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 Post subject: Magnavox A531-01-00 amplifier
PostPosted: Mar Thu 12, 2020 10:18 pm 
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Location: Warner Robins, GA
I have a Magnavox A531-01-00 amplifier that I forgot I had and would like to get it working again just to see how it sounds.

I do not have a schematic handy.

Does anyone have just the schematic?


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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Fri 13, 2020 3:49 pm 
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Location: Rochester NY USA
Sams 669-8, don't have it, though I could get a copy next week.

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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Fri 13, 2020 6:07 pm 
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Location: Warner Robins, GA
Thanks for your assistance.

I found the SAMS

https://www.samswebsite.com/en/photofac ... msid/669_8

I went ahead and bought the sams since it was only $15.

When I first got the amp I did recap it. I borrowed the rectifiers for something else though, but I should have some to replace them with.

It basically involves replacing the diodes, powering the amp up, connecting a pair of speakers, connecting a source (cellphone as it has a volume control) and seeing how it sounds.

In a way it reminds me of the amp circuit Sylvania used in the RM-300X so I suppose this amp should sound just as good only it will have a bit more power.

One thing I did back then was to disconnect the balance control as I saw no point in it.

Never did quite understand why Magnavox often put the balance control on the amp chassis unless its because they expected it to be something that rarely if ever needed adjustment when set to the desired position.

I will be replacing the carbon comp resistors though as my experience has been that most I've come across have been out of tolerance or very close to being out of tolerance.

The electrolytic caps I'll see how the amp performs then decide if I want to upgrade any of them. I used larger values initially as back then I thought that would be better to keep the voltage more stable under load, but I might rethink that idea and replace some with lower values if not the stock value. I also noticed four black caps I did not replace which I assume are electrolytics. Those will be replaced as well.

What's nice is the sams also has a phono stage schematic so if I ever wanted to make this to work with a ceramic cartridge record changer I can do so.


Last edited by Tube Radio on Mar Sat 14, 2020 8:26 pm, edited 1 time in total.

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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Fri 13, 2020 6:12 pm 
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Joined: Sep Tue 15, 2015 1:16 am
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Location: 18424 PA
Phase shift compared to what? Hope you are using full range speakers cause you have phase shift in your crossovers.


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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Fri 13, 2020 6:43 pm 
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Location: Warner Robins, GA
The phase shift is explained here.

https://www.antiqueradios.com/forums/vi ... 2&t=372298

Its more of getting the phase relationship between the mains and powered sub to be the same versus absolute zero phase shift.

Here's the schematic.

Attachment:
Magnavox A531-01-00 amp.jpg
Magnavox A531-01-00 amp.jpg [ 314.32 KiB | Viewed 873 times ]


Here's a couple pictures.

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Attachment:
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I tested the amp on a couple old two way car audio speakers I had laying around and the amp sounds quite good as is so I may leave the capacitor values alone and just replace the carbon comp resistors.

So without the balance control in the circuit both 33 ohm resistors need to be reduced to 24.81 ohms which will round up to 25 ohms.

I would then put the emitter bypass cap across the single series resistor.

I also plan on adding a pair of 1/4" jacks for the speakers and either a stereo 1/4" jack for the input or RCA jacks. The current RCA jacks are too close together for most any quality RCA cable to properly fit without the two plugs touching each other to the point where one is hard to insert.

So far I am well pleased with the amp and am really surprised with how good it sounds.

I have two 8 ohm 30 watt non-inductive resistors I'll use for a load when testing the amp.

I did notice that the B+ does vary a few volts as the amp is played loud.

Normally that would mean I need to upgrade the power transformer to one with more current, but that's only if the amp is going to be used full range. If used with a powered sub and active crossover, tghe stock power transformer will be just fine.

EDIT:

Here's a couple pictures showing the jacks and volume control.

Attachment:
20200313_215857.jpg
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When I went to wire the power plug I grabbed some of the original wire and found it was solid. So I used more of the wire for the other additions to keep it more original looking underneath.

I kept the original speaker terminals as they were a convenient place to wire in the 1/4" jacks plus it makes the jacks optional.

I'll eventually clean the amp up and I'll either clean the metal transistor shield and paint it satin black or I'll completely remove it.

I'll also make a metal bottom cover for the amp and put some stick on feet on it.

EDIT:

I connected one of my altec 811B horns to one channel with no signal going to the amp and I heard absolutely no hiss. I played some audio at a low volume to be sure the horn was connected and I heard audio from it so I know the amp really is that quiet.

Unfortunately there's people out there who would trash an amp like this in favor of a better name of 25WPC amp that may have a little hiss and they will do it because of the name on the amp.

That's fine. I'll take these magnavox amps and other quality amps that a lot of audiophiles thumb their noses up at and I'll get the same if not better sound quality.


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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Sat 14, 2020 8:51 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
Interestingly enough while looking for something else I found the tone/volume/balance control board I had pulled from a defunct Samsui 4000 (wasn't worth restoring) several years ago.

If so desired I could put it in its own box and use that with the amp.

Years ago when I first got the amp working, the first stage of this amp confused me due to the small value of input coupling cap used so I used a 100uF cap as that's what I had on hand.

I now know that some feedback configurations with a transistor actually cause the input impedance of the transistor to be much higher so I will reduced that cap value considerably.

