Author Topic: 1964 Zenith console with drop-in tuner  (Read 3405 times)

electra225

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Re: 1964 Zenith console with drop-in tuner
« Reply #15 on: June 13, 2018, 12:32:00 PM »
I know you've already considered this, Dave, but you might change the orientation of the cabinet with the internal twin-lead connected to see if there is any improvement.  Magnavox has that issue.  The internal antenna is extremely directional at times.  Even an external antenna will not help.  If it helps to change the orientation of the cabinet, then you can use an external antenna to advantage.
I'm great at multi-tasking.  I can listen, ignore, and forget all at the same time.

Motorola Minion

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Re: 1964 Zenith console with drop-in tuner
« Reply #16 on: June 14, 2018, 11:52:07 AM »
I know you've already considered this, Dave, but you might change the orientation of the cabinet with the internal twin-lead connected to see if there is any improvement.  Magnavox has that issue.  The internal antenna is extremely directional at times.  Even an external antenna will not help.  If it helps to change the orientation of the cabinet, then you can use an external antenna to advantage.

The garage I'm working in has some odd reception quirks, with a big Pontiac 6 feet away, I definitely need an outdoor antenna. I have a dipole up at the ceiling but that seems only a bit better than a 30-inch clip lead I hang near the tuner. Even a 1966 Imperial chassis I'm working on has poor reception at the low end of the band, the only FM I'm interested in keeping on for more than a few minutes ::)

Putting the "shielded" cover back on the bottom of the chassis helps too. I'm not quite ready to drop it in the cabinet yet.

My 1960 Magnavox Symphony reception seemed odd this way too. This Magnavox had a 57-01 tuner having work done to it and a few new tubes. I could not get it right as it was "re-aligned" such that only the most powerful stations (commercial ones) came in clear. I suspect some one knew what they were doing generally but did not follow a few key rules like keeping shields in place and "maligned" it instead. :(

Luckily, I had a 57-03 tuner from a recent swap meet, complete and un-tinkered with. That one has not been aligned but the tuning cap's "copper fingers" are scratchy, and keeping from being all it can be. It has a very tight area below 90 Mc, but still can get a medium station next to a strong one.
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Dave

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Re: 1964 Zenith console with drop-in tuner
« Reply #17 on: June 14, 2018, 12:08:32 PM »
What is really holding this console back from sounding great is the 10 inch OEM Zenith speakers, 49-950. They seem worn out and one is especially fuzzy with a rubbing voice coil.

Sure I may be able to wet and realign the cones but in this case its better to upgrade them. I saw a post about an RCA (it was not Larry) having questionable woofer-driver speakers and the successful replacement of same with new ones for less than $50.

After looking on the website and comparing about 6 possible replacements, I found a speaker that is right in the 6-8 ohm impedance range AND 96 dB sensitivity . I ignore the power rating of 100 watts and look at everything else, sheet attached.

And, like the RCA owner did, I can enclose/isolate the speaker compartment on this model. Even with only a single-ended 6BQ5, sound and power could be comparable to a PP 6V6 amp. Even though its not one of the expensive Zeniths, I put effort into the chassis and want it to sound like one.8)

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Dave

TC Chris

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Re: 1964 Zenith console with drop-in tuner
« Reply #18 on: June 14, 2018, 11:27:37 PM »
Sounds like you're having fun.  I bought a couple 10" speakers to fill a Heath console and they are still sitting on the kitchen floor, awaiting time and opportunity.  It's summer now and there are summer things to do, so the electronics are deferred. 

But if you find time, post something about your rubbing voice coil repair method.  I've got a couple E-V reflex speaker enclosures built from kits long ago and using E-V LT-12 speakers, if memory is correct.  One developed a rub.  I added a shim under the basket and torqued the screws enough to make it stop, but that's a pretty crummy solution.

