Magnavox Imperial model 1ST233

[electra225]

Something I have noticed. I get more bass on FM than with the tape input. Maybe that might be an area of investigation.....

What is the difference in the input from the FM tuner than from the tape input?

And, for the record, Bill's IMperial has the same output transformers, speakers and the power transformer has the same part number, although the two are not the same.

[Hydrolastic]

Hello Greg, I wish I could understand most of what your saying there. My CG will rattle the windows in the entire house it has bass and more. I actually turn the bass down one click. It wasn't always that way. I now use a phono pre amp and a high quality bluetooth adapter. By bringing up the line voltage on the tape side of the preamp it fills out the entire scale. Run some Dom Dolla for a test.

[electra225]

You may be onto something, Hydro. So, you are saying that before you put a little extra boost on the tape input of your CG, it exhibited strangled bass, right? This is exactly what I have been experiencing with this Imperial chassis. What if there is too much of an impedence match or mismatch between output of a CD or tape player and the input of the amplifier? There is a drifted-low high value, 1.5 megohm resistor across the tape input to the volume control. I think I need to measure or somehow know the impedence of the output from a CD changer or tape deck. There is a 1.5 meg high value resistor in the FM tuner output to the volume control. The 1.5 meg resistors in the tape input have drifted down to about 700K ohms. There are several high-value resistors in the tuner I need to replace yet. I don't have the value I need.

[electra225]

In the tuners used for the 88- and 93-series amps there is no high-value resistor between the tape input and the volume control. These amps have way better tape input performance than do the bi-amps. I wonder what would happen if one were to shunt the high-value resistor between the tape input and the volume control?

[electra225]

I am making a world-wide fool of myself fumbling around with this. I'm not sure I understand what I know about high-Z input. I've read where hifi equipment typically uses input impedence in the 100K ohm range. Magnavox bi-amps, both on the Imperial and on the Concert Grand use high-value, over 1 megohm, resistors in the tape input. Hydro's comment about raising the line voltage is what helped how his CG performs. I'm ready for someone who knows more about this than I do to jump right on in here and bail me out. That includes anybody, since I know nothing about this....

It looks to me like eliminating the high value resistor might help with raising the input line voltage....

[William]

I wish I could help, Greg, but I understand even less than you do.

Bill

[electra225]

I have consulted my wife, who knows absolutely NOTHING about this stereo, doesn't really care, but is concerned that she can't run the CD changer with her music on it until I get this figured out. So, we have decided to bite the bullet and just go for it. I'll get a quad and a pair of matched 6V6's, set the bias correctly, then start tinkering. I have some resistors to change yet in the tuner, so that is the project for this afternoon. After I have documented that all the components, resistances, voltages are withing 5% of spec, then I'll change certain component values and note their effect. I will make charts for future reference. The more I read, the more confusing this gets. I just read an article with a bunch of math and silliness on it. I get along better when I don't know too much. Fumbling around in ignorance is when I learn the most.....

[William]

Fumbling is what I do best too, and I learn a lot that way. Of course, will I remember is the next thing. My Imperial was a lot of fumbling and experimenting as was the little Silvertone tabletop, an RCA 45 player, and my last project which was the VM 570 and its matching speaker/amp the VM102. Now I have the VM 801 to fumble and experiment with and with time, I will get there with that one too.

Bill

[electra225]

I've decided to quit driving myself nuts trying to fix something that may not be broke. I'm going to go thru the changer, do a few mop-up operations, put new wheels on the cabinet, then put the guts back in and see what I've got. I have this chassis set up as close to how yours is as I can get it. When I get the tubes, I may find that makes a big difference. Let's hold a good thought.....

[electra225]

The casters on the cabinet are not in horrible shape. They won't roll on tile. They were made by the Nagel-Chase Manufacturing Company out of Durnee, Illinois. They are still in business, believe it or not. I don't know what I can do to make them better. The casters are held on with corner brackets, they are loose. I think I'll try to create something to put on the casters so they will roll easier, then clean and lube them up real good, and tighten the screws on the brackets. I may run by Harbor Freight to see if they have something I can use.

[hermitcrab]

The latest Magnavox I got from my niece is sitting in the basement... the casters are so touchy on the cement floor, I got to push it up tight to the wall because it rolls so easy , my Imperial astro sonic in the dining room rolls Ok but it is on vinyl plank floor and takes some effort to push it around.

[William]

Greg, are the castors on the Imperial rubber? Is it possible that the rubber, like a drive tire in a changer, is so hard that they can't grip your tile? Just a thought and my 2 cents.

Bill

[electra225]

No, Bill, they are like a really hard plastic. They call them carpet casters. I have a couple of options I can see right off. First, I can clean and lube them up well, which will let them work better. OR I can cut the axles off the wheels, then install some softer rubber wheels that will roll on tile better. I'll get some pictures so you can understand better. Magnavox got chintzy with cabinet finish. The wood is bare on the bottom side of the cabinet.

Bill, since I have forgotten to ask, how do you adjust the thingy that holds the lid to the changer compartment open? I remember you said you adjusted yours, and would advise when the time came to do mine. That time is here....

[electra225]

I went to AES this morning and picked up the output tubes. A matched pair of JJ 6V6S's and matched quad of JJ 6V6S's. The guitar guys say these tubes are a mix of 6V6 and 6L6. These will allegedly handle 450 volts on the plate. Wish me luck. I'll populate the amplifier with the new tubes, then check bias. I'll know in a couple of hours whether matched output tubes are a scam or not. I would never believe, not in a million years, that I would buy expensive matched tubes for a console stereo......

The guy at AES asked me what kind of guitar amp I was putting these tubes into. When I told him I was not working with a guitar amp, but with a Magnavox bi-amp, he thought I was daft......

