Classic Hifi Care

I just found this Zenith upstate from the original owner's son. Of course, I have two other amps just like it to fix . It is a front-loader, so hard to find around here.

Repairs pay for my collectibles, only way I can stay in business as a collector

The next year model in 1961 also had "extended stereo" but used horn tweets plus a 4-tube reverb chassis unlike Motorola equipment, along with the extended stereo PP EL84 amp 7F33, the next version of the 7F31 I have now.

I like anything that has front access. It leaves the entire top open to display one's other toys.

Bill

Their OK as long as you don't have a bad back

hermitcrab wrote: ↑Fri Nov 25, 2022 5:24 pm Their OK as long as you don't have a bad back

Very true indeed. I think this preference faded quickly, based on designs of the later years.

I actually got to play with it too An acquaintance of one of my best customers mailed me his 7F33 amp from a Zenith SFD2560.

I was going over the re-cap work done by another guy he bought it from and found just one (1) mistake in all the capacitor replacements, a .022 where a .22 needed to be. I did catch about 17 OOT resistors, all half watt with silver or gold bands, 10-20% too high. Many restorers assume Zenith and Motorola used better quality parts, but all carbon resistors increase in value just sitting around. I have tossed over half of the spare, unused NOS resistors I measure, so I order new ones at 1% metal film. These are not expensive but time-consuming to replace, in order to thread the small gauge wires onto tube pins and terminal strips.
Because this amplifier is not a typical stereo P-P 6BQ5/EL84 amp, it has different feedback for each channel, completely different tone, balance, volume and an expansion (extended) stereo control that ranges from mono to stereo to extended. Hence, the off-tolerance resistors can really exacerbate balance and tone issues between L + R channels.

The only problem was testing the amp, because the tuner-preamp was not sent along. Yet, a power-up showed voltages a bit high, as the amp shares its power supply with the preamp/tuner. I decided to check schematics between my 1960 preamp-tuner 9D24 and 1961 preamp-tuner 9F24, seeing a few differences mostly due to the 1961 SFF2560 model also having reverb!

With these options, I tried to plug in the amp anyway and first my tuner preamp in my SFD2565 has a bad power switch, very scratchy volume control and neither AM nor FM was very good at all. I used a CD player connected to tape input. It did present the effect of expansion (extended) stereo control very well. I can tell you this is not reverb but it is a Zenith exclusive design that no other manufacturer offered to my knowledge. It was on only 1960-62 models and reverb was only offered on the 1961-62 models. At any rate, it is a very different reverb circuit using 3-4 tubes on a separate chassis.

In operation on my tuner/preamp, the balance control only silenced one channel. The presence control (Magnavox "timbre") did nothing. Bass and treble seemed to work well. But extended stereo control made up for it by creating an effect I can only describe as ambience and sound staging that could be overdone with a control just like reverberation. I was using a stereo source so I'm not sure how a mono channel could benefit from this effect.

I mailed the amp back and the customer says he is happy with it, so I now want to restore this 1960 and 1962 model MH2635 I have with reverb AND FM stereo but a single-ended 6BQ5 amp .

Dave my '61 (?) Zenith with 1 tube preamp and no tuner has a "presence" control iirc. How in the world can it have: Bass, Treble, Volume, Mode selector, and "Presence" controls on that little preamp, all out of 1 lonely dual triode 12ax7 ?!?

Dave you do beautiful work under the chassis.

electra225 wrote: ↑Tue Dec 06, 2022 12:56 am Dave you do beautiful work under the chassis.

I think I should credit a previous owner for the cap work, which even included the dubious-quality "22" disc caps dipped in wax. The new owner is color-blind, so I mostly took on the resistor work.
After I replaced resistors, it got real colorful under there

Firedome wrote: ↑Tue Dec 06, 2022 12:24 am Dave my '61 (?) Zenith with 1 tube preamp and no tuner has a "presence" control iirc. How in the world can it have: Bass, Treble, Volume, Mode selector, and "Presence" controls on that little preamp, all out of 1 lonely dual triode 12ax7 ?!?

I'm grasping at straws here, when stereo furniture (SF) models were introduced by Zenith, users had to compensate for a variety of recording issues, playing mainly earlier mono records. New stereo records were more consistently equalized.

Using a stepped equalization control like on my Harmon Kardon A300, you select RIAA, NARTB, as provided "factory set" equalizations. There are many caps and resistors associated with these preset options. I bet this was done initially before users felt more able to use a variable control rather than incremental settings. Presence and Timbre seem to affect only mid-range, but also affect sound staging somewhat.

The phono-only preamp feeds into the 7B31 amp, which has a second high-gain preamp stage, inverting and push pull outputs. With only one 12AX7 configured for high voltage gain of a 0.3 volt input into the RC network formed by the volume and tone controls. In radio models, this input stage is on a tuner-preamp chassis.

Interesting, so one of the preamp stages is in the amp chassis itself, and equalization was more simple and built-in by '61?
I can't recall what cartridge the record changer came with (or even what changer it is... senior moment alert!)... whether it's an earlier Shure M3 or M7 or a ceramic of some kind (will check when we get home next week), but if was a MM cart I assume it would need more preamp gain than a ceramic.

Now that I think on it, the console mine came out of had a Cobra-matic, I guess those were VOM built?
Still can't recall the cartridge or whether it was MM or ceramic, did Cobramatics use either type in different consoles?
Wonder what the differences are between amps 7B31, 7F31 and 7F33?

All the Cobra-matic changers that I have seen were built by VM. I'm not sure if VM produced the tone arm as well, or if Zenith supplied it.

Bill

I've read here that Zenith used a belt-drive changer at some point. Maybe somebody who has studied Zeniths can give a short history of their changers, with their names, mechanics (belt vs idler-wheel), makers, etc.

Chris Campbell

Zenith had in-house made belt drive 2G Microtouch
changers from 1962 to 1968.

Two Chris Cuff (R.I.P.) videos he made about them.

https://www.youtube.com/watch?v=nOGfkrqOaC4

https://www.youtube.com/watch?v=SVODx5PxDSw

As far as I can determine, Zenith made their Cobramatics
of the late 1940's to mid 1950's,
some with Stroboscopes.

Strobe: https://www.youtube.com/watch?v=e2SkspAPWso

Two Zenith in-house changers had a chain drive (1949-1950).

I found a 1950 Sams 112-15 that appears to be V-M made
Zenith idler drive changer with a Cobra tonearm.

The V-M later 1950's Cobra changers had Zenith special knobs, other
changer base accessories and the Cobra style arm.

Good info. Thank you. Let's clarify that the "chain drive" refers to the changing mechanism only. Ive got an old Zenith tabletop 78 rpm radio/record player with the chain drive mechanism. I'm pretty sure the platter is idler-wheel.

Chris Campbell