Zenith 6D525 (6-D-525) Restoration

The Zenith 6D525 from 1941 is a 6-tube AC/DC Superhet with an untuned RF amplifier stage and Wavemagnet loop antenna.  It only receives the Broadcast band.  The cabinet is solid walnut, and affectionately known to some collectors as "the toaster" due to the slots in the top and sides. The schematic for the 6D525 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 - all of the original parts (except tubes) were still in place.  I decided to try to maintain the set in its original condition to the extent possible, yet get it working.

My antique radio restoration logs

Condition As Found

This radio was purchased on eBay.  The cabinet finish, knobs, and grille cloth were original and in excellent condition.  The cord was cut off, and the radio sold as not working.  The often missing back cover was present, but the spacers between the chassis and Wavemagnet antenna were missing.  Unfortunately, the seller did not properly pack the radio for shipment, and there was shipment damage. Here is what the radio looked like when received, and the packing box.  Only a single layer of small bubble wrap was used for protection, and also the chassis bolts were missing, which allowed the radio chassis to move freely inside the cabinet.  Fortunately, the damage was not serious and was repaired with no repairs visible.

Previous Repairs

• All but two tubes were the original Zenith engraved base types with date codes consistent with a 1940 manufacturing date.  The 35L6G and 35Z5G tubes had been replaced with GT types.

• The line cord had been cut off, likely because it was unsafe.

• The wooden spacers between the chassis and Wavemagnet antenna were missing.  Likely these were removed when the tubes were replaced.

• All original wiring, resistors, and capacitors were fortunately still in place.

Survey

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 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.  

• The AC power switch was bad (it measured a high resistance) - dirty and/or oxidized contacts were likely.  It responded to a shot of Big Bath cleaner followed by repeated cycling and then worked correctly.  The volume control was also difficult to turn.  It responded to light oil on the bushing and the Big Bath spray used to clean the AC switch, then moved freely.
• The speaker cone had a repairable tear, perhaps damaged in shipment.
• The output transformer was OK.
• All coils and the IF transformers were OK.  
• The pilot lamp was OK.
• All the tubes tested good.  This was great since many were the originals and some of them are hard to find (12J7G, 12K7GT, 12SA7G).  The 35Z5G and 35L6G had been replaced with GT types.
• Some of the wiring in the radio was rubber covered and the insulation was falling off.  All of this damaged wiring would have to be replaced.  
• All the original resistors were probably close enough to work, although some were more than 20% out of tolerance.  All resistors were either wire wound or dogbone types, so I did not want to replace one unless absolutely necessary.
• The dial cord was off its pulley, and the spring had been stretched.
• The chassis bolts were missing.

Repairs

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 oscillation) if wiring is re-routed or lead dress is not the same as the original.  

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 top and sides of the chassis were then cleaned with GoJo hand cleaner and a tooth brush.  

The AC line cord stub was replaced with a new vinyl cord.  Some of the rubber covered wiring was disturbed when components were replaced, and insulation would fall off.  This wiring was replaced.  I always use "unrated" type cloth covered wire available from RadioSupply.Com (their 600 volt rated wire is not appropriate for radio restoration).  I have not found a closer replacement for the old Zenith #22 gauge rubber covered solid wire.

The dial cable spring was straightened up as much as possible, and the original dial cord reinstalled.  It was not obvious how the cable was originally secured to the tuning capacitor pulley.  There were a couple of small holes in the pulley where perhaps tabs projected to engage the spring.  But the tabs had broken off.  So I used a piece of thin buss wire to secure the spring to the pulley - almost invisible.

The missing Wavemagnet antenna spacers were fabricated from 5/8" wooden dowel stock.  The center hole was drilled using my small Unimat lathe.  The exact length of the spacers was unknown, and queries on Antique Radio Forums as well as the rec.antiques.radio+phono newsgroup did not yield an answer.  But looking at available photos of this model from the back, with the back cover in place, gave me some idea of the location of the Wavemagnet antenna in relation to the back cover.  I finally settled on a length of 15/16".  The diameter was also unknown, but there were some indications on the chassis that gave me a clue as to the original diameter.  NOTE: Later I acquired another example of this radio that had its original spacers intact.  They were made of laminated cardboard rather than wood.  Their length was just over 15/16" and diameter was just over 5/8".  So my guesses were close!

