Sparton Equasonne Radios

39

 

The model 39 (manufactured 1928-1929) was a battery set.  It is a nine tube set.  The battery arrangement used in this set depends on the nature of the tubes used. 

Download schematics 

 

  

 

Photos courtesy of Bill Turner 



49

 

 

The model 49 (manufactured 1928-1929) was a battery set.  It originally sold for $76 (tubes were extra).

If you have a nicer photo of this model, please e-mail me.

Download schematics

 

 

 

Photos courtesy of Tommy Epps.

 





69

The model 69 (manufactured 1928-1929) apparently came in an AC version only.  It uses the same chassis as Models 79-A and 89. The 69 is an 8 tube radio.  This is the least amount of tubes found in the Equasonne line of radios.  It uses a single 585 power output tube rather than the usual paired arrangement of two tubes in push-pull. The RF Amplifier unit is the same as that found in the 930, which differs very slightly from the 931 type, common to most models.   The tuning selector unit is also of the 930 type.  Models 69, 79-A and 89, did not come with an internal speaker.  Rather they featured a speaker jack for connection to an external speaker, like the Model 29 cabinet speaker shown in the photo.

The Model 69 originally sold for $147 (complete with tubes).  The Model 29 cabinet speaker sold for $52.50.

 Download schematics

A Model 69 equasonne (left) sitting atop an external Sparton Model 29 cabinet speaker.  Photo courtesy of Doug Houston.

 

79A

 

 

 photo courtesy of Ralph Pugh

The model 79-A (manufactured 1928-1929) apparently came in an AC version only.  It uses the same chassis as Models 69 and 89. The 79-A is an 8 tube radio.  This is the least amount of tubes found in the Equasonne line of radios.  It uses a single 585 power output tube rather than the usual paired arrangement of two tubes in push-pull. The RF Amplifier unit is the same as that found in the 930, which differs very slightly from the 931 type, common to most models.   The tuning selector unit is also of the 930 type.  Models 69, and 89 differ from the 79-A.  These models did not come with a built in speaker.  Rather they featured a speaker jack for connection to an external speaker.

 Download schematics

 

 

 

 

 

  

 

 

 

 

 

 

  Photos of this Model 79-A Equasonne photo courtesy of Larry Wanicki

 

89A

 

The model 89A (manufactured 1928-1929) uses the same RF amplifier unit and selector unit as the 930, but differs in its power converter.  The model 89A converter incorporates a pair of 281 half wave rectifier tubes rather than a single 280 full wave rectifier tube.  The 89A differs from the Model 89, in that it included an electrodynamic speaker.  The Model 89 came with a speaker jack for an external speaker.

 Download schematics

 

 

89

 

The model 89 (manufactured 1928-1929) apparently came in an AC version only.  It uses the same chassis as Models 69 and 79-A. The 89 is an 8 tube radio.  This is the least amount of tubes found in the Equasonne line of radios.  It uses a single 585 power output tube rather than the usual paired arrangement of two tubes in push-pull. The RF Amplifier unit is the same as that found in the 930, which differs very slightly from the 931 type, common to most models.   The tuning selector unit is also of the 930 type.

Download schematics

 

 

 

 

 

 

 Sparton model 89 cabinet.  Photo courtesy of Diana Symons Howell

 

 

 

 

 

 

 

 

99

The model 99 (manufactured 1928-1929) is one of the very few models of Equasonne that included a built in turntable.  Many of the other models simply included a phono jack through which you can connect an external phonograph.  This model possibly came only in an AC version, as I have yet to uncover any information on a DC version.  The Model 99 is an 11 tube set.

Personal taste perhaps, but this is the most beautiful Equasonne I have ever seen!

Granted, the model 99 did not use the tuning selector unit with the additional 485 tube like the Model 235 Equasonne radio/phonograph combo, but it did incorporate a module with additional stage of audio frequency amplification, which the Model 235 did not.

Download schematics

A Model 99 Equasonne.  This model is a combination radio/phonograph.

Photo courtesy of Doug Houston.

 

101

No information available

 

 

103

The model 103 (manufactured 1930-1931) is one of the very few models of Equasonne that included a built in turntable.  Many of the other models did however include a phono jack through which you can connect an external phonograph.  This model possibly came only in an AC version, as I have yet to uncover any information on a DC version.

