SK111W-FMA restore

[stbasil]

Great! I'll pick some up tomorrow...

[William]

I use that wax on tone arms too as well as the level arm and the changer base. It works good on cars too.

Bill

[stbasil]

Sucess!
Finished recapping amp, but still short one 6V6, so I just tubed up L & R channels with small test speakers, and fired up with the dim bulb (100W) tester, and everything looked ok, so I took out the bulb and shorted the socket for this picture. Feeding the inputs with an old cellphone headphone jack and Prime Music, it sounded fine. Then, I moved the left tubes to the center channel and all works OK, too. Even on my little speakers, you can hear the bass emphasis on the center amp.

So, 137 watts for just the amp seems OK, since total draw for the SK111 is supposed to be 240 watts according to the back panel sticker. (that's at 120V, my line is 125V) Interesting, that's much higher than the Sam's datasheet which says 175 watts.

amp powerup.jpg (154.84 KiB) Viewed 12362 times

[William]

It looks good, do you have source for the tubes?

Bill

[stbasil]

One of the forum members is sending me some 6V6's along with a 45 adapter, so I'm all set!

[Brettzky99]

Let me know if this is what you need and I can try to take a clean photo

[stbasil]

Well, thanks for that shot, mine is slightly different in the upper right connections, but I can actually just use this photo, along with some Photoshopping to recreate something workable. So, please don't mess with trying to get a better shot!

[stbasil]

Recapping, DeOxing and cleaning pretty much finished, for the electronics at least. FM stereo works great, lots of stations show up, even with a crummy antenna. AM weak, unless I am touching the loop antenna, then it works great, so gotta look into that. On my small , very messy bench, using some test speakers, the bass channel sounds too boomy, hopefully that will be corrected in the cabinet with the correct OEM speakers.

The mpx chassis did not agree with the schematic in Sam's, I found a schematic supposedly for the correct chassis number, but even it was not the same. Motorola apparently messed with that design a lot in the early days of FM stereo!

The phono sounds good, new stylus from VM for the Sonotone 9T. Now, the tape input audio from my Bluetooth adapter, and the phono audio are about the same volume, but the radio is much louder. I tried bridging each radio audio input at the preamp with a 22k resistor, and that brings the volume down, but kills the radio's bass, an impedance mismatch thing, I guess. So maybe there's a way to cut down the output of the mpx? I'd really like to fix it so you don't get blasted out when switching over to the radio! Any suggestions are welcome!

Changer cleaned and lubed, finally got that guy working correctly, I'll post more info later on that in the changers section, since I did at least one unusual thing there!

After scratching my head about how to convert the mono tape input to stereo, I finally decided to just replace the wierd OEM rotary selector switch (it shorts left and right when in the tape position) with a traditional 2 pole, 3 position one, and add another RCA jack for the left channel tape input I'll mount the Bluetooth adapter to the upper back of the cabinet after I finish that part of the restoration.

And, lest I forget, special thanks to HiFiMogul for sending me the scans of the Sam's manuals for the console and the changer, as well as selling me a 45 adapter and a couple of 6V6's; and to my neighbor Martin (Brittex) for loaning me a 6v6 from his Capehart to tide me over till I get HiFiMogul's package!

[William]

Wow, you have made great progress in a short time. Congrats.

Bill

[electra225]

Magnavox Multiplex adapters have level controls. Some models have separate controls for each channel. It is also possible to use the wrong input for the MPX, which can overload the amp.

[TC Chris]

As to the volume level/impedance mismatch issue, maybe use resistors in both series and parallel? The parallel one will reduce the impedance being seen and act as shunt.

Chris Campbell

[stbasil]

Well, adding a resistor T-pad attenuator, instead of just a shunt resistor, on the preamp radio input still destroys the bass. Checked all power supply output voltages, they are all as expected for my 125V line. Tried fiddling with output circuit of mpx chassis, same result, any attenuation seems to kill bass response. BTW, there's no adjustments for output levels on it, or the tuner. However, there are two mono outputs from the tuner, one to use with an mpx, the other just straight mono to use if you don't have an mpx. I tried using the mono-only one to feed the mpx, and the output level was lower than the "stereo" one, but it has a .005mf cap to ground, probably to filter out the 38khz stereo carrier, so no stereo reception using that jack. But, I cut that cap out, and used that output for the mpx, which is still a little louder, but not nearly as much, the bass response is still ok, and stereo FM works as it should. So for now, that's my fix!

[Motorola minion]

Apologies for coming in late, this is a very interesting thread highlighting one of the few issues with 1963 Motorola consoles. Once you figure out the volume disparities, hope channel volumes are balanced. Thanks for such a thorough analysis!