Chris Campbell

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Re: 1964 Zenith console with drop-in tuner
« Reply #19 on: June 15, 2018, 08:41:53 AM »
I pulled the speaker out yesterday, the voice coil does NOT move freely. EIA code 343 was the OEM, and not their best work.

The cone is OK but the coil may have been, well cooked :o, causing all the friction. I swear I had a '68 solid state Zenith console with the same issue on the 10"drivers, I know those were baked 8) by a friend of mine, who showed its 2g record player and orange&yellow illuminated dials , ....when his parents were away.
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Dave

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Re: 1964 Zenith console with drop-in tuner
« Reply #20 on: July 12, 2018, 08:02:47 AM »
The FM problem is solved and I learned (or was reminded) of something important about tubes.

I had only one other FM-osc-mixer tube to confirm with*, and it did fix it. In one Substitution handbook, no sub is listed. In another, the 6JK8 has two subs and four alt subs that "may not work in all circuits", obviously having a 9AJ base diagram-pinout.

Many dual-triode RF tubes fit this pin-out and the 6BC8 6BK7-6BQ7-6BZ7 all worked in this tuner  ;) when I tried out NOS boxed of these, which any TV repairman had tons of!

 *6JK8 was subbed with another found in a 9H20Z tuner in an other Zenith console.  a 6JK8 was made with one triode using a "frame grid". 312 coded means Sylvania custom-made this tube for Zenith to meet a tough spec for sensitivity, no surprise. 
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Dave

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Re: 1964 Zenith console with drop-in tuner
« Reply #21 on: July 12, 2018, 09:52:39 AM »
The other part of this console, I'm working on is the speakers. I got a pair of 10", "200 watt" which sound far different from the originals which seem to be more sensitive than the new ones, spec sheet claims 96 dB.

While the mechanical issues like rubbing voice coils and loose cones (coming unglued at basket/gasket) are gone, so is the "lower-volume bass" these Zeniths are noted for.
Even the bass-boost acts weird using these speakers. I'm sure frequency response shaping RC networks of this amp are dialed in to the specific speaker used.

This explains why Magnavox AMP-175-xx and others use several suffixes on their own amps, often with a chart (factory schematics only, not Sams) or notes showing different resistor and caps values in de/pre-emphasis networks. All this allowing the same rugged amps in the various models' speakers and cabinets.

The originals are not great but they are same part number as in another console I have. Out of four, I plan to use the wet-dry cone-reshaping method on the worst one and compare. The cones look great otherwise and have no rips-tears. This exercise actually discourages replacing and makes a great argument for re-coning of originals :)

More to come, as the repaired chassis' are put back in the cabinet, speakers re-shaped and boxed-in.
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Dave

TC Chris

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Re: 1964 Zenith console with drop-in tuner
« Reply #22 on: July 12, 2018, 08:11:28 PM »
Interesting account of the RF tube substitution.  It's great to have a bunch of tubes, either stored on a shelf of residing in their devices.

But once again, share your speaker-cone repair methods.  Most of us know that sad sound of a rubbing voice coil.

Chris Campbell

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Re: 1964 Zenith console with drop-in tuner
« Reply #23 on: July 18, 2018, 09:04:02 AM »
The FM tuner is now working as well as it ever has, no drift and stereo light on. The 6JK8 was substituted with a 6BQ7A/6BZ7, which works the best overall.

I tried other dual triode tubes used as RF amp/osc and most worked: 6BK7, 6AQ8/ECC85 that the Germans and Fisher used.
The 6AQ8 tested OK yet it did not produce a strong signal and the tuning had to be moved about .3 Kc, as an oscillator, differs in operating characteristics from the common 6BQ7. 

With no static from silver-mica disease on AM or FM :D, I can finally put this in the cabinet and re-visit the speakers.

 
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Dave

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Re: 1964 Zenith console with drop-in tuner
« Reply #24 on: July 18, 2018, 09:20:43 AM »
After doing the soak and set on the originals, the mushy sound is gone ;D! The tone controls and bass boost operate as intended. I greatly underestimated how good the original speakers are and how tailored this amp is to drive them. The replacement speakers I got will have to be used in another project. Their specs, if accurate, were lost on this amp.