 

 

 



The 103 is a 13 tube radio.  It uses 9 484/485 amplifier tubes, two 281 half wave rectifier tubes, and a pair of 586 power output tubes arranged in push-pull.  The chassis of the 103 Equasonne is identical to that used in Model 578.

 

 



The 103 uses a slightly modified version of the 484 tube containing selector unit.  This unit contains an additional resistor that is not found in any other model.

 

 

 

 

 

 

 

 

 

 

 

Pictures courtesy of Shawn L Van Horn

Download schematics 

109 Deluxe

 

 

The model 109 Deluxe (manufactured 1928-1929) apparently came in an AC version only.  It's power converter unit seems to be unique to this Model only.   This model was a 10 tube radio.  The RF Amplifier unit is the same as that found in the 930, which differs very slightly from the 931 type, common to most models.   The tuning selector unit is also of the 930 type.

If you have a picture of this model please e-mail me. 

Download schematics

 

 

 

 

 

 

110

The model 110 (manufactured 1929-1930) apparently came in an AC version only.  It uses the same chassis as Model 111. The 110 is a 12 tube radio.
It was the most expensive model in the Equasonne line and sold for $395 (including tubes).

There appear to have been two slightly different versions of the 110 chassis, which differ only in the construction of the power converter.  Type 1 models had an additional resistor and capacitor in the power converter circuit (R15 and C19), and differ in a few internal connections.

The RF Amplifier unit is the same as that found in most models (ie. 931 type).   The tuning selector unit is also of the 931 type, and is shared by several other models.   The Model 110 and 111 differ from the 111-A which was slightly more luxurious and used the tuning selector with the additional 484 tube.  The power converter unit of the 111-A also has some minor differences from that of the 110 and 111.

Download schematics for the Model 110 Type 1

Download schematics for the Model 110 Type 2

 

 

 

 

 

111

 

The model 111 (manufactured 1929-1930) apparently came in an AC version only.  It uses the same chassis as Model 110. The 111 is a 12 tube radio.

There appear to have been two slightly different versions of the 111 chassis, which differ only in the construction of the power converter.  Type 1 models had an additional resistor and capacitor in the power converter circuit (R15 and C19), and differ in a few internal connections. 

 

Model  111 Equasonne.  Photo courtesy of Doug Houston 


The RF Amplifier unit is the same as that found in most models (ie. 931 type).   The tuning selector unit is also of the 931 type, and is shared by several other models.   The Model 110 and 111 differ from the 111-A which was slightly more luxurious and used the tuning selector with the additional 484 tube.  The power converter unit of the 111-A also has some minor differences from that of the 110 and 111.

Download schematics

 

 

 

 

111A

The model 111-A (manufactured 1930) apparently came in an AC version only.  It uses the same chassis as Models 574 and 870. The 111-A is a 13 tube radio. The RF Amplifier unit is the same as that found in most models (ie. 931 type).   The tuning selector unit is of the type which houses an additional 484 tube.  This particular tuning selector was also used in the majority of the other Equasonne models (103, 235, 301-A, 564, 570, 574, 578, 600, 610, 620, 737, 740, 750 and 870).  Models 111-A, 574 and 870 differ from most other Equasonne models in the use of an additional audio connector unit which houses an input audio transformer and a pair of 226 tubes.  This unit provides an extra stage of audio frequency amplification.

In case you are wondering, the 111-A is essentially a deluxe verion of the model 111 which used the selector unit without the additional 484 tube.

If you have a picture of this model please e-mail me.

Download schematics

 

235

The model 235 (manufactured 1930-1931) is one of the very few models of Equasonne that included a built in turntable.  Many of the other models did however include a phono jack through which you can connect an external phonograph..  This model possibly came only in an AC version, as I have yet to uncover any information on a DC version.

The 235 is a 10 tube radio.  It uses 7 484/485 RF amplifier tubes, a single full wave rectifier tube, and a pair of 183 power output tubes arranged in push-pull.

The 235 uses a slightly modified version of the 931 type RF amplifier.  This amplifier contains an extra capacitor, and a connection to the radio/phono switch.  This is not present in any other model.  The 235 uses the selector unit containing an additional 484 tube which is used in the majority of other Equasonne models.  The power converter used in this model appears to be unique.

