1973 Magnavox. One dead channel.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Now, does the amplifier work properly once more...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Now, does the amplifier work properly once more...

Well... it kind of worked. I noticed the right channel was kind of there now, not 100% dead like before. I pushed down on the volume knob a bit and the right channel came around loud. A month ago, before I took this thing apart, I sprayed all the pots with D5. I know for SURE this wasn't the case before, but I don't think I would have been able to tell since the right channel was 100% dead. Its kind of like a double stacked pot. The pot nearest the knob is the right channel, I was brave enough to take that half apart and found it to be really dirty. Soaking it a bit to clean the left channel.

TA DA....



Uh oh...

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

well, how does it sound, and did you pull the 22 ohm resistors?

If you're fine with how it sounds (make sure you also check music with the volume down low), then don't bother pulling the 22 ohm resistors. it'll keep the transistors cooler at the expense of distortion.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

I need to step away for the night. Even after cleaning the volume pot I still need to screw around with the volume knob to keep the right channel working. Sometimes you have to press it a bit to one side, or pull up a bit.. I don't believe there is anything I could replace it with since it has all the extra taps?

When working, everything sounds great. The thing can get pretty dang loud, so I would be up for ideas on how to replace it even if I have to give up the top half of the volume range.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Looks like a very custom and log pot. Main problem is that it's a PCB mount it seems? I guess you'll need to do what you need to do. Great that it sort of works now, but any qualitative information about it, including how the left channel sounded prior to the change (the right channel probably sounds infinitely better than before, of course)?

I find my ears get annoyed when in a closed room near a stereo with only 2 watts of power per channel. I'm sure at 20WPC they'd hurt. I've never tried pumping my Denon up to its rated 70WPC, then again I don't have enough speaker(s) for it to probably shake down my house...

Incidentally I was doing some calculations and that Denon's quiescent power draw is immense due to the bias needed... IIRC like 60+ watts when quiet, and it doesn't help with 5 channels...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Left channel is just as strong as before. Right is very strong when the loudness pot isn't causing it to kick out

Can I skip the stuff tied in at 150,151 and 152,153 and just tie in a audio pot? The pot also bolts onto the front, so I could put another pot in there and rotate it so the pins face up, then wire it to the PCB however I wish.

For example:
PC3b pin 1 > pot > point 93
PC3a pin 7 > pot > point 112

What does all the stuff in PC3b and PC3a seem to do?

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Based on your replies it "sounds" you're not an audiophile either and don't care how it sounds as long as it's somewhat close, like me... so in this case yeah you can probably just not connect the stuff hooked up to the taps. The taps try to emphasize certain parts of the spectrum at different volumes so without then, likely things will sound different and you may need to do the same to the left channel to make things sound balanced.

Again IANaA.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Based on your replies it "sounds" you're not an audiophile either and don't care how it sounds as long as it's somewhat close, like me... so in this case yeah you can probably just not connect the stuff hooked up to the taps. The taps try to emphasize certain parts of the spectrum at different volumes so without then, likely things will sound different and you may need to do the same to the left channel to make things sound balanced.

Again IANaA.

100% correct, I just want to play some Christmas records and maybe some Dean Martin . It will most likely live most of it's life at mid-low volume, I don't mind adjusting the knobs to get the sound right.

What factors do I need to look out for with pot sizes? For example, if the pot doesn't have a high enough range would that mean I wouldn't be able to mute it? The face of the radio doesn't have any sort of volume scale printed on it, so I don't mind if "max" comes before a full turn. EDIT: I am a noob, now that I study the diagram a bit more, the 10K and 50K values are for the taps (won't be used), the pot range I need appears to be a 100K, two gang, audio, correct? Eying up 652-PDB182-B230K104A.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

If the slider can be adjusted to 0 ohms to GND then it will mute just fine, but again the sound and behavior of the knob will likely be different as you adjust it through the range. Chances are what used to be 50% will be something else.

I also would think as you're going to be omitting the PC3 circuitry that 100K may now be too high and the lower part of the taper might not produce enough volume change. But this is just a guess based on the circuitry...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

If the slider can be adjusted to 0 ohms to GND then it will mute just fine, but again the sound and behavior of the knob will likely be different as you adjust it through the range. Chances are what used to be 50% will be something else.

I also would think as you're going to be omitting the PC3 circuitry that 100K may now be too high and the lower part of the taper might not produce enough volume change. But this is just a guess based on the circuitry...

Got it, I have a few cheap ones laying around, I'll try a few values first.

I freaked out a bit when I saw how crazy the stock pot was, I thought I was stuck after all this work. The old non-tube stereo consoles aren't worth much of anything, but I wanted to get it working. I need to practice on this cheap stuff first.

Think I should remove PC3? I wonder if it will do anything odd left floating.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

no idea what else is on PC3...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

It only has what is outlined in both. So, 7 total pins, 1,2,3,4 for the left channel 4,5,6,7 for the right channel, pin 4 is a shared ground pin.

Either way, it was easy to remove so I left it out for now. I hooked up the 100K pot up and it doesn't work. Tried it on left and right. To make it even harder, it seems the diagram doesn't match the board 100%. My best bet might be to draw out the physical board and confirm I am not blind.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Best I can tell, the bass section of the diagram is correct, but it's placed in the wrong order. The correct order is Timbre, Treble, Bass, Loudness. So I don't feel as crazy now.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Great news:

With PC3 removed, the factory pot works. It's still only a single channel, but that confirmed that PC3 does something that I might not even care about.

Using jumper wires, I connected the factory pot in like a standard three lug pot (works like a 100k pot) No extra taps. It works as it should and I moved it between the two channels.

When I posted the other night saying it didn't work, I had it wired it the same way except I was using a cheap pot I had laying around. Something that most likely came in a Arduino hardware kit.

Forgive the messy work:

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

Oh...it's a hybrid...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

The parts show up tomorrow

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

So, it was working PERFECT... then I removed it from the cabinet to clean up a few things and noticed the right channel was a bit odd again.

This is starting to come together like a CSI episode...

I believe there might be a bad solder joint between the back panel board and the main board. The speaker output passes through that board first, along with a bunch of BS, then comes back down. If you lightly press on that board the channel comes back. The volume knob is located on the edge of the main board and that rear board.. I bet the last owner(s) fix was to apply pressure to that volume pot (like I did) and make it work for a bit. Over years It wore out the bushing and then messed up the carbon trace... Also, I am sure it spent a lot of time with that channel open, causing it to blow out the transistors..

I am going to re-flow the joints between the board. If that fails, I will just bypass that whole section. It's function is for the 2-Channel/4-Channel switch and 4 channel decoder. It also has the External speaker lugs.


In the attachment below, on the left side, 123 and 127 ish are the output caps. I really don't care about the stuff in the middle. I would tie the speakers in at 123/127, call it good.

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

Well well well.. got her running perfect now. See attached.

[eccerr0r]

Re: 1973 Magnavox. One dead channel.

hooray for broken solder joints...

[JunkForLess]

Re: 1973 Magnavox. One dead channel.

hooray for broken solder joints...

Thanks a lot for all your help. Learned a bunch. I'll be making a demo video of the console soon.