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The Philco 70B Cathedral from circa 1932 is a 7-tube superhet with
an RF amplifier stage. This example is the early version, without
AVC.
The radio had seen minimal servicing in the past - all of the original parts were still in place. I decided to try to maintain the original above and below chassis appearance and to reverse all previous repairs to the extent possible. The schematic for this radio can be found on-line at Nostalgia Air. |
My antique radio restoration logs
This radio was purchased on eBay. The cabinet was in very good original condition, as were the knobs and grille cloth - just the usual wear, dings and scratches (and nails through the top holding the brace underneath - has no one heard of glue? This is my second Philco 70B with those same "repairs"). There was no evidence that the radio had been restored. Even the line cord was original, and a nasty break resulting in a dead short would have prevented plugging it in to "test it", thus avoiding potential damage. I always avoid knowingly purchasing a radio that has been restored by a collector, as many take shortcuts such as removing the original Philco bakelite block capacitors and filter capacitors. There was some damage to the base moldings on both sides that occurred in shipping. The seller did not adequately pack the radio, and used a moving box rather than a shipping box - the box was not much bigger than the radio! It was fortunately to have survived without major damage. The damage to the base moldings was repairable.
Most if not all tubes had been replaced. About half were globe types, as would have been installed originally. And even though some tubes, including globe types, were branded Philco, their date codes indicated vintage 1935 or 1936. So even these were replacements. Only one tube, a Philco type 24 globe (RF amplifier) looked original and had a date code of 18, which could be 1931. Some tubes had service shop test result tags, also indicating mid 1930's in most cases.
Two paper capacitors had been tacked in. One of the lugs on one bakelite block capacitor (part 12) had been removed to facilitate using a replacement capacitor for one of the two 0.09mfd sections. Fortunately, the original bakelite block was still in place.
The two original copper Mershon filter capacitors 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. If major and 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.
At this point I made BEFORE photos of the chassis 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 oscillation) if wiring is re-routed or lead dress is not the same as the original..
All tubes and tube shields were removed. The tuning capacitor and dial assembly was removed for cleaning and replacement of chassis grommets. The two copper Merchon filter capacitors were removed for polishing and restuffing. All non-original parts were then removed.
The top and sides of the chassis was cleaned with GoJo hand cleaner and 00 steel wool. Since this process may leave small steel wool fragments that can cause problems later, I follow up with a thorough vacuuming and go over everything with a small magnet and masking tape to pick up any stray fragments.
The tuning capacitor grommets were replaced by GLg-Tuner grommets from Renovated Radio. The tuning capacitor disassembled and then was cleaned with soap, water, and toothbrushes.
The rotary power switch was removed from the radio and cleaned using Big Bath cleaner. After this cleaning switch worked well.
The volume control on non-AVC Philco 70s is a ganged wire wound control. The front section is about 250 ohms and is used to control the bias of the RF and IF amplifier tubes. The rear section is 5000 ohms and shunts the antenna coil primary. The rear section was open. The control was removed from the set and the rear section removed for inspection. The control is held together with a tough fish paper retainer, and the two sections are linked internally by a fiber insulator. The rear section was removed by removing a nut and washer on the back and carefully removing the fish paper retainer. The break in the windings was between two sections with different wire sizes and types. The coarse wire provides a slower change in resistance on the low end of the control. The wire sections were likely originally linked by welding. The weld had likely broken. I have had repair success in the past by using a thin piece of sheet brass - almost paper thin, and used for construction of model railroad equipment. A piece was first sanded until bright and then cleaned. A piece about 1/8"x3/8" was cut using scissors. This piece was then inserted between the resistance wire and the outer insulation - across the gap between sections. The repair was successful - the rear section now measured about 5000 ohms.
The primary of the first detector RF transformer was open. Examination indicated "green disease" - corrosion of the wire, which is very common in Philco radios. The first thing I tried was to start unwinding the wire until a break was found and then to repair the break. But the wire was too far gone and was falling apart and further breaks would occur as it was unwound. So the primary was rewound. I first estimated the number of turns needed and the wire size. Also noted was the winding direction. There were about 50 turns on the coil (the exact number of turns is not critical). The wire was about #38 cotton covered. Fortunately I had a roll of suitable wire available. I fabricated a jig to hold the coil form level and permit rotation, so that the wire could be wound evenly. The jig consisted of a piece of PVC thin wall plumbing pipe that just fit inside the end of the coil form (about 12" long). The other end of the pipe was chucked into my Unimat small lathe. The end of the pipe with the form attached was supported using my lathe's steady rest. The drive belt was removed so that the lathe could be turned by hand. The source wire spool was supported using a wooden dowel with wood screws in each end, supported by a piece of strapping screwed down to my work bench. This permitted me to rewind the coil evenly with no gaps between turns, and no overlaps. Once complete, the winding was secured using some melted rosin salvaged from servicing RCA early superhet catacombs!
All original Philco bakelite block capacitors were removed from the radio, their contents removed, cases cleaned, and restuffed using modern film capacitors. One other bakelite block capacitor had a wire wound resistor inside in addition to a bypass capacitor. This resistor was reused.
The tone switch capacitor was removed, the insulator salvaged, the capacitor replaced, and the case resealed using melted rosin left over from restoration of RCA superhet catacombs. This wax melts at low temperatures and will not damaged new components. Some collectors use tinted hot glue or even caulk for resealing components.
The two metal cased bypass capacitors 29 and 36 were restuffed using new film capacitors. I was able to salvage the original lead wires and reuse them. The original insulator inside was also retained and reused. These types are very easy to rebuild by straightening the metal tabs, unsoldering the ground lead from the case, freeing up the contents by passing a thin blade between the insulator and metal case, and pull out the contents. The tar covered capacitor inside was discarded after removing the original lead wire for reuse.
Both original Mershon copper filter capacitors (6mfd) were still in place. Some collectors have been able to remove the tops of these capacitors in order to restuff them. I have never been able to do this easily, and did not want to risk damaging the very nice looking copper capacitors. To restuff, the capacitors were chucked in my Unimat lathe and their cases scored about 1" up from the bottom. The cuts were then completed using a hobby razor saw and cleaned up using an Exacto knife. This leaves only a thin line on the case - hardly visible. The original contents were then removed and the capacitor case cleaned inside and out (using Brasso). The original positive foil was removed and the stud was cut short and then drilled to accept a ground lug and 4-40 screw and nut. The positive lead of a replacement 10mfd/450volt capacitor was attached to the ground lug. The negative lead of the new capacitor was routed though a small hole drilled into the hard rubber base and attached to the original ground lug.
The radio was re-fitted with all globe type tubes, with the exception of the 47 output tube. I did not have a good 247 in stock, and these are very expensive. I did have a weak 247, and tried it, but the audio was quite weak with it in place. One replacement 24 was a NOS Philco type 24 globe!
The cabinet only needed a good vacuuming inside and then cleaning on the outside with GoJo and 00 steel wool, plus removal of the deteriorated chassis washers. I did not have replacement chassis washers in stock. I tried all the washers available at Renovated Radios, but none of them worked. So I simply omitted the chassis washers. The control shafts were properly centered with no washers installed. A previous owner had used small nails to secure the cabinet arch to its support. These were hardly visible, so I left them in place.
Once the radio chassis 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 radio powered up and worked. The radio was then aligned. The radio performs well, is quite sensitive and has very good tone.
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