 

 

 

Photos courtesy of Paul Chomiak

Download schematics

 

 

 

 

301

 

The model 301 (manufactured 1929-1930) came in an AC version and a DC version.  The 301 uses the same "tubeless" selector unit as the 591, 593 and 931.  The Model 301-A is slightly different than the regular 301 in that it used the tuning selector unit with the additional 484/485 tube that is common to most Equasonne models.

The 301AC model is a 10 tube radio.  The DC version lacks the pair of 281 half wave rectifiers, and is thus an 8 tube radio.

The RF amplifier unit of the 301 is of the 931 type and is common to virtually all Equasonne models.

The power convertor unit of the 301 AC differs from most Equasonnes in that it utilizes a pair of 281 half wave rectifier tubes as opposed to the more common use of a single full wave rectifier tube.  This convertor was also used in models 740 and 750.  As usual, the convertor houses the power output tubes, which are a pair of 250s in push pull.

It should be noted, that radios labelled as Model 301 are occasionally encountered in radically different styled cabinets to that shown.  It does appear that Sparton installed 301 chassis in several different style cabinets and sold them as model 301. 

Download schemtaics for the 301 AC

Download schematics for the 301 DC 

 

301A

The model 301-A (manufactured 19??) as far as I know, came in an AC AC version only.  The Model 301-A is slightly different than the regular 301 in that it used the tuning selector unit with the additional 484/485 tube that is common to most Equasonne models.

The 301-A AC model is an 11 tube radio. 

The RF amplifier unit of the 301 is of the 931 type and is common to virtually all Equasonne models.

The power convertor unit of the 301-A AC differs from most Equasonnes in that it utilizes a pair of 281 half wave rectifier tubes as opposed to the more common use of a single full wave rectifier tube.  This convertor was also used in models 301, 740 and 750.  As usual, the convertor houses the power output tubes, which are a pair of 250s in push pull.

If you have a picture of this model please e-mail me.

Download schematics

 

564

The model 564(manufactured 1930-1931) possibly came in only a DC version The 564 DC is a 9 tube radio.

The DC 564 contains the same chassis as 570, 600, 610, 620, 740 and 750 DC Equasonnes.

The 564 uses the 931 type RF amplifier, common to virtually all Equasonne models, and uses the selector unit containing an additional 484 tube which is used in the majority of other Equasonne models.

If you have a picture of this model please e-mail me.

Download schematics

 

570

The model 570(manufactured 1930-1931) possibly came in only a DC version, as I have yet to uncover any information on an AC version.

The 570 DC is a 9 tube radio.

The DC 570 contains the same chassis as 564, 600, 610, 620, 740 and 750 DC Equasonnes.

The 570 uses the 931 type RF amplifier, common to virtually all Equasonne models, and uses the selector unit containing an additional 484 tube which is used in the majority of other Equasonne models.

If you have a picture of this model please e-mail me.

Download schematics

 

574

The model 574 (manufactured 1930-1931) apparently came in an AC version only.  It uses the same chassis as Models 870 and 111-A. The 574 is a 13 tube radio. The RF Amplifier unit is the same as that found in most models (ie. 931 type).   The tuning selector unit is of the type which houses an additional 484 tube.  This particular tuning selector was also used in the majority of the other Equasonne models (103, 111-A, 235, 301-A, 564, 570, 578, 600, 610, 620, 737, 740, 750 and 870).  Models 111-A, 574 and 870 differ from other Equasonne models in the use of an additional audio connector unit which houses an input audio transformer and a pair of 226 tubes.  This unit provides an extra stage of audio frequency amplification.

If you have a picture of this model please e-mail me.

Download schematics

 

578

The model 578 (manufactured 19??) is one of the very few models of Equasonne that included a built in turntable.  Many of the other models did however include a phono jack through which you can connect an external phonograph..  This model possibly came only in an AC version, as I have yet to uncover any information on a DC version.

The 578 is a 13 tube radio.  It uses 9 484/485 amplifier tubes, two 281 half wave rectifier tubes, and a pair of 586 power output tubes arranged in push-pull.  The chassis of the 578 Equasonne is identical to that used in Model 103.

The 578 uses a slightly modified version of the 484 tube containing selector unit.  This unit contains an additional resistor that is not found in any other model.

