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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Tue 05, 2018 3:23 am 
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 Post subject: Re: Motorola SK14M...Story...and Schematic Needed...(Sigh)
PostPosted: Jun Thu 21, 2018 3:14 am 
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i tacked in some electrolytics this evening and hooked up the cd player to the tape inputs.

oh yes, does it ever sound nice with great power and punch.

it certainly sounds excellent even before restoration. i would say it is a good solid 15 watts per channel.

i can't wait to get to it, hopefully this weekend.

steve

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 Post subject: Re: Motorola SK14M Restore
PostPosted: Jun Sat 23, 2018 6:05 pm 
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cathode bypass capacitor question:

I have never seen a 200 mfd cathode bypass capacitor, but then again, I never seen four cathodes being bypassed thru one capacitor.

on the parts list and on the schematic, the electrolytic can shows that the cathode bypass capacitor is 20 mfd value, as seen below in my pics.

on the metal can itself in the amp, it says 200 mfd on it, and the capacitor inside the can measures 200 mfd. I did not take pics (take my word for it, :) ).

this amp is completely untouched and all is completely original.

I plan to replace the cathode bypass capacitor with a 200 mfd as per the can's printed value and the capacitor's measured value, but what about the schematic and parts list ? why the difference of 20 and 200 mfd ?

explanation will be appreciated. all I can think of is perhaps it was a typo ?

thanks.

steve


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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jun Sat 23, 2018 11:17 pm 
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20uF would be the design value -- it rolls off the bass starting at 88Hz, probably to reduce acoustic feedback from the speaker to the turntable in the same cabinet. But it would reduce the deep bass somewhat, so a better value might be 47uF. 200uF is much larger than needed.

-David


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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jun Sat 23, 2018 11:38 pm 
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Technically speaking, a push-pull amplifier should not need a bypass cap on a shared cathode resistor.
The audio component needing bypassed to ground should be equal and opposite.

However, I see many manufacturers which did use a cap. Perhaps because of unequal balance of the push-pull stage in reality?

This one, having both channels sharing a cathode resistor, perhaps they experienced too much cross-talk between the channels and squelched it with a 200 uf cap?

Just an idea of why this is… otherwise, I would think 20 uf would be plenty.

-Steve

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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jun Sun 24, 2018 4:54 am 
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i did not scope anything out, but using a 22 or a 220 makes no difference in tonality.

also, whether or not i use a capacitor, there barely a difference, but the 220 is installed as per what was on original capacitor can.

i restored the amplifier today, every part. it sounds amazing with extremely full audio, powerful audio, deep undistorted punchy bass, and crisp treble. despite it being run hard, all the resistors were well within tolerance. i installed 1 watt carbon films since some of the 1/2 watters got a bit warm.

i did not do the preamp/control box yet. that is sunday's project.

i must say, motorola had it over zenith, magnavox, and rca of that era. i've heard, owned, repaired many zenith, magnavox, and rca hifi sets. they sound good and i like them, but any motorola tube amp from this era was waaayy over the top of them in comparison.

my test bench speakers for smaller amps consist of two zenith allegro 3000's. this amp thumps them like there is no tomorrow. one would think i am powering them with a 25 watt solid state amplifier, it is that good.

as any 6BQ5 that i ever tested in these types of amps are weak to marginal. unfortunately, the four in this motorola test ok at first, but they suddenly decline in emission to being barely marginal. they will have to go.

with the weak tubes, this thing pounds. i can't wait to get the preamp box restored, fix up the cabinets of the main unit and speaker unit, and put it all together.

it is going to sound great.

steve

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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jun Mon 25, 2018 12:22 am 
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Dutch Rabbit wrote:
. . . here it is, in its entirety, porch swing included.

steve


Was that porch swing dark green when you got it? I'm in Central Pa., too, and damn if it doesn't look a lot like the one we gave away.


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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jun Mon 25, 2018 4:34 am 
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no, the swing has been in the family for decades.

i restored the preamp/control chassis today. it was very well packed with ceramic discs and resistors.

the only thing i replaced was two .02 "goodall" paper capacitors and the 10 mfd electrolytic.

all the resistors were very well close if not on value, thank goodness.

i don't see the need to replace the ceramic discs.

steve


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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jul Sun 01, 2018 6:09 am 
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both chassis are now done. on each 6BQ5, I installed 1 ohm precision 3 watt 0.5% tolerance resistors on the cathodes to the 82 ohm cathode resistor.

the original 6BQ5 tubes are not visibly cooked like so many Magnavox, fisher, and zenith tubes usually look. these look good and i thought i might get away with not having to replace them. unfortunately, they test terribly as every barn fresh 6BQ5 tube has ever done for me. at first, they have a nice swing into the "good" range, but they quickly mess the bed under load when the emissions or gM test is thrown.

they are run stock at 32mA with a plate voltage around 315v for a static idle dissipation of 10-11 watts. I thought they would have been run much hotter than that because of the sound and solid power that they produce, but I believe we can attribute that to the large output transformers.

a wonderful friend fixed me up with a matched quad of JJ EL84 since I don't want to dip into my pile of NOS American and Soviet Military 6BQ5 and 6N14p-K tubes. I can't believe how the price of new/used old stock 6BQ5 tubes and the Soviet Military 6n tubes have increased over the past year or two as in wow, simply wow :shock: .

with the original weak 6BQ5 tubes, this amplifier thumped. with the new JJ Yugoslav tubes, it pounds. it beats my bench Zenith Allegro speakers (for smaller amps) to death. it just might have to be hooked up to the Realistic Mach Ones and pound them for a few days during my "after restoration burn-in before cabinet reassembly" period.

