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#31
08-16-2010, 12:25 PM
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you know andy you hit it on the head. I finally swapped the outputs since I could not figure out what was going on, bingo, both channels come back with no distortion at all. I still have a nagging hiss, its after the volume so it has to be in the finals, driver or amp. I think its the finals. I will try swapping around to see if I can isolate. I tested them for leakage and they had some, but on finals I think some leakage is ok. I tried swapping out the drivers with some good PNP germainium, no help. Maybe a leaky cap? well moving transistors around should test that theory. sure sounds like a leaky transistor.
Not a big deal since I have already swapped the tuners out, I just dont like unsolved puzzels. 
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#32
08-16-2010, 08:53 PM
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well the stereo light is acting up. it comes on weakly, and the transistor that drives it get hot. gonna have to look into this a bit more. the tuner sounds great with the new coupling caps to the drivers and a couple new audio amp transistors (no more hiss at all).
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#34
08-16-2010, 10:20 PM
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I lioke that Stereo chassis.
It is the "second best" of the Zeniths. I have about 4 of the various complete chassis like that lying around, (including a 26XT20, and a 26NT20) and a complete console stereo with one. It is NOT as nice as the "big one" with the separate amp in the bottom and 15" 3 ways- (like the 6AT24 amp and 29AT24 tuner chassis--(a console set I ALSO have--and may get another one locally soon..) --but is is pretty good. Probably about 20W/CH or so, I would say. There was also ONE "horizontal version" of that chassis--laid out somewhat differently--but electrically very similar--the 26YT21--I have that chassis too. I LOVE the "look" of ALL of them--nice "eye candy" to me. I LOVE old SS stereo chassis-- and have a LOT of the Zenith ones a few RCA's and a couple GTE's.
I personally think the old Zenith SS stereos were THE BEST--MUCH better than Maggys or even RCA sets--though the Sylvania's of the time frame(1965-70) or -- were not bad--either....their TOTL and next in line anyway.
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#35
08-16-2010, 10:52 PM
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yea its a sweet chassis, its amazing to think of the hand wiring that went into that set AND the fact that it was done here in the the USA.
The only thing I dont like about it is the dumb soundess contour built into the volume pot. Kills the highs rather than pumping the lows, at least until you crank it up. I would love to just disable that tap but I respect the thing to much to start re engineering it. I may check some of the componets in that area to see if anything has drifted making it worse that it is. You are rigth about it in comparison to the maggie (the only other SS tuner I have), it has better sensitivity, and very low distortion at higher power settings. I seems to have a decent PS for an early SS unit. I need to seset the opt transistors, the heat sink paste is all crumbly but at least those expensive PNP outputs are good. there is a slight hum that is not present on the ebay find so I will need to go thru the caps. Funny how one set has bad caps and the other does not. I think it has something to do with the storage enviroment, maybe the useage as well. The Ebay find was pefect, but for a slight bend in the tuning shaft. I dont know how that could get bent and not break the tuning knob. It has a slight over center feel as you turn it, Not a big deal but just enough to bug me. The tuner that came in the TV console has a perfect tuner shaft.
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#36
08-17-2010, 07:35 PM
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new bulb fixed the MPX light, guess the #47 was messing up the bias. It now comes on like a switch and is nice and bright. Before it would always glow dim and just get a bit brighter (but still not very bright) when the MPX was detected. Not to mention the transistor was getting warm that drives it (warmer than it should be).
Only thing left is the SPDT on off. Not sure how that is going to go. my guess is a burned contact. It closes when the button is pressed, press again and its off, but will come on if the turntable is on.
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#37
08-18-2010, 06:33 PM
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progress, on off switch on stereo tuner fixed, took it apart and vigorously rubbed the contacts. seems ok now, but time will tell.
Now the decison, to I leave the replacement stereo in or put back the orig. The replacement has a slightly different face (mostly the way the channels are layed out). Its a nothing job to replace it. But then again the switch may act up. I think I will just use the orig and test the switch. I just got in some HV caps that I needed to finish off my 20Y1C38 zenith so I will button it up, leave the CRT for now (its the one with a weak reg gun but not too bad after it warms up), and then I will pull the chassis on this console and go over it. Its a filthy mess so I will start by a gentle cleaning, then will check the B+ for for shorts and leaky caps. If that goes well then will pull the HOT and VOT and do a slow power up to see if the filter caps are ok. after that the reg test.
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#38
08-19-2010, 10:07 AM
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the MPX light still varied somewhat with signal stength. for fun I swapped out a few and found a transistor that would act much more like a switch either on or off. For fun I tested it and the Beta was nearly 10 vs 5 of the other ones that while worked, did have some dimming on weaker stations.
The transistor checker is kinda nice since I can use it to match transistors.
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#40
08-19-2010, 04:20 PM
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while playing with the tuner I checked the phone input, seems like a lack of separation going on, seems like there is some signal leakage.
I am thinking decoupling or some other power supply problem. I will put a scope on the B+ supplies and see how much signal modulation is there if any.
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#41
08-19-2010, 05:43 PM
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my ctc17 combo lost stereo separation too. I use the tape input for the cable box.
I figured it was a burnt or failed ground somewhere but you bring up a good point. O, btw... on both my zenith combos with the same tuner, the bass is mono and the high end is stereo. I never questioned it because thats the way systems with subwoofers are setup. There should be little to no stereo information in the bass and it makes sense to have it monoed out.
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#42
08-19-2010, 08:56 PM
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its goofy. I put in a signal 400hz into the left channel thru the phono, then I scope the base of the left transistor and compare to the base of the right. WTF the signal is about 1/3 the Left.
there is a network of resistor that connect the two bases togehter on mine its a 56k/22k/56k. I removed the transistors from the sockets and check the resistance, total was about 160k which would make since after accounting for drift there was nothing else in the circuit but for the disc coupling caps. I dont know why this would be the case, but I can clearly see the 400hz on the base and of course the collector of the right channel. I wonder if there would have been any reason to tie the two channes together?
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#43
08-19-2010, 09:14 PM
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I will check the replacement tuner if it does the same thing then between us thats 3 that lack decent separation, and maybe its just the design.
If so I may be inclined to see what I can do to break that connection and see what happens. Worst case is I muck up a few transistors, but since they are in sockets, big deal.... what bugs me is to go thru all the effort in the mpx to separate the channels just to throw them back together in the amp.
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#45
08-19-2010, 09:49 PM
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my osc is just a single hrz, I may have anoher will have to check around
BUT I did disconnect the singe resistor that connect both the transistors bases, other than the bias voltage being a bit further off than before (it tied them together) I could hear no difference AND much better separation. about 10 to 1 where it was about 3 to 1. clearly that resistor was the signal path between the two channels, whats left is prob un decoupled from the B+. I am going to listen the to radio all day tomorrow and see how it does. I may go thru my stash of transistors and see if I can match some (I have a transistor beta checker). Just to see how close I can get the bias.
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