Grunow Model 700 (Chassis 7A) Restoration

The Grunow (General Household Utilities) Model 700 (chassis 7A) from circa 1934 is a 7-tube AC Superhet circuit radio that receives the broadcast band and one short wave band. The radio had been serviced in the past, likely multiple times, but most of the original parts were still in place. I decided to try to reverse all prior servicing and to restore the original chassis appearance if possible. No recent restoration work had been done based on the vintage of replacement components used.

The schematic and a parts list for the Grunow Model 700 can be found on-line on Nostalgia Air (there is a separate document on alignment). There are no identifying symbols or part numbers called out in the schematic - only values - and in the case of capacitors, the type of container used. (tubular, block, or small can).

My antique radio restoration logs


The radio was purchased on eBay and was sold as not working. Its original cabinet finish was in excellent condition, as was the grille cloth and chrome.  The knobs were all present and original. 

Servicing this radio is very difficult due to the use of a laced cable harness.  This harness blocks access to nuts and other fasteners holding parts that must be removed for servicing.  It appeared that major parts such as the filter capacitors, filter choke, tuning capacitor and the antenna and detector coils were attached and THEN the laced harness was installed and connected. Many parts requiring servicing were riveted to the chassis or to coil shields. The speaker cable was connected to the speaker using soldered terminals, which had to be disconnected in order to remove the chassis from the cabinet (the cable was long enough for minor servicing or alignment without disconnecting it). Several metal cased capacitors containing multiple parts are used: one contains 7 capacitors, one 3, one 2, and one with a single 0.05mfd capacitor (audio coupling capacitor from the second detector to the volume control). All of these capacitors were sealed in tar, making restuffing them quite messy and difficult.

All the small resistors used were old-style "dog bone" types.  Most were installed on a terminal board, which would have to be unfastened from the chassis in order to change the resistors if out of tolerance.  A large wire wound Candolm type resistor was also used, having four sections.  Two of the filter capacitors, as well as the tuning capacitor, were contained by outer shields.

The power supply is quite unusual. There are two input filter capacitors with a normal filter choke between them. Their negative terminals then connect to one end of the speaker field.  The speaker field has a tap which provides the negative bias to the type 42 output tube. The negative end of the speaker field is then connected to a wire wound resistor with two taps which provide other smaller negative bias voltages.  So the power supply has chokes (or the field coil) in both the positive and negative sides of the supply.  A final filter capacitor connects between B+ and ground.

Previous Servicing

I always attempt to avoid purchasing radios that have been "restored" by collectors or flippers, and am looking for either all original examples or those which have been "lightly serviced" in the distant past by radio service shops, rather than peppered with new film capacitors. This radio had received prior servicing, but most of the original parts were still in place. There was originally a dual 0.1mfd small metal can type capacitor riveted to the top of the antenna and detector coil shields.  This capacitor had been forcibly removed, which bent and damaged the coil shields. Remnants of the metal mounting tabs remained, along with the rivets. This capacitor was used for the first and second AVC line filter capacitors, at the cold ends of the antenna and first detector coils. It had been replaced by a couple of tubular paper capacitors attached to the opposite end of the coils. This capacitor must have been a problem part.  Looking for an unrestored example for reference, a member of the Antique Radio Forums provided a pre-restoration photo of another model 700 chassis.  The identical capacitor had been removed in his example also! 


The chassis was very dusty, but not rusty. All tubes and tube shields were removed. The dust blown off, top and bottom, using an air compressor. After removal of the three filter capacitors (for restuffing), the tuning capacitor shield, and the filter choke (for access) the top of the chassis was cleaned using using old tooth brushes and a vacuum to remove dust from the crevices. Parts of the chassis were then cleaned with GoJo (white) hand cleaner and 00 steel wool, keeping the steel wool well away from the tuning capacitor.


The Riders schematic for this radio has no part numbers or references to resistors or capacitors - only values and types. Three types of capacitors were identified on the schematic: tubular, block, and small can. The schematic shows that the block capacitor contains six capacitors and has 6 leads.  But once removed from the radio for restuffing, it was found to contain 7 capacitors and had 7 leads. The 0.05mfd tubular capacitor (code A) that was used to bypass the B+ going to the primary of the second IF was actually now a part of the block capacitor, and had a red lead.  Obviously this was a production change. Before starting restoration I made photos of the chassis bottom for reference. I then annotated a copy of the schematic, as well as the chassis photograph, with reference part numbers (R1, R2, C1, C2, L1 etc.).