Now it drives a 4 ohm load fine, but I'm only getting 8.41 watts at 400Hz which I know cannot be right, although at full power both channels driven right before the point of distortion as seen on a scope with a 4 ohm load on both channels at 400Hz the B- drops to 29.33Vdc.

The normal no signal B- is 37.54Vdc.

So a power transformer upgrade may be necessary like I have done in the past.

Depends on if I have one handy.

Either needs to be 60 Vrms CT to use the stock full wave rectifier scheme or needs to be 30Vrms and a bridge rectifier will then be used.

The stock one is 56Vrms CT according to the schematic, but it is much easier to find a 60Vrms CT or a 30Vrms transformer.

EDIT:

Found a transformer from a defunct soldering/desoldering station that looks like it can do several amps. It puts out 28.2Vac so that will work fine.


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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Sun 15, 2020 10:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20612
Location: Warner Robins, GA
Well I've ran into a potential deal breaker.

As I increase the frequency to 16KHz I noticed the output dropping on my scope. I'm not sure where it happens exactly but if I compare 8KHz to 16KHz it is very noticeable.
I tried disconnecting the ceramic disc caps thinking they were the problem, but no change.

I did notice, however that at 16KHz the waveform between the primary of the driver transformer and ground looked quite bad with the cap in series with the 10K resistor was disconnected. Reconnecting that cap made the waveform much better.

EDIT: (pictures are upside down here, but once the picture is clicked it is right side up on the page you're taken to)

Here's some waveforms.

The top waveform is the output across a 4 ohm load.

The bottom is the signal at the collector of the driver.

Looks like I have a 180 degree phase shift which I don't think is right and could just be the scope.
Attachment:
500Hz.jpg
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1KHz.jpg
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2KHz.jpg
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4KHz.jpg
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Attachment:
8KHz.jpg
8KHz.jpg [ 314.73 KiB | Viewed 778 times ]


Attachment:
16KHz.jpg
16KHz.jpg [ 317.47 KiB | Viewed 778 times ]


Now I disconnected the load from that channel and it became readily apparent what those ceramic disc caps are for.

They keep the amp from breaking into oscillation when no speakers are connected.

I also discovered that with no load, the amp is pretty much flat to 20KHz without the cap across the 1K resistor in the feedback circuit connected.

If I connect that cap the output is only down slightly at 16KHz.

I still get the increased signal at the collector of the driver transistor which might be due to the .001uF cap between the collector and ground of the emitter follower.

That both helps stop the amp from breaking into oscillation and it may also boost the treble some to compensate for the other three disc caps which keeps the amp stable.

Now I don't quite understand why the upper treble is reduced when I connect a 4 ohm load to it.

May take the amp to work tomorrow where I have better test equipment so I can do a proper analysis of the amp as all I can do at home is guess at the waveform amplitudes plus I'm limited to an audio generator on my computer.

Plus I can see how the amp works on a power supply capable of 3 amps to see what if any effect the current capability of the power supply has on the amp.

I may also add a .1uF ceramic disc cap across all filter caps.

EDIT:

I definitely found some close to being out and out of tolerance resistors.

I've replaced what I could with what I have.

The only resistors I haven't replaced are these as I don't currently have them.


.47 1W QTY 4
47 1W QTY 1
680 1W QTY 1

Measuring at the - end of the speaker coupling caps I get the following voltages. Channels will be labeled top and bottom corresponding with the schematic location of the two channels.

No signal applied
TOP -18.0614Vdc
BOTTOM -17.4952Vdc

Full signal right before clipping 400Hz into 4 ohm load both channels driven

TOP -17.880Vdc
BOTTOM -16.4986Vdc

I suspect that one of the .47 ohm resistors are out of tolerance causing more of a voltage drop.

I did make sure that when I removed the output transistors (to clean the amp and replace the insulators) I put each transistor back in its original location as I figured they could have been matched so I didn't want to take the chance of making things worse.

The only thing I don't like about the amp is there's no adjustment for the bias.

I'm running the amp at 38Vdc B+ from an external power supply.

Now my Lafayette LA-375 amp says to set the voltage at the speaker coupling cap to 1/2 the B+ so I figure the same should apply to this amp given the output stage is similar in that it takes the output from the junction of an emitter and collector of a pair of transistors. Of course I could measure my Sylvania RM-300X radio and see what voltage is at the + end of the speaker coupling cap since its output stage is the same type as the Magnavox amp.

I'll measure the .47 ohm resistors tomorrow and see what their values actually are.


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 Post subject: Re: Magnavox A531-01-00 amplifier
PostPosted: Mar Mon 23, 2020 3:33 am 
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Posts: 20612
Location: Warner Robins, GA
I have a major update.

I decided to post about this amp in a solid state amp builders group on Facebook and was advised to switch to silicon transistors.

After much help, work and transistor matching (painstaking process), I finally got it done.

I am extremely well pleased with the results so far.

I still have a ways to go in getting the amp finished.

I figure this will be good for those who have Magnavox Astro Sonic consoles with this particular amp or any console that uses a similar style of amp with the same output transistors as they will have a proper option for transistor replacement.

Attachment:
Magnavox A531-01-00 amp (modified adjustible bias 2).jpg
Magnavox A531-01-00 amp (modified adjustible bias 2).jpg [ 342.61 KiB | Viewed 664 times ]


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