If you have a picture of this model please e-mail me.

Download schematics

 

589

The model 589 (manufactured 1930-1931) came in an AC version only.  It uses the same power converter unit as the 591, 593 and 931 AC models, and uses a big 2 section cylindrical electrolytic capacitor in the filter circuit. The 589 is a 10 tube radio. The RF Amplifier unit is the same as that found in most models (ie. 931 type).   Electrically, the 589 only differs from the 931 in the fact that it uses a tuning selector unit which houses an additional 484 tube.  This particular tuning selector was also used in the majority of the other Equasonne models (103, 235, 301-A, 564, 570, 574, 578, 600, 610, 620, 737, 740, 750 and 870).

 

Download schematics

 

591

 

 

 

 

 

 

 

Photos courtesy of Ron Wentz 

 The model 591 (manufactured 1930-1931) uses an identical chassis to Model 593. These two models are virtually identical to the 931, and differ only in the inclusion of a single capacitor connecting one side of the power mains to the grounding post on the selector unit!  This capacitor is not used in the 931. 

The selector unit is of the 931 type, which does not include a 484/485 tube.  The RF amplifier is also of the 931 type which is common to the vast majority of Equasonne models.  The power supply also appears to be identical with that of the 931.

The 591 requires an external speaker for operation.

To the best of my knowledge there was no DC version of the 591.

Download schematics

 

593

The model 593 (manufactured 1930-1931) uses an identical chassis to Model 591. These two models are virtually identical to the 931, and at most differ only in the inclusion of a single capacitor connecting one side of the power mains to the grounding post on the selector unit!  This capacitor is not used in the 931.  Some model 593 sets used a slightly different power converter to the 931, but it seems that even Sparton eventually did not see the sense in this, the Sparton parts lists (Service Bulletin 32)  notes that "Model 591-593 converter interchangeable with 931 converter. Some model 591, 593 sets were equipped with 931 converters/ 931 converters are identified by Amplifier terminal block located on end.

The selector unit is also of the 931 type, which does not include a 484/485 tube.  The RF amplifier is also of the 931 type which is common to the vast majority of Equasonne models.  


To the best of my knowledge there was no DC version of the 593. The 593 appears to be a late model set, and the one known specimen contains the extra amplifier stage found in some model 931 sets. The cabinet seems to be identical to a 931, but without the doors. The inside lip at the front of the cabinet has been framed in where the door hinges would be. Curious if this is the original state of the set or not. If so, then the 593 Equasonne is really just a model 931 with no doors. If you also have a 593, please send me some pictures, and perhaps we can solve this mystery.

So, when all is said and done, a model 593 appears to simply be a model 931 without the doors! Probably a last ditch attempt to sell off remaining 931 sets by making them appear to be something new.

Photos of the 593 Equasonne courtesy of Myounki Kim. 

 

Download schematics

 

600

 

 

 

 

 

 

 

 

 

The model 600 (manufactured 1930-1931) contains the same chassis as the models 610 and 620.  All three of these models came in both AC and DC versions.  The DC versions were designed to operate in areas where household current was supplied as direct current.  The DC versions of these three models therefore have different power converter units than the AC versions, but use the same RF Amp and tuning selector.  The 600, 610 and 620 AC circuits are very similar to that of Model 589, they share the same tuning selector unit (with the extra 484 tube) and the RF Amplifier units with the 589.  These same RF Amps and the tuning selector units were also used in many other Equasonne models as well. However, the 600 series of Equasonnes differ from the 589 in that they used a Magnavox speaker with a 5000 ohm field coil, instead of 2500ohms.  The power supply incorporates a secondary winding of 375 volts to the center tap, rather than 300 volts as used in the 589, presumably to accommodate for this greater resistance.  Thus, although the power converter unit of the 600 series looks a lot like that found in the 589 (also 591, 593 and 931 AC models),  it incorporates a different power transformer.  Model 235 uses the 600 series power transformer.

Black and white Image courtesy of Jeremy Hopkin. Color images courtesy of Czeslaw Kijanka.

If you have a picture of a restored version of this model please e-mail me.