Let's keep our fingers crossed.........

[electra225]

This may be wishful thinking, but it sure seems to me like this thing sounds better. I won't know for sure until I get everything back into the cabinet, running better speakers.

I spent 30 minutes trying to figure out why the treble amps weren't working, only to find I had two dead horns. Those things are now on the other side of Rittenhouse Road, over there where they are building new houses..... Maybe somebody will run over them with heavy equipment. Note to self.....NEVER use horns for test. NEVER.....

I have some really hot biasing on the treble amps. All the voltages are dead on. All the resistances are within my 5% tolerance. My mathematical calculations say I have 85 ma plate current. The plates should be red carrying that kind of current. The bass amp tubes are all about 37ma, give or take a tenth. They all have 11 watts +/- of plate dissipation, right on the old nose. The treble amp tubes figure out to 27 watts of plate dissipation. No way are they that hot. I need to figure out what I did wrong in my calculations. I'm running between 318 and 322 volts on the plates, so that is good. Screens are all good.

I connected the MPX so I could get an accurate wattage draw on all three pieces. The MPX has decided to not pass B+ to the tuner. The power switch still works, but something is definitely wrong. I need to sort that yet.

[electra225]

I printed out the schematic for the 70-series Multiplex adapter. Sorting that is my next step so I can install the fuse and figure out what value power resistor I need to drop line voltage to the chassis. I still have work to do in the tuner. It involves the PCB, so I've been putting it off. I did the bias calculations wrong on the treble amp tubes. Not sure yet what mistake I made.....

[electra225]

While I'm not out of the woods yet, by any means, I'd like to share some observations I have made during this process.

First, Magnavox built this stereo with "sizzle, but no steak". Built for a home you would see on Perry Mason, with tall ceilings, wool carpet, heavy draperies, classical or easy-listening music. Built with beautiful hardwood cabinetry (although not finished on the bottom of the cabinet). The bean counters were hard at work killing Magnavox when this stereo was built. They used a 15 or 20-watt amplifier, so boomy bass is not likely because of the flea-power amp. Second, Magnavox bought and used the cheapest garbage parts they could find, utilized generous tolerances, producing a "sloppy and slushy" amplifier. Enthusiasts complain about wimpy bass. In reality, what this one was producing was wimpy FIDELITY. Wimpy bass was only a part of that. When this stereo was new and all the cheap parts were new, it probably sounded pretty good, good enough to get paid and get it out the door. The last 64 years has not been kind to this old stereo. It was a high-hour set when I got it and I have used it heavily myself.

90% of the resistors I have changed during this process were within the factory tolerance of 15%. They were mismatched between channels. The reason I bought the resistor kit was so I could have multiple resistors of the same value so I could find two that closely matched. The "big three" plate load, grid load and cathode bias resistors were matched between channels and are all less than .1% off spec and less than that different between channels. There are four resistors in the amp chassis that are still original. I even replaced the resistors in the feedback circuit so they would be the same. I selected resistors so the "difference" in value between channels would be "on the same side" of the specification. If one was 30 ohms high, the other is similar. I didn't put one high and one low.

I believe, at this point, my recommendation for any Magnavox bi-amp is matched output tubes. If you have a CG, you will need two sextets and one quad of matched JJ 6V6's about $480 worth. This Imperial has a quad and a pair of matched tubes, and I asked that the quad and the pair be matched. They recommended that the treble amp tubes need not be as stout as the bass channel tubes. The treble amp tubes are matched at 29ma, the bass amp tubes at 37ma. There are mathematical equations about audio that I don't understand. From a "mechanical" perspective, I am holding my own, I guess.

I love my Magnavox stereos. I believe Magnavox was king of the hill in console stereos and audio in general in their day. They have beautiful cabinetry, cad-plated chassis that look like new 64 years after they were built. The wiring is sloppily oriented, but it appears to be excellent quality, resting on hot tubes for decades with no apparent damage. They have features and doodads galore, but by the time this stereo was built, they used really cheap parts that have not aged well. If you want your old Magnavox stereo with bi-amps to sound as good as it can, you need to do a component-by-component restoration of the electronics, keeping the tolerances to 5% or less. Then you should spring for some balanced tubes. Output, audio and rectifiers. Keep the component tolerances the same between channels. Match resistors between channels. You essentially need to do what Magnavox did when they built it. Do it part by part.......

[electra225]

There is a plastic tang or finger on the Molex connector on the Multiplex adaptor chassis that has broken. This allows he Molex connector to retreat back into the chassis, not allowing the plug from the tuner to make good contact. I have a spare adapter, but it is not very good. I used it for parts for the one with the broken finger. My cure is to take the bottom off the MPX, make sure the tuner plug is properly seated, then put the bottom back on the MPX chassis. The MPX and the tuner will be as one, tied by their connecting wiring. I may never have to remove the MPX chassis again. I will make notes on my service documentation about this so I'll know next time.

[electra225]

The original MPX is not going to work. There is an intermittent in the Molex connector. It may be easier to get the spare going, then work on the original. I took some pictures of the grand mess that is the Imperial chassis and related wiring. It's a thing of beauty. The JJ tubes aren't very pretty running since they don't have clear tops. The "JJ" insignia is lined up with the key on the tubes, so they are all in the same place, and that LOOKS pretty cool. I ran this outfit while I listened to Western Red. It is still working, so I reckon now I have "burnt in" output tubes.....

[William]

Those photos bring back memories of when I had mine sprawled out all over the bench. It looks great, Greg, and it is playing so you are way ahead of me with my current VM project. Keep up the good work and before you know it your Imperial will be singing like you have never heard it before.

Bill