Zenith 6R631 Restoration

The radio was purchased on eBay. Externally it appeared to be all original, complete and in good condition. The knobs and push buttons were slightly warped (typical for this radio). The back cover was present but had some damage. The radio features a tuned RF amplifier stage and 3-gang tuning capacitor in manual tuning mode, so it should perform well. However, the RF stage plate load is a 15K resistor rather than an actual transformer primary, which limits the gain of the RF stage. In automatic (push-button tuning) mode, the first detector stage is untuned. The radio also features a cut down version (5 buttons) of the Zenith Radiorgan tone control system. The circuit is quite complex, with a high parts count. The chassis is small, and access to parts is limited and difficult. Zenith radios of this vintage use rubber covered wiring which by now has deteriorated and the insulation will fall off if the wiring is disturbed. Both the automatic tuning unit and tone control panel are permanently connected to the chassis. Removal of either switch panel requires removing interconnecting wiring. To make matters worse, the automatic tuning switch has a cardboard cover, limiting access.

The eBay seller published a Youtube video of this radio working. While there was noise as the radio was tuned, there was reception so obviously there were no major problems likely.

Previous Servicing

I always attempt to avoid purchasing radios that have been "restored" by collectors or flippers, and am looking for either all original examples or those which have been "lightly serviced" in the distant past by radio service shops.

Cleaning

The chassis was very dusty, but not rusty. The dust was blown off with an air compressor. The power transformer and IF transformers were cleaned using GoJo (white) hand cleaner and 00 steel wool and old tooth brushes.

Survey

In Zenith schematics, all resistors and capacitors having the same value have the same part number call out. So for example, there may be multiple R2's or C5's on the schematic. Before I start work on the chassis I annotate the schematic so that all parts have unique identifiers. I usually add an alphabetic suffix, so that the part numbers are thus R1A, R1B, etc. I then annotate the chassis photo with these unique part numbers with a red felt-tip pen. I then removed all non-original capacitors, documenting their locations and connections. Since the radio actually worked, there were no major problems suspected.

I normally measure the values of all resistors as part of the survey process. But in the case of this radio, there are two cases where loops exist which prevent accurate measurement of all affected resistors unless the loop is broken. Otherwise all resistors in the loop will measure lower than true resistance due to the shunting effect of parallel resistors. The two loop paths are shown below (I added a suffix "a" or "b" to indicate two parts with the same value and part number which are unique parts in the radio):

Restoration Strategy

Since almost all of the original parts were still in place I decided to try to maintain the original chassis appearance to the extent possible. All original capacitors would be rebuilt in their original cases (restuffed), including the original filter capacitor (but the terminal board would have to be replaced since the original terminals had been cut off). Any parts replaced in servicing would be replaced with original parts if available. Any out of tolerance resistors would be replaced with the same types if available. When I replace a component, I always remove the original part completely from a terminal. Other good components connected at the terminal are protected from heat using old medical clamps (hemostats). Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.

I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original components if possible). I do not replace mica capacitors, but may test them in place if possible (usually this requires disconnecting one end of the capacitor).

Repairs

Tuning Capacitor

This is what the tuning capacitor looked like as found. It was tilted to the right and the pulley was rubbing against the speaker frame and the back of the dial. The tilt was MORE pronounced than shown in the photo with the dial cord in place. Removing the capacitor was a complex and difficult task, involving first removing the dial cord (the stringing is quite complex) and dial assembly.

I recently restored a Zenith model 6D2615 which had the same problem.

The capacitor was not attached to the chassis with the usual bolts and nuts. There were three studs welded to the capacitor frame which ended in tabs which were passed through a flat washer, the chassis hole grommet, and an eyelet on the other side. These tabs were then simply bent over flat, and in one case, soldered together.

The capacitor was then reinstalled using new 5/16" (groove diameter) vinyl grommets. I added an additional flat washer on the two studs on the right side. When the dial cord was replaced I made sure that excessive tension was not used for the cord driving the dial pointer. Excessive tension in this cord likely caused the tuning capacitor to tilt right as found. The original tension spring had been shortened, perhaps to overcome the friction problems with the deteriorated pot metal hub. The dial cord was original. It was found that the pulley still contacted the back of the dial frame. This was remedied by loosening one of the screws that secured the dial to the speaker.

Resistors

The radio used two older style "dogbone" type resistor as well as carbon composition resistors (most of which were 20% tolerance). One "dogbone" resistor (R13) was out of tolerance and would be replaced with the same type resistor. I keep a stock of NOS and used "dogbone" resistors, and buy all I can on eBay and at radio swap meets (when reasonably priced)! Of course, most of these resistors, even NOS resistors, have also drifted in value and no longer have their marked values. My solution is to find a replacement resistor of the correct value and size as measured (ignoring the markings), and then repaint it to the needed value codes using enamel hobby paint! In the case of this radio R13 was replaced with a 600K1/4 watt "dogbone" resistor that now measured 643K (-5%). It was repainted as a 680K resistor using hobby enamel paint (only the end had to be repainted). Two carbon composition resistors (R2 and R4) were out of tolerance and were replaced with modern carbon composition resistors (10% tolerance, since 20% resistors are not available).

The two resistors inside the 2nd IF transformer (47K and 470K) are not listed separately in the parts list. Both were in tolerance (20%).

Filter Capacitor

The filter capacitor was a dual twist lock unit: 15mfd at 350 volts and 20mfd at 250 volts. The original positive lugs had been cut off. It was very difficult to remove this capacitor from the set for rebuilding since the oscillator coil is right above it (the coil's mounting lugs are soldered to the chassis!). I can see why the repair person simply cut off the lugs rather than removing and replacing the original! I found a suitable replacement terminal board having two lugs in my junk capacitor box. The original capacitor was rebuilt using 15mfd/450volt and 22mfd/250volt radial electrolytics. The crimp around the base of the unit was uncrimped using an old pair of diagonal cutters. The ground terminal ring was then removed. The insulator with two positive terminals was then removed by prying. In this case, the contents were removed along with the terminal board as a unit. In some cases I have had to clip the connections between the back of the terminal board and the contents and then remove the contents separately. In some cases, heat will release the contents from the can. In other cases I have had to remove the contents using a spade bit in my drill press and other tools!

The aluminum case was then cleaned. Small holes were drilled in the replacement terminal board close to the two positive lugs and near one of the ground lugs (on the metal ring). I use small numbered drills that are only slightly larger than the lead wire that passes through the terminal board. The two replacement capacitors were then mounted to the back of the terminal board.

Their positive leads were insulated using small spaghetti tubing and then passed through the terminal board and connected to the two positive lugs. A common lead was passed through the terminal board near a ground lug but not attached until after the capacitor was re-mounted in the chassis and the mounting lugs twisted. The terminal board and ground lug ring were then reinstalled (there is a tab on the ground ring which lines up with a slot in the terminal board). The crimp around the base was restored using a small tack hammer. I did my best to smooth things out but the repair is visible.

Paper Capacitors

All except one of the original Zenith branded paper capacitors were rebuilt in their original cases using modern 630 volt axial film capacitors in order to maintain the original under-chassis appearance. Zenith tubular capacitors are easy to restuff. My restuffing process for these types is documented here. One original Zenith capacitor had been replaced: C5. This capacitor was replaced using the correct original Zenith dud from my stocks, which was also restuffed. One capacitor C10 was rated at 0.00075mfd/600 volts. Normally, capacitors in this value range are mica capacitors. But this one was a conventional paper capacitor. There was just enough room inside the original case to install two 0.0015mfd/630 axial film capacitors in series, end to end, yielding the correct value.

Dial Pointer Hub

The hub that was fastened to the dial pointer pulley was made of pot metal and was falling apart. As received, it was held together by a strip of electrical tape. The original construction retained the dial pointer hand but allowed it to be moved for adjustment purposes. The stud holding the assembly (attached to the center of the speaker) was 1/4" in diameter. I used the brass bushing from an old volume control to fabricate a new hub. The inside diameter was correct and a good fit on the stud. I removed some of the back of the bushing and turned it down on my small Unimat lathe until it would fit snugly in the dial pointer hole as well as the pulley. I also had to remove some of the length to allow for replacement of the C-clip that retained it on the protruding stud. I used epoxy to attach the bushing to the dial pulley and the dial pointer, first making sure the slot in the pulley and the dial pointer were correctly aligned. Since the dial pointer can no longer be moved or adjusted, it may be quite a challenge to adjust the pointer position with the dial cord reinstalled! There was an existing felt washer behind the pulley, but it was slightly too thick and there was too much tension on the pulley. I replaced the washer (not visible) with a thinner felt washer from my junk box.

Other Repairs

The GT type tubes were replaced with the correct types: 6K6G and 6X5G. Other defective tubes were replaced with the same types (loctals of various brands).

Two rubber covered leads to the second IF transformer had insulation falling off, risking a short. The transformer had to be removed in order to replace all of the leads. This was VERY difficult to do. The IF transformers are secured to the chassis with spring clips, which are then soldered to the chassis! And there is very little room to work. All the leads were replaced with similar color cloth covered wire, except that I had to use black instead of gray (that color is not a standard available color).

The cardboard dust cover around the automatic tuning unit was falling apart and in several pieces as received. It was repaired by gluing on strips of file folder material on the inside. In order to remove or reinstall this cover, three wires need to be temporarily disconnected from the automatic tuning unit (bottom switch) and attachment screws removed or loosened temporarily. Fortunately, the connection points are accessible.

Almost all of the rubber covered wiring that was disturbed had to be replaced. Fortunately, the most of the wiring to the tone switch panel was OK. Only one wire needed to be replaced. All of the wiring to the automatic tuning unit was OK. One filament power lead had to be replaced.

Cabinet

The cabinet was in good shape. It was cleaned with GoJo hand cleaner and 00 steel wool. The knobs were cleaned in my old Heathkit ultrasonic cleaner using dilute ammonia followed by soap, water, and an old toothbrush. No attempt was made to straighten the warped knobs or push buttons. The control shafts were squeezed to make future knob removal easier. Removing the knobs originally was very difficult. The push buttons were cleaned using GoJo hand cleaner and an old toothbrush and Q-tips. I did not attempt to remove the buttons from the switch shafts, as that would have likely destroyed the buttons or the switches.

The dial cover was re-secured to the cabinet using staples. It had shrank somewhat, leaving a gap in places.

Two ribs on the back cover were broken off. One was found inside the cabinet. It was reattached to the back.

The two control shafts were squeezed a small amount across the slot in front in order to make future removal and replacement of the knobs easier. It was VERY difficult to remove the knobs originally without damaging them. They were removed by removing the loop antenna and chassis bolts, then pulling back on the chassis until the knobs were free. This method allowed the knobs to be removed without prying them off, which would have risked breaking the knob or damaging the cabinet.

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 and the wattmeter. The radio came alive and worked, but only in automatic tuning mode. It did not work in continuous tuning mode. It was later discovered that one contact on the tuning mode switch (bottom push button) was bent and no longer making contact with the moving element. Also, operation of the push buttons was noisy and intermittent, indicating that cleaning was needed. Access to the contacts for cleaning is very limited by the construction of the switch panel. During cleaning the contacts with D5 and operating the switches, the sliding member of the bottom automatic tuning switch broke in half! It was repaired with epoxy and could then be operated. Unfortunately one of the sliding contacts originally attached to the slider was lost and could not be found. This meant that the bottom automatic button did not work.

The radio was quite sensitive. The tone controls worked well. However, there was some speaker rattle and distortion or muddiness on low bass on certain stations. I have had this same problem on other Zenith radios. Perhaps there was less bass when these radios were designed? There was no obvious problem with the speaker cone. It just appeared to be excessive cone excursion. The solution is to simply depress the BASS tone button or to reduce the volume when this happens. Some collectors have modified the tone control circuit to reduce this tendency.

Restoration Results

Most of my restoration objectives were met, but not all. There was no intention of restoring the set to factory new appearance! My objective is usually to reverse any prior servicing and make the radio appear to have never been repaired. I do not go so far as to artificially "age" solder joints, as do some collectors! Here are some of my "misses":