Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jun Tue 28, 2022 10:15 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 29 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Magnavox amp question
PostPosted: May Sat 21, 2022 8:18 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
I have the amplifier from the following Magnavox Astro Sonic console.

Attachment:
Magnavox model.jpg
Magnavox model.jpg [ 432.92 KiB | Viewed 1103 times ]


1RP621 is the model I think.

Here's the amp.

Attachment:
Chassis 1.jpg
Chassis 1.jpg [ 886.67 KiB | Viewed 1103 times ]


I'm looking for a schematic for the amp.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sat 21, 2022 10:01 pm 
Member
User avatar

Joined: Jan Tue 31, 2012 1:55 am
Posts: 20526
Location: Mountains of Mourne. Ireland.
The chassis number is on a paper tag attached to the frame/heat-sink near the three spade terminals, prefixed with the letter R.

Random example shown - - not yours. lol


Attachments:
x.png
x.png [ 224.42 KiB | Viewed 1087 times ]
Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sat 21, 2022 10:18 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Oh ok that will have to wait until I get the amp as the tag in the picture isn't clear enough.

EDIT:


Just messaged the seller and am seeing if he can give me the numbers on that tag before he ships it.

That way I can go ahead and order the electrolytic caps.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sun 22, 2022 2:07 am 
Member

Joined: Dec Mon 23, 2019 5:52 pm
Posts: 807
Going by looks alone, this one is in Photofact set number 1473, folder number 4.


Attachments:
Magnavox stereo amp 001.jpg
Magnavox stereo amp 001.jpg [ 708.2 KiB | Viewed 1060 times ]
Magnavox stereo amp 002.jpg
Magnavox stereo amp 002.jpg [ 671.53 KiB | Viewed 1060 times ]
Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sun 22, 2022 2:14 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Thanks.

Would that photofact also cover the 50 watt version of that amp?


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sun 22, 2022 2:58 am 
Member
User avatar

Joined: Jan Tue 31, 2012 1:55 am
Posts: 20526
Location: Mountains of Mourne. Ireland.
Set-1473 Folder-4 covers these 1975 chassis'
https://www.samswebsite.com/en/photofac ... h&s=Search

None of them are mentioned anywhere online.

The complete (hard to find) Set-1473 can be found on eBay for seven bucks.
https://www.ebay.com/itm/183896441638


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sun 22, 2022 3:07 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Thanks for that.

Went ahead and ordered it.

For the schematic I'll most likely take a photo of it and email it to myself.

That way if I make any changes I can put them on the schematic.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Mon 23, 2022 12:09 am 
Member

Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2168
Location: Great Bend KS
Be careful! I serviced a similar Magnavox amp like that and the main filter capacitor was positive ground!! The metal outside cylinder case was positive, and the replacement in the parts list was a typical negative ground!


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Mon 23, 2022 12:31 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Will be careful.

Gonna first power it up then measure voltages.

EDIT:

When I replace electrolytic capacitors I always look at how the stock caps were installed and ensure the new caps are put in the same way.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Mon 23, 2022 2:36 pm 
Member

Joined: Mar Tue 03, 2009 11:12 pm
Posts: 2168
Location: Great Bend KS
I meant the multi-section capacitor in the power supply. Somehow Magnavox got the supplier to make the capacitor with the can positive ground. But the Sams photofact supplied replacement part numbers with ordinary negative ground. Somebody at Sam's wasn't paying attention, and my boss at the time, replaced a cap in a Magnavox amp with a positive ground canned cap with a regular negative ground cap and he couldn't understand why the new cap was boiling hot!


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Mon 23, 2022 2:43 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
I had a Magnavox amp once with a B- supply which used PNP transistors.

Was real careful when I recapped it so that I got the capacitors connected properly.

Can't remember if it had can caps or if they were under the chassis.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Mon 23, 2022 10:02 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 29170
Location: Detroit, MI USA
Magnavox was one of several manufacturers back in those days that used some can type electrolytics with the can positive. If you aren't aware of that and not expecting it, disaster can result.

Someone else posted a few days ago about a Packard Bell unit with glaring errors in the Sams schematic regarding polarity of electrolytics. Apparently at the time one or more people drawing the Sams schematics and specifying replacement parts were not aware that those electrolytics with positive cans existed.