I found a good Zenith branded 35L6G tube in my stock of tubes.  It had a "1" date code (1941?).  I was able to purchase a good Zenith 35Z5G tube from a member on the rec.antiques.radio+phono newsgroup.

Capacitors

The original power supply filter capacitor C7-C8 was removed and restuffed.  It was a 20+20mfd @ 150 volt FP twist tab type capacitor.  It was restuffed using two 22mfd 160 volt axial capacitors.  The chassis is very compact, and there is hardly any room to install tubular capacitors under the chassis.  My procedure for restuffing FP type can capacitors is as follows (there are many discussions and examples with photos on Antique Radio Forums):

• Remove the cardboard cover.  In this case, it pulled off without difficulty.  Sometimes I must use a heat gun to release the glue or tar holding it to the can.
• Uncrimp the bottom using a small screwdriver and diagonal cutters (try to minimize damage to the outside of the can)
• Remove the mounting ring, the terminal board and the gasket behind, cutting the aluminum leads to the capacitor body to free up the terminal board.
• Remove the can contents using a heat gun to release the tar, followed by a thorough cleaning using mineral spirits.  In this case, the foil roll could be removed without heating, and the remaining tar removed by mechanical means.
• Connect the replacement capacitor leads to the original terminals by drilling small holes through the terminal board close to the terminals, passing the leads through, and soldering them to the terminals on the outside. The common ground lead is passed through a small hole drilled near one of the capacitor ground/mounting tabs.  This lead is not soldered to the ground lug until after the capacitor is secured in the chassis mounting wafer (otherwise the solder would prevent the lug easily passing through the hole in the mounting wafer or could damage the wafer - ASK ME HOW I KNOW THAT).
• Reinstall the terminal board and mounting ring and restore the crimp around the base.
• Reinstall the cardboard cover (it was not glued - just held by friction).  The cover hides any signs of the bottom crimp being disturbed.  Be aware that the capacitor body is electrically HOT (connected to one side of the AC line), so the cover is an important safety feature.  The chassis itself is isolated from the AC line by a capacitor.

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.  My re-stuffing process is as follows:

• The original capacitor is removed from the radio, and the required lead length and any spaghetti sleeving use noted.
• The low melting point wax from each end is melted and removed using an old 25 watt soldering iron.
• The original wire leads are removed, as well as any remaining wax.
• While the wax is still molten, a small screwdriver is used to push out the original paper-foil roll.  In some cases, the contents comes out when the leads are pulled out.
• The original cases are then cleaned out, and any wax and dirt on the outside removed by gently heating the body over a small alcohol lamp and wiping with a paper towel while still molten.  At this point, the cases look like this:

• If the required lead length is longer than that of the replacement capacitor, a piece of bus wire is attached before restuffing.
• The replacement capacitor is wrapped in a narrow strip of paper towel in order to keep the new capacitor centered and to keep it from falling out.  Here are the finished capacitors before sealing the ends.

• 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).  

• The finished capacitor is then sealed with melted rosin (salvaged from early RCA Superhet catacombs, and donated by or purchased from members on Antique Radio Forums).  I do NOT recoat the outside of the rebuilt capacitors with wax (I'm not sure what was originally used - probably beeswax).

Cabinet

The cabinet needed a good vacuuming inside and then cleaning on the outside with GoJo and 00 steel wool. No further treatment was needed.  The dial cover was loose from the cabinet when received, due to shipping damage.  It was stapled back to the cabinet.  The wooden grille cover was glued back to the cabinet.  The repairs are invisible.  Fortunately, the knobs were not damaged in transit!

Testing and Alignment

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 came to life immediately and worked well.  The radio was then aligned.  There are no alignment instructions in Nostalgia Air that I could find.  Instruction are in Riders Volume 12, Zenith page 12-9.  Alignment is very simple, consisting of the IF transformers (peak at 455kc), wave trap (minimum output at 455kc), and antenna (1400kc) and oscillator (1600kc) trimmers.  For the IF and wave trap alignment, the signal generator is connected to the grid cap of the 12J7G RF amplifier.  For the oscillator and antenna adjustments, the output of the signal generator is fed into a 1-2 turn loop of wire placed close to the Wavemagnet antenna.

Restoration Results
Chassis Bottom Before and After Restoration