Restoration Strategy

I assume that all paper and electrolytic capacitors are leaky and thus should be replaced (I always "restuff" the original components if possible). I do not replace mica capacitors, but may test them in place if possible (usually this requires disconnecting one lead of the capacitor).

Since almost all of the original parts were still in place I decided to try to maintain the original chassis appearance to the extent possible. Normally I would rebuild all original wax-paper capacitors as well as the filter, block, and can capacitors in their original cases (restuff them). 

When I replace a component, I always remove the original part completely from a terminal.  Other good components connected at the terminal are protected from heat using old medical clamps (hemostats).  Excess solder is then removed using a solder sucker in order to expose terminal holes for reattachment of the rebuilt or replaced component.  

The Candolm type wire-wound resistor would have to be replaced by discrete wire wound resistors on some sort of terminal strip or strips.  The two open sections were exposed to B+, and it would be dangerous to simply jump them with new power resistors.


Wax/Paper Tubular Capacitors

Four original paper/wax capacitors were tubular cardboard types and could be restuffed.  My re-stuffing process is as follows:

Filter Capacitors

Before servicing the filter capacitors, careful notes were made of their positive terminal lug orientation, confirmed on the under chassis photo.  This is important since this radio uses a laced wiring harness, and many leads from this harness connect to the filter capacitor lugs. The three filter capacitors were normal aluminum screw based wet type capacitors rated at 8mfd/500 volts DC.  One (the final or output filter) was mounted out in the open, its case grounded to the chassis.  Two others were mounted inside a metal cover which was lined with fish paper.  These capacitors were insulated from the chassis, and sat at more than 100 volts negative! The metal cover protected any wandering fingers from this voltage. The visible capacitor (output filter) was restuffed using an 8mfd/600 volt electrolytic obtained from Just Radios in Canada. My process for restuffing these types of capacitors is as follows:

The two input filter capacitors are hidden under a metal cover and are not visible. The capacitors were restuffed in a manner similar to the output filter, again using 8mfd/600 volt electrolytics::

Block Capacitor and Other Metal Clad Capacitors

The line bypass capacitor consisted of three 0.01mfd capacitors.  It was cleaned out and three 0.01mfd/630 volt film capacitors installed as indicated in the schematic.  The tar was not replaced, since the back of the capacitor is not visible. Another small can type capacitor contained a single 0.05mfd capacitor. This capacitor couples the cold end of the second IF transformer to the volume control through a 400K resistor.  Originally it was sealed with tar and riveted to the side of the chassis.  After drilling out the rivets, disconnecting all connecting wires and components, and removing the capacitor, the tar and contents were removed and a 0.047mfd/630 volt capacitor installed.  The tar was not replaced.  The capacitor was reattached using 6-32 screws and nuts.

The block capacitor containing seven capacitors was removed from the radio.  The nuts that hold the capacitor also secure the resistor board. Several connections were temporarily disconnected from the resistor board and it was moved back to allow access underneath, and for replacement of any out of tolerance resistors. After removing the block capacitor, each lead was identified and its length measured in order to maintain the original lead dress.

The cardboard cover was removed by bending back the retaining tabs. I first tried using a heat gun to free up the contents.  That did not work.  So I resorted to mechanical means to remove as much tar as possible. Once down to the level of the capacitors, I was able to free up one capacitor roll and remove it.  Once it was removed, the remaining capacitors could be removed. There was a common ground buss soldered to the can, which was disconnected.  The case was then cleaned up by removing as much tar as possible mechanically (small screwdrivers) then soaking overnight in mineral spirits. Final cleanup was done with lacquer thinner.  New wire leads were attached to the seven replacement capacitors, wired per the schematic.  The replacement capacitor assembly was wrapped in strips of paper towel to secure and center it in the can. The sides of the can were insulated with fish paper to prevent shorts. The can was then filled with melted rosin to hold everything in place. The cardboard cover was reinstalled.  Here are the restuffed block and metal cased capacitors:

AVC Filter Capacitor

One small can type capacitor had been removed and replaced by two tubular paper capacitors.  The original had been attached to the antenna and detector coil shields with rivets, and had been roughly removed - damaging the coil shields.  I had no clue what the original capacitor looked like.  Originally it contained two 0.1mfd capacitors, which were the first and second AVC line filter/bypass capacitors (from the cold end of the antenna and first detector coils to chassis ground).  In a photo of another unrestored chassis provided by an ARF member, this same capacitor had been removed.  But in his example, the original wire leads were still in place.  They were routed through holes in the tops of both coil shields, near where the original can type capacitor would have been mounted.  Tubular capacitors were connected to the wire stubs in his chassis.  This helped me determine how the connecting leads were routed originally. 

I have a collection of used metal can type capacitors.  I did not have an original dual 0.1mfd metal can type capacitor with two lugs.  However I did have a metal can type Micamold capacitor which was a dual 0.05mfd/600 volt oil filled capacitor with three lugs.  Its mounting tabs matched the mounting centers of the missing capacitor exactly!  So I opened up this capacitor (the back cover was soldered on) and removed its contents.  I also removed the center connection lug, since I only needed two lugs. I mounted two 0.1mfd 630 volt film capacitors inside, each grounded to the metal case.

In order to mount this capacitor on the antenna and first detector coil shields, I had to remove both coils from the radio - a difficult and dangerous task, since all connecting wires and components must be removed from the coil lugs.  Once the coils were removed, I attempted to straighten the shields as much as possible (they were riveted together and also riveted to the chassis). The replacement small can capacitor was then attached to the shields using 6-32 pan head screws from the inside of the shields, with star washers and 6-32 nuts on the outside. Before tightening the screws, the connecting leads were attached to the capacitor and routed through holes in the top of the shields. The leads were left long so that they could be attached to the correct coil lugs once the coils were reinstalled in the shields.

Metal Clad Power Resistor (Candohm)

Two sections of the four-section Candohm (metal clad) resistor were open.  The 14.7K open section previously  had been jumped with a 10K 1 watt dogbone resistor.  The following 17.5K section was now also open. The two low resistance sections (26 and 46 ohms) were OK.  I hesitated to simply jump the two open sections with appropriate wire wound resistors. The open sections could at some point reconnect, or even worse, short to the chassis.  The 14.7 and 17.5K sections form the screen grid divider, and thus are exposed to full B+.  I decided to fabricate a suitable replacement having the same terminal spacing as the original resistor.  This is important since the wiring in this set is mostly in the form of a laced cable, and thus the leads to the resistor have definite positions. 

A piece of insulating material was used for the base (IIRC, salvaged from a scrapped Tektronics Scope!).  Flat head screws were countersunk in the reverse side, and solder lugs attached to the top with nuts.  Appropriate wire wound resistors were then attached to the solder lugs.  The resistor base was sized so that the original mounting holes and screws could be used, and the spacing between solder lugs was made close to the original so that the original set's wiring would reach.  I piece of insulating material (fish paper) was placed under the base to prevent shorts to the chassis.  

The values of the four sections of the resistor were documented on the schematic, but not the power ratings! And also, no voltages are documented. Using a tube manual, I used a B+ value of 250 volts and RF screen voltage of 100 volts (type 78 tubes). I used -15 volts for the 42 output tube bias. This voltage drop through the 47 and 27 ohm sections of the candohm plus the documented field coil resistance allowed calculation of the power dissipation in the 47 and 27 ohm resistors.  I replaced them with a 47 ohm 2 watt and 27 ohm 3 watt wire wound resistor. In order to calculate the power dissipation in the 14.7K (I used 15K) and 17.5K screen divider sections of the candohm, I used the tube manual screen current value listed as 1.7ma each (for the three 78 tubes). This allowed calculation of the required power dissipation of the two resistors: 1.63 watts for the 15K resistor and 1.37 watts for the 17.5K resistor. I used 15K 6.5 watts and 17.5K 5 watts.  When later tested, the screen voltage was quite close to the estimated 100 volts.  Solving for these power dissipations required use of two equations and two unknowns - High School Algebra? At 115 volts input, B+ was 235 volts (est. 250), B+ Screens was 97.6 volts (est. 100), 42 tube bias was -15.56 volts (est. 15), and B- was 103.8 volts (est. -100).  After about 1/2 hour of operation, the power resistors were barely warm.

