Restoration of a Magnavox Concert Grand model 1ST800F

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Do you want to share my restoration of a Concert Grand?

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Wed Jun 03, 2015 11:53 pm

I haven't measured the MPX exactly, but it is about a foot long, three inches wide.  The chassis is about an inch and a half thick, with two shielded tubes,three IF transformers and a filter cap on top of the chassis.  Approximately, you understand. Very Happy

The official nomenclature for this unit is "The Magnificent Magnavox KO 597B Stereo Adapter." Available either as a factory option on all models equipped with a MPX adapter connection on the radio chassis or as an aftermarket factory accessory. I have the factory documentation giving installation location for the various cabinet styles and chassis variations, along with troubleshooting guide. Mine is a model 7002-00 and has a 6EA8 tube and a 12AT7. The addition of a later adapter built after the stereo was built would be expected with a pre-mid-1961 instrument. It could be safely assumed that few, if any, 1959 or 1960 Magnavox instruments came from the factory with an MPX adapter, since the final version of FM stereo was not approved until April 1961 with broadcasting starting in June or July 1961.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Bill Cahill on Thu Jun 04, 2015 3:44 am

So, how do you "Measure up " to fixing this??
I'll give you a hint. If that capacitor on MPX unit opens up, you will get ehoes instead of stereo sound.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by 75X11 on Fri Jun 05, 2015 5:16 pm

If I were going to mount the unit within the cabinet, I would make up an L shaped bracket the length of the assembly with 1 wall having enough room to mount the bottom of the chassis. the other wall I would have extend an inch or two above the chassis top. I would then put 2 or 3 evenly spaced keyholes for mounting screws clear of the chassis top and mount the entire thing with the open wall flat against the top of the receiver area. Then you could loosen the mounting screws in the keyholes and move the converter out of the way easily. The chassis would face outward toward the rear.
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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by jerryhawthorne on Fri Jun 05, 2015 5:31 pm

We are pulling for you WC, as Bill said:

So, how do you "Measure up " to fixing this??

I'm certain you are! Looking forward to your progress. And of course confusing you!
Jerry

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Sat Jun 06, 2015 12:19 am

Thanks for your interest, Jerry. Very Happy

75, your idea is pretty close to what I had in mind. From what I can see, the MPX chassis will either need to be mounted with the tubes straight up or mounted with its "input" side down. The side with the cables and volume controls. I was thinking about making a wooden shelf like the tuner is mounted on, only not as big. Then use "L" brackets and fasten it somewhere above the power supply chassis. I want it to look like it belongs there, even though it doesn't. The Magnavox engineers may have designed provisions to wire the MPX into the console, but failed to make provisions of how to mount it into the cabinet.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Bill Cahill on Sat Jun 06, 2015 9:29 am

Then, mount it on the bottom with ends pointing forward, and, back.
Have it as far away as you can from the power chassis as possible.
That's the way Magnavox did it on their later tube sets with stereo MPX. The filter on MPX chassis is near the front of cabinet, but, spaced in middle between front, and, back.
They used wood screws with hex head.
Hope this helps.........

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Wed Jun 10, 2015 2:17 am

Pending my being able to post a schematic for this instrument, I invite you all to use your minds' eye to share my progress today.

I have two schematics for this instrument. A genuine Magnavox service manual, dated 8-58 and a Sams schematic dated 12-60. Keep in mind that my Concert Grand is allegedly an "early serial number" 1959. We know now that tube coding puts its manufacture in the last few weeks of 1959 or the first few weeks of 1960. Not only are these two schematics, both by Sams, drawn differently, but there are components that are different as well, primarily in the crossover network. Perhaps Magnavox was chasing the same performance problem that I'm concerned with? Curiously, the "external amp socket" used on the AMP-148 (?) chassis mono Concert Grand models in 1957 and 1958 is still in place. It is on the top of the chassis as it sits in the cabinet and is inaccesible for use. One could suppose that a schematic drawn in December 1960 would be useful for instruments built in 1961. This schematic shows the 6E5 eye tube,when factory sales documents for the 1961 Concert Grand models specifies a tuning meter, replacing the eye tube. My instrument appears to be wired like the August 1958 schematic. I am trying to find a Sams manual printed in late 1959, but so far no luck. I would like to compare schematics and see when the changes to the crossover circuit were made. Capacitors C69 an C70 in the 1958 schematic are .047uf and .033uf respectively. The values are reversed in the 1960 schematic. These caps are in the tone circuit following the "tone choke" in the input of the amplifier. There is a 150K ohm resistor added to the grid circuit (noted as not on all instruments!) of the 12AX7 amplifier on the 1960 schematic that is neither on the 1958 schematic nor my instrument.

