Magnavox Imperial model 1ST233

[Conelrad]

Modern solid-state hi-fi gear is designed to work into a 10K unbalanced bridging load, more or less. If you have enough input voltage to allow adjustment of volume, that is fine.

It will never have that conjugate match one finds in pro gear.

DG

[electra225]

Thanks, Dennis. I have plenty of volume. The fidelity is what is lacking. I have been tinkering with the music conversion software I use. I believe I can add some bass and lower midrange when I convert a record to digital that may help some.

This project has been slid to the back of the bench last couple of days. I got the wheels greased on the cabinet and cleaned it real good to keep the peace on the home front.

[electra225]

I have two broken MPX's. The original one won't let the tuner come on. It kills one branch of the filaments in the tuner. The three 6EU7's and the eye tube comes on, the rest of the tubes in the tuner are dead, along with the dial lights. The spare MPX kinda sorta works, but not great. IT seems weak with poor fidelity. I have a lot of B+ in the spare MPX. All the element voltages are running 50 volts high. I should be getting 320 volts B+ at pin 3 on the main power Molex plug, and it is spot on. That goes into the MPX, thru two 2-watt power resistors for the two branches of B+ in the MPX. The 175 volt branch has 225 VDC on it and the 55 volt branch has 68 VDC on it. Do we have any MPX mechanics in the house?

[William]

I'm not an MPX mechanic, but the Imperial will work without the MPX, right?

Bill

[electra225]

Yes, it will. I need to get it fixed, though. I need to work thru this. I may cheat and put the two together to make a good one.

[electra225]

I cheated. I admit it. I need a better service document than what we have here in Downloads if I want to dig into the MPX. I almost got it. I got the volume better, but it won't lock on the stereo composite. It works okay for a bit, drifts, then sounds fuzzy. I may order the Sams for the MPX, then post it here. All we have is the schematic, better than nothing, but I need alignment instructions. I also may need a working scope in order to do a proper alignment. I used the good harness off the wonky MPX and put it on the good MPX with the wonky harness. So far so good. Next job is to sort the changer.

[electra225]

Do any of you guys have service and alignment instructions for Magnavox Multiplex adapters you could post on Downloads?

[Ken Doyle]

In case it helps, I found these:

https://pacifictv.ca/schematics/magna700110data.pdf

https://pacifictv.ca/schematics/magna700300data.pdf

[electra225]

Thanks, Ken. I'll print these out and see what I can do.

[electra225]

I worked for a couple of hours on the spare Multiplex adapter. I have it close. The audio is clear, no extraneous noises. The audio output is still weak. They want me to set the output of the Multiplex adapter to a level slightly higher than mono FM. I can't get it as loud as mono FM, by a considerable margin. I have checked every component, recapped it completely, checked all the voltages. The only thing I see amiss is that I have about 50 volts too much B+. I have no idea why. I'd say this MPX is usable, certainly for test or if you don't have anything else and you need one.

The electronics go back into the cabinet tomorrow. I wanted to get that done today, but I ran out of energy. I have never gone over anything in the detail I used on this one in my life. I have no idea whether I accomplished anything until I get everything back in the cabinet. I'm holding a good thought that I made improvement in fidelity and bass response.

[TC Chris]

Is it possible that the problem is on the main chassis?

Chris Campbell

[electra225]

No. The problem that caused me to combine two MPx's to make a good one is definitely in the Molex connection on the MPX, the one that the tuner plugs into. I used the wiring from the spare unit to fix the original unit. Then I put the wonky wiring onto the wonky spare MPX. I rigged a jumper from the amp chassis directly to the tuner so I could have filament voltage to the tuner. THEN I went thru the spare unit to see could I figure out what ailed it. There is a 200pf ceramic cap between Check Point Charlie and a coil that I highly suspect of being partially shorted. I'll do more research to see what that cap does. I have no way to check it.

[William]

Greg, maybe just replace that cap, just because.

Bill

[electra225]

I should not post while I'm working on something. I am editing this post since I originally posted something fairly pessimistic, which may be unnecessary. I haven't run it enough to get a thorough assessment of bass response, but I can say it is not fantastic in the tape input mode. I have a dead horn that I need to investigate. I suspect a bad connection. I also suspect that wimpy bass in tape input mode may be a result of an impedence mismatch, rather than a defeciency in the amplifier. The fidelity is definitely better.

[electra225]

I needed to perform surgery on the Molex connector for the speakers. I set the treble on the speakers. The fidelity of this thing is incredible. "I'll Follow The Boys" by Connie Francis with that big kettle drum roll at the beginning will send chills up your spine. Hydro made a suggestion on maybe upping the input on the tape input a bit, so I think I'll follow that suggestion and see what I get. I havent got the changer done yet, so I can't say how the fidelity is on it. FM sounds pretty good as far as bass is concerned. AM is AM, some good, some not so good. The tape input is fed directly into the volume control, just like on a record player. There must be an impedence mismatch somewhere. The suggestion has been made that tape players and CD players have a 10K ohm output impedence. This exercise may not have been a total waste of time, as I first thought. IT is fair to say I haven't found any more bass with the system set on tape input.

[William]

So, you are saying it's all back in the cabinet and it sounds good, but the bass isn't really any better? Are the speakers enclosed, or is that still to come after you investigate the horn? The mere fact that you stated the fidelity is incredible makes me happy. With a CD player playing through the tape input on mine, it sounds great, but the best sound is with a record and that is where my bass is incredible. FM sounds the worst, but I do not have an external antenna hooked up as I never listen to FM on my Imperial.

Bill

[electra225]

The speakers are totally enclosed. With the exception of the power transformer and the resulting higher B+, my Imperial is as close to yours as I can get it. When I play it all day, I'll be able to make a better assessment. I'm a ways out on the changer. I need to figure out a way to power it up not involving the Imperial. Some way to power it up on the bench. Any ideas?

I've been looking at the schematic for the 57 series tuners like is in the Symphony. It sounds good with a CD player. The input is exactly the same as on the Imperial, direct to the volume control. Since I am nearly certain that the issue is in the CD/tape player and not in the stereo, per se, I can live with it. Now that I know nothing is "wrong" in the stereo itself, I at least feel better......

[William]

I am happy for you, Greg. It has been a long journey getting the Imperial sorted and getting it back to factory specs, you should be proud of what you have accomplished. Will it go back into the house once done? I hope so because getting it backed up to a wall will make a difference with sound too. Since I need to work on my Collaros, maybe we can do it together one baby step at a time. How does that sound? Again, I am happy for you and now you will have years of listening enjoyment.

Bill

[electra225]

The cabinet never left the house, only the guts. It is still in my "laboratory" where it's always been. "Living A Lie" by Al Martino is on now and it sounds better than it did. The thing that gets me is how "alive" this thing sounds now. We'll run it a lot today and see what we have. I have discovered that some of my homemade CD's are not the best anymore. Would I spend the money on matched output tubes again? IN A HEARTBEAT! The 6BQ5's in the Stereo Theater are still Radio Shack tubes. I wonder what a set of matched JJ's would do for it?

[electra225]

I may also have a problem in my big CD player. One channel is not as strong at the other. I reverse the patch cords, the problem moves to the other channel. So I need to try another set of cords. I also noticed that the "EQ" setting on the music conversion software I used is turned "off". I have no idea how long it's been like that. That will wash the bass out somewhat.

"My Hometown Sweetheart" and "I'll Follow The Boys" are almost "boomy" now!

There is a lot of difference in how much bass is in recordings. I need to figure out how I can get the bass the same on all my CD's.