Magnavox MX 1581 information please

ADKBob

Well-Known Member
I picked up this receiver for 6 bucks plus change and wondered if anyone has any information to share. A user manual or schematic would help. When I get it on the bench with the DBT, I'll post some pics. As I understand, this is just before the lab series. Thanx in advance.
 
Dim Bulb Tester - no smoke

I hooked up the 1581 to my DBT and the light went from bright to a low glow quickly - nice. Then I plugged her right directly into the power. I warmed her up and took DC offset measurements: Right = -2.5 mV and Left = -2.5 mV. Since I don't have a schematic, I'm not sure if the outputs are capacitor coupled or not. From the fact that the dc values don't "float around" and are very stable after warmup, I'm guessing it's not cap coupled. Thanks to AK, I follow the process of carefully bringing these vintage pieces up. After quite a while, I heard a "click" - maybe a relay. When powered off, another "click".
 

Attachments

  • Magnavox_MX_1581_Front_DC_Offset.jpg
    Magnavox_MX_1581_Front_DC_Offset.jpg
    77.6 KB · Views: 90
Nudies

I plugged headphones into one of the two jacks and nothing. No hiss, no hum, no "crackle / scratch" when turning the volume all the way up / down. So, time to look inside for obvious "broke" parts. This is a modular build with edge connectors, so I suspect there may be loss of power somewhere. Without a schematic, it's next to impossible to troubleshoot. If I have to, I'll do the schematic "reverse engineering" this winter. The build quality is nice, but it may have seen dampness and corrosion is not our friend.
 

Attachments

  • Magnavox_MX_1581_cover.jpg
    Magnavox_MX_1581_cover.jpg
    95 KB · Views: 65
  • Magnavox_MX_1581_top1.jpg
    Magnavox_MX_1581_top1.jpg
    111.4 KB · Views: 76
  • Magnavox_MX_1581_top2.jpg
    Magnavox_MX_1581_top2.jpg
    117.4 KB · Views: 64
  • Magnavox_MX_1581_top3.jpg
    Magnavox_MX_1581_top3.jpg
    121.3 KB · Views: 62
  • Magnavox_MX_1581_top4.jpg
    Magnavox_MX_1581_top4.jpg
    114.5 KB · Views: 64
  • Magnavox_MX_1581_top5.jpg
    Magnavox_MX_1581_top5.jpg
    117 KB · Views: 77
Last nudie

Limit 6 - seventh
 

Attachments

  • Magnavox_MX_1581_top6.jpg
    Magnavox_MX_1581_top6.jpg
    119.1 KB · Views: 53
David Bowie sings Rock and Roll Suicide

I was listening to headphones while turning the power on / off. With an MP3 player into AUX1 playing Ziggy Stardust, I heard the music when the power switch was turned off! It took about 20 seconds to discharge the caps. If I cycle power on and off, sometimes the power on results in music! So, when power on gives tunes, the DC offset is: Left = -16 mV and Right = -6 mV. That makes more sense than before. Not knowing the power in wpc, I slowly turned volume up on the Criterion 4Xs and found the sound to be very nice. There's little hum or hiss between songs. The protection module does not have delay - instant on when it works. Running for a while, the big heat sink gets a little warm, not hot. It looks like a rebuild of the Protection Module is in order. Since it just plugs into an edge connector, it should be fairly easy to restore if parts are available. I surmise the power output is in the order of 20 wpc. FM tuning is nice and clean. From what I have read and since no one has offered up any information, this is a pretty rare unit only made for a year or so.
 
I can appreciate the post but not many of us have knowledge of that unit. A pic of the pcb might help.
 
Binkman:

Thanks. I realize this baby is "way out there" and information is just not there. In the previous post, there is the top1 pic that shows a small module with the white relay. I tried to pull out the module, but it seems to have some latch I can't see. I don't want to pull too hard and break it. There are a couple of caps and transistors with the relay and I'm going by what I've seen in other receivers. I have to update my power estimate since the MP3 player probably does not have enough drive to fully modulate the amp. In FM on the bench speakers, it is much louder than the AUX and seems louder than the Sansui 2000 playing FM I just had on the bench. Perhaps a CD player with a higher line level would be as loud as the FM. Maybe in the 60 to 80 wpc.
 
Probably 50-65 watts per channel would be more like it seeing as it looks like it would be in the same ballpark as some of the Pioneers like the SX-1000s which were made around the time this was made and I know that the SX-1000 Pioneers were between 50-65 watts per channel.
 
1581 Latch released

After looking at the protection module and other pwbas, I figured out how the latches worked and voila - pulled out. Those engineers did not want any of those modules coming out no matter how much vibration was applied. This design smacks of industrial strength. California quality - a little quake shake and the tunes keep a coming. Now I need to reverse engineer the circuit and see if I can rebuild the board. The Criterion 4Xs are supposed to put out 100db spl at 16 wpc (BASS-01-11-7308), so at about 2 on the volume with a good strong FM signal they are about there. Maybe 80 wpc. All of the potentiometers are scratch free with only the Dolby FM switch a little intermittent. That switch probably got little use. After about 10 cycles it cleaned up and worked. Born in the USA - wish it was still that way.
 

Attachments

  • Magnavox_MX_1581_Protection_Module1.jpg
    Magnavox_MX_1581_Protection_Module1.jpg
    40.2 KB · Views: 44
  • Magnavox_MX_1581_Protection_Module2.jpg
    Magnavox_MX_1581_Protection_Module2.jpg
    60.1 KB · Views: 39
  • Magnavox_MX_1581_Protection_Module3.jpg
    Magnavox_MX_1581_Protection_Module3.jpg
    31.5 KB · Views: 37
  • Magnavox_MX_1581_Protection_Module4.jpg
    Magnavox_MX_1581_Protection_Module4.jpg
    34.3 KB · Views: 38
I've been looking for the CDE 682-891b relay in the picture above with no luck. Does anyone have a source for older relays or a substitute?
 