_________________
Dennis

Experience is what you gain when the results aren't what you were expecting.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Tue 24, 2022 3:48 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Photofact shipped yesterday.

Amp will ship today.

Will probably use this cap for the main filter cap.

https://www.mouser.com/ProductDetail/KE ... 2O5g%3D%3D

Other electrolytic caps will be upped in value as needed to reduce phase shift in the amp provided doing so doesn't negatively affect the amp.

EDIT:

If this amp can handle 4 ohm speakers I may look into getting another at some point and making a pair of monoblocks with a balanced input.

EDIT:

For the balanced input I may use an Edcor XSM15K/15K balanced/unbalanced to balanced/unbalanced transformer.

I'll install two 1/4" jacks on each amp with one wired as balanced and the other wired as unbalanced. I'll add a DPDT switch so that I can select between the balanced and unbalanced input. That way I can use the amp with both balanced and unbalanced sources relatively easily.

I could always go with an OP-AMP phase splitter, but that requires a +/- supply which the amp doesn't have that I know of.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Fri 27, 2022 2:13 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Not sure when the amp will be here.

An update yesterday morning said it had left the facility then the update right after said they didn't dispatch the trailer on time. So did it leave the facility or is it still there? Can't be both.

Checked several times today and same status.

EDIT:

Looks like the amp may be here tomorrow.

If so I'll do a basic once over, wire the power cord straight to the power transformer power it up and see how it sounds.

Tuesday I may take it to work and do further testing. I'll take an Edcor balanced/unbalanced to balanced/unbalanced transformer to work as well so I can test the amp as a monoblock with an 8 ohm load. Will only be 30 watts, but will give me some sort of idea if it can handle being run bridged.

If it indeed can then I'll see if I can find another identical amp.

Once the photofact comes in I'll see if it covers the 50 watt version. If so I'll order what parts are needed to mod the amp to the 50 watt version. I'll of course need a better power transformer, but I know an AnTek toroidal power transformer will fit the bill quite nicely.

Now if the amp can handle being bridged I'll eliminate the speaker coupling caps if there's 0Vdc between the + speaker connections. I'll then get some magnet wire and wind it on the power transformer so that I have a winding completely isolated from the amp's supply to power a speaker protection circuit. That way if there's DC on the speaker the relay will open.

EDIT:

The amp arrived today in good condition.

Packaging was one of the best I've ever seen. Told the seller that and he said he likes to use memory foam.

Also the amp model was on the PC board so I looked it up.

According to this topic

https://antiqueradios.com/forums/viewto ... 1&t=311619

The photofact I ordered based on what rcadanny identified the amp as indeed is the correct one and it also covers the 50 watt version of the amp.

So later today I'll fire the amp up and see what it does.

When the photofact comes in I'll order what's needed to make it the 50 watt version.

Aside for three wirewound resistors and two carbon film resistors, the amp uses carbon comp resistors so the carbon comp resistors will be replaced with 1% metal film resistors.

The two trimpots I'll most likely replace them with multi-turn trimpots as that will provide a more fine adjustment than the single turn trimpots.

Pretty sure I have something I can cut to make a metal base for the amp as the circuit board is completely exposed underneath. That will protect the board and keep it from shorting against anything plus it will act as shielding. I will however put some tape or other insulative coating on the side of the metal that faces the PC board to ensure nothing can short against the metal piece.

EDIT:

Amp powers up just fine.

The cap for the tuner B+ I had to disconnect as it's a 25 volt rated cap and has 51 volts across it given there's no tuner plugged up to the amp.

Will solder a couple RCA cables to the inputs and connect the speakers then see how the amp sounds.

I'm sure it will sound good, but once recapped will sound better.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: May Sat 28, 2022 3:49 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Amp sounds decent enough.

I know it will sound better with new electrolytic caps and resistors.

Measured the voltage between both + terminals of the output coupling caps and got around 160mVdc.

Once I upgrade to the 25 watt version and replace the resistors I'll measure it again.

If it's zero or a few millivolts I'll know both amp sections are balanced enough to where I can run it as a monoblock without output coupling caps.

However making it a monoblock will come later given I first have to find a second identical amp.

I'm surprised at how loud 15 watts is from the speakers I built.

