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 Post subject: Motorola wafer?
PostPosted: Jan Wed 23, 2019 12:09 am 
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Location: Wayne, NJ
Hi all,

Does anyone know of a way to replace this K3 "wafer"cap? Got a slightly weak left channel on this Motorola HS-899 three channel amp. Started testing with the preamp and signal is good into the volume control, but starts to fall as it goes into the 12AX7A and then remains steadily weaker......


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 12:38 am 
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Location: Near Brandon, Iowa
That's called a "couplate", an early attempt at reducing assembly cost by aggregating parts, sort of like a passive integrated circuit. You can make your own from scratch using discrete components mounted on
a piece of "anyboard" cut to size. It's all audio frequency so layout is not critical.


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 2:04 am 
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lorenz200w wrote:
You can make your own from scratch using discrete components mounted on
a piece of "anyboard" cut to size.


Jeez! I wouldn't know where to start!

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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 2:11 am 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 4178
Location: USA
addyboy wrote:
Jeez! I wouldn't know where to start!

Think of it as an improvised circuit board. You cut a small perforated circuit board or poke holes on a stiff piece of plastic sheet. Mount/solder the components (4 capacitors, 2 resistors) together on to the board based on the schematic.

Are you certain this couplate is the problem with your set? This looks like something that changes tone as you change volume.


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 4:32 am 
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Location: Mountains of Mourne. Ireland.
Four capacitors - two resistors. (use 1/2 watt resistors).
Image
100mmf equals 100pF
50mmf equals 50pF
0.0025μF equals 2500pF
0.008μF equals 8000pF

Image
image: Philip Nelson

Image
image: Philip Nelson

From→ Replacing the Vertical Stage Couplates. (Predicta Television).
https://www.antiqueradio.org/PhilcoH341 ... vision.htm


Capacitor uF - nF - pF Conversion Chart
https://www.justradios.com/uFnFpF.html


Greg.

oh... your Couplates may be OK... :)


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 5:31 am 
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Joined: Dec Wed 25, 2013 7:57 am
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Location: USA
Don't kill the couplate yet. May not be the cause of low volume.


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 6:04 am 
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Posts: 28437
Location: Detroit, MI USA
I'd be very surprised if that was the problem, but it's not impossible. Couplates in that type of circuit are very reliable.

As already demonstrated, they are very easy to duplicate when you know all the parts values.

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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 3:25 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2096
Location: Lafayette, CO
Why not disconnect it entirely? Then see if the audio fades. Craig


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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 4:10 pm 
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Posts: 555
Location: Wayne, NJ
egg wrote:
Four capacitors - two resistors. (use 1/2 watt resistors).
Capacitor uF - nF - pF Conversion Chart
https://www.justradios.com/uFnFpF.html
Greg.
oh... your Couplates may be OK... :)



Wow! Thanks!

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 Post subject: Re: Motorola wafer?
PostPosted: Jan Wed 23, 2019 4:13 pm 
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AJJ wrote:
Don't kill the couplate yet. May not be the cause of low volume.


No, not yet.
Was figuring out my options, going to disconnect, bypass, bass treble section to see if thats dragging down the audio.

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Sometimes discussing the wrong path leads you down the right one.....


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