|The Emerson model Q157 from 1937 is a small 4-tube AC/DC TRF
circuit radio with a Plaskon case. The front of the radio
resembles a clock face, and is sometimes called the "Clockette"
(perhaps wrongly so, as this term is normally applied to the Kadette K25
Clockette). The radio had seen some servicing in the
past. Several parts had been changed.
The radio's circuit is very similar to the many low-cost "depression" sets of the early to mid 1930's, except that this radio used an octal 25L6G tube instead of the usual type 43 output tube. I suspect the 25L6G was used simply because it has a smaller envelope (this is a VERY compact set).
The schematic is available on line at Nostalgia Air. Any part number references will be relative to that schematic.
My antique radio restoration logs
The radio was purchased in the flea market at the Charlotte AWA Radio Conference in 2012. As acquired, the Plaskon case had no obvious cracks or breaks. Later, I found that the dial pointer was not original, and that there was a small hairline crack. The metal screen over the speaker had some damage and staining, but was acceptable. The resistance line cord ("Curtain Burner") had been wrapped with what looked like white coiled telephone cord! A phono jack was dangling from a piece of zip cord connected to the radio. The knobs were present and were original. There was one cabinet foot missing and one extant foot was missing its spacer.
My usual restoration procedure is to first make a complete survey of the condition of all components before starting any repairs. The survey results guide my restoration strategy. If major and unique components are defective and cannot be restored or replaced, I may elect to sell the radio for parts rather than to restore it. I 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 good until testing proves otherwise.
The dial pointer was not original. I obtained an original pointer from David Frush, but when checking its fit, discovered that the plastic stud that holds the pointer had been broken off! This was a potential showstopper!
A phono input jack had been installed. Fortunately, it had been installed at the end of a short piece of AC zipcord - no holes had been drilled into the cabinet or chassis. The cord was connected between the chassis and PLATE of the 6C6 detector tube. I'm not sure how well this would have worked. And in addition, this is a HOT CHASSIS radio, so the phono hardware could have been a shock hazard. And this connection method would have placed B+ voltage on the phono cartridge (through a 500K resistor) with no isolating capacitor (unless one was installed in the phonograph).
The 25L6 cathode resistor R8 (110 ohms, 1/2 watt wire wound) had been replaced by a 150 ohm 10 watt resistor, and a small electrolytic capacitor was placed in parallel (I suppose to increase the gain of the output stage for the phono function).
The original filter capacitor had been replaced. All other capacitors and carbon resistors were original.
The speaker field coil, cone, and output transformer were OK.
Both RF coils were OK.
Almost all resistors were out of acceptable tolerance range (+/-20%). There was one wire-wound metal clad resistor (the pilot lamp shunt R10) that measured 52 ohms (should be 40 ohms). The smaller carbon resistors were an older style that were the size of modern 1 watt carbon composition resistors.
It was assumed that all paper-wax capacitors would be leaky. There was also a molded 0.1mfd 400 volt capacitor used as the AC line bypass (brand was Solar). It was measured and was leaky.
One cabinet foot was missing. A spacer was missing from another foot. The feet are threaded and actually hold the cabinet and chassis together.
The cardboard speaker surround was damaged and in several pieces, but all parts were present.
The dial drive cord was OK.
The pilot lamp was OK.
All the tubes were branded Arcturus, and were likely the originals. All except the 25Z5 were good. The 25Z5 tested bad on one side and also indicated a short.
The original dial pointer was attached to a stepped stud on a plastic pulley which was driven by a cord from the tuning capacitor and tuning knob. The tip of the stud was broken off, and the original pointer had been replaced by one that did not require the stud. I was able to obtain an original pointer, but needed to find a way to attach it to the pulley. The diameter of the stud needed was about 5/32"+ inches. I found a steel rod in my junk parts stock which was a suitable size. The plastic pulley was removed from the radio and carefully chucked into my small Unimat lathe. The end of the remaining stud (larger diameter portion) was cleaned up and filed flat. A center drill in the lathe tailstock was used to precisely locate the center of the stud. A suitable hole was then drilled into the pulley while still mounted in the lathe. It is VERY important that the stud be exactly centered. A 1/2" piece of the steel rod was then inserted into the pulley - held only by friction. The dial pointer could then be installed onto the steel rod and secured by a set screw in the pointer.
