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The Zenith 5D625 from 1942 is a 5-tube AC/DC Superhet with Wavemagnet
loop antenna. It only receives the Broadcast band. The cabinet
is walnut veneer with photo-finish (picture of wood) trim. Other models share the same
chassis 5B01 including the 5D610 and 5D610W bakelite sets. The schematic for the 5D625 can be found on-line at Nostalgia Air. Any references to part numbers refer to that schematic. The radio had seen typical repair shop type 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 been cut off and the radio sold as not tested. The often missing original back cover was present but very fragile.
All the tubes were replacements. Two were metal types, not used by Zenith.
Three wax/paper capacitors had been replaced: C2, C3 (line bypass capacitor) and C7. All replacement capacitors were CD branded, but were different types and ages. The original filter capacitor was still in place.
All of the original wiring and resistors were still in place.
The volume control and switch had been replaced.
At first observation, the radio was a typical 5 tube AC/DC superhet using octal tubes. Like in the previous year Zenith models (e.g. 6D525), it used a 12K7GT (with a grid cap) rather than a 12SK7GT in the IF amplifier stage. When I pulled the chassis, I could not find an output transformer! It was not mounted on the speaker nor under the chassis. The speaker was original (per the parts list) and was a field coil type. But there were only two leads to the speaker. One lead was from the 50L6 output tube plate, and the other from B+. The leads to the speaker went to the field coil! So somehow, the speaker field coil not only magnetized the core, but also acted as the output transformer. On the schematic diagram, there is an arrow pointing to the usual position and connections of the output transformer, but labeled "speaker field". The field coil in this case is NOT used to assist with B+ filtering - the radio used resistors for this purpose. I have never seen this before used in any radio. When I asked about this circuit on Antique Radio Forums, someone recalled seeing at least one other example of this circuit.
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 usually never apply power to a radio before restoration, even through a "dim bulb tester" or variac "to see if it works". But there was signs of damage where the speaker leads enter the combined field coil/output transformer, and there were bare areas or the wiring exposed. Perhaps someone was trying to measure the field coil resistance? I had to know if this speaker was working. So I applied power through a watt meter and variac and monitored the B+ using my DVM. I slowly applied AC power in increments of about 10 volts, allowing plenty of time for the capacitors to reform before increasing the voltage. At about 70 volts input, the radio started making noises and some stations were received. But the B+ started dropping - a symptom of leaky capacitors. So I stopped the test, satisfied that the speaker was working. I did learn that the volume control and tuning capacitor were "scratchy" - useful information.
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.
All tubes were removed. The chassis was very clean, with no rust - just very dusty. It was first cleaned off with a vacuum cleaner. The chassis was then cleaned with a tooth brush and vacuum cleaner to remove any remaining dust. The tuning capacitor was not removed for cleaning, since it is mounted using bent-over tabs through rubber grommets. The tuning capacitor was cleaned by spraying it with GC Big Bath spray cleaner.
Wiring
The AC line cord was replaced with a replacement vinyl cord with molded plug. All of the rubber covered wiring was replaced using cloth covered stranded and solid wire available from Radio Daze and other sources. I use their "unrated" wire rather than their 600 volt rated wire (which is much too large for antique radio repair work). It is available in 8 colors. One of the colors used in Zenith radios (a grayish green) is not available. I normally use green or black to replace this wire. Their solid and stranded wire is 20 gauge, which is really too large for Zenith repairs (original Zenith solid wire is about 24 gauge). But that is all I have been able to find in cloth covered wire.
One of the leads to the speaker field/output transformer was replaced using a similar type wire from my junk box. It was spliced to the lead on the speaker after cutting and stripping it close to the field coil. Afterward, both speaker leads were reinforced using several layers of GC Service Cement, followed by a layer of cloth tape, and do not like to use modern plastic covered wire.
Tuning capacitor pulley repairs
The hub of the tuning capacitor pulley was pot metal. Originally it was inserted through a hole in the pulley and then peened over to retain it. But wear had allowed some rotational motion. A previous owner and attempted to solder the hub to the pulley. But that did not work, since one cannot solder to pot metal! I did not want to risk damaging the pot metal hub by banging on it with a hammer! So I cleaned off all the solder I could. I then drilled a small hole through both the pulley and the edge of the hub using a numbered drill and Dremel Moto Tool. I then inserted a tapered clock pin into the hole and clipped it short. I then applied epoxy to the area around the pin as well as on the back side of the pulley in order to secure the pin in place. That seemed to work. I'm not sure how long this repair will continue to work, but it did eliminate backlash when tuning.
Filter Capacitor
The original power supply filter capacitor C9-C10-C11 was removed and restuffed. It was a 30+20+20mfd @ 150 volt twist lock type capacitor. It was restuffed using two 22mfd 160 volt radial capacitors and a 22mfd/160 and 10mfd/160 volt capacitor in parallel (I did not have a 33mfd/160 volt in stock, and the 33mfd/350 volt I did have was too large to fit in the can). My procedure for restuffing FP type can capacitors is as follows (there are many discussions and examples with photos on Antique Radio Forums):
Wax/Paper Capacitors
Three original Zenith wax/paper capacitors had been replaced in previous servicing: C2 (22-829, .05mfd/200v), C3 (22-1017, .05mfd/200v) and C7 (22-243, .01mfd/400v). 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 original Zenith duds for C2 and C7. But I did not have a Zenith 22-1017 in stock, and was forced to use a Zenith 22-212C instead (.05mfd/400v).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.
The cabinet was cleaned using GoJo (White) hand cleaner, applied and then wiped clean using Toolbox White Rags (soft, smooth paper towels made from rags). The knobs were cleaned using my old Heathkit ultrasonic cleaner and dilute ammonia, followed by rinsing and drying.
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. The tuning capacitor was scratchy when adjusted. I first thought it had plates that were rubbing. To check this, I temporarily disconnected any leads from both stators. I then connected a 9 volt battery and pair of antique radio high-impedance head phones in series between each stator terminal and the capacitor frame. As the capacitor was rotated, no sounds were heard. This meant that the plates were not touching. The other possibility was that there was dirt and/or grease between the rotor shaft and the brass grounding fingers. The fingers were riveted to the capacitor frame, and thus could not be removed for cleaning. So instead I used pipe cleaners and lacquer thinner to clean the contact areas as the shaft was rotated. After this treatment, the scratchiness when rotating the capacitor was gone. My normal procedure is to remove the capacitor from the radio and clean it using an ultrasonic cleaner using dilute ammonia solution. But in this case, I did not wish to remove the capacitor since it is not mounted using threaded studs, but rather bent-over tabs through rubber grommets.
For alignment I isolated the chassis using an isolation transformer (since my signal generator is grounded and this is a hot chassis radio). But alignment of the IF transformers was difficult. There was considerable hum and distortion, and also the set could break into oscillation as the IF transformers were peaked. So I got the IF transformers close to their correct frequencies and then disconnected the generator and peaked the transformers using a broadcast station. The antenna and oscillator trimmers were peaked using this same method - a known broadcast station at 1400 kHz on the dial.
Most restoration objectives were met. I was able to restore the radio to its original condition prior to servicing except for one wax/paper capacitor. That capacitor Zenith 22-1017 was replaced with another Zenith part with the same value (different voltage rating). The non-original volume control and switch were left in place, since they were good, and good original parts are likely not available.
