My antique radio restoration logs
||The RCA Model 110 is a 5-tube AC non-AVC superhet circuit radio. It receives the standard broadcast band and the old police call band. The circuit is somewhat unusual, in that it has an RF amplifier stage but no IF amplifier stage. The schematic can be found on-line at Nostalgia Air. Any part numbers will refer to numbers on that schematic.|
The radio was purchased on eBay. It was described as working with its original parts (I was skeptical)! I downloaded the photos in eBay and enlarged them as much as possible. It did indeed appear that most if not all original parts were still present, although I did see some tacked-in filter capacitors. When the radio arrived, I confirmed that all the original parts were present. No parts had been replaced, but several filter capacitors had been disconnected and newer tubular capacitors installed. It appeared that the radio had been serviced multiple times, since there were several generations of replacement filter capacitors present. Once it was determined that there were no B+ shorts, I slowly powered up the radio using a fused variac and watt meter, with a DVM monitoring B+. One resistor had broken loose from the volume control, and the antenna lead had broken off from the antenna coil. Once these were repaired, the radio did indeed work as advertised! This is truly amazing on original parts (one of three filter capacitors was original). The cabinet was in great original condition, as were the knobs and grille cloth. The power cord had been replaced by a newer cloth covered type. All of the grid cap leads had been replaced with new wire (correct cloth covered type). From prior experience with this vintage GE/RCA, most of the original grid cap leads were rubber covered and the insulation would be flaking off, resulting in shorts to the tube shields.
I decided to try to reverse any previous servicing to the extent possible and to restore as much of the original top and bottom chassis appearance as possible. My objective was that any repairs or restoration would not be obvious, and no new parts visible. The objective was NOT to return the radio to factory new condition! This same chassis is used in several RCA and GE radios. I had previously restored a very similar chassis, the RCA R-28P. But that radio had seen extensive servicing and many original parts had been removed. This one was largely original.
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 and cannot be restored, I may elect to sell the radio rather than 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). Since this radio actually worked, it was assumed that all major parts were OK. I did check the power transformer for balanced high voltage across the center tap and wattage draw with no load. It was OK, even though there were signs of tar or wax leakage on the chassis. This was likely due to overheating and high current draw due to leaky capacitors in the past.
One original tube shield had been replaced by one that did not properly fit. Luckily, I actually found a correct original shield in my junk box!
The volume control (a 4.5K wire-wound control with switch) was open at some point. A common failure mode of these controls, which use several sections of different size resistance wire, is a break between sections due to failed welds or other methods of joining the nichrome wires.
The power switch (on the volume control) measured high resistance - likely due to dirt and corrosion.
The tubes were stated as tested and replaced as needed. All tubes tested strong, except for the 2A7 which tested weak (but did work).
Six resistors were out of tolerance. All were old style "dogbone" resistors in various sizes.
The chassis is very compact and there is really no way to service many of the components without removing others for access. Here is what the chassis looked like before the start of restoration. The loose 14K resistor is supposed to be connected to the volume control rheostat:
First, all non-original parts were removed after taking careful notes of where they connected. Several parts that originally had been connected to one lug of the two-section filter capacitor were moved back in the vicinity of the lug (as found, they were hanging in mid air and connected to tacked in filter capacitors). I then took more photos of the chassis, since this would likely represent the original condition prior to servicing. In order to gain access to the capacitors and resistors that needed replacement, I removed the tuning capacitor, the volume and tone controls, the speaker (chassis top), the filter capacitors, the detector plate choke L9, and the antenna and RF coils (on top of the chassis, L1-L4). One must be VERY careful with the speaker leads, which are sleeved and quite stiff. They are easily damaged, and a tight fit through a hole in the chassis which has no grommet.
All paper capacitors were rebuilt using modern 630 volt axial film capacitors in order to maintain the original under-chassis appearance. There were two types of paper capacitors used in this radio: the first type was a cardboard tube containing the paper-foil roll with its ends sealed with tar (not wax). These are very easy to restuff. First the component leads are pulled out of each end, which also takes much of the tar with them. Then the foil roll is pushed out one end using a screwdriver. The cardboard tube is then cleaned of any remaining tar. I also clean the outside of the tube by heating with a heat gun and wiping away the wax, tar, and dirt. A new axial film capacitor is then inserted. Before restuffing, the component's original lead length and any insulating sleeving use is noted. In some cases longer leads must be first attached to the replacement capacitor before restuffing (I use #20 or #22 solid tinned bus wire to extend the leads).Here is the method I use to rebuild cardboard tube type capacitors.
The other type of paper capacitor used in this radio has no cardboard tube. These look like a block of tar wrapped in a thin fish paper wrapper, and are generally oblong in shape rather than round. I came up with a way to restuff (more correctly to reproduce) these RCA capacitors. See Restuffing Early 1930's RCA Paper Capacitors. The tone control and quality capacitor C20 and C30 was this type, and contained two capacitors (.035 and .005mfd). It was reconstructed using a .0047mfd (the quality capacitor) and a .015mfd and .022mfd capacitor in parallel (the tone control capacitor). Two capacitors in parallel were used because a .033mfd was too thick, and because of the shape of the original capacitor. The original outer fish paper wrapper (complete with part number) was reattached using hot glue. Capacitor C13 (2400pf) appeared to be a paper capacitor rather than mica. It was in a very difficult position to remove it (under the band switch), and would be difficult to reproduce. I disconnected one end and measured leakage and capacitance. Leakage measured more than 40 megohms DC (infinity) on my DVM, but the capacitance measured 4200pf using my DVM's capacitor measurement function (a higher reading usually indicates a leaky capacitor). Since it was shunted by a 20K resistor and was only exposed to low voltages, I left it in place.