Line Cord Resistor
The original line cord resistor R9 (185 ohms, 17 watts) was severely damaged and not usable. In order to reduce the heat that would be dissipated inside the cabinet if an equivalent resistor were installed, I decided to use a diode to reduce the line voltage to the filaments. A diode in series with the tube filaments will reduce the voltage by a factor of 0.707, or to about 85 volts. The tube filaments and pilot lamp together require 68.6 volts. This means that only about 17 volts needs to be dropped, at 300ma. Thus an additional 59 ohms resistance is needed. The power dissipation is only 5.1 watts as compared to 17 watts without the diode. Only a 10 watt resistor would be needed. Checking my stock of power resistors, I decided to use two 130 ohm 5 watt resistors in parallel. I did not have a 60 ohm 10 watt resistor in stock, and did not wish to pay shipping for a single item at Mouser! Also, there was really no space under the chassis for a 10 watt resistor, but plenty of space for two 5-watt units stacked on top of each other. Of course, the presence of the diode meant that the radio would no longer work on DC (but today, that is not an issue).
All five original paper-wax capacitors (called "roll" capacitors in the parts list) were restuffed with new metal film 630 volt axial capacitors in order to maintain the original under-chassis appearance. I reseal the cardboard tubes using rosin salvaged from RCA catacombs (it melts at a low temperature and will not damage the replacement capacitors). All "roll" capacitors were Micamold branded. The Solar brand bakelite molded line bypass capacitor (0.1mfd 400 volts) was also rebuilt. The case was sliced open using a Dremel cut-off disc and a hobby razor saw. The contents were removed, and a 0.47mfd 400 volt radial film capacitor was installed inside - it would just fit inside the case (the only capacitor in my stock that would fit). The case was then reassembled using epoxy. The original appearance was thus maintained, although the slice in the case is visible (I made no attempt to conceal it). I made careful notes of the original capacitor lead lengths used and any insulation sleeving (almost every capacitor had red spaghetti tubing installed to prevent shorts, since the chassis was very cramped and compact).
Evidence on the chassis indicated that the original filter capacitor was likely a tubular clamp mounted unit with wire leads. There was an unused hole on the right front side of the metal front plate holding the speaker, near the top, and signs that something had been mounted there. There was almost no other space on the chassis for a filter capacitor. The original capacitor was listed as a dual 16 mfd unit at 100 volts (likely a mistake in the Riders schematic - it would have to be at least 150 volts).
|I found a dud capacitor in my dud stocks that was clamp mounted. I restuffed it using two 22mfd 160 volt capacitors, adding wire leads. I then fabricated a new label with the Emerson part numbers and original values. The clamp was then reattached and secured to the capacitor with a small screw (the original rivet was drilled out in order to remove and relocate the clamp). The capacitor was then mounted on the front panel using the clamp and a screw that protruded from the speaker mounting - the likely original mounting hole was not used.|
The metal clad pilot lamp shunt resistor R10 measured 52 ohms - spec was 40 ohms. I decided to leave this resistor in place pending completion of testing (the original pilot lamp was OK). Once the radio was restored and tested, I felt that the pilot lamp was MUCH too bright at power on, and would soon fail. So I added a shunt resistor (hidden in sleeving) to bring the resistance down to the 40 ohm specification (I used 160 ohms, 1/2 watt). The lamp is still too bright at turn on, IMHO. The simple 40 ohm shunt does not provide the soft start function common with later sets that use a 35Z5 rectifier (which has a pilot lamp filament tap).
I had nothing in my resistor stocks that looked anything like the carbon resistors used. They were rated 1/4 watt, but were the size of modern 1 watt carbon composition resistors. I decided to use modern 1-watt carbon resistors but to paint them to resemble the original resistors using hobby enamel paint. Originally there were several coding schemes used on the various resistors. I decided to repaint the replacements using the scheme of the only good original resistor left in place: body, end, stripe for the first two digits of the value and the multiplier. For example, 2 megohms would be red (body), black (end), and green (stripe).
R8 (110 ohms, 1/2 watt, wire wound) had been replaced by a 150 ohm 10 watt resistor. Since this resistor had been replaced, I had no ideal what the original looked like. But it was likely a flexible type wire wound resistor, which are usually rated 1/2 watt. I had no suitable flexible wire wound resistor in stock, so I used a small wire wound resistor that measured 101 ohms which already had an insulating sleeve covering it, thus resembling a flexible resistor.
After completion of restoration, the radio was connected through a watt meter to a variac and the power gradually applied while monitoring the B+. A 50' indoor antenna was connected. The radio worked immediately, and picked up lots of stations using only my indoor 50' antenna strung across the basement ceiling. The radio was then aligned. There are only two trimmers on the tuning capacitor. These were peaked when tuned to a station at the high end of the dial. While the radio works very well, it does require a longer than usual warm-up time, like due to the filament dropper resistor being slightly larger than ideal (66 ohms vs. 59 ohms).
Chassis Before Restoration
Chassis After Restoration