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The Zenith model 7S634R (7-S-634R) (1942 model year) is a
large tabletop
7-tube AC superhet circuit radio.
It receives the standard broadcast band and two short wave bands, and has
"automatic" or push-button tuning and the Zenith Radiorgan tone
control system.. This example had seen minimal servicing in the past. I decided to try to reverse any previous repairs to the extent possible. The schematic for the Zenith 7-S-634R can be found on Nostalgia Air. Any part numbers will refer to numbers on that schematic. |
My antique radio restoration logs
This radio was purchased on eBay. The cabinet was in good original condition, as were the knobs and grille cloth - just the usual wear, dings and scratches. The radio was sold as working. There was no external evidence that the radio had been restored other than the line cord which had been replaced. I always avoid knowingly purchasing a radio that has been restored, as many collectors take shortcuts such as removing the original capacitors and filters. Unfortunately, the radio had been very poorly packed and there was shipping damage. Here's what greeted me when I opened the package:
The tuning shaft was broken at its weakest point (the groove where the retaining C-clip is installed), likely due to the tuning knob having taken a sideways blow during shipping:
The 6AC5G tube was broken (it was an original Zenith). Fortunately, there was no other physical damage. The seller refunded me $50 for the replacement parts. Fortunately, I was able to obtain a replacement tuning shaft from the friendly and helpful people on Antique Radio Forums (from a junker chassis) for the cost of mailing.
I had previously restored a Zenith model 7S633 radio, which uses a very similar chassis (see Restoration Log). But that radio had had extensive repairs including a replacement power transformer. It had been badly hacked and the back cover was missing. This radio was complete with only minimal servicing and had its back cover.
The Zenith 7S634R is a 7-tube AC superhet circuit radio. It has a tuned RF amplifier stage for the broadcast band only. The Wavemagnet loop antenna is also only used for the broadcast band. Short wave reception requires an external antenna. The radio features the Zenith Radiorgan tone control system with five adjustments, and also has automatic or push-button tuning.
A 6X5GT was installed (should be 6X5G). Only a few tubes were branded Zenith, so they had likely been replaced.
Only two paper capacitors had been replaced: C14 (1st AF grid coupling) and C21 (quality capacitor).
All resistors were original.
Two tacked in tubular electrolytic capacitors had been installed to replace two sections of the filter capacitor C17-18-19. The original filter was still in place, and one section was still being used (C18).
The power cord had been replaced.
The dial drive cord had been patched and part of it replaced with string
Several original capacitors had leads cut and then tacked back together - I suppose to test them.
My usual restoration procedure is to first make a complete survey of the condition of all components. The survey results guide my restoration strategy. If major or unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio rather than restore it. I always assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original containers if possible). Any mica capacitors are assumed OK until testing proves otherwise. I did not test the radio prior to restoration, and especially since there was shipping damage. But I am confident that the radio would have worked prior to shipping. So I did not suspect any major component problems.
The automatic tuning unit (push button assembly) was first disconnected in order to prevent damage to the fragile coils during handling of the chassis. Fortunately, only four wires had to be disconnected. I found:
All tubes and shields were removed. I removed the tacked in filter capacitors and moved components that were originally connected to the lugs of the filter capacitor back close to where they were originally. I then took a photo of the chassis bottom so that routing of wiring and component placement could be restored. Lead dress is often critical in radios. At this point, the chassis bottom looked like this:
I then removed all the non-original components, documenting their locations and connections. The volume control was removed for access. The tone control panel was disconnected (its wiring would have to be replaced anyway). The tuning capacitor was then removed for cleaning access to the chassis, and to replace the mounting grommets (I used standard 5/16" rubber grommets). The triple filter capacitor can was removed for restuffing. When I replace a component, I always remove the original part (or wiring stubs left over from past component replacement) completely from a terminal. Other components connected at the terminal are protected from heat using old medical clamps. Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.
The top of the chassis was cleaned with GoJo hand cleaner and 00 steel wool. This process often leaves behind metal fragments which can cause shorts, so I then went over the chassis with a small magnetic screwdriver to retrieve them. The tuning capacitor (previously removed) was cleaned in an old Heathkit ultrasonic cleaner with dilute ammonia. After drying, the bearings were lubed with Lithium grease.
All the original Zenith paper capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. I reseal the cardboard tubes using rosin salvaged from servicing RCA Radiola Superhet catacombs (it melts at a low temperature and will not damage the replacement capacitors. Since two original Zenith capacitors had been replaced (C14 and C21), I searched my stock of old Zenith parts for the correct part numbers. In one case, I did not have the original Zenith dud part in stock for C21, (22-1119, .005mf, 1000 volts). So I used a Zenith 22-1016E shell (also rated at .005mf, 1000 volts) and restuffed it with a 0.0047mfd 630 volt film capacitor (this was C21, the tone quality capacitor). I did have an original dud for C14 in stock (.004mfd, 600 volts).
Some of the components that had to be replaced were buried beneath the band switch. These were VERY difficult to get to without damaging other parts, but I managed.
The original filter capacitor C17-18-19 (15+5mfd @ 450 volts and 15mfd @ 350 volts) was rebuilt in its original case. It was restuffed using new 15mfd/450, 15mfd/350, and 4.7mfd/450 volt radial electrolytic capacitors. The original can was restuffed using the following technique:
The three 47K 1/2 watt carbon composition resistors that were out of tolerance were replaced. I had no 20% tolerance resistors in stock (and they are no longer available new), so I had to use 5% and 10% tolerance replacements, which are similar to the Zenith originals but have 4 color bands rather than 3.
Some of the original rubber covered wiring had to be replaced. The insulation would crumble and fall off if the wiring was moved even the slightest amount. If the insulation was intact and not disturbed, I left it in place. Several wires that were in danger of shorting to adjacent components were replaced. The cable to the tone control panel had to be replaced, as it is subject to movement during servicing, and already had several wires that were bare in spots. The most difficult part of this repair was threading the new leads through the braided shield! Unfortunately, no available replacement wire looks similar to the original type, so I was forced to use new cloth covered hookup wire to replace it (using the same or similar color if available).
A new 6X5G tube was installed to replace the existing 6X5GT. A NOS 6AC5G tube was purchased to replace the one broken in transit. The 7B8 tube was weak and was replaced. The remainder of the tubes were good.
The volume control and switch was cleaned with GC Big Bath spray cleaner. The push-button tuning contacts were also cleaned using Big Bath followed by lacquer thinner on Q-tips and pipe cleaners.
The dial cord was replaced. The original spring was reused.
Once the radio was reassembled and the tubes installed, power was brought up slowly using a variac. AC power consumption was monitored using a watt meter, and a DVM monitored the B+. The set came alive and worked immediately. The set was then aligned - no surprises. The push buttons were adjusted to local stations without difficulty.
The set worked well on all bands, and the Radiorgan tone control operated correctly.
Deviations: cloth covered wire vs. rubber, 5% and 10% modern resistors vs. 20% originals, vinyl grommets vs. gum rubber, one wrong capacitor part number.
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Chassis Before Restoration |
Chassis After Restoration |
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