Karola Unknown 5-tube AC-DC Radio

This Karola is a 5-tube AC-DC radio using all loctal tubes.  The model number and year are unknown (likely 1946 or later).  There are no schematics or other information available for Karola radios, and very little is known about the brand.

The radio had been serviced several times in the past, judging from the age of the replacement parts used.  I decided to attempt to reverse all previous repairs to the extent possible and restore the original above and below chassis appearance.  

My antique radio restoration logs

Condition As Found

This radio was purchased on eBay.  The combination wood and metal cabinet was in good original condition. The grille cloth did not look original.  The knobs were not original, since they did not have set screws and one original shaft was plain round with no splines.  The radio was sold as not working. The line cord had been replaced based on the eBay photos.  I always avoid knowingly purchasing a radio that has been restored, as many collectors take shortcuts such as removing the original capacitors and filters.  The chassis was very dirty and corroded as found.  The plywood speaker baffle is original, and has been observed on other Karola radios.

Karola Corporation was located at 922 Washburn Avenue. Minneapolis, Minnesota. 1946 was the first year that Karola Corporation appeared in the Minneapolis city directory.  At that time Robert C. Brame was the president.  Thus this radio was likely made in 1946 or later. I have seen one other Karola radio which had a red ink stamp on the chassis: "Brame Mfg".  So Brame Mfg. may have made products other than Karola radios.


The radio has a standard 5-tube AC-DC superhet circuit using all Loctal tubes.  The circuit is often called an AA5, or "All American 5".  The tube line-up is 14Q7 (Converter); 14A7 (IF Amplifier); 14B6 (Second detector, AVC, first audio amplifier); 50A5 (Audio Output); 35Y4 (Rectifier). The radio uses permeability tuning (variable inductance) vs. a variable tuning capacitor and fixed inductance coils.  An external antenna is needed (no loop antenna).  Only a minimum number of parts are used.  As wired, the chassis  was "hot" - it was connected to one side of the AC line through the power switch - a very dangerous situation. There was no sign that the radio ever had a back cover.  The hot chassis is NOT  insulated from the metal sides of the cabinet.  The chassis and metal cabinet sides and top is "hot" even when the switch is off (through the tube filaments).  But this method of AC line switching is quite common in AA5 radios, since hum is reduced (the switched AC line is connected to the chassis). I constructed a rough hand drawn schematic prior to starting restoration.

Previous Repairs

The radio had been serviced perhaps several times.  This was a "well loved" radio, and someone had paid a lot of money to keep it running!  This is VERY strange since this is a cheap, crappy radio with poor performance.


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 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.  


Before starting repairs 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 or motor boating if wiring is re-routed or lead dress is not the same as the original.  

The adhesive tape tube location labels were removed and the tape residue removed as well as possible using lacquer thinner. The top and sides of the chassis were cleaned using GoJo hand cleaner and 00 steel wool.  Since this may leave behind metal fragments, I follow up this cleaning with a good vacuuming, a small magnet and masking tape.  

The speaker cone was repaired as well as possible using service cement and paper towel material.  The output transformer leads were reattached to the voice coil terminals on the speaker.

The broken oscillator coil was repaired.  First the hookup wire brace (original - seen on other Karolas) was removed to provide access.  Some of the winding was removed.  The two parts of the coil form were aligned using the tuning slug.  The parts were then joined using melted rosin salvaged from servicing RCA Radiola Superheterodyne catacombs!  A thick layer was attached to the break as  well as the base of the coil, which was loose.  The winding wire that was removed was then replaced on the form (not easy with the form in place!) The hookup wire brace was not reinstalled.


All out of tolerance (more than +/- 20%) resistors were replaced using similar type 1/2 watt carbon composition resistors.  The replacements were either 5% or 10% tolerance, vs. the 20% tolerance originals (which are no longer available).  20% tolerance resistors would have been cheaper at the time of manufacture.  The 3meg AVC time constant resistor was replaced by a 2.7meg 1/2 watt resistor that measured a little over 3meg.  I did not have a 3meg resistor in stock, and 3meg is no longer a standard value for 10% tolerance.

