Author Topic: Motorola SK28M  (Read 415 times)

John m

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Motorola SK28M
« on: July 22, 2017, 10:16:07 AM »
Hello all, just found your site and am floored that there is so much info on these old tube amps.
I have found a Motorola sk-28 console model that was a phono only unit. Its in fair to good condition ..We have confirmed that all three channels produce sound, some louder than others. (All three are not very loud at all). The record changer works properly and all the lights come on...I think this unit was produced in 59 or 60/61 according to the low SK- 28 number (please correct me if you know for sure )

The man wants 70 bucks for the unit and its 125 miles from me ..Looking for advise as to what it might take to get it right?
as a novice collector if its something i can do myself? (can solder pretty well thats it)
 Can i get a schematic?
Will parts be expensive or difficult to find?
Any help would be greatly appreciated..

Thank you all in advance
John M



 

HiFiFun

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Re: Motorola SK28M
« Reply #1 on: July 22, 2017, 02:03:06 PM »
Welcome John !
Before I start, PLEASE ask the seller to not turn
on the stereo anymore !
If the filter capacitors should fail, then
the power transformer could be damaged/destroyed.
These capacitors (unless new ones in it) are almost
60 years old, past their service life.

To delineate the basics for electronic
restoration/servicing that must be done:

1. If you can read a schematic, or are willing to learn,
you can do the replacement of the basic components.
This will include filter electrolytic capacitors, wax/paper
capacitors and any out of spec resistors.

2. I have found the chassis schematic in Sam's Photofact
database. It is # 491-11. So, it's available to you.

3. The record changer (V-M 1201) should be cleaned of all old grease,
and possibly install a new idler wheel.
That service folder is also available, as your record changer
is made by The Voice of Music Corp. for Motorola.
This can be a bit of a learning curve, but there are many
online tutorials on these very common V-M changers.
You may want to replace the stereo cartridge if it tests
bad. Modern ceramic cartridges are good and are not
expensive.

4. Parts are available from many sources, and are not
expensive (e.g. capacitors and resistors )
Tubes should be checked, and are readily available.
We will suggest sources where prices are reasonable.

5. Suggest you buy a good digital or analog multimeter,
so you can do some basic check-out of your phono
circuits.

6. Ask the seller if he would take a bit less, as there is a
good deal of time to invest in the electronic and record
changer servicing. But, $70 is still a good price if seller
won't come down.
Yes, that stereo was made in late 1959 or early 1960.

7. I have owned one of these units, and when they are
restored, they have a very wide and open sound that can't
be reproduced by modern stuff.

8. Most important and not last, is if you have no electronics
repair experience, you must learn the safety rules for
working with high voltage components.
This usually means the electrolytic filter capacitors and
anytime the circuit is energized for testing.

You won't be disappointed with that Motorola,
HiFiFun

p.s. Check for an antique radio club in your area.
The members will be able to help you in person,
which can be a big help when starting out.

picard

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Re: Motorola SK28M
« Reply #2 on: July 25, 2017, 07:05:13 PM »
That looks like one for sale in my area. The veneer looks damaged and no tuner. Those were the reasons I don't own it.

Motorola Minion

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Re: Motorola SK28M
« Reply #3 on: July 27, 2017, 01:36:54 PM »
If the SK28 does not get very loud, Id guess it needs output tubes in addition to all that HHF mentioned and this is especially true if they are 6BQ5's.
Mine is insanely loud like these beasts all should be, even using the original 6V6 output tubes. the tubes tested just barely into the good zone but my tester is a standard emission tester, so they are probably weak.

The Motorola I'm playing now was fairly straightforward to re-cap* and the resistors tested within tolerance, so I left them alone.

Your record changer looks like the one in a 1961 SK70 I sold 2 yrs ago. The cartridge was an Astatic 710, and I got a new one from V-M for $28 plus shipping.

* The electrolytics were in bad shape according to my cap tester. Good news you only have about 4 in that amp, and two in the preamp chassis.
Tubes - Magical - Tubes

Dave

Motorola Minion

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Re: Motorola SK28M
« Reply #4 on: July 31, 2017, 11:38:31 AM »
I confirmed the SK28 was one of the early three-channel consoles. It has four 6BQ5/EL84 output tubes. Two for the center channel bass and left and right single-ended output. Also, the cause of low volume can easily be the  rectifier tube.
Tubes - Magical - Tubes

Dave