Almost has me rethinking making it the 25 watt version.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: Jun Thu 02, 2022 2:47 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Brought the amp into work to test today.

It indeed has phase shift in the bass as expected.

Replacing the input coupling caps with 10uF caps took care of some of it.

Caps C9 & C10 on the board (not sure where they are in the circuit as the schematic isn't here yet, but should be arriving today) are 220uF. I had to parallel a 1,00uF and a 2,200uF cap to get zero phase shift to 20Hz measured before the speaker coupling cap. So I will need the next standard value above 3,42uF. Makes the amp take slightly longer to come up though which is fine as I plan on adding a DPDT relay and pair of 100 or 220 ohm resistors so that when I upgrade the output coupling caps to 15,000uF there won't be a pop in the speaker and the caps will not require as high of an initial charge current given the resistors. I figure a delay of 5 seconds will be enough. When I order the parts I'll look on mouser and see what time delay relays they have at a reasonable cost that can operate on about 60Vdc.

Also there was a little reduction in output as I went lower in frequency which could explain why the bass didn't seem to sound as good.

What I'll do is measure across the top output transistor emitter resistor as I adjust what I think is the idle current adjust. Will measure the channel that ran cool at no signal input first and will set the other channel to match it.

Once the schematic comes in I'll see what all is different between the 15WPC and 25WPC versions and will order the necessary parts.

Also as expected the power supply does drop nearly 10 volts with both channels driving a 4 ohm load. So that means I'll need a beefier power supply.

I did mount the amp to a wood base so what I may do is see if there's room to mount a toroidal transformer to the base and will use a bridge rectifier and at least a 10,000uF filter cap if not two. My preference is for amplifiers to have a very stout power supply. I also like for my amplifiers to be able to do full undistorted output with a sinewave input (whatever frequency puts the most load on the power supply and amp circuitry) into its rated impedance without failing when run for 8 hours. I'll then test the amp several days for 8 hours just to be sure.

What I may do is connect the amp to a 3 amp variable power supply and see how much current the amp actually draws and will compare power outputs between the stock supply and the regulated supply.

EDIT:

Here's the numbers.

Both channels driven at 400Hz into 4 ohms with output set to right before visible clipping.

47.3Vdc supply (highest the variable 0-50V supply goes before the crowbar circuit kicks in) at about 2 amps.

11.297Vrms 31.9 watts

Stock power supply. 53.4 Vdc unloaded 41.6Vdc fully loaded.

23.76 watts.

Heatsink does get quite warm.

I however do not have two 8 ohm loads so I could only test one channel at 8 ohms.

I at least know the amp can handle a 4 ohm load, however I at least with the stock circuit and an upgraded power supply I would not run it at high volume levels with a 4 ohm load due to the transistors tending to get real warm and possibly needing a better heatsink, however I will try the 4 ohm load test once I upgrade to the 25 WPC version and see how well it does.

The amp is flat to 20KHz though.

I checked the idle current of the channel I did not have to adjust and across the emitter and collector resistors on that channel's output transistors (there's a test point at the emitter of one transistor and collector of the other transistor so I assume that's where idle current is measured) and measured 20mV. So I set the other channel to match that. Not sure it's right, but they are at least matched and before the adjustment the amp did seem to sound good.

I did notice a slight difference in output between both channels. Not sure if it was the volume control I added (if so I'll get a quality volume control that is better matched) or drifted carbon comp resistors.

More than likely I'll see what resistors I have and start changing them.

Only real gripe about the amp is the component leads were not cut off properly on the solder side of the board. Maybe it was easier not to cut them off or there was no need since the amp board would be sitting on a wood console bottom or maybe it was done to provide an easy way to connect test leads for troubleshooting.

EDIT:

Concerning the idea about making the amp into a monoblock I don't think I will do that.

1. The amp wasn't designed for it.
2. I wouldn't use the coupling caps meaning both channels would need to be perfectly the same so there's a minimum DC offset voltage between the + speaker outputs.
3. I'd need to find another amp for stereo.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: Jun Sat 04, 2022 7:13 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Just ordered the rest of the parts I didn't have.

Because I'm upping the value of the main filter cap to 10,000uF I am also replacing the rectifiers so that I know they can handle the inrush current.

After what work I've done I think the amp may sound a little better.


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: Jun Mon 06, 2022 7:46 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
Here's the stock schematic.

