Author Topic: Motorola SK16W troubleshooting  (Read 737 times)

willtaylor

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Motorola SK16W troubleshooting
« on: January 10, 2017, 12:41:11 PM »
Hey all,

I'm excited to be a part of all this. About a year ago my wife and I purchased a 1960 SK16W with external cabinet from a local thrift shop. It's served us well for about a year, but recently it stopped delivering sound to the speaker. I think I've got the problem diagnosed to the rectifier, a 5U4GB. I ordered a new one off amazon and will be giving it a try on Wednesday. This thing is absolutely gorgeous and sounds terrific. I hope that I'm doing the right thing and I would love any feedback, especially if it turns out that I need to look under the amplifier and look for further problems.

Thanks,

WT

Motorola Minion

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Re: Motorola SK16W troubleshooting
« Reply #1 on: January 11, 2017, 08:32:07 AM »
That is a very clean console :D. I'm putting off repairs to my 2 Motorola tubed consoles until I can actually enjoy them. I did restore a SK70 and sold it. It had four 6BQ5 outputs and while those tube sockets did not need replacing, they did need to be cleaned thoroughly. If yours has 6BQ5's clean the sockets/tube pins with De oxit.

If you are replacing the 5U4 rectifier tube, watch carefully as you fire it up. If you see any sparks or hear hum, yank the plug. If not done already, a replacement of power supply and coupling capacitors on the amp chassis is in order. Refer to Phil's tutorial on capacitors https://antiqueradio.org/recap.htm#check

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Dave

HiFiFun

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Re: Motorola SK16W troubleshooting
« Reply #2 on: January 30, 2017, 07:26:26 PM »
Also agree with Dave, that you should look into replacing the old
filter electrolytic and wax/paper capacitors.
Also, check out the values on all the resistors.

Your model Motorola is found in Sams Photofact 445-8 from 1959.

That amp looks like the usual nice Motorola.  Dual 6BQ5 power
tubes for each channel. [ : )

1. Has the 5U4 rectifier been tested in a tube tester ?
1.a. Have all the other tubes checked.
2. Supplying you with a thread from antique radios re: rectifier short causes.

http://www.antiqueradios.com/forums/viewtopic.php?t=32086

HFF

willtaylor

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Re: Motorola SK16W troubleshooting
« Reply #3 on: January 31, 2017, 07:47:57 AM »
Many thanks to the both of you!

I received the 5U4 in the mail the other day and it appears to have fixed my problem of not having any sound. However, upon closer inspection of the unit as a whole, I found 2 issues with the speaker wiring:

First, while the larger speaker is working, the three smaller speakers in the console are not. There is an orange cylinder wired between the large and small speaker that appears to be the problem. I've yet to remove it and look up where to get a new one. But, that should be my first action, right?

Second, there is a switch installed on the back of the console for the external speaker (right side of the amp) which upon switching it back and forth, appears to be causing the left side to cut in and out.

All in all, I'm thinking that a complete rewiring of the speakers should be in order. Does that make sense to the both of you? Excuse my inexperience and lack of knowledge with this thing. I'm learning as I go.

HFF, I really appreciate the information about Sams Photofact. That looks like a great resource! I'm going to look into having the rest of my tubes checked as well.

Motorola Minion

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Re: Motorola SK16W troubleshooting
« Reply #4 on: January 31, 2017, 10:51:52 AM »
Lemme see if I have the Photofact and post it. You need that schematic to wire those speakers right.
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Dave

HiFiFun

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Re: Motorola SK16W troubleshooting
« Reply #5 on: January 31, 2017, 01:42:47 PM »
Will, can you please post some sharp photos of the inside rear of both cabinets ?
Each picture is worth a 1000 words as the saying goes.

Quote
First, while the larger speaker is working, the three smaller speakers in the console are not. There is an orange cylinder wired between the large and small speaker that appears to be the problem. I've yet to remove it and look up where to get a new one. But, that should be my first action, right?

The orange cylinder is most likely a crossover capacitor b/t the bass speaker and the
other mids-tweeters. I have not found them to be a problem in the Motorolas I own.

1.Look for any breaks in all of the wiring. If none, then consider re-heating the
speaker wiring solder points to obviate any cold solder joints.
Replacing all the physical wiring may not be necessary. I've never had to do it.
You can also get a digital multi-meter with a continuity checker and check
that the wiring has continuity from point to point.

Quote
Second, there is a switch installed on the back of the console for the external speaker (right side of the amp) which upon switching it back and forth, appears to be causing the left side to cut in and out.

2. Buy yourself a spray can of Deoxit D5 or  CRC QD electronic cleaner and spray
into the switch. Then work switch some, spray again.
Oxidation can be a big culprit in these 50-60 year old hi-fi's.
And you can use the cleaner on your controls' potentiometers and switch if they
develop scratchy noises when rotated.
HiFiFun


TC Chris

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Re: Motorola SK16W troubleshooting
« Reply #6 on: January 31, 2017, 06:53:25 PM »
I agree with the comments.  The big orange thing is likely a capacitor that keep bass frequencies out of your tweeter.  The wiring is likely not bad--wires are pretty durable things, as electronic components go--but it sounds like the speaker selector switch needs to be cleaned up.  Your s probably one of those Motorolas that used three channels--a center, combined bass channel (because bass sound is largely non-directional) and higher-frequency channels for left and right. 

It's likely that your big problem is just the contacts in the speaker switch.

Chris Campbell

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Re: Motorola SK16W troubleshooting
« Reply #7 on: February 01, 2017, 11:22:41 AM »
It might be an easy fix. One reason I like Motorola so much, they're built like tanks and quite simple to restore, much like Zenith, RCA and Magnavox.
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Dave

heats00

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Re: Motorola SK16W troubleshooting
« Reply #8 on: June 26, 2017, 08:02:01 AM »
Hate to revive an old thread but, check to make sure that your socket for the 5u4 is okay.  I just picked a SK16W up and the socket was loose on mine, and I had to wiggle the rectifier to get it to light up.  That'll be the first thing I fix when I recap this amp.

willtaylor

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Re: Motorola SK16W troubleshooting
« Reply #9 on: June 26, 2017, 10:14:50 AM »
You may be right on the socket. Actually, I've had to wiggle it a few times to get it to light up over the past months. I think installing a new socket would be a little tricky. Agree?

heats00

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Re: Motorola SK16W troubleshooting
« Reply #10 on: June 26, 2017, 11:44:23 AM »
I don't think it'd be that hard to replace, maybe a few wires to remove and re-solder and some new rivets/machine screws.  It's the rectifier tube so it shouldn't have a ton of stuff connected to it.