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 Post subject: Motorola SP29E Portable center channel static
PostPosted: Aug Mon 30, 2021 7:31 pm 
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Joined: May Sun 16, 2021 1:18 pm
Posts: 28
I have been working on a very nice Motorola SP29E portable that has a very inconsistent static noise coming from the center channel. The unit has been recapped and the changer restored but this static issue is troubling. The unit plays well and sounds great until this static appears. It comes/goes with varying degrees of intensity, with no particular pattern except there appears to be a static "dissipation" when the unit shuts down.
I've tested other changers and speakers to eliminate these items as the culprit, but the static is still present.
Any help on where to look would be greatly appreciated.


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Photofact-596-7-1081100 for SP29E suitcase player.pdf [4.13 MiB]
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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Aug Mon 30, 2021 8:47 pm 
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Joined: Jan Wed 16, 2013 12:04 am
Posts: 2542
Location: 77001
thermofin wrote:
I have been working on a very nice Motorola SP29E portable that has a very inconsistent static noise coming from the center channel. The unit has been recapped and the changer restored but this static issue is troubling. The unit plays well and sounds great until this static appears. It comes/goes with varying degrees of intensity, with no particular pattern except there appears to be a static "dissipation" when the unit shuts down.
I've tested other changers and speakers to eliminate these items as the culprit, but the static is still present.
Any help on where to look would be greatly appreciated.

1. Clean the vacuum tube pins until shiny

2. Clean and crimp the tube sockets.

See Edit of this in next post.
"Don't use a product such as De-Oxit directly on
the phenolic socket plastic, as it may degrade."

3. Using a non-conductive probe, such as a wood chopstick,
power up the amp and see if you can find a cold-solder
joint. Use dangerous high-voltage safety rules.

4. Have you tested the tubes ?
4.a. Are resistors WNL ?

5. Clean amplifier wire connectors/plugs.

6. Is the static independent of the tonearm
playing on a record ?

Hotwax


Last edited by Hotwax on Aug Mon 30, 2021 11:49 pm, edited 1 time in total.

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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Aug Mon 30, 2021 9:49 pm 
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Joined: May Sun 16, 2021 1:18 pm
Posts: 28
The static stops when the tone arm is off the record.
I'll start with the tube pins, connectors and crimp the sockets. I had thought the problem was resolved when I replaced the 12ax7A since the static seemed to be gone. After about 30-40 min it started again slowly and randomly
I may have used de-oxit on the sockets, could this have ruined one or more of them? This is good advice, I wasn't aware it could damage the phenolic.
Just to review, the tapping will cause noise when there is a cold joint?


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Aug Mon 30, 2021 11:46 pm 
Member

Joined: Jan Wed 16, 2013 12:04 am
Posts: 2542
Location: 77001
1. Check tonearm wire connections and clean
connectors to cartridge pins.
If tonearm connections to the amp input are
RCA cables, clean those connectors.

If tonearm wires to amp are soldered on a terminal
strip, you can heat and reflow the solder points.

2. I believe I incorrectly stated that Deoxit
may degrade phenolic plastic.
My reading is that It is a potential problem with dual phenolic wafer
switches. In that its residue may promote a conductive
pathway b/t the 2 layers.
Caig says their products are safe for plastics.
Someone else may have more information.

3. Yes, wood chopstick probing on solder joints
will induce noise on a bad joint.
Hotwax


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Aug Tue 31, 2021 1:03 am 
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Joined: May Sun 16, 2021 1:18 pm
Posts: 28
I cleaned the tube pins and the four 6BM8 tubes had a dark residue on them that I removed. I pinched the pin tabs in and cleaned the RCA jacks to the changer. Interestingly enough the static seems to be worse and now is steady vs intermittent.
The static appears to be associated only with the center bass channel and when I disconnect the speaker, the rt/lft have no static. I did attach another speaker and the static was still present.
My next step is to try the tap test to see if I have a cold solder. I have to believe that the pinching of the tabs likely caused the issue to worsen by touching or moving a connection.
Can a resistor cause an issue like this? Since this appears to be associated with the center channel, are there any suggestions on where to start the search?
I don't believe the problem is with the cartridge since I have attached another changer with similar results.
I mentioned that when the unit shuts off, there is a distinct strong static sound almost like a discharge of some sort (on the center only). Any thoughts what would cause this?


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Wed 01, 2021 10:25 pm 
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Joined: May Sun 16, 2021 1:18 pm
Posts: 28
I suspect the problem with this unit is the K3 "couplate" that is associated with the center channel. Can anyone lend some advice on how to check this component and determine which lead is which? I'm struggling with the schematic and the 7 leads are only identified as 1-7.
I would plan to replace these resistors and caps individually but I'm struggling with identification.


