Author Topic: Motorola Stereophonic restoration at last  (Read 557 times)

Motorola Minion

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Motorola Stereophonic restoration at last
« on: June 19, 2017, 12:34:12 PM »
Quality sound is needed to serenade all the flooring and painting work. So I got this http://vintagehifi.net/index.php/topic,1559.0.html from CL almost 2 years ago and has been sitting in the rec-room attic of an addition I started about that time. Also at that time, I began working on a 1964 Fisher Custom Electra for this event, but since moved it to the garage because the Italian Provincial cabinet needs to be stripped. I also need to repair its Garrard changer and power switch though its amp and tuner are all cleaned up and recapped.

Back to the Motorola. ONE bit of advice if you work on one is to LABEL ALL PLUGS and SPEAKER WIRES before you attempt to disconnect anything. And you have to undo the whole business to work on any one chassis. I will add some photos of how I used "brady tags" to assign numbers to all these plugs and leads. Unlike Magnavox, RCA and Zenith, Motorola seemed to separate all the components. Placing the preamp in its own sealed chassis is pretty neat though.  ;)

The 3-channel amp has the power supply on it as well. The power is distributed to a tuner, a preamp and the FM multiplex adaptor. Speaker leads and an RCA plug are on the verbaphonic reverb unit, which is made by a supplier to Hammond Organ. Looking at the Sams schematic does explain all the routing but when you actually look inside one of these consoles, you will be impressed and intimidated. I swear there is more wiring (though neatly routed and tied) than in a VW Beetle  ???

After labeling all the plugs including speaker wires, I disconnected them and put each chassis into its own flat box. This will allow each to be carried down to my bench and I can place all related hardware, cables, spent parts in the box as I work on it. First one up is the amp.

I also gave the cherry veneer finish (not a Drexel cabinet) a good cleaning of dirt and old wax using 000 steel wool and turpentine. The drawer hardware was removed and will probably end up in a bag inside the cabinet, since early American style needs all the aesthetic help it can get.

Last night I decided to pull out all five speakers to check physical condition and to test them and the crossover caps on my audio oscillator. The 12" woofer was pretty even (by ear) from about 36 Hz up to 500 Hz. The 8-inch midranges made noise from 400 to 6000 Hz and the 3.5" cone tweeters were clean from 5K until my ear's limit of 12K. I put them back in and wonder if maybe that 12" woofer should have its own air-suspension box built into the cabinet.  :-\

Ill post all the electronic restoration here rather than in repairs because I do not expect anything unusual.
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Dave

TC Chris

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Re: Motorola Stereophonic restoration at last
« Reply #1 on: June 19, 2017, 07:29:48 PM »

Last night I decided to pull out all five speakers to check physical condition and to test them and the crossover caps on my audio oscillator. The 12" woofer was pretty even (by ear) from about 36 Hz up to 500 Hz. The 8-inch midranges made noise from 400 to 6000 Hz and the 3.5" cone tweeters were clean from 5K until my ear's limit of 12K. I put them back in and wonder if maybe that 12" woofer should have its own air-suspension box built into the cabinet.  :-\



If you seal up the woofer, won't you change its resonance and mess up the overall balance designed into the system?  If it's easy to do it wouldn't hurt to try, but it's likely designed for infinite baffle/open box use.

Chris Campbell

Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #2 on: June 20, 2017, 04:13:12 PM »

[/quote]

If you seal up the woofer, won't you change its resonance and mess up the overall balance designed into the system?  If it's easy to do it wouldn't hurt to try, but it's likely designed for infinite baffle/open box use.

Chris Campbell
[/quote]

Sounds like maybe the cabinet is all it needs to be. This is not a Drexel or Heritage cabinet like the more expensive units but these are the exact same components that would be used in the high-end furniture.
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Dave

Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #3 on: June 20, 2017, 04:16:31 PM »
Some pictures

Hardware removed and cleaned up, making it look a bit better

The Multiplex adapter next to the amp, look at all those wires - tags are mandatory unless you like figuring it out yourself from the schematic.

All apart on the floor, the amp is now on the bench for tomorrow. Ill take more pictures of just that.

Early summer mornings it is cool in my shop, and the only way to get uninterrupted time to work on this fun stuff.
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Dave

AirChief

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Re: Motorola Stereophonic restoration at last
« Reply #4 on: June 20, 2017, 09:05:56 PM »
Don't you love when you actually have free time to devote to restoring these wonderful music machines.  6 6v6GTs is going to sound awesome.  I think Motorola's are well-built, have style, and look great. Good luck and let us knowhow it turns out!.

Harbourmaster

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Re: Motorola Stereophonic restoration at last
« Reply #5 on: June 21, 2017, 07:58:34 PM »
The finials on the top of that cabinet are pretty unusual for something of this vintage.
-- Aloha, Ken

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Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #6 on: June 23, 2017, 04:52:09 PM »
First, I measured the resistors and found all to be within spec. I also checked the primary resistance on all three output transformers, hoping for the best and all were close to resistances on the schematic.  ;D

Most Motorola products work as found due in no small part to the extensive use of ceramic disc caps in place of wax-paper caps for values .001 through .05 in. But due to effects of temperature and vibration on capacitance, ceramics have limitations in coupling and emphasis'/de-emp networks. Mylar film caps will replace them as I have done with two of my Magnavox amps.