Attachment:
1 Stock schematic.png
1 Stock schematic.png [ 486.97 KiB | Viewed 462 times ]


Here's the modified schematic showing stuff I did to fix what I felt were some deficiencies with the design. Also I lowered the amplifier's gain somewhat by replacing R8 & R7 with 100 ohm resistors as it only took a 60mVrms input signal to drive the amp to full undistorted output with no load. Now it requires 196mVrms for the same output voltage.

Also lowering the value of C7 & C8 extended the amp's frequency response from 17KHz (above that the + peak started to not look like a sinewave) to 36KHz with the amp still seeming to be just as stable as it was with the 100pF caps there.

With the stock resistors to set the gain the response was 15KHz which might explain why the amp didn't sound so good in the upper treble.

Also I upped the value of C1, C2, C10 & C9 so that the amp has zero phase shift down to 20Hz from the input to the + of the speaker coupling caps.

Attachment:
1 Magnavox amp.png
1 Magnavox amp.png [ 512.61 KiB | Viewed 462 times ]


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: Jun Mon 06, 2022 9:28 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 4924
Location: Boston, MA USA
This seems like a pretty decent amplifier. But it is DC-coupled throughout, with a single global feedback loop for both DC stabilization and AC feedback. And that loop is controlled by R7/C9 and R8/C10. You are REALLY LIVING DANGEROUSLY mucking around with those values, and you risk de-stabilizing the amplifier in very unpredictable ways. Already you effed-up the frequency response by trying to change the gain by changing R7 and R8, which you had to correct by changing C9 and C10 to completely unreasonable values.

I recommend you return all components within the feedback loop to their original values, and play around all you want outside it.

-David


Top
 Profile  
 
 Post subject: Re: Magnavox amp question
PostPosted: Jun Mon 06, 2022 10:04 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 21936
Location: Warner Robins, GA
The frequency response was 15kHz with the stock value of feedback resistor.

Changing the feedback resistor lowered the gain which would actually make the amp more stable.

C9 and C10 were changed because there was phase shift at 20Hz and also the output reduced in level some as I went closer to 20Hz. However the phase shift was there when I got the amp before doing anything to it. I subbed a 1000uF cap paralleled with a 2200uF cap and temporarily connected it across the stock value of 220uF just to see what effect it had. All it did was eliminate the phase shift. No other effect that I could see.

I wouldn't consider 3900uF unreasonable as it's what was needed to get the desired result.

Of course with the 100 ohm feedback resistors I can use a lower value for C9 and C10.

I always do extensive testing when making mods to a solid state amp.

Usually involves running the amp with no load and no signal and seeing if there's any sort of waveform on the output. If the trace is a straight line as though the scope isn't connected to anything. If it passes that test I feed a sinewave to the amp at full undistorted output from 20Hz to 20KHz looking for issues and I'll go beyond 20KHz looking for problems.

If it passes that I'll do a full load test for 7 hours and do it several days at various frequencies looking for problems.

I do need to get the equipment to do bode plots though.

That way I'll know for sure the right way how stable an amp is.

I had a switch mode power supply designer on Facebook help me modify a 25 watt amp kit to produce 93 watts into 4 ohms and I learned a whole lot about solid state amps from him helping me with that one amp project.

If not for him I wouldn't have known as much about what I'm doing with solid state amps and probably would have made some majorly wrong changes.

Also I failed to mention that one change I'm making is to replace the bias transistors with a TO-220 transistor so I can mount it on the heatsink for each channel so that I have proper thermal compensation. However there is a hole in the heatsink and if it's the same size as a TO-92 transistor I may just stick the stock transistor in the hole and see if that works.

Had intended to change the amp to the 25WPC version as it only involves changing a few parts and populating some empty parts of the board, but I decided against that as it also requires a different power transformer.

I do intend to add a delay relay to eliminate the speaker pop when the amp is turned on. That way the speaker coupling caps charge initially through a 200 ohm resistor per channel.

There's a method to my madness and it will result in a very good sounding amplifier.

Now before I use the 3900uF cap I will test the amp since I upped the feedback resistor and that may require a lower value cap to eliminate phase shift to 20Hz.


Top
 Profile  
 
Post New Topic Post Reply  [ 29 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 3 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB