Zenith Model 8S661 (8-S-661) Console Restoration

This circa 1942 Zenith console radio was found in an antique shop at a reasonable price, which is rare! The finish was original and in excellent shape. This is the condition after only cleaning with GoJo and 00 steel wool. The grille cloth is original and not torn, but has considerable staining (it was left AS IS).

The circuit is an 8-tube superheterodyne that receives the broadcast band and two short wave bands. It has a tuned RF stage for broadcast band only. It uses mostly G type octal tubes with only one loctal. It has flywheel tuning and features the Zenith Radiorgan tone control panel and push-button tuning. The large solid cabinet, push-pull 6F6G output tubes and 12" speaker deliver excellent tone!

The radio had seen some servicing in the past but had not been hacked excessively. This being the case, I decided to try and retain the original top and bottom chassis appearance if possible.

The schematic and parts list for the radio can be found on Nostalgia Air. I also ordered a "Service Manual" from A G Tannenbaum but it turned out to be only a copy of Riders information.

My antique radio restoration logs

Survey

My usual restoration procedure is to first make a complete survey of the condition of all components in the radio, then replace any defective parts before any testing is done. To me this is simpler than diagnosing problems piecemeal and replacement of parts. The survey results guide my restoration strategy. If major and unique components are defective and cannot be restored or replaced, I may elect to sell the radio rather than restore it. I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original components if possible). And most original resistors would have drifted and be out of tolerance range.

The 5Y4G rectifier tube socket had been rewired to accept a 5Y3 or 5V4 tube. A 5V4G was installed.
All other tubes except the 6A8G (which was weak) were original, or at least branded Zenith. The 6F6G output tubes were weak.
The dial drive belt was stiff and would not operate the tuning control. The seller had ordered a replacement belt from Adams Manufacturing which was with the radio, and was used in the restoration.
The glass dial cover was missing.
The station selector buttons were warped, as if exposed to excessive heat.
The volume control knobs was also warped, and the metal decorative insert was missing. Fortunately, the steering wheel type bandswitch knobs and tuning knob were present.
The power transformer was OK. It drew almost no power (10 watts or so) unloaded at 120 volts input. All output voltages were correct, and the high-voltage winding was balanced on each side of the center tap.
The speaker field and output transformer were OK.
All RF coils and transformers were OK.
The original rubber power cord was usable (the plug was not original).
Much of the rubber wiring, including wiring to pilot lamp sockets and Radiorgan tone control panel had deteriorated and was crumbling. If a wire had intact insulation and it was not necessary to move it, I generally left it alone.
The tuning capacitor mounting grommets had deteriorated.
All tube shields were in place.
Six original Zenith tubular capacitors had been replaced in previous servicing (the replacements were branded Sprague).
Both pilot lamps were burned out.
Five resistors were out of tolerance. One (R18, the 6F6 cathode bias resistor) was a replacement, which was open. The original is listed as a 470 ohm 1.5 watt wirewound. One defective resistor was a dogbone type, but the rest were standard carbon composition types.
The volume control (R10) with its two taps was good (fortunately) as well as the attached switch. This control would have been difficult to replace, and its value is critical since it is part of the complex tone control system.

Cleaning

I normally clean the chassis before starting restoration. I first blew off the above and below chassis dust with an air compressor. The chassis was then partially disassembled for access and cleaning. The tuning capacitor, dial drive mechanism and flywheel assembly was removed after unsoldering the leads and ground braids. All these parts were cleaned prior to installation.

The chassis and top components (coil shields) were cleaned using GoJo, steel wool, and toothbrushes.

Repairs

Before repairs were begun, a photo of the underside of the chassis was made and printed. All parts (resistors, capacitors, coils, trimmers) were noted on the photograph (using arrows from notations in the margin). Since the parts are not unique (i.e. the schematic has several resistors noted as R1 - all with the same value) all parts were made unique by adding suffixes such as R1a, R1b, C1a, C1b etc. The schematic was then annotated the same way. Defective and non-original parts were highlighted on the photo.

All alignment trimmers were also noted on the photo, since I was unable to find any documentation for this set that indicated where the trimmers were! The trimmers and broadcast padder were identified by tracing the schematic.

