Author Topic: Motorola Stereophonic restoration at last  (Read 1969 times)

Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #15 on: July 12, 2017, 12:46:22 PM »
All four chassis' are on the floor and connected together behind the cabinet for the past two weeks. As I decide how to add supplemental lighting for the controls, checking for overall performance on power-up before bolting everything back in seemed a good idea. I'm glad I did. The first issue was with tube sockets.

At first power up, there was no sound from one of the channels and the other would not do much until I touched the phono input, with hum from center and left channels. It was then I noticed one of the IF amp tubes was not lit. Even though tube sockets were cleaned and re-tensioned before re-inserting all 22 tubes, a few of the molded sockets were not making good contact. I would say this may be a common issue with Motorola based on some other observations on this forum and my own experience. After a second cleaning, the tuner began working on AM and FM, minus sound from one channel. 

What I found was adjacent terminals on a 12AX7 in a phenolic wafer socket, which uses the wide versus the skinny terminals on the 7/9 pin molded sockets, were touching and shorting grid to cathode. Removing the short was easy, seeing it was not but wiggling the right tube and then hearing LOUD crackles is the only way I found it. Wiggling of tubes in 8-pin octal sockets for 6V6 outputs including the 3DG4 rectifier turned up no issues. 

Still not done, the stereo neon lamp indicator was not lighting on a strong local station and when I pushed fm-stereo, one channel dropped out again. I worked the selector push buttons without change, that eliminated the switch as a cause. It turned out the cause was loose RCA plug socket on the HS-996 mpx chassis.

Next, after getting both channels and bass booming on a clear FM station, I decided to test the reverb (vibrasonic). After hearing little effect turning up the control, I decided to check the third tube on the pre-amp chassis, labeled the vibrasonic amp. Funny thing but the Sams schematic for HS-1051A called it out as a 12AU7 but the tube label on the preamp shows 12AX7. A similar issue was the case on the tuner chassis for the first audio after the ratio detector and AM detector. Being that pre-amp chassis is HS1051B, tuner chassis is a -B also, later versions reworked for use with the FM multiplex HS-996 chassis, tube types were changed. The tube chart on the chassis is nearly ALWAYS correct, so I swapped two 12AX7s in for the two 12AU7's which were original Motorola 188(GE) and 322(tung-sol) coded tubes.

First, the tuner output level increased and second, the vibrasonic effect now became apparent. Normally a low setting like 2 or 3 is plenty on some models, but this one has a moderate effect even turned up to 10.  What was really fun  ;D was cranking up the volume to 6 ro 7 and seeing the compression lamps on the reverb chassis begin to glow orange in response to the bass notes!

The only chassis that had no post-recapping/contact cleaning issues was the amplifier/power supply. The 6V6's are OEM Motorola, with a Tung-Sol 322 code. Even though these all tested marginally good on a simple emission tube tester, replacement with some Russian-made tube could really turn this thing loose. 8)

Working through any High Fidelity rig like this is much easier after eliminating capacitors as a potential suspect. Tubes are eternal suspects though. ::)
Tubes - Magical - Tubes

Dave

TC Chris

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Re: Motorola Stereophonic restoration at last
« Reply #16 on: July 12, 2017, 09:26:59 PM »
Isn't the major difference between the 12AX7 and 12AU7 its gain curve?  I don't have my tube manual at hand now.

Thanks for the lessons from experience.

Chris Campbell

Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #17 on: July 13, 2017, 10:01:30 AM »
Yes indeed, its the 12AX7/6AV6 amplification factor is 100 in the RCA RC-30 tube manual. A 12AU7, 12AT7, etc are more general use triodes with factors of 19 and 60, not really optimized for audio.

The ratings for the 12AX7 differ significantly from the 12AU7, also really evident in the much higher plate resistance. I guess the fact the basing diagram is the same is just a coincidence:) Unfortunately - simple tube testers like mine cannot tell much difference, so a Hickok is worth the extra dough. I prefer to test these kinds of tubes in the set itself.
Tubes - Magical - Tubes

Dave

Motorola Minion

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Re: Motorola Stereophonic restoration at last
« Reply #18 on: July 17, 2017, 04:13:37 PM »
The record player was very dirty like the remainder of the console. instead of Isopropyl alcohol (IPA), I used lacquer thinner to remove the dirt encrusted in the grease. An aggressive approach was needed to remove the gunk, more so than most Ive seen. 

Fortunately the idler pulley was in good condition :D, so I proceeded to set up the speed control and found no other adjustments were needed. The player still has its original Sonotone 9t cartridge and stylus, which looked OK. After playing a worn record and then a good one, I determined the stylus is OK. I have a replacement in case. I also changed the foam rubber damping blocks inside the springs.

After dropping the changer back in the cabinet, with the components connected but out and on the floor, I was impressed by how well it worked initially and by how well it did not feed-back as it was cranked up. Unlike Magnavox changers, which regain all function without much work, I usually need to play around with Voice-of-Music units like this one.

The next few posts will be about the awesome tuner, improving the overall lighting and re-assembly of all parts back into the cabinet.
Tubes - Magical - Tubes

Dave