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 Post subject: Motorola HS-544 Schematic?
PostPosted: Sep Tue 29, 2009 6:35 pm 
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Location: Richmond, Virginia
Hi,

I am working on an amp from Motorola phonograph 67 HFK2. The amp chassis is HS-544.

If anyone has a scan of this schematic that they could email me it, it would be a great help. I believe it is in Photofacts 345, folder 8.


EDIT: I found a schem at http://techpreservation.dyndns.org/schematics/Motorola.htm


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 Post subject:
PostPosted: Sep Wed 30, 2009 2:51 pm 
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Location: Berkley, Michigan
That is a nice console. The output transformer is almost as big as the power transformer.

I have a '57 version that is almost identical. It shakes the floor joists.

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 Post subject:
PostPosted: Sep Wed 30, 2009 7:41 pm 
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Doug,

Thanks for the reply. Yes, it has a huge OT! I have not yet fired up this one but I hope to shake some floor joists, too.

One thing about the schem that I questioned was the position of the volume control at the input, before the grid of the 6AU6. I was thinking of moving it to a position after the tone controls , replacing the 470k resistor that's before the grid of the driver tube. Do you think that is a good idea? I would like to have the full signal hitting the first two triodes.

Richard


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 Post subject:
PostPosted: Sep Wed 30, 2009 8:26 pm 
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Location: Albuquerque, NM 87123
I believe the correct Sams would be 349-6.


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 Post subject:
PostPosted: Oct Thu 01, 2009 2:03 am 
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RWood wrote:
...One thing about the schem that I questioned was the position of the volume control at the input, before the grid of the 6AU6. I was thinking of moving it to a position after the tone controls , replacing the 470k resistor that's before the grid of the driver tube. Do you think that is a good idea? I would like to have the full signal hitting the first two triodes.
Richard

The four voltage amp stages give this amp a lot of gain. Running the input stage wide open could bring up the hum and noise level quite a bit. I learned not to second guess the pros a long time ago. I've modified amplifiers like this only to find that when I compare a modified one to an original in top shape, the original is hard to beat.

I experimented with replacing the Jensen P15R woofer with a newer and much better C15NF. The original had a medium size alloy magnet and paper surround. The replacement had a heavy ceramic magnet, Flexair surround and a much bigger voice coil. The result was less than spectacular. The original speaker sounded better all the way around.

One thing that really did make a huge change for the better was replacing the 5 mfd non-polarized crossover cap with a Jensen A20 2-way crossover network. It really cleaned up the high end and brought it to life.

Image
Image

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That warm tube sound can usually be overcome by turning up the treble.


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 Post subject:
PostPosted: Oct Thu 01, 2009 4:11 am 
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Location: Richmond, Virginia
Thanks for the tips - I'll try it stock.

Beautiful set - yours looks like new!


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 Post subject:
PostPosted: Oct Fri 02, 2009 4:40 am 
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Here is the one I am working on :shock:

Image

Image

Image


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 Post subject:
PostPosted: Oct Fri 02, 2009 9:53 am 
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Location: Berkley, Michigan
Now I understand your question about the position of the volume control on the grid of the first AF amp. Your interest in this amplifier is about how it distorts. Mine is about how it does not.

Musical instrument amplifiers tend to have a narrower bandwidth. They tend to be designed for power and a specific instrument being played back on them and also for being moved around from stage to stage.

High fidelity phonograph amplifiers like this one were designed to have the widest possible bandwidth to reproduce recorded music accurately, everything from a double bass violin to the sleigh bells, Bach to Humble Pie. The amp is not supposed to add anything to the music or take anything away.

For your purposes, things like the loudness compensation network on the volume control and the frequency compensating network on the phono input circuit become unnecessary, even the huge output transformer becomes unnecessary unless you intend to distort a low frequency instrument.

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 Post subject:
PostPosted: Oct Fri 02, 2009 5:15 pm 
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Location: Baltimore, MD, USA
I have built that amplifier from scratch before. That circuit is in various Motorola models. The hard part is the Compentrol. I have only one and it is dual (stereo). If one can find a five-lug volume control, duplicating the Compentrol Couplate is not hard.

I intend to build another one of these amps very soon. I have the parts (sans Compentrol) and have bought the chassis also. They sound very good. I suspect about 10 watts RMS. Good on ya'!

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 Post subject:
PostPosted: Oct Sat 03, 2009 2:09 am 
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I see that the speaker is not a PM speaker. Did you provide B+ for it? I don't think that the Motorola amp used that type of speaker.

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 Post subject:
PostPosted: Oct Sat 03, 2009 6:44 pm 
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That is correct, it is a Quam "Electrodynamic" speaker, which I believe originally came from a Lowery organ. I swapped it out for one with a permanent magnet, so that I could test and listen to the amp.

If I were to use this speaker, would I interrupt the B+ line as with a choke, before the first filter cap?

Do they draw much current?


Richard


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 Post subject:
PostPosted: Oct Fri 09, 2009 2:41 am 
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RWood wrote:
...
If I were to use this speaker, would I interrupt the B+ line as with a choke, before the first filter cap?

Do they draw much current?


Richard


If I recall, the choke (EM) is usually between the first and second capacitor. It would usually contribute a lot to the filtering. You will see that many amplifiers that have an EM speaker have very little filtering done by capacitors.

I don't think that there would be much current draw done by the coil. However, I am not an engineer and the experts may want to contribute some facts here...

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