Grunow 460 Small Cathedral Restoration

The Grunow 460 is a small 4-tube non-AVC superhet circuit cathedral style radio with a distinctive curly maple veneer cabinet. Grunow was a trade name of the General Household Utilities Co. The 460 only receives the broadcast band and requires an external antenna (and ground). The radio had seen minimal servicing in the past and most original parts were still in place. This being the case, I decided to try to reverse any previous servicing and retain the original top and bottom chassis appearance if possible. The radio was purchased on eBay. The seller stated that it "needs work" - that was a bit of an understatement!

The schematic and a partial parts list for the radio can be found on Nostalgia Air. Any part number references in the text below reference that schematic.

My antique radio restoration logs

Previous Servicing

The original filter capacitor bank (a cardboard cased unit mounted under the chassis) had been removed and several modern capacitors tacked in. The original was a dual 8mfd unit at 350 volts. The replacement were four 4.7mfd @ 450 volts (two 4.7mfd in parallel on each side of the speaker field).
All resistors and paper-wax capacitors were original
The line cord and plug were original and even usable with repairs.
Three of the tubes were branded RCA or RCA/Cunningham and may have been original. The 6A7 was a replacement.

Survey

This radios is difficult to service with limited access to several parts due to the rather small chassis and the tuning capacitor being mounted under the chassis. My usual restoration procedure is to make a complete survey of the condition of all components and repair all items before the radio is tested. It may not be possible to test high value resistors because of leaky capacitors. In this case, one end of the resistor must be disconnected or leaky capacitors disconnected. The eBay seller made no statement about the electrical condition of the radio. Like in Zenith radio schematics, all parts having the same part number use the same call out in the schematic. So there are multiple occurrences of R7, C11, and C14 on the schematic. Before doing the survey I first make these occurrences unique by adding a suffix letter (A, B, C etc.)

The power transformer was defective! The primary winding was open, and the high voltage secondary measured a very low and asymmetrical resistances across the center tap: 8.3 ohms on one side and only 88 on the other side. The high voltage winding obviously had shorted turns. There were no external signs of burning or leaking tar. However during cleaning, the material on the chassis around the power transformer was very difficult to remove even using lacquer thinner. It may have been some sort of wax that had leaked from the transformer.
The unusual tapped speaker field coil, cone, and voice coil were OK. It is very difficult to replace a speaker having a tapped field coil, since in this case the tap location determines the output tube grid bias.
The antenna and oscillator coils as well as the IF transformers were OK (all were tested for resistance and/or continuity).
The original cloth covered power cord and plug were OK but had one area that would need to be repaired to be safe.
The original speaker cable was OK.
There were two wire wound resistors (one a tapped Candohm type unit). Both were OK. All remaining resistors were old style dogbone types. All were significantly out of tolerance,
All supplied tubes tested good (the 6F7 triode section was marginal).
The wire wound volume control was bad. Its total resistance was unstable and had only intermittent contact from the center terminal to either end terminal as the control was rotated.
The power switch was initially bad, but responded to a good spray of GC Big Bath cleaner through holes in the housing, followed by repeated operation.
The 6F7 grid cap lead from the first IF transformer was frayed and had deteriorated. The shield would have to be removed in order to effect correct repairs (vs. simply sliding on spaghetti tubing or shrink tubing).

Cleaning

The dust was first removed using an air compressor. After removal of the dial assembly and tuning capacitor, the tacked in filter capacitors, the antenna coil and its shield and power transformer, the chassis and remaining top components were cleaned using lacquer thinner, GoJo (white) Hand Cleaner, 00 steel wool, and toothbrushes.

Restoration Strategy

Since almost all of the original parts were still in place, I decided to try to retain the original top and bottom chassis appearance to the extent possible. All original capacitors would be rebuilt in their original containers (restuffed). Any out of tolerance resistors would be replaced with the same types if available. The power transformer would have to be replaced, hopefully with a similar unit. I would attempt to reproduce this missing filter capacitor block - chassis mounting holes and a shadow on the chassis indicated its size and shape.

Repairs

Power Transformer

The original power transformer was removed from the chassis for inspection. It was hopelessly burned inside!

The primary winding was open, and the high voltage secondary indicated shorted turns - one side of the center tapped winding measured only 8 ohms, and the other side 88 ohms. It was not clear what caused the original failure, since the filter capacitors had been replaced recently. There were no shorts from B+ to ground, even though all the paper-wax capacitors were original. There were no shorted or gassy tubes. It could be that the previous owner replaced the filter capacitors without first testing the transformer. This is apparently quite a common practice with new or inexperienced antique radio collectors.

The original transformer supplied three 6.3 volt tubes plus a pilot lamp (1.75 amps total), and a 5 volt 2 amp rectifier. It was a horizontal mount unit. I just happened to have a parts chassis for a Stewart Warner model 3043 in stock. The transformer in that chassis was good, and was a horizontal mounted unit also. It supplied four 6.3 volt tubes plus a pilot lamp, and a 5 volt 2 amp rectifier. The current draw on the filament windings for both sets was identical even though the tube count was different (the Stewart Warner used a 6K6 output tube, and the Grunow uses a 42). So I decided to install the replacement transformer to replace the original. The fit was acceptable, although the replacement transformer was slightly smaller than the original unit.