Here is the replacement Candohm:

Small Resistors

Nine small resistors were out of tolerance by more than 20% (some as high as 50%).  All were 1/4 watt dogbone resistors.  One original dogbone resistor previously had been replaced by a modern type (the oscillator tube grid resistor).  Two of these dogbone resistors were in spaghetti tubing and thus not visible.  In these cases, I normally replace them with standard carbon resistors. I collect NOS as well as used dogbone resistors just for this purpose, and buy all I can find on eBay and at swap meets.  I did not have replacements available that were in tolerance.  In these cases I attempted to find a replacement that was the correct size and had the correct measured value (within 20% tolerance) but not the correct markings!  I then repaint the resistor with the value required using hobby enamel paint.  In the two cases where the original resistors were hidden inside spaghetti tubing, I used standard 1 watt  carbon resistors (both were 250K). In the photo below are some repainted resistors:


All of the original tubes were re-used, since they were good or only slightly weak.

Other Repairs

The original cloth covered line cord was worn and frayed and not usable.  However, a long piece of vintage cloth covered AC cord had been used as an antenna, and connected to a short piece of shielded wire.  This wire was cut in half and used to replace the original cord.  It also served as a replacement for a future radio, since it was about 15' long.  An old style brown bakelite plug was attached.

The tone capacitor was original, but one end was connected to two leads extending from the laced cable.  One lead went to the plate of the 42 output tube, and the other to the speaker cable (to the output transformer).  But the tone capacitor was found just hanging in space, and right above the power transformer terminals (see Chassis Before Restoration photo).  It was hard to believe that with the high build quality of this set that Grunow would have done this.  Photos of another chassis provided by an ARF member were similar.  One hint was that in my set, there remained part of a terminal lug on the tone capacitor lead.  So the capacitor must have originally been attached to some other part nearby.  There are no holes in the chassis nearby, or evidence of where a part was attached to the chassis. My guess is that the tone capacitor as well as the connecting leads were originally connected to a lug on the AVC block capacitor, which had been removed in both my set and the other reference chassis.  I mounted a terminal strip on one screw of the power transformer and used this as a junction point.  This way the tone capacitor would not be flopping around and risk the junction contacting one of the power transformer terminals.

The lacing on the speaker cable was removed down to the break, then tied to new lacing cord.  The cable was then re-laced from the break all the way to the speaker terminal attachment point.


The cabinet was vacuumed then cleaned using GoJo (white) hand cleaner and 00 steel wool. Nothing else was done to it.

Testing and Alignment

Once the radio had been reassembled, the radio was powered up slowly using a fused Variac.  This allows the filter capacitors to reform.  A DVM monitored the B+ voltage.  The radio came alive and worked well on both bands. But I did note that there was an intermittent drop in volume. During alignment, the radio would oscillate and/or motorboat if the antenna and first detector trimmers were adjusted for maximum output.  The oscillation and motorboating would go away if certain parts were moved, and especially the antenna and first detector coil shields. These shields (aluminum) were riveted to the steel chassis in several places.  But when I measured the resistance from the shields to the chassis I measure between 8 and 15 ohms - and it varied with the positioning of the shields. Since the cold end of both the antenna and first detector coil were grounded to the chassis through the small block capacitor I added to replace the missing capacitor, and since this capacitor was grounded through the coil shields, this would certainly explain the oscillation and motorboating. I soldered a piece of buss wire to the AVC capacitor can and connected the other end to an existing ground lug (where the antenna shielded cable was grounded).  This seemed to work, and the oscillation went away. The radio was then aligned per the instructions in Riders.

Unfortunately, I do not think this is the end of any intermittent problems. There are many aluminum parts such as tube shield bases attached to the chassis with rivets.  I suspect there are potential bad connections.  But for now the performance is excellent, with excellent sensitivity on both bands and great tone quality due to its large speaker and solid cabinet.

Restoration Results

Chassis Before Restoration

Chassis After Restoration