Classic Hifi Care

I got a call from a visitor to the antique mall asking if I would work on a Zenith console. I do make more money servicing rather than selling, which is standard cover for curating a collection. This model was a rather budget priced for 1961 with one feature making it special among all others. The Zenith service info shows this model as having a 20 watt stereo amplifier with extended stereo effect on a three-way mono-stereo-extended selector. See model for sale here https://www.bing.com/shop/productpage?q ... rowse=true

Making a housecall to inspect and estimate, I found a well-preserved but inoperative set. It was found the 2-amp fuse (good move Zenith) was open. That explained why it stopped working after moving it to present location, so I removed amplifier, tuner and Cobramatic style VM player. No way did I want this console in my garage now, its bench season!

The player was fitted with a "power point" style cartridge, which has an integrated stylus/ceramic element that "plugs in", I was surprised to find the player in good condition with serviceable idler pulley and other rubber parts. What I did not expect was to see the factory wiring mistake on the stereo phono cart wires. The left side was grounded and right side connected to both preamp leads! This never sounded good from new Im sure! I have found wiring mistakes in Motorola and RCA chassis too! I swapped out for a EV5632-HLF and Tetrad plug, did not even need to change terminal lugs. I assume this was the worst it could get, as the VM players always need a good bit of work to restore operations, but it was the easy part compared to this 5F29 amp. The player sounded excellent and played right on speed.

After replacing 14 wax-papers, four electrolytics and one cathode resistor. I was ready to power up this AM and it blew both a 3 amp and then 2 amp fuse. Maybe time for a dim-bulb tester but I suspect the typical Zenith troublemaker, a bad power transformer! I see no wax drips, smell or sign of overheating which normally accompanies a shorted primary winding.

Being that the 5Y3 tested OK, this may have been caused by a voltage surge as can happen when an electrical system loses its Neutral Conductor and current imbalance on your two lines of 120 causes one line to drop while the other line surges the same amount, approaching 240 volts! This is WHY we place X2 and Y2 capacitors across the line, where there is a very good case made on https://www.justradios.com/safetytips.html

Hey Dave. I have that exact model, minus the tuner, sitting in storage with the same issue. Mine too has a blown PT which I have posted about a couple of years ago and I believe you even offered one you had that would work in my Zenith. My cabinet is not in that good and one of the woofers cones is all torn so not high on my priority list for repair. If you still have that PT you offered me maybe it will get this unit going for your customer.

Bill

William wrote: ↑Wed Feb 19, 2025 4:32 pm Hey Dave. I have that exact model, minus the tuner, sitting in storage with the same issue. Mine too has a blown PT which I have posted about a couple of years ago and I believe you even offered one you had that would work in my Zenith. My cabinet is not in that good and one of the woofers cones is all torn so not high on my priority list for repair. If you still have that PT you offered me maybe it will get this unit going for your customer.

Bill

I agree that a transformer using a standard 5Y3 or possibly 6CA4 is best fit in this case.
This is not a push-pull like the 7-tube 7B30 and later 40-watt stereo amps. Those use a 5U4 or 5BC3 as a full wave rectifier.

Really: All you need is 6.3 volts AC 2.5 amp transformer and any transformer with a 120 VAC primary and secondary of 150 VAC (in which case you build a doubler with two 1N4007 and stacked 100 mf caps) or 550 VAC center tapped to use a tube full-wave rectifier. I have an eye on one of the spare Zenith chassis for it.

Yup, single ended, 5 tubes total and "stereo expander" model amp. Smallish output transformers too. Not sure where they get the 20 watts and only driving 10" woofers and one pair of 5" tweeters so 4 speakers total.

Bill

Pictured at right is a clean-looking transformer that once AC input hits 53 volts (on the way to 120, in theory), the HV center tapped secondary winding begins to arc internally @300VAC and a very acrid odor is silently and invisibly emitted, with absolutely no load on any of the other windings

The current meter on my variable power supply rises erratically past 1.2 amps as I advance past 55 volts input. That 2 Amp fuse did not save this transformer, failing under normal service as its dielectric strength via varnish on windings and paper separating case broke down. This overall rating is normally 1000 volts AC.

As Bill has observed based on his own unit's amp, the 524-prefix transformer (Triwex) fails again, so I have a more standard (for Zenith) 138-code OEM Stancor transformer* that is an exact match. I put it in last night and tried it this morning, all works as intended even thru my bench speakers. Just need to hook up the cobramatic with its new cart and see how extended stereo affects a stereo source, not just the mono radio source.
*For this trans I had to part out a MK2606, 4K21 amp chassis a third time, having removed the concentric volume control for a deserving SF2560 1959 Blondie Zenith. It already donated its 10" woofers to another Z on which I had bollixed a cone repair.
It has a nice mpx AM-FM drop-in tuner (on board PS) that needs a home, then its just a phono and some output transformers.

Running to a stock of spare parts like this amp for Zenith, Magnavox, RCA has become routine but not so much with others.

The official replacement transformer is a near match, I would trust this
100% enough to guarantee a repair but my customer is OK with used at half the price.
https://www.ebay.com/itm/145587467067?_ ... R9KFgdekZQ

Zenith had an issue with using wimpy power transformers in their pre-war equipment. One would have thought they'd have that issue sorted by the time this stereo was built....

I just found a second wiring mistake from factory. It seems the three sources 375 volts from 5Y3, 300 volts after power resistor and 260 volts after second resistor in voltage divider were swapped around so 260 was too high and likely loading the 375 volt rail.

After connecting the record player and hearing the end result of switching between mono, stereo and extended stereo on a stereo LP, its finally working and sounding in a way Zenith intended. Extended stereo seems to add to any echo in original recordings, some LP's could sound over-processed, whatever that means. There are plugs on the amp for a reverberation amp, as if the added gimmick wasn't enough. Standard stereo amps in this series had a bass-boost switch that altered the feedback from OPT secondaries. Its effect is equally interesting, depending a great deal upon the record or input material chosen.

Maybe I just cant imagine spending $300 (in 1960 $) on a "stereo phono console" and getting a mono phono and reduced bass due to wiring errors from the factory. These must have been flying out of the sales "salons" of old, maybe it was marked down as the inevitable sound would not have compared well to a similarly equipped one wired correctly. You can bet the TV guys had to work on these, as many were installed in BW TV combinations (no Zenith color until M.Y. 1962)

Thanks for sharing, Dave, and from what I have read in your thread I am glad I did not hurry up and overhaul my same model Zenith. It may never make it to the bench.

Bill