Tuning Capacitor

The tuning capacitor was cleaned using my old Heathkit ultrasonic cleaner and dilute ammonia. It would not all fit in the cleaner at once, so several cleanings at different angles were needed. I normally remove the mica trimmer insulators before cleaning in order to avoid damage. Before removing the mica, I note the original position of the trimmer screws on the clock, then note the number of 1/2 turns to fully tight. This is done so that the trimmers can be returned close to their original settings after cleaning. The capacitor was then cleaned using soap, water, and toothbrushes. After drying with a heat gun, the bearings and grounding fingers were lubricated using automotive distributor cam lubricant.

Volume Control

The volume control was removed from the radio and disassembled. It was a wire wound type with a fixed resistance mechanical stop. The wiper and spring were first removed. The rivet holding the center terminal was drilled out and removed. On the inside, this rivet along with a flat washer retained the resistance element. The end terminals are not physically attached to the resistance element - the connection relies on pressure supplied by the retaining rivet and washer. Any dirt or corrosion will result in an intermittent connection. The resistance element was then removed and carefully cleaned using lacquer thinner and a tooth brush. The wiper, spring, and center rotating contact were also cleaned. The resistance element was then reinstalled and the rivet replaced using a 4-40 x 1/2" brass round head machine screw, plus the original flat washer. A 4-40 nut retained the original center terminal. One must carefully note the orientation of the resistance element as well as the wiper during disassembly and reassembly, otherwise the minimum resistance and taper will not be correct (ask me how I know that!) After these repairs, the total resistance was OK. The minimum resistance measured about 400 ohms, which seems reasonable (specification is 350 ohms).

Resistors

The radio uses two wire wound resistors, both of which were OK. All five other resistors were 1/4 watt "dogbone" type carbon resistors. ALL of these original resistors had drifted out of acceptable tolerance range, and would have to be replaced. In most cases, I would replace an original resistor only if it was not within +/- 20% (or marked tolerance). Any "dogbone" resistors would be replaced with the same type resistor. I keep a stock of NOS and used "dogbone" resistors, and buy all I can on eBay and radio swap meets! Of course, most of these resistors, even NOS resistors, would have also drifted in value and no longer have their marked values. My solution is to find a replacement resistor of the correct value and size as measured (ignoring the markings), and then repaint it to the needed value codes using enamel hobby paint!

I was able to find suitable replacement resistors in my stock of "dogbone" resistors. Here are the repainted replacement resistors:

Filter Capacitors

The original filter capacitor block had been removed. The mounting holes, plus a shadow on the inside rear chassis, indicated its size. And the Riders documentation indicated its location and height relative to the power transformer and other parts. The original was a dual 8mfd @ 350 volt unit with a common positive lead. The capacitor apparently used wire leads. The revised schematic lists the wire colors used. I was able to fabricate a cardboard box using cardboard from the back of a writing tablet. Two 10mfd 450 volt capacitors were installed inside the box. The radio requires a common positive lead and two separate negative leads. The box was spray painted, and a label fabricated using the correct Grunow part number and capacitor values. Here is the reproduction capacitor:

Paper Capacitors

The paper capacitors used have no markings other than the Grunow part number (which agree with the schematic) and a "ground" symbol, which I assume means "outside foil". They were likely made in-house. All paper capacitors were rebuilt in their original cases using modern 630 volt film capacitors in order to maintain the original under-chassis appearance. I remove the capacitor from the radio by unsoldering its leads. The lead lengths and any insulating sleeving lengths are then noted. I take notes of which lead goes where in the radio, and identify the capacitor using marking tags (which note the part identification and the location in the notes where it was removed). Here is my restuffing process for this type of capacitor. And here are some examples of restored Grunow capacitors from this radio:

Other Repairs

The pilot lamp was replaced with a #40 bulb (6-8 volts, 0.15 amp, screw base). There was no marking as to type on the original bulb.

The line cord had one area that required repairs. The outer cloth insulation was missing and the inner lead insulation was starting to fray. The cord was cut at this point, spliced back together, and insulated with heat shrink tubing.

The 6F7 grid lead from the second IF transformer was frayed and had to be replaced. The grid cap was removed followed by the IF transformer shield. The transformer was then removed from the shield - its connections were not disturbed. A new lead was attached to the coil and the lead routed through the grommet in the shield. The coil was then returned to its shield, the shield reattached to the chassis, and the grid cap reattached to the new lead.

Cabinet

The cabinet was in excellent shape. It was cleaned with GoJo hand cleaner and 00 steel wool.

The grille cloth was slightly loose. It was re-positioned and reattached to the cabinet to eliminate sag. The type of cement used allowed careful localized removal of the cloth in the affected areas. The cloth was stretched then simply pressed back against the cabinet. The original glue grabbed it and retained it.

Testing

After the radio was completely reassembled, power was applied through a wattmeter and fused Variac. Power was brought up slowly while monitoring the B+ voltage and the wattmeter. The radio came alive and worked - no assembly errors! The radio was then aligned.

Restoration Results

Most of my restoration objectives were met, but not all. There was no intention of restoring the set to factory new appearance! My objective is usually to reverse any prior servicing and make the radio appear to have never been repaired. I do not go so far as to artificially "age" solder joints, as do some collectors! Nothing gives away a restoration faster than bright and shiny solder joints. Here are some of my "misses":