This little foray into my Concert Grand concerns the Channel 1 8200 series amplifier. The 220K ohm grid resistors that are typically found drifted high are that way in this amplifier. They were found in the 262K to 269K range. Although these are out of tolerance even at 20%, they are not as high as I typically find. This adds credence to my theory that this instrument is fairly low houred. The 100 ohm resistors that tie the grids together were found at 89, 92, 90 and 90 ohms respectively. The 220ohm, 2 watt cathode bypass resistor on the treble amp was found burned and drifted up to about 320 ohms. This is the second dual-frequency amplifier that I have found a burned cathode by pass resistor in. So all cathode bypass caps will be changed with both voltage and capacity increased in all four amplifiers. The same for the bypass resistors. No need to take any chances. This seems to be a trend, at least with my instruments. In summary, I will change all the electrolytic capacitors, nearly all the resistors, and all the coupling capacitors in both amplifiers as a start. I'm not convinced that the hum I have is connected 100% to that burned cathode resistor. That shouldn't put hum in the bass amp, nor on both channels. The hum is common to both channels. It's not as bad as the hum I had in the Imperial, but it is there and I need to get to the bottom of that situation.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Bill Cahill on Wed Jun 10, 2015 10:10 am

Sams rarely showed updates. Occasionally show a few updated schematics in a folder marked say 99A, or, 108A.
As for Magnavox, they rarely did things the same way, and, didn't show all the changes, either. Neither of the schematics I have for the amp in my 1947 Magnavox shows the tweeter, and, mid range speakers mine actually uses.
Also, inmost cases, Sams tended to draw schematics about a year after the equipment came out. Another mistake Sams made, along with drawing mistakes, was claiming they only drew schematics for the most popularly used equipment.
That simply wasn't true...

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Wed Jun 10, 2015 11:11 am

Magnavox can be "interesting" to investigate and attempt to service. Some of the updates I was considering have been drawn into the 1960 schematic.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Thu Jun 18, 2015 9:17 am

I finally got the amp schematic scanned.  Hopefully so you can see it this time.   Very Happy



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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by neali on Sat Jun 20, 2015 2:24 pm

Wildcat,

For an excellent discussion of the effects of changing the value of the cathode bypass cap, watch this video.

It uses a high mu triode in the example, but the same principles apply exactly to Beam power tetrodes and pentodes.

https://www.youtube.com/watch?v=HrkYUPmEUhk

One does not want to pass signal to the loud speaker that is lower in frequency than the speaker can reproduce - say 30 Hz. It makes the output transformer unhappy.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by jerryhawthorne on Sat Jun 20, 2015 3:54 pm

Great video neali. I also watched some other of his. They helped clear up a lot of things I wasn't certain of. The guy does a very nice job of explaining parts of audio amplification. I wish he would do some on RF and IF.
Thanks, Jerry

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Sat Jun 20, 2015 5:48 pm

The originals are 20uf at 25 volts.  That is on the 6V6's. After watching his video, I have an observation.

The example he gave was working with a 12AX7 preamp tube. The bypass resistor is 1.5K and the grid resistor is 100K. On my amp, the bypass resistor is 1.5K and the grid resistor is 220K. He was giving as an example a bypass cap value of 20uf. The one in this amp if .047. Quite a difference. Thanks, Neali. That was good stuff. I did not find the link he referred to in his text. This is getting interesting.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Sat Jun 20, 2015 8:21 pm

I googled it and it came right up. Very interesting.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by neali on Mon Jun 22, 2015 12:35 pm

Thanks Jerry, I have to make more time to watch more of that guys videos. He is a gifted teacher.

Hey Wildcat,

Here are some thoughts of your 12AX7 simulation.

While interesting, I do not think that simple example relates well to the first AF stage of your amplifier. You have a lot of things going on that aren't in his example.

The first is the low pass filter immediately in series with the input. This is comprised of L21, R74 and C69. L21 blocks high frequencies and C89 shunts them to ground. If I knew the inductance value of L21, I could tell you the 3 dB corner frequency for the filter. So this amplifier, unlike the one in the video, is not going to see a flat audio spectrum. The low frequencies are going to dominate. Think of this low pass filter as part of your crossover network.

Now lets discuss what is going on with the three triode pins.