All original Zenith wax-paper capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance.
All non-original capacitors were removed. The correct Zenith parts were then located in my dud capacitor stock (I have a good collection of branded wax-paper capacitors just for this purpose). Zenith capacitors are all identified with part numbers on the schematic, so the correct parts could be identified. The correct Zenith parts were then restuffed.
A replacement plastic dial cover was ordered from West-Tech Services.
A reproduction volume control knob was ordered from Alan Jesperson.
The replacement dial drive belt from Adams Manufacturing was installed.
The rubber wiring to the Radiorgan tone control panel was crumbling and had to be replaced. The harness was laced like the original. While disassembled, the contacts were cleaned of old dried grease and tested.

Audio Control Panel Before Restoration	After Rewiring

The main filter capacitor was a 20+20mfd 450 volt FT type twist lock can capacitor. It was replaced by a NOS 20+20/450 and 20/25 volt FT type capacitor which was reformed and tested. This capacitor reformed very quickly and had minimal leakage afterward. It was however about 1" shorter than the original. To hide that fact, I placed a black cardboard cover over it (I have not decided which looks better - a shorter than original filter capacitor or the black cover).
I was able to acquire two good used, tested and closely matched 6F6G output tubes at the Charlotte, NC AWA conference. These were installed, along with a NOS 6A8G and 5Y4G.
For R18 (the 6F6 cathode bias resistor) I installed a large 3 watt dogbone resistor which was repainted for 470 ohms using hobby paint (it was originally a 500 ohm unit, measured 503 ohms). Since the original R18 had been replaced, I had no clue as to what it looked like (probably a flexible wire wound unit). R15 (220K 1/4 watt dogbone) was replaced with another repainted dogbone resistor that measured within 10% of 220K. The remainder of the defective resistors were standard 1/2 watt carbon composition types.
I replaced the tuning capacitor mounting grommets with standard rubber grommets. That worked OK, although they are really too stiff.
C21, the line to ground capacitor (0.005uf at 600 volts) was a Micamold type, and it was of course leaky - not good! I sliced it open using a Dremel tool and cut-off disc, hollowed out the inside with milling tools, and inserted a 0.0047uf/630 volt film capacitor. The two halves were then reattached using epoxy. The resulting repair/restuff is not obvious.
The old, stiff grease on the station selector unit switches was removed using lacquer thinner and Q-Tips.

Push Button Station Selector Panel

The push buttons on the original station selector panel were warped and distorted. Perhaps they were exposed to heat. The panel itself was not damaged. I obtained a replacement tone control panel on eBay that had the correct buttons which were in good shape. I originally planned on replacing the individual buttons from the donor unit. I then noticed that the station selector panel was the same as the tone control panel, except turned upside down! So I was able to simply cut the metal bracket that held the tone control switches with a Dremel tool and also slice off the plastic switch tabs from the back of the buttons. But this left the tone control labels on the switches visible (and upside down). There were remnants of existing stations labels installed. I was able to use these as a pattern and fabricated some stations selector labels from paper, cut them out, and inserted them into the push buttons. The station adjustments were later tuned to the indicated local stations.

Testing

After the radio was completely reassembled, power was applied through a wattmeter and fused Variac. Power was brought up slowly while monitoring the B+ voltage. The radio worked the first time. The radio was then aligned. Since several resistors in the phase inverter circuit had been replaced, I then wanted to make sure the output tubes were close to balanced (the 6F6G tubes were measured at 1980/2000 and 2000/2000). I applied an audio tone from my signal generator to the volume control and measured the voltage on the plates of the 6F6 output tubes. Initially, the voltage was about 10% different. I then played around with the 6F6 grid resistor using a resistance substitution box until the voltage was about equal on both plates. I then installed a suitable resistor. It turned out that the needed value was 346K (the schematic called for 330K 10%). The existing resistor had drifted to about 369K. I found a suitable dogbone resistor and repainted it as 330K using hobby paint.

The radio has great tone and is very sensitive.

Chassis before restoration	Chassis after restoration (C21 not yet installed)

Chassis Top Before Restoration

Chassis Rear After Restoration

Rear View of Cabinet