capacitors that are the problem. If the woofers are good there is little reason the tweeters wouldn't be. I suspect they are probably small paper-pleat surround types, but might also be small horns. I'm not very familiar with the Zenith consoles of any vintage..

Finally all of the electrolytic capacitors in the power supply and amplifier should be replaced. There may also be small electrolytics and paper caps in both the tuner and amplifier/pre-amplifier that will also need to be replaced. The supply electrolytics and coupling capacitors to the output tubes are the most critical and should be replaced soon..

Sounds like the TT might have been an upgrade and must have had a magnetic cartridge of some sort given the phono pre-amplifier. Early M3D and M7D Shures are valuable these days if that happens to be what is/was on that table.
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Old 29th September 2011, 07:39 PM   #10
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More pictures. Get a picture of the backs of the drivers and i can possibly help there.

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