Between the FMA and the mono tape input, I still have done nothing to my SK112 beyond initial recapping and cleaning of pots, switches and RCA jacks. I swapped several disc caps in the signal path out for mylar caps, though not absolutely necessary.
The pots are a bit scratchy but not
The radio is a bit louder than either the lame tape input or the record changer, which may be the tired but good sounding Sonotone 9t.

I have probably restored 20 Motorola so far, now learning something new If there is a common two pole, three position switch available, my customers will want to know.

[stbasil]

I ordered the switch off Amazon,
uxcell 2P3T 2 Pole 3 Position Selectable Single Deck Band Channel Rotary Switch Selector 8Pin https://a.co/d/iVXbbWv

But, it's not an exact replacement compared to OEM: shaft is shorter so I'll have to put something inside the knob to make sure it doesn't scrape on the control escutcheon, shaft flat portion required grinding down to fit inside knob, detents are stiffer so I lubed and bent the detent mechanism to lighten that up a bit. Also, OEM version had a bunch of unused pins serving as a terminal strip for incoming power components, I moved all that stuff off the switch to its own terminal strip, as seen in earlier photo.

[stbasil]

Acting like AM signal very weak, touching loop antenna increases signal, but still weak according to the tuning eye. Swapped the 6BA6 RF amp with one from the FM IF path, no change. I suspect some loose connection (I've wiggled lots of stuff looking for that) or a drifted component value maybe. I doubt this is an alignment issue, it's too drastic. Besides, I don't have a signal generator, scope or tube tester (yet!). I'm just going to leave this alone for now since AM is not a personal priority (most stations are just trash "talk radio" nowadays).

[stbasil]

One last piece to test, the vibrasonic seems to work ok, but I'm not completely sure. It has two springs, but they are not quite identical as I first assumed. The upper one is larger, and when I tap it, you can definitely hear the effect! The smaller spring is barely audible when tapped. The two springs' output transducers are wired together and feed a single channel into the preamp. The resistors, input transducers and "compression lamps" ohm out ok (about 65 ohms series) but it's hard to hear the effect on music with my test speaker setup, even though it's connected as intended (across the hot side of R & L speakers). Like the mpx, Motorola apparently changed this design often, I'm guessing to make it cheaper as new models were introduced. In this version, a single 12AX7 makes up the preamp and only feeds the center bass channel, mixing with the filtered R & L bass material in the power amp.

Anybody have an idea if this is behaving normally? Hopefully it'll work as intended once I put everything back in the cabinet!

Now, on to cabinet restoration and getting rid of it's cigarette habit!

[stbasil]

Besides the abuse this cabinet has suffered (lots of scratches, gouges, ripped off veneer), it also had a virtually opaque factory finish. To remove the scratches/gouges, I sanded the top and sides, revealing a nice grain-matched top veneer, with the raw veneer on the sides being normal-looking (brown) walnut. The top, however, was much lighter, so I adjusted my staining accordingly, no problem.

Before

After sanding and staining, before laquer

But the front, wow. A mix of

[stbasil]

Oops, accidentally posted before I finished this entry. Can't see any way to edit, so new post...
Anyway, after sanding, it was evident that they used a mix of poplar for the frame and molding (some large olive green streaks typical of poplar) so I needed an opaque treatment for the front. Even the nice louver assembly had nicks exposing a lighter finish which would not accept stain for touch ups.

I suspect they used parts of that louver assembly on several different cabinet designs.

I'm letting everything dry overnight, and tomorrow I'll finish up with a final laquer treatment. Hopefully in a few days, I'll post pictures of the final result!

[William]

Looking good, and I really like the coloring of the top, it has a Danish natural look to it. It sounds like you are almost there and then it is kick back and enjoy time.

Bill

[stbasil]

Well, everything's back in the cabinet (I'll post some pix soon), but I have a few questions:

1. OEM power cord is two prong, someone temporarily replaced it with another 2-prong polarized. Wondering if I should put on a modern 3-prong grounded power cord, and connect green ground to power amp chassis. What do you guys normally do?

2. The Sonotone 9TA tracks ok at about 4 grams, but bass seems weak. Feeding a Bluetooth adapter into the newly added stereo tape input sounds fabulous, no bass boost needed! Since that's essentially electrically the same as the ceramic phono input, I'm wondering if the cartridge is not up to par or wired incorrectly, or if it just has weak bass. I would think that Motorola would not have designed that kind of performance for records, especially since the FM sounds great, too. Does the phasing look correct in these photos?
I hesitate to pull off that connector. BTW, that's a new stylus from VM Gary.