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Thu 02, 2021 1:18 am 
Member

Joined: Jan Wed 16, 2013 12:04 am
Posts: 2542
Location: 77001
thermofin wrote:
I suspect the problem with this unit is the K3 "couplate" that is associated with the center channel. Can anyone lend some advice on how to check this component and determine which lead is which? I'm struggling with the schematic and the 7 leads are only identified as 1-7.
I would plan to replace these resistors and caps individually but I'm struggling with identification.

Yes, couplates can be bad actors.
It won't be possible to ID each part of the
couplate, since they are inside it.

But, the schematic gives you all the info you need to
create this resistor/capacitor network.
Then connect each section of the network per
the schematic.

1. Start by identifying the wire connections
that exist from the couplate to the plates and grids of the
two 6BM8 tubes V3 V4.
Both triode and pentode sections. The tube pin numbers give
you that information.

ID the couplate wire that connects to a B- point,
and the 195 Volt source at the C3(D) 40mF capacitor.
This is also on the schematic.

Then I would imagine how much space
will be there, when the couplate is cut out of circuit.
Gather the parts you need, and
think how you could place terminal strips
to connect the components.

Or, some folks use plastic perforated board
to construct the network.

Take your time and repeat observe the wiring
until you see mentally what's going on.
Hotwax


Attachments:
1-Center Channel K3 Couplate.png
1-Center Channel K3 Couplate.png [ 400.71 KiB | Viewed 408 times ]
1-Electrolytics.png
1-Electrolytics.png [ 347.1 KiB | Viewed 408 times ]
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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Thu 02, 2021 2:35 am 
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Joined: May Sun 16, 2021 1:18 pm
Posts: 28
I struggle with schematics, so this has been challenging.
V3 and V4 have 9 pins and 10 terminals which was making it very confusing to initially identify the pin #’s. I’m confused as to what the 10th terminal is used for. With that being said, let me know if my identification below is accurate.
The couplate has 7 leads, #3 is the ground and I believe #5 is a 220K resistor connected to the 10th terminal on V4

V3
-Pin 9 is connected to the .02 MF cap LEAD #4
-Pin 3 is connected to the 680K resistor LEAD #1

V4
-Pin 1 is connected to the 1meg resistor LEAD #2
-Pin 3 is connected to the 680K resistor LEAD #7
-Pin 9 is connected to the .01 MF cap LEAD #6

I’m not sure how the 600K resistor comes into play. I also don’t understand the schematic listing the 680K 3 times and the 220K 2 times.


Attachments:
couplate ID.jpg
couplate ID.jpg [ 684.2 KiB | Viewed 405 times ]
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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Thu 02, 2021 4:34 am 
Member

Joined: Jan Wed 16, 2013 12:04 am
Posts: 2542
Location: 77001
The chassis looks very crowded unfortunately.

1. RE: resistors.
Remember that all these caps & resistors are
internally connected in a network within the couplate.
The 600K, 680Ks, and 220Ks are inside the
couplate.

2. The 7 leads off of the couplet correspond to
their connections on the tube sockets, the B- point,
and the electrolytic cap.
It appears you have located these points correctly.

3. The chassis is too crowded for me to see the
"10th terminal" you mention.
I'm not familiar with such a configuration.

A 220K resistor in the couplet connects to Pin 9
of the tube socket on each of the 6BM8s.
Check if there is continuity b/t the "10th terminal"
and pin 9 socket terminal of 6BM8.

Hotwax


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Thu 02, 2021 3:06 pm 
Member

Joined: May Sun 16, 2021 1:18 pm
Posts: 28
The 10th terminal can be seen in the photo and can also be identified by the pinched socket since no pin runs through it. There is continuity on V4, pins 9 and 10 with a reading of 227K. This is a SP29E and does not have the V6 12AX7A tube. All the tube sockets have this 10th terminal.
I have taken readings on the couplate while it is still in circuit and all the leads are giving both OHM and MF readings.
I would like to replace all the leads with individual components but really have no idea which components to connect to the pin terminals. If someone could help determine what to replace each lead with, it would be very helpful. I feel there is enough space in the chassis for multiple replacement components.
Below is a list of the leads and which pins they are connected to.
This is over my head!


LEAD Socket PIN
1 V3 3
2 V4 1
3 GROUND
4 V3 9
5 V4 10
6 V4 9
7 V4 3


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 Post subject: Re: Motorola SP29E Portable center channel static
PostPosted: Sep Thu 02, 2021 5:32 pm 
Member

Joined: May Sun 16, 2021 1:18 pm
Posts: 28
On another note, I would be interested in paying someone to build this couplate replacement.


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