Author Topic: My Maggie: O, how she sings! sometimes  (Read 80 times)

AlanInSitges

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My Maggie: O, how she sings! sometimes
« on: September 06, 2019, 02:49:53 PM »
I've posted in a couple of other threads about (probably) the only Magnavox console in Europe, and how it's MINE!, finally, a 3ST657 Danish Modern with the 9800 series amp and 100 series tuner.

I've finished the repair and restoration of the power amplifier (98-04-20) a few hours ago and been listening with glee. I changed all the filters and bypass electrolytics (along with the dropper resistor for the B+ supply to the preamp/phase inverter tubes), changed the crap Elmenco coupling cap on the phase inverter side (the other one had already been replaced at some point in the past but it got changed too) and for the moment left the ceramic coupling cap on the preamp side. Also replaced a weak rectifier and tracked down a cold solder joint on the ground side of the output bias resistor.

I powered up the amp directly from a digital source and, well, you already know: it's glorious. The bass ain't all it could be, but I'm using some 8-ohm Klipsch speakers on the bench so it's not running with the correct load. I did jump in some .0047 film caps in place of the preamp coupling caps and I'm not 100% sure I could tell the difference in a blind test. Things may change when it's running into the right load with those big-ass woofers. I may also put a scope on it and see what response looks like below 100Hz with and without the higher value in there.

Anyway, a couple of questions and an observation:

- I have also done a cursory recap of the preamp/tuner (79-09-10), replacing the bypass caps on the audio output tube and the two filters. Most everything in the audio path is a couplate (ain't nobody got time for that), along with some .01uf ceramics that I haven't changed. Running the same source through the preamp's tape inputs and then into that power amp and all the magic goes away. It's flat, and muddy, and the highs are rolled off...ick. Anyone have experience or advice to offer here? Should it sound better than that? What's missing?

- Interested in others' thoughts on swapping out those .0015 ceramic caps for a higher value; there's been lots of discussion on AK and elsewhere but always in the context of using a pulled amp, without the correct original speakers. I've got some replacement poly .0015s, as well as some higher values. Is this worth it? Bad idea?

- I was surprised and fascinated to realize that this tuner/preamp runs in MONO(!) below about 200Hz! The bass control is a single potentiometer. I guess it makes sense, especially in light of the weird asymmetric 225Hz bass roll-off in the power amp; still, I've never seen that before.

I still need to clean up the tuner/preamp and recap the mpx adapter. Then it will be on to that turntable.

electra225

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Re: My Maggie: O, how she sings! sometimes
« Reply #1 on: September 07, 2019, 12:06:01 AM »
The Symphony series, in all its various forms, is THE BEST stereo Magnavox ever built, period, hands down.  Best in every conceivable measure of performance.

My best advice is:
1.  Don't follow any advice you may get on AudioKarma. 