The amp chassis is pretty shallow, so installing electrolytics under it requires some careful placement and the use of a 5-lug strip. this should be pretty clean when its done and I will label the caps with half-circle, square, triangle and dash to match the 4-section can designations, also shown on the Sams schematic.

Typically, I solder all the film caps across the ceramics, then cut them out when trimming all the leads. It ensures I double check old and new values before removal.

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Dave

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Re: Motorola Stereophonic restoration at last
« Reply #7 on: June 26, 2017, 03:34:35 PM »
I started on the Preamplifier chassis right after the amp was done. Only the 10 mfd cap on the power supply rail going into the preamp was bad. The other 4 caps used for signal coupling looked like better grade than the ceramic discs in the amp, tested good and within tolerance, so I left them in place rather than replace them. Pictures coming.

Ill post a picture of how the power supply caps fit into a small corner on the amp. I may need to check the 3DG4 (like a 5U4 with lower max ratings) tube socket based on Will's experience with the SK16's rectifier tube socket. When 3 amps of filament current needs to flow through tube pins, trouble can happen.

The amp chassis also has a 3-amp resettable circuit breaker for line protection, which I left in place. I did however install two 2.7 ohm 10 watt resistors to drop the 124 VAC line voltage we have down to the design value of 115. I did this on three of my Magnavox tube amps and added 2-amp line fuses, as there was no primary over-current protection on the power transformer. :o
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Dave

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Re: Motorola Stereophonic restoration at last
« Reply #8 on: June 28, 2017, 09:06:06 PM »
In both the pre-amp and the multiplex chassis, only the B+ lines from the amp to the pre-amp and another from tuner to mpx chassis have 20 and 50 mf electro capacitors, one of which is for the only transistor at the mpx input stage, supplying a low collector voltage  :)
Both that oem Motorola PNP-Ge transistor and the phase diodes (marked Toshiba)showed 0.2 volts bias drop using the Fluke's diode setting.

Coupling caps are the same early film types in the MPX chassis as the pre-amp, look like a better grade of original. Much better in these low-level stages than a ceramic type.

I attempted to upload pics also of the reverb lamps and foam replacement but lost the whole post. Now I am behind a few pics. :(
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Dave

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Re: Motorola Stereophonic restoration at last
« Reply #9 on: June 29, 2017, 04:16:47 PM »
I am now working on the tuner. An electrical contact cleaner, that leaves a lubricating residue was used on the selector pushbuttons. After many operations, each now stays down until another is pushed. 

Also, the Motorola-branded electrolytics and multi-section cans have failed tests in the mpx-tuner and preamp sub-chassis. I was curious how these tested on my EICO 950B as I never check them. I normally just replace capacitors I find.

Ill attempt some pictures here.

 
The vibrasonic unit and lamps needed only some light cleaning and the damping foam blocks replaced with A/C bug strip foam.

The preamp underneath and the layout diagram

The mpx adapter underneath showing only one suspect cap
 

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Dave

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Re: Motorola Stereophonic restoration at last
« Reply #10 on: June 29, 2017, 06:20:35 PM »
Dave, What is the purpose of the "compression lamps"?
-- Aloha, Ken

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TC Chris

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Re: Motorola Stereophonic restoration at last
« Reply #11 on: June 29, 2017, 08:58:08 PM »
Dave, What is the purpose of the "compression lamps"?

I was wondering the same thing--maybe some sort of variable resistance?  I was impressed by the diagram showing the tube key position.  How many times have we all tried to plug a miniature tube in, reaching blindly and trying to make the @#$%&*!! thing fit the socket. 

Chris Campbell

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Re: Motorola Stereophonic restoration at last
« Reply #12 on: June 29, 2017, 09:57:35 PM »
Yes, getting the tubes right is pretty miserable unless tuner-preamp is taken out. Its the worst kind of tube socket' the wafer type. 
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Dave

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Re: Motorola Stereophonic restoration at last
« Reply #13 on: June 30, 2017, 10:30:46 AM »
Dave, What is the purpose of the "compression lamps"?

I will hazard a guess, since many reverb circuits do not have them. A particularly-high impulse may overload or otherwise saturate the primary coil and the lamps dissipate this energy in a manner unlike a resistor.
The lamps measure 12 ohms, operate on 12 volts and are located in the low-impedance primary coil of the reverb "tank or can", making this an intuitive guess at best.

Someone knowledgeable in organ circuits can probably add something.
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Dave

TC Chris

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Re: Motorola Stereophonic restoration at last
« Reply #14 on: June 30, 2017, 06:13:22 PM »
I think you've got it.  Bulbs have low resistance cold and high when hot.  So at low voltage and current they don't do much.  When they are conducting enough to warm up the filament, the resistance rises to limit current.

Chris Campbell