In addition to tube-type communication gear, I occasionally enjoy working on antique entertainment radios. This Zenith model 9H988R LP console radio/phono was interesting because of the combination of the old and new FM bands and the old 78 and new 33 RPM (introduced by CBS in 1948) phonograph speeds. Even the phono circuit is unique. (see below) The turntable has separate tone arms with Zenith Cobra cartridges for each record type. The red Cobra cartridge has a 3 mil needle for 78's and the green has 1 mil for microgroove 33.
Confession time: OK, I'll admit it. This thing got almost no bids at an auction. There was a warning note on it not to plug it in. I felt sorry for it and bought it even though I had no room for a console (does anyone?). And I WAS curious about the separate tone arms, the dual FM bands and the unique phono circuit. The only advantage to this console as compared to some others was that the top is not opened, therefore things (such as the XYL's Dicken's Village) can be placed on top.
One obstacle in repairing this set was getting the chassis out for service. The knobs were very difficult to remove. I used two straps of belt webbing to get under the knobs for pulling upward without damaging them. Heating the knobs with a hair dryer also helped.
As suspected, the input electrolytic cap was shorted. A missing 5Y3 rectifier was replaced as was the flaking power cord. After verifying that the power transformer was in good condition, I replaced the electrolytic with a carefully reformed new-old-stock cap. Several other caps were also replaced. Some of the wiring was of dried rubber covered insulation. The replacement wiring was chosen to match the original color code. The set came to life and performed quite well after repairs. As expected the knobs were difficult to replace despite having squeezed the control shafts a bit. A bit of Deoxit on the knob shafts provided just enough lubricant to allow the knobs to mount without excess force.
Both tone arm Cobra cartridges work OK, but I need to check out the turntable mechanics. At the moment it only turns at 78 RPM. Updates later. I'm not desperate to play my 33's on this unit although it appears to be more gentle than other phonos of the era.
"Radionic" phono circuit
The schematic for this set shows a 6J5 tube as "phono oscillator". Apparently the Zenith Radionic Cobra cartridges vary the "Q" of a tiny inductor to modulate a 2.5 MHz oscillator. This is fed directly to the receiver which detects and amplifies the signal. Unusual to say the least.
Ads for Zenith proclaim "Zenith announces the New Cobra Radionic Tone Arm", "A Sensational New Way to Play Records, Only Zenith Has This".
While the exact model number as shown on the chassis is not listed in Riders, the schematics and information in Rider Volume 19 Zenith pages 22-30 for model 9H888R are nearly identical to this set. The Rider info is available free online from Nostalgia Air. See the homepage for a link. Since this chassis is mounted with the tubes horizontal, it has a heat shield between the power tubes and the electrolytics. That shield is not shown in the Rider info or on the interior label.
The AM broadcast performance is better than I expected. The sound quality with its 12 inch speaker and push button tone controls is excellent. The FM is working up to par for a 1948 set. Its sensitivity is well below that of a modern top-of-the-line FM receiver, but the Zenith will give the AM section of that typical receiver good competition.
12-20-06; addl pix 6-11-08
An Erres European Bakelite radio was the previous item on the bench.