The wet can type electrolytic capacitor C21 was rebuilt in its original can. The original was 10mfd, voltage unknown. The can was deeply scored about 1" up from the base using my small Unimat lathe and the cut completed using a hobby razor saw. In this case, it was completely dry - with no electrolyte inside (otherwise, the process can get messy!) This cut would be hidden by the capacitor's mounting clamp. The original contents were removed (positive electrode foil and insulation sleeve) and the case was cleaned. The positive center stud was cut short and a hole drilled in the end in order to attach a ground lug using 4-40 hardware. The positive terminal of the 10mfd/450 volt replacement capacitor was soldered to the ground lug. The negative lead of the replacement capacitor was extended, insulated with spaghetti tubing, and routed through a hole drilled into the side of the capacitor. The negative lead would eventually be clamped between the capacitor can and the mounting clamp and thus hidden. The two halves of the can were joined using 1" PVC plumbing couplings and epoxy cement. I also apply a few layers of masking tape to the PVC coupling to take up excess clearance with the can. The original mounting clamp was very rusty. It was removed from the chassis by drilling out two rivets. It was then cleaned up using wire brushes on my bench grinder and using a Dremel tool with a small wire brush. It was then reattached to the chassis using 6-32x1/4" hardware. The negative lead of the capacitor (hidden by the top of the clamp) was routed through the capacitor mounting hole and clamped under one of the mounting nuts to ensure a good ground connection.
The cardboard cased block capacitor C22/C23 (2 x 4mfd at 300 and 150 volts) was opened up and restuffed using two 4.7mfd/450 volt electrolytic capacitors. First the backs of the three hollow rivets were ground off using a Dremel Mototool with a grinding stone. The rivets could then be removed from the top with minimum damage to the cardboard case, and could be reattached later. The stiff bottom cardboard base was then removed using a heat gun to melt the glue and wax. The capacitor case was then carefully opened, again using the heat gun. The capacitor inside was then removed after severing the foil connections to the back of the external terminal rivets. Excess foil was then removed from around the terminals from the inside using a sharp Exact knife and #11 blade. Small holes were then drilled through the center of the rivets holding the terminals and through the cardboard on the back side. Two 4.7mfd at 450 volt capacitors were then mounted inside, and their positive leads were routed through the drilled holes. The leads were then wrapped around the terminals on the front side and soldered. The negative leads of both capacitors were joined to a piece of bare bus wire, which was insulated using small spaghetti tubing, routed through the drilled hole in the common external lug, wrapped around the base of the lug and soldered. The capacitor was then reassembled and held together using hot glue. The bottom cardboard base was reattached using hot glue. The three rivets were then reinstalled. Two are held in place by the capacitor's mounting screws. The one that did not have a mounting screw to hold it in place was secured using a small dab of epoxy.
Here are the restuffed filter capacitors:
All the mica capacitors were left alone. Any of these that could be tested by lifting one lead (in the process of replacing another part) were tested. All that I tested were good.
All original resistors significantly more than 20% out of tolerance were replaced. I used dogbone type resistors as were used originally. I purchase all the NOS and used dogbone resistors I can find on eBay or at antique radio swap meets. I picked out NOS and used dogbone resistors from my collection that were either in tolerance or had drifted to the correct needed resistance and then repainted them to match the original resistor's color codes using hobby enamel paint. The replacements may continue to drift, as would most new carbon composition type resistors. But to me, maintaining the original look is more important than long term reliability of the radio. And besides, most modern collectors would simply rip out all the vintage parts and replace them with new parts without even testing them!
|Here are the replacement dog-bone
resistors, ready for installation in the radio. Three are NOS or used parts that are still in tolerance (the 60K, 1 Meg, and 20K). The other two
have been repainted using hobby paint. The NOS 14K 1 watt resistor was
originally a NOS 10K, but now measures 14.03K. The 200K 1/2 watt was originally
100K but now measures 186K.
The volume control was intermittent in operation (the resistance jumped around) and measured open circuit at some point of rotation. I originally suspected a break in the resistance wire - a common failure mode. The control was removed from the radio and was disassembled. It was found that there was no break in the resistance wire - apparently the control only needed cleaning! The C-clip retaining the shaft was removed and the shaft and wiper removed. All parts were then cleaned using lacquer thinner. A Q-Tip and lacquer thinner was used to clean the resistance wire contact area. The wiper showed some wear and corrosion. The contact area was smoothed using 600 grit emery paper and then cleaned using lacquer thinner. Upon reassembly, the control operated smoothly throughout its range. The resistance measured 5.6K ohms rather than the specified 4.5K ohms. The minimum resistance was 115 ohms (not specified in the Riders documentation). The control was marked "RCA-VICTOR CO." so it must have been original.
Once the radio was reassembled and tubes installed, power was brought up slowly using a variac. AC power was monitored using a watt meter, and a DVM monitored the B+. The radio came alive immediately and worked. Warm-up is slow, likely due to a weak 2A7 first detector tube.
The set was aligned - no surprises. It picked up lots of stations using my basement 50' antenna. It even picked up some stations on the "police" band in the middle of the day (including the local low-power traffic station at 1610. Tone was OK, but there was some distortion which is characteristic of the type of second detector used.
I was able to successfully reverse previous repairs and to restore the radio to its unserviced appearance. No replacement parts are visible except for the power cord, grid cap leads, and the antenna and ground leads. It is likely that some of the components were not in exactly the same position as installed at the factory, since the radio had already been serviced when I received it. And there is a slight difference in appearance between the tar originally used to seal the paper capacitors and the rosin that I used.
Before Restoration (Non-Original Parts Removed