|The Zenith 6D612 from 1942 is a
6-tube AC/DC Superhet with a tuned RF amplifier stage and Wavemagnet
loop antenna. It only receives the Broadcast band. The cabinet
is mottled brown bakelite. There is also a white version (6D612W) - I
The schematic for the 6D612 can be found on-line at Nostalgia Air. Any references to part numbers refer to that schematic.
The radio had seen minimal servicing in the past. I decided to try to reverse all previous servicing to the extent possible, yet get it working.
My antique radio restoration logs
This radio was purchased on eBay. The cabinet and knobs were in excellent condition. The power cord had deteriorated, and the radio sold as not tested. The often missing back cover was present.
All but two tubes were likely the original Zenith brand with date code 14 consistent with a 1941 manufacturing date. They did not have an "R" suffix and thus were OEM and not Zenith replacements.
The 35Z5 and 12SA7 tubes likely had been replaced, since they were not Zenith. All tubes were GT type, even though the schematic called for G type tubes for the 35L6 and 12J7. But since these two tubes were both GT types and likely original, I assumed that the change to GT types was a production change or a change forced by pre-war parts availability.
Three wax/paper capacitors had been replaced with two disc ceramic capacitors and one orange drop capacitor. C6 (22-1049, 0.03mfd 400 volts) had been replaced with a .01mfd (disc). The original filter capacitor was still in place.
All of the original wiring and resistors were still in place.
My usual restoration procedure is to first make a complete survey of the condition of all components. The survey results guide my restoration strategy. I never apply power to a radio before restoration, even through a "dim bulb tester" or variac "to see if it works". If major and unique components are defective or missing and cannot be restored or replaced, I may elect to sell the radio for parts rather than to 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.
Before starting repairs I made BEFORE photos of the chassis top and bottom. I use these photos to ensure that replacement parts and wiring are placed as close as possible to their original positions. Some radios are subject to problems (such as hum pickup or oscillation) if wiring is re-routed or lead dress is not the same as the original.
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 C4's on the schematic. Before I start work on the chassis I annotate the schematic and under chassis photo 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.
It was discovered during my component survey that my radio did not match the published schematic found in Riders:
All tubes were removed. The chassis was very clean, with no rust - just very dusty. It was first cleaned off with an air compressor. The chassis was then cleaned with a tooth brush and vacuum cleaner to remove any remaining dust.
The AC line cord was replaced with a new vinyl cord. I was able to make all repairs without disturbing any of the rubber covered wiring, which would have meant replacement of the wires.
The original power supply filter capacitor C7-C8 was removed and restuffed. It was a 20+20mfd @ 150 volt FP twist lock type capacitor. It was restuffed using two 22mfd 160 volt axial capacitors. My procedure for restuffing FP type can capacitors is as follows (there are many discussions and examples with photos on Antique Radio Forums):
Terminal boards being attached using epoxy.
Replacement capacitor leads passed through the terminal boards and soldered to the terminals on the outside.
Three original Zenith wax/paper capacitors had been replaced in servicing. Two were part 22-243 (0.01mfd/400 volts) and one was a 22-1049 (0.03mfd/400 volts). I maintain a stock of branded (Zenith, Philco etc.) as well as generic (Sprague, CD etc.) dud capacitors just for this situation. I was able to find two Zenith 22-243C capacitors in my dud stock. For the 0.03mfd, I did not have an original Zenith 22-1049, but did have a Zenith 22-836E (0.03mfd/400 volt, same value and voltage) in stock. All of the original Zenith paper capacitors as well as the Zenith duds were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. My re-stuffing process is documented here.One capacitor, C5 was a challenge. It was a 500pf 600 volt capacitor. There was not enough room in the original case for a silver mica or ceramic disc capacitor rated at 600 volts. So I used two 0.001mfd 630 volt capacitors in series. There was just enough room for two capacitors placed end to end (but only if a certain brand of capacitor was used - some commonly available film capacitors are too large for this application). 500pf/630 volt axial capacitors are now available from http://www.justradios.com/capacitors.html in Canada. I will be using these type capacitors for future restuffing of this particular capacitor which is common in Zeniths.
The cabinet was cleaned using GoJo (White) hand cleaner, applied and then wiped clean using Toolbox White Rags (soft, smooth paper towels). A coat of Johnsons wax was then applied.
Once the radio chassis was reassembled and the tubes installed, power was brought up slowly using a variac. A DVM monitored the B+. The radio came to life immediately and worked well.