I can't wait to assemble the unit into the case and hook up the original speakers and the companion cabinet this week.

chassis restoration pics to come

I've repaired and restored many of the similar zenith and rca sets from the 50s and they sound very good with decent power, but every Motorola or Fisher from the same era beat the other brands out by a couple big steps in both tonality and output power. Motorola and Fisher always, and I do mean always have/had the extra kick, punch, and muscular tonality.

steve

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 Post subject: Re: Motorola SK14M Restore-Cathode Bypass Help pg 2
PostPosted: Jul Wed 04, 2018 5:30 am 
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amp is all done.

I upgraded the 82 ohm cathode resistor with two 39 and a 4.7 ohm resistor. the 7 watter in there just got too hot pulling 4 cathodes thru it.

I also added precision 1 ohm 0.5% tolerance resistors to the cathodes in order to be able to check and match cathode current on the output tubes.

doing the math, they run at 10.5 watts static so i don't have to worry about any cherry plating. they are not pushed to or over their limit of 12 watts static dissipation. both sets of tubes are matched within 2-3 mA of each other.

it sounds great just purring away on the bench. I hope to get it all together over the weekend.

for now, I am going to shine up the TT and let it set in the unit. it is a simple VM and is easily restorable--someday.

I want to use this in my audio room hooked up to my computer's music collection.

steve


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 12:37 pm 
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That is wonderful!
I think fate smiled upon you that Saturday :D

-Steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 2:04 pm 
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it sure did !

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 2:20 pm 
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Steve,

That looks like a real gem!

Very good timing to test it out today with some John Philip Sousa marches!

Enjoy! Tim


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 5:04 pm 
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Nice one, Steve. I'm sure you had fun tweaking it. Nothing boring about that unit :lol:

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 5:43 pm 
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i've had it playing on the bench hooked up to my Realistic Mach Ones bench speakers.

honest to god, it sounds like a mosfet solid state 25 watt stereo amplifier. the tonality is just so robust, rich, clear, deep, and sparkling.

i have not hooked it up to the speakers in the cabinets yet. today, i wiped down the main cabinet with rejuvinating stain. it came out not perfect, but it looks very nice and is quite displayable. it looks 10 times better than the original as found pics at the beginning of this thread.

i hope to get to the other speaker cabinet this week and have it all put together except for the turntable, which will be shined up and dropped back inside the cabinet.

steve

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Last edited by Dutch Rabbit on Jul Wed 04, 2018 7:53 pm, edited 1 time in total.

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Wed 04, 2018 7:52 pm 
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question:

since this is a stereo unit, why did they pull all the cathodes thru one resistor that became mighty hot ?

why didn't they use a cathode resistor for EACH channel (two PP tubes) instead of using one for both channels (two pairs of PP [4] tubes) ?

i know "stereowise" it does not matter.

all i can think of is that it was a cost saving matter by not using one more resistor and a capacitor.

:?

also, i did the final tube measurments today. the 6BQ5 tubes have 285 on the plates and are pulling 35 mA thru the cathodes.

they are run very nicely at 10 watts static dissapation as 12 is the recommended max. i'm just glad they are not beat to death like some of the units of this era do. no cherry plating to worry about.

usually in the magnavox and zenith sets of the era, i would increase the cathode resistor from 33 to 47 ohms to take a slight edge off the current draw on those poor little beat-to-death 6BQ5 tubes. when i did that, the differential on the output between both values was completely unnoticeable and the tubes had a small sigh of relief.

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Thu 05, 2018 4:41 pm 
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See? Now you too have joined the small elite group of people who like these weird Motorolas. I have 4 of them in my house now- one belongs to the museum and will be sold, one is a huge one in the living room with a 15" woofer and lots of little speakers, and then two units that someone gave me- just the guts. I have one in the office and one in the garage. The one in the garage powers a RCA LC-1 speaker as the bass and there's 2 Dynaco speakers for the right and left.

For whatever reason these are real easy on the tubes. I have both the 6BQ5 and the 6V6 variations and amazingly they all use their original tubes. The one in the garage is on all the time, usually every single day. After 4 solid years of use I tested the output tubes: Still all tested as very good.


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Thu 05, 2018 4:49 pm 
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i absolutely love the 6V6 motorola amps too.

i salvaged a fisher project that did not end well.

did a homebrew with the fisher cabinet and a motorola amp.

the power and sound quality is simply amazing.

viewtopic.php?f=13&t=304362

steve

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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Thu 05, 2018 5:01 pm 
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Interestingly enough I too had the same experience with Fisher. Someone gave me a very nice, top of the line Fisher stereo console 5-6 years ago. It was ridiculously complicated using a hybrid solid state and tube powered amp. It was difficult to restore and when done it wasn't very reliable. That and despite its high cost it to me sounded sort of so-so. My Motorolas will blow those things out of the water, are amazingly simple and basic- as you seem to allude to as well- and they just work and I've yet to have a failure.


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 Post subject: Re: Motorola SK14M HiFi Stereo Restore
PostPosted: Jul Thu 05, 2018 5:18 pm 
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exactly, simple and basic

they just have that extra kick over other brands from the same era/models.

believe me, zenith, magnavox, rca, and the like sounded wonderful. i've owned, restored, and have/had experience with many, but the motorolas just have some extra kick punch.

steve

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