Grid:
R75, 220 K in the common grid resistor as you noted. Also present is C70 - coupling cap from Low Pass Filter, and R76 (although I can't tell if it is deleted and shorted out on the schematic - seems to be.) If R76 was present, it would lower the gain evenly across spectrum by about 40%. Simple voltage divider.

Cathode:
Here is where it gets hinky. While C(bypass), C72, 0.047uF and R(bypass) R78 1.5K are easy to identify, the frequency vs. gain behavior is going to be largely affected by the 16 ohm tap of the audio transformer connected through the long tailed pair, R86 and C75. This provides GNFB - Global negative feedback.

Plate
Plate resistor is 220K R77. C71 680 pF would damp any hyper sonic parasitic oscillation and C73 is the coupling cap to the next stage.

Just wanted to advise that this a bass amplifier, not full spectrum and the GNFB will have considerable effect on the gain/frequency plot.

Some people build amps with no feedback, either local or global, but PPP amps tend to oscillate because of the summing of the Miller capacitance so I would be leery of going open loop.

I am following your thread with great interest and anticipate your testing of the stock restored amps and then possible modifications. Eventually, when your magnum opus is complete, please feel free to delete my chatty conversational posts so your thread will be a useful archive for future CG restorers.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Mon Jun 22, 2015 5:11 pm

The value of L21 is 380 ohms. R76, a 150K ohm resistor, is there. So far, everything is stock. There are only three resistors that I have not replaced so far. Both cathode by-pass resistors R80 and R101 have been replaced. Both caps have been replaced. So far, so good, just tedious. After I get all the components replaced that need it, I will check all the resistances to ground to make sure they are all okay. Thank you for your input, Neali. I very much appreciate it. I am going to finish the Channel 1 amplifier, power it up and make sure nothing smokes, then go to Channel 2. There is really nothing hard about this, just takes a lot of time.

WC

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by 75X11 on Mon Jun 22, 2015 6:09 pm

Looks like you have some room to work in, in there.
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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Mon Jun 22, 2015 6:18 pm

The inductance of L21 is 540 microhenries (uh).

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Tue Jun 23, 2015 3:43 pm

The Channel 1 amplifier is done and is sitting back in the cabinet.  I powered it up on the bench with my dimbulb, Variac and iso transformer.  No smoke or anything weird happening.  I have good B+ and the voltages are approximately where I would expect.  There was no input signal.  I kluged speakers up for a load for the output transformers.  I ran all the resistances per the chart, and everything looks good there.  I replaced every component in this chassis with the exception of transformers, chokes, the wiring and 11 resistors.  Current draw with no signal was 1.6 amps, with spec at 1.8.  Pretty close.  So far, so good. The final test will take place after I get the other amplifier done and power up the entire instrument.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by 75X11 on Tue Jun 23, 2015 4:45 pm

Good luck!
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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Tue Jun 23, 2015 5:09 pm

Thanks, 75. I'll need some. There is still a long way to go.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by jerryhawthorne on Tue Jun 23, 2015 5:15 pm

Sounds like good progress WC, add some pictures as you progress.
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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Tue Jun 23, 2015 5:37 pm

I forget to take pictures then think of it later. I have some I can post once I get them on my computer.

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by Wildcat445 on Thu Jun 25, 2015 6:27 pm

Channel 2 amplifier is done and back in the cabinet.  It passed the "smoke" test. I will call amp draw a "short" 1.6 amps, with 1.8 being spec. It reads about a half mark shy of 1.6.  No signal, of course.  I have good B+ and no hum.  I replaced all the filter caps, all the coupling caps, both cathode bypass caps and resistors.  I replaced all the resistors except for 13, which were still within 5%.  More evidence that mine is a low-hour instrument.  Although I replaced all the 220K ohm grid resistors, I actually found two in this chassis that were still okay.  These are known to drift, so I always replace then just like I do coupling caps.  I installed a 2 amp fuse just for good measure, as I did in Channel 1 amp.  Next up is to power everything up and make sure I have audio from both channels.  I will power up each amplifier individually with 100 watt dimbulb, Variac and iso transformer, although the latter is not really needed.  No choice in my case.  I still need to restore the radio chassis and power supply to eliminate the possibility of audio problems there.  Those components will come out next if all is well with the amplifiers.

WC

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Re: Restoration of a Magnavox Concert Grand model 1ST800F

Post by jerryhawthorne on Thu Jun 25, 2015 7:33 pm

Great WC, we are waiting with bated breath!  Crossing our fingers and waiting for pictures with you with a smile on your face. Wink
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Re: Restoration of a Magnavox Concert Grand model 1ST800F

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