2. You won't need to re-engineer the 93 series amp or any of its various versions.  There was never a better 15 watt per channel stereo amp ever produced, anywhere, anytime for any amount of money.  They are total sweethearts.  Replace any 220K ohm resistor you find in that amp.  Don't measure them, just replace them with a 1% metal film resistor.  This is especially critical in the grid circuits.  They are cheap components to start with and they drift high with impunity.  You may find you'll have to replace 80% of the resistors in that amp due to drifting.  I replace all the resistors as a matter of course.  Also replace both the cathode bypass cap and the cathode bias resistor without fail.  I use 47uf at 50 volts since I have plenty of those on hand.  I also like to use 525 volt filter caps, and I increase the capacity of the output filter cap, the one following the choke to 47uf.  Replace the rectifier and the audio output tubes with JJ's.  I do not like NOS tubes, I never trust the originals.  The 6EU7's should be carefully tested for shorts.  They are not very good and are problematic.  Carefully clean all the tube sockets in the amp and in the tuner.  I restuff the can on the tuner, since there is not room under the chassis.  I either restuff the can on the amp, or use a terminal strip under the chassis.  I also install a 1.5-amp Slo-Blo fuse in the primary of the power transformer in the amp.  You won't need to change ceramics.  Do not judge the output of that amp or tuner depending on the input of a device in the tape input.  Make your judgment based on phono input, using the original changer and cartridge.  I use the 525 volt filter caps to compensate for the increased line voltage we see nowadays.  Be sure to set the bias voltage for the output tubes per the schematic.  IIRC, it calls for 11 volts, but don't trust my memory, check the schematic.  This voltage is set with the hum control on the chassis.  Not super-critical, but it drives the output tubes really hard if it is not set right.  Sometimes that hum control is intermittent and I've heard reports of them shorting.  I have never had one short, but I clean them if they are intermittent.  Leave them alone if they are okay.  The bass on the 93 series amp is the strongest of any Magnavox builds.  Use the original Magnavox speakers before you judge bass output.  I have NEVER EVER heard of anyone complain about weak bass from a 93 series Magnavox amp.  Don't mess with any couplets.  There are several in Magnavox tuners.  I've never had to mess with one.  There is a double electrolytic cap in the tuner, small value.  I don't have the schematic in front of me, and I should have to comment on them.  In any case, these need to be replaced with two caps.  Then just the paper caps and test those pesky 6EU7's.  I'd bet your performance issues will be found to be due to a wonky 6EU7 or a dirty tube socket.  Clean CAREFULLY AND THOROUGHLY both the function switch and all the tone and volume controls.  Don't forget to clean the external speaker switch. 

Some versions of the 93 series amp have the balance control on the amp chassis, some have the balance control in the hollow shaft of the bass control.  Whichever version you have, you set balance by playing a mono source, record, radio, whatever, then, using a VTVM on each speaker, set the balance control until the speakers have equal volume/voltage.  Not the most convenient method in the world, and you may not get it right the first time, depending on what record you are playing.  It is not tough to do, and if it is not perfect, no harm is done.

The tuner is all mono with stereo capability.  You may notice the "FM MPX" position on the function switch.  This is for use with a Mulitplex adapter that you need to receive FM stereo broadcasts.  Yours should have an "FM" "FM MPX" and an "FM AFC" position on the function switch.  You have the adapter, so you should be all set there.  Replace the 30uf filter cap in the MPX and there are also a couple small-value electrolytics in it as well.  And it may have another 6EU7, depending on the model MPX you have.  Be sure to clean the tube sockets in the MPX and be careful not to bend the pins in the Molex connectors. 
 
Antique Electronic Supply has the best prices on JJ tubes. 

Replace the resistors, all the paper caps, all the filter caps, amp, tuner and MPX, don't overthink the project and connect all the original components in the cabinet before you make any judgments on performance.  The electronics are tuned to the speakers that are tuned to the cabinet.  Yours may be new enough it has horns replacing the six paper cone speakers my Symphony has.  Mine is an early 1961 with no FM AFC.  When you tune the tuner on FM, start in the "FM" position, find the station, switch to "FM AFC" then to "FM MPX" if the broadcast is in stereo.  Magnavox tuners without AFC tend to be drifty at times. Mine has the eight tube, six knob "series 200" tuner in it.  I believe yours has the six-tube, five knob tuner. 

I don't get into scoping and "roll off" or any audiophile activity when I'd dealing with Magnavox..  I just put them back like the factory built them as best I can and let the chips fall where they may.  I'm certainly not smart enough to come up with much in the way of improvements.  You are dealing with a 15WPC amplifier.  That is relatively tiny as far as stereo amps go.  When you set back and listen to Patsy Cline on a Symphony, my goodness, how in the world could anything sound better than that?

I can say here, without doubt or hesitation, that the two Magnavox instruments I own with the 93 series amp are amazing devices.  I have traditionally said my Symphony was king of the hill.  That was before I got the Stereo Theater, and it only has 12" speakers.  You could put 4" clock radio speakers on a 93 series and it would damage drywall, it is just that good.  My wife plays the Symphony then calls the Concert Grand a "300 pound clock radio".  That might be a touch of over-dramatization.  My Symphony makes me grin every time I use it, and I'm betting yours will too.  The best of luck with this project.   :) :)
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