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 Post subject: Motorola SK14M HiFi Stereo as Done as It Will Get
PostPosted: Jun Sun 03, 2018 5:47 am 
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Location: Central PA 16801
I've stated in many of my threads over the past year or more that I have downsized and am not taking in anything more. I have lost interest due to being in this hobby/profession for nearly 40 years since age 10 in 1980. my enjoyment over the past year or more has been to just sit back play, and enjoy my 40 or so pristine pieces, which is a number has been downed from hundreds.

well, skip to today, Saturday about 12 noon...

I was on the porch swing today, kicking back, relaxing, doing absolutely nothing (planned on it for many hours), not bothering a soul, and darn near asleep when my dad's truck pulled in with a Motorola Stereo Hi Fi with the matching speaker.

it is the Motorola SK14M main unit and the S14M matching speaker.

I am now the "proud owner" :roll: of this thing and I don't have the heart to part it out or make it incomplete, which means I feel obligated to RESTORE it :| .

it uses two 6BQ5 tubes on each channel. the amp and control board are separate and it looks like someone removed the capacitors going to the tweeters.

so, since there is plenty of room in a completely cleaned out shop, I am going to put the dog-gone thing in the corner, cover it up, and work on it sometime.

any thoughts on this piece ? any experience or knowledge on its performance ?

I have a few restored mono Motorola 6V6 PP HiFi amps from the mono units of this era and they are just over the top amps.

I was never a fan of the 6BQ5, but since this has a pair on each channel, will this unit pound like it's 6V6 mono counterparts that I have lots of experience with ? if this amp were a stereo version of the 6V6 PP mono amp beasts that i have, I most likely would be on this set tomorrow, :P

reason I ask all this is that I'm not going to power it up with the old can in there b/c I want my interest to be sparked to restore it, not the transformer. the restoration process could be quite some time from now.

in the meantime though, does anyone have a schematic hard copy or a good resolution PDF scan for this unit ? if so, PM me for my email.

also in the meantime, anyone remember if the missing capacitor to the three mid/tweets 5 mfd ? according to my 6V6 PP mono units' schematics of this style with the exact same speaker lineup, it is 5 mfd.

here it is, in its entirety, porch swing included.

thanks for any schematic help.

:arrow: don't worry folks, it won't get chopped up, parted out, or "messed up". I plan to give it a good home and restore it, but certainly not today, tomorrow, next month, or in the near future.

steve


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Last edited by Dutch Rabbit on Aug Thu 16, 2018 5:19 am, edited 6 times in total.
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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 7:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: parkersburg west virginia
That 6BQ5 amp has A little more power output then it's 6V6 powered predecessor. the 6V6 developed around 10 watts as where the 6BQ5 amp could go up to approximately 20 watts peak per channel with 12-13 watts nominal.
that amp is well regarded in the audiophile community.
being that you got the companion speaker, that thing will rock the house quite nicely.


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 12:40 pm 
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Good for Dad... :)

Here is the 1959 service information, courtesy of Bill J.
http://www.antiqueradios.com/forums/vie ... 1#p2091381

Greg.


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 1:04 pm 
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Beautiful and you have an awesome dad, good luck with this beautiful piece and be sure and remember your dad on Father's day. :wink:

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 1:54 pm 
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hi guys,

that is great news to hear that the amp is a good one. i am extremely impressed with the Motorola 6V6 PP mono amps of this era, very impressed. with that said, if the 6BQ5 amp is as good as or perhaps better as stated here, that is extremely encouraging.

i am going to get it down to the shop today if it quits raining and i guess i can say that i have "one in the queue".

i haven't said that in quite a while.

i will get it done sometime.

steve

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 2:09 pm 
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Sams 453-9. I'm currently 300 miles from my Sams folders, or I'd offer a scan. It appears that radiomusum.org has it (non-members can download now).