The more I cycle the power (with speakers off), the better the receiver turns on. I am wondering if there is a delay in the Protection Module or does it just protect against things like speaker shorts? Also, playing a CD through the AUX input is just as loud as the FM - MP3 player did not have the line drive. Sound is clean, quiet between songs and it punches the bass.
 
You seem determined.. Slow down a bit..

By powering on and off.. you getting the circuits working but I would not delve further until you diagnose the circuitry and you need to clean it up, if you want to work on it.

I don't know your skills but this is not flip amp with an easy solution.
 
Binkman:
I have worked as a electronic technician and electrical engineer for about forty years and have multiple patents that I reduced to practice by building the circuits from my designs. I have repaired smt pwbas under a stereo microscope. However, I never heard the term flip amp. As I power on the 1581 to measure voltages, currents and temperatures I am very slow (cautious) since this is quite rare and do not want to mess anything up. My question was whether or not the protection module may be designed to come on immediately or have a delay. I have seen and worked on both types. I should be able to reverse engineer the circuit to better understand its features and functions and any information on the parts would help me better understand it. Without any technical documentation I have narrowed the troubleshooting to the protection module since the rest of the circuits appear to be performing as expected. I suspect the relay contacts have been idle and had to be cycled to get reliable contact. Power supply values are not available, but appear proper with virtually no ripple ("hum"). Thank you for your help.
 
The circuitry is behaving as it seems to be intended. The initial power on is turning on the light between the two headphone jacks (assuming this indicates a fault). By turning off the power and turning the power back on the light is out and everything works beautifully. Without the schematic, I can only surmise the initial inrush of power trips the circuit. This may be due to drift in components on the main board, but I cannot analyze the circuitry further without documentation. I vacuumed the inside while brushing the parts. The only hum detected is when my lab is totally silent - that huge transformer has a slight but expected hum for its huge size. Time to find a couple of feet for the front of the bottom plate and clean the exterior. I'm listening to CDs from the thrift store find pile - great sound.
 
I have the later Magnavox designed Philips receivers and on both the electronic protection circuit design and relay circuit design there is a turn on delay. Never measured the time but for the former maybe 10 seconds and the later closer to 25. Lights come on immediately but sound muted.
 
Just for general info, most Japanese Protection circuits are designed with a 4-7 second delay built in based on how long the capacitor take to charge and energize the transistor that controls the relay. Magnavox's I've seen take generally 10-20 seconds. Again that may be a cap variance. Or the controlling transistor is getting weak.

Another just for general info. Instead of dry switching to clean contacts, why not try some DeOxit D-5 cleaner on all of the control's, and follow it up with G-5 gold. Softens up the sulfates from the environmental pollution and then friction from exercising them will cut a nice clean path for the contacts. Experience has shown me that the contacts stay clean longer with this treatment than if not done by 2-4 years. You might find that cleaning the edge connectors with D-5 & G-5 helps greatly.
Note that if any of the controls are sliders the use of Faderlube is mandated vs. D-5. D-5 is too agressive on sliders.

Over on the YAHOO Magnavox Group Alan Maier and Jim Murphy probably know more about this particular unit and may have access to at least schematics.


Larry
 
Thank you all for the help. I use DeoxIT DN5S-6N to clean the pots, switches and connectors in equipment I fix. I then spritz them with F100S-L2 to re-lubricate. The protection module uses a triac to engage the relay, with the triac driven by two transistors. I appreciate the hint to check out the YAHOO group - will do. I have a question about the power supply capacitors. They are 4 lead NICHICON 35mm x 100 mm 6800 mfd at 50 WV. The 4 leads are 3 wires with a negative ear. Any recommendations on replacements?
 
The two extra leads are just for stabilization during transit. Not really needed. replacements in 6800uf/50V will be approx 25mm by 40-50mm. 6800uf/63 are slightly larger @ 25mm by 50mm. If they are on a board, no problem. If clamped, wrap a piece of window foam weatherstrip around it and clamp in.

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oi3QhMTkogUYQ= $5.86 each. These are 63v units to give you a bit more surge and circuit voltage headroom due to increased wall voltage.

You could go up to 8200uf @ 63v but not sure if the Maggie can handle the additional capacitor startup surge. Plus you get in to snap in at that size, which means you might have to drill (arrrggggghhhhh!!)
 
Thanks for the tip from your experience, Larry. Looking at the main circuit board from underneath, I can see the two traces that connect positive and negative. The original height of the capacitors was such that there were circular 1 inch foam pads on top of the caps that fit into depressions in the top cover. Probably to stabilize those tall caps. During cleaning, the foam disintegrated, but I don't think it will be needed now. Since the "Hunting / Gathering" season for vintage stereo is near an end (October), it's time to start ordering parts for the many projects to be done this winter. I have over fifty pieces to fix up. As others have stated here, the hobby has become quite intense - loving it. If it were not for people like you at AK, I wouldn't have all of this fun. I did sign up for the Yahoo Magnavox group. I read it takes quite a while for the owner to get back to me, but no big rush - winter lasts a while.
 
I picked up this receiver for 6 bucks plus change and wondered if anyone has any information to share. A user manual or schematic would help. When I get it on the bench with the DBT, I'll post some pics. As I understand, this is just before the lab series. Thanx in advance.
You got a heck of a deal. They were 650 bucks new. I designed the audio, power supply and more. All but the receiver.
 
Back
Top Bottom