Bypass and Coupling Capacitors

The original Chicago brand wax/paper capacitors were restuffed using modern 630 volt film capacitors.  The one CD branded capacitor (.02mfd/400 volts) was also restuffed, since I did not have any Chicago branded duds in stock.  The radio as found did not have a capacitor installed across the output transformer primary.  Most radios do have such a capacitor.  Another Karola I restored (a 32 volt set) had a 0.01mfd/600 volt capacitor in this position.  I installed a restuffed .01mfd/600 volt Cornell-Dubilier branded capacitor in this position. I assume the original may have failed and simply been removed vs. replaced.

Filter Capacitor

The original dual filter capacitor had been removed and had been replaced by two tubular units.  The original was replaced by a NOS tubular 3-section filter capacitor rated at 50/30/20mfd at 150 volts.  This capacitor was reformed and tested good afterward. The third 20mfd section was used to bypass the output tube cathode bias resistor (vs. simply clipping off the lead).  The capacitor was installed using the original clamp, which was still in place.


The extant line cord was black, very thick and had no plug.  The seller had spliced on a replacement plug for "testing" - with exposed bare wires!  The cord was replaced using a standard brown vinyl cord with a polarized plug.  The original power wiring was revised in order to make the radio safer to use.  The neutral lead (wide blade of the plug) was connected directly to the chassis, and the hot lead was switched.  This makes the radio safe if the AC outlet is properly wired (no guarantee of that, of course).  

The antenna lead was frayed and had been taped.  It was replaced.


The original tubes that were good were reinstalled.  The 14B6, 14Q7 and 35Y4 tubes were replaced.


The cabinet only needed a good vacuuming inside and then cleaning on the outside with GoJo and 00 steel wool.  It turned out definitely presentable without refinishing.  The grille cloth was left in place, since I had no idea what the original cloth looked like.  The knobs that came with the set were re-used, although not original.  But I have photos of another Karola with the same knobs!

Testing and Alignment

Once the radio chassis was reassembled and the tubes and tube shields installed, power was brought up slowly using a variac and isolation transformer.  The radio powered up OK but there was very little reception - just some whistles and weak distorted signals with considerable hum modulation (tunable hum).  When I switched the isolation transformer out of the circuit, the hum modulation improved somewhat.  So obviously the radio needs a chassis ground to work.

There was considerable hum, even though the AC ripple on the B+ line was measured at only a few tenths of a volt.  There was several volts of 60Hz AC going into the grid of the 50A5 output tube, thus originating in the 14B6 first audio amplifier.  When the grid of the 50A5 was shorted to the chassis, the hum went away.  When the grid of the 14B6 was shorted to ground, the hum also went away.   I then suspected that the hum was originating in the volume control, since I had rewired the AC line switching in order to make the radio safer to use.  I was forced to return the power wiring to the original as found condition in order to eliminate the hum.  The hot side of the AC line fed the rectifier plate and filament string.  The cold side (wide blade of the polarized plug) was routed through the AC line switch and then grounded to the chassis.  This eliminated the hum, but the shock danger is back (chassis is hot through the filament string when the radio is OFF).

The severe hum modulation was improved when the 14Q7 tube was replaced.  But the whistles (when tuned on station) remained.  They were worse on  weak stations, or in the evening when stations switch to low power.  Another thing noticed was that our local highway information station at 1620kHz also comes in weakly at about 730kHz! I noticed that this is about double the Intermediate Frequency away, and thus was likely an image frequency that was not being rejected by the tuner!  This is thus the likely source of the whistles.  I checked the local oscillator frequency using a frequency counter and was correct at several frequencies across the dial.  So the radio IS tracking properly.  I tried many things to eliminate the whistles, including detuning the IF transformers and adding additional bypass capacitors at various points.  Nothing helped.

The radio was then aligned, although I did not attempt to adjust the tracking of the permeability tuner.  Only the IF transformers and antenna trimmer were peaked up.  The IF frequency was 455kHz.  The speaker repair seemed to work OK - at least there was no rattle.

Restoration Results

I was able to successfully reverse some previous repairs and restore the likely original appearance of the radio under the chassis.  Of course, I did not know the exact appearance of some of the original parts such as the filter capacitor.  Exact placement of components and wiring was also unknown. The non-original volume control was left in place.  But the performance of the radio is dismal!  There is still some hum modulation present, and the whistles are still there, and worse on weak stations or in the evening.  Since all parts check out OK, this may have been the original characteristics of the radio.  If used in a strong signal area, or with a longer antenna, it may have performed acceptably. 

Chassis Bottom Before and After Restoration