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 4:01 pm 
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hi tom,

keep me in mind when you get back.

i downloaded these from the link in the old thread that was listed above. the links are in a post, second from the bottom:

http://www.antiqueradios.com/forums/vie ... 1#p2091381

https://app.box.com/s/otst9nnq3bdkwkdf2paf

these are good, but if you want to send better...i'm in no hurry.

thanks a bunch.

oh yes, forgot to mention, i have a wonderful dad. (he has a wonderful son too, :) , i never gave them any trouble.)

steve

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 4:14 pm 
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A link to a high resolution OCR scan of Sams 453-9 has already been posted to this thread!

Greg.


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 5:41 pm 
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I wouldn't say the 6BQ5 can handle more power than the 6V6, but its darn close to being equal!

Most amps I see with them are in the 12-14 watt range such as the EICO HF-12 (12 watts mono) and HF-81 (14 watts per channel Stereo)

6V6 amps seem to top out about 15 watts Class A.

In my experience, 6BQ5 amps do VERY well! They are very comparable to 6V6 in sound.
That said, what I have seen is the 6BQ5s don't seem to last as long. I see more cooked to death 6BQ5s than I see cooked to death 6V6s.
I guess their smaller size means they don't dissipate the heat as well?

Nice find, even though you weren't really wanting it. :wink:

And yes, as you know, I'm VERY familiar with the Motorola 6V6 amp you speak of. It is one of my favorite amps!

-Steve

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Sun 03, 2018 9:27 pm 
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Beautiful walnut cabinet by Drexel. Decent VM changer with Sonotone cart. Jensen speakers with 15" woofer. Not boomy but fairly accurate sound. If that appeared at my door I would gladly welcome it home. My parents purchased similar but mono with FM tuner around 1958.

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 2:12 am 
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yes, i am getting the urge to work on it someday, perhaps this summer when school is out and i send the little darlings home in a week or two...

i had a fisher where they peaked 28-30 watts from a PP pair of 6BQ5 tubes. zenith and magnavox used to squeeze as much as they could and cook them too, but not like that fisher did.

every time i see a 6BQ5, my thoughts are "that poor little tube".

top notch motorola 6V6 PP amp below

steve


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 2:29 am 
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Here is one of mine.
The second one I got. I've not done anything with it yet.
It still has the original knobs…. covered in white yuck...

-Steve


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 2:33 am 
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azenithnut wrote:
That said, what I have seen is the 6BQ5s don't seem to last as long. I see more cooked to death 6BQ5s than I see cooked to death 6V6s.
-Steve

I use a bucking transformer when running anything with 6BQ5's in them. The reduction in heat coming off the chassis is significant. Most mfgs were already pushing them close to their limits back when line voltage wasn't as high as it is today. A bucking transformer evens out the playing field, so to speak. My 14 tube 1958 Fisher is running a pair and the power (rated 35W peak, mono) is excellent.

Larry

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 2:49 am 
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if the tubes are pushed really hard, which they usually are, i will increase the cathode resistor 22 to 33 ohms. this too will take the edge off and will have hardly noticeable difference in output.

the plate voltage goes up a hair, but it's nothing to worry about.

steve

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 3:04 am 
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True, but I run a bucking transformer as much for the chassis components, including the transformer, as for the tubes. Transformers can sometimes be ridiculously difficult and/or expensive to find and replace. Bucking transformers are inexpensive to build and are plug & play with no mods to the chassis.

Larry

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 3:23 am 
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yes.

steve

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Last edited by Dutch Rabbit on Jun Thu 21, 2018 3:11 am, edited 1 time in total.

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 3:40 am 
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You will have to get the chassis numbers, either an inked rubber stamp or a paper tag.

Greg.


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 8:55 pm 
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just like on the manavox tube amps/pieces ?

steve

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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Mon 04, 2018 9:32 pm 
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I've worked on a few of these Motorolas including the more upscale 3 channel stereo units. These sound fantastic and I use one as the main sound system for our TV. As far as the 6BQ5's, well I wouldn't worry about modifying anything. Those tubes can take a severe beating and of the versions of this amp ( they made 6V6 powered ones too ) most were still on their original set of output tubes. These are really great sounding systems and you'll probably be surprised at how well it sounds.


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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Tue 05, 2018 2:12 am 
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excellent, i am eager to begin this summer..

Steve

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