Now to how the speakers were "fixed": What I did was spray water onto both the cones from the front, being careful not to let the water run past the voice coil felt pad, then flipping them over and wetting the cone thoroughly from the back.

A local radio guy that posts on Videokarma clued me and several others into this trick. I have used on several speakers to date and I think I know what de-formed the cones.   

Most consoles live in a nice house but then moved to garages, basements and worse than that, out-buildings and barns. Rarely are they found in attics like old radios, heat does other nasty things to old rubber and plastic.

The speaker is mounted in such a way gravity is acting on the coil/cone assembly. Humidity causes the paper cone to deform and "droop" pulling the voice coil out of alignment.

What the wetting procedure does is make the cone pliable, then drying/setting with speaker in a vertical plane and allow the voice coil and spider assembly to "fall" back into alignment..
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Dave

Bill

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Re: 1964 Zenith console with drop-in tuner
« Reply #25 on: July 18, 2018, 09:39:04 PM »
I would never thought of that.  Too me water and paper on speakers would not mix.  This is one for the memory banks, just in case I ever need a speaker fix. 

Bill

ed from Baltimore

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Re: 1964 Zenith console with drop-in tuner
« Reply #26 on: July 19, 2018, 07:36:24 AM »
         There is also a difference between 6BK7 and 6BK7A. Usually the difference between A and non A is controlled heater warmup time for use in series string filament chassis but I think in the case of the RF amplifier dual triodes, the difference is that the two triode sections are not identical. One of the sections has a remote cutoff characteristic so that Automatic Volume Control voltage can be applied to its grid from the detector output and the gain can be reduced without cutting the current off, which avoids cross-modulation distortion on strong FM signals. The tube might be labelled 6BK7A/6BC8.
         6DJ8 is two identical frame grid sections with higher transconductance than the other dual triodes. Old Tektronix oscilloscopes used them all over the place in balanced differential amplifiers, and selected ones got a Tek part number and went into more critical sockets than the rest  of the scope. Some old Fisher FM tuners used them and they had really odd numbers like Z10000 or something odd like that printed on the RF subchassis.
         The FM RF amplifiers had such selective tuned circuits that the least bit of difference from one type to another would require at least  some realignment. So many- different tubes had the same pinout that just about any dual triode will work at least some, but the exact tube called for is best------and then some adjustment is still needed after 50 years !!
   My HH Scott FM tuner has 6BK7/6BC8

Motorola Minion

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Re: 1964 Zenith console with drop-in tuner
« Reply #27 on: August 30, 2018, 10:29:54 AM »
After weeks of playing this console while I work on a Motorola phono-only tube console, I have gotten the amp and tuner to the point of diminishing returns. Putting them back into the cabinet, now that Im satisfied with the speaker box-ins.

I still have the record players for both these to do but playing the radio on a few great non-commercial jazz-classical and independent stations is fun without handling records. But a console IS a record player first, receiver second.

The 6BQ7A now in the tuner does not drift, stereo reception below 91 MC is possible with two 20" antenna wires (a too-short dipole), crystal clear with just a bit of weak-reception hiss. It is hard to beat a older tube Zenith tuner unless you have a Fisher 8) of the same era. An rooftop FM directional yagi antenna could be awesome, but not required to get the best FM anymore. Out of all the distant FM possible from Philadelphia and Baltimore, nothing beats the local translators. 

The bass boost switch does odd things to sound of this amp, but Zenith went all SS after this model, and bass-boost was not a .

]I measured the bias (meter in the photo) of 11.3 volts on the 6BQ5 cathodes, meaning these are probably low hour tubes. I can hear a huge difference between a weak output tube as the cathode voltage hovers around 6 to 7 volts.   
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Dave