Download schematics for the 600 AC

Download schematics for the 600 DC

 

610

The model 610 (manufactured 1930-1931) contains the same chassis as the models 600 and 620.  All three of these models came in both AC and DC versions.  The DC versions were designed to operate in areas where household current was supplied as direct current.  The DC versions of these three models therefore have different power converter units than the AC versions, but use the same RF Amp and tuning selector.  The 600, 610 and 620 AC circuits are very similar to that of Model 589, they share the same tuning selector unit (with the extra 484 tube) and the RF Amplifier units with the 589.  These same RF Amps and the tuning selector units were also used in many other Equasonne models as well. However, the 600 series of Equasonnes differ from the 589 in that they used a Magnavox speaker with a 5000 ohm field coil, instead of 2500ohms.  The power supply incorporates a secondary winding of 375 volts to the center tap, rather than 300 volts as used in the 589, presumably to accommodate for this greater resistance.  Thus, although the power converter unit of the 600 series looks a lot like that found in the 589 (also 591, 593 and 931 AC models),  it incorporates a different power transformer.  Model 235 uses the 600 series power transformer.

B & W Image courtesy of Jeremy Hopkin, color photo courtesy of Rene Caouette (Note: center leg in photo is non-original.)

 Download schematics for the 610 AC

Download schematics for the 610 DC

 

620

 

 

 

The model 620 (manufactured 1930-1931) contains the same chassis as the models 600 and 610.  All three of these models came in both AC and DC versions.  The DC versions were designed to operate in areas where household current was supplied as direct current.  The DC versions of these three models therefore have different power converter units than the AC versions, but use the same RF Amp and tuning selector.  The 600, 610 and 620 AC circuits are very similar to that of Model 589, they share the same tuning selector unit (with the extra 484 tube) and the RF Amplifier units with the 589.  These same RF Amps and the tuning selector units were also used in many other Equasonne models as well. However, the 600 series of Equasonnes differ from the 589 in that they used a Magnavox speaker with a 5000 ohm field coil, instead of 2500ohms.  The power supply incorporates a secondary winding of 375 volts to the center tap, rather than 300 volts as used in the 589, presumably to accommodate for this greater resistance.  Thus, although the power converter unit of the 600 series looks a lot like that found in the 589 (also 591, 593 and 931 AC models),  it incorporates a different power transformer.  Model 235 uses the 600 series power transformer.

 

Download schematics for the 620 AC

Download schematics for the 620 DC

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

737

The model 737 (manufactured 1931) came in an AC and DC versions.

The chassis of the DC versions are all the same.  The AC versions have two different possible chassis.  Chassis with serial numbers 6502 and higher use a slightly different version of the power converter unit, which incorporates a different power transformer, and several additional resistors.  These later production 737s probably utilized left over power transformers from another Sparton radio model, the resistors have been added to the circuit to compensate for the differences.  Other than the power converter differences, high and low serial number Model 737 AC Equasonnes are identical.  The low serial number 737 chassis is identical to that found in AC Models 600, 610 and 620.


The AC version is an 10 tube radio.

The RF amplifier unit of the 737 is of the 931 type and is common to virtually all Equasonne models.

The selector unit is of the type which houses an additional 484 tube, and is common to most Equasonne models.

 

Photos courtesy of the RadiolaGuy 

Download schematics for the 737 AC Model. Serial No. below 6502.

Download schematics for the 737 AC Model. Serial No. 6502 and up.

740

The model 740 (manufactured 1930-1931) came in AC and DC versions.

The chassis of the 740 and 750 models have identical chassis.  These models the tuning selector unit with the additional 484 tube that is common to most Equasonne models.

The AC version is an 11 tube radio, the DC version is a 9 tube radio.

The RF amplifier unit of the 740 is of the 931 type and is common to virtually all Equasonne models.

The power converter unit of the 740 AC differs from most Equasonnes in that it utilizes a pair of 281 half wave rectifier tubes as opposed to the more common use of a single full wave rectifier tube.  This converter was also used in models 301 and 750.  As usual, the converter houses the power output tubes, which in this case are a pair of 250s in push pull.

 

Download schematics for the 740 AC

Download schematics for the 740 DC

 

 

 

 

750

The model 750 (manufactured 1930-1931) came in AC or DC models, which contained different power converter modules. The AC model is an 11 tube radio, the DC model is a 9 tube radio.  The 750 chassis is identical to that of the 740.

The AC model contains the same power converter as models 301 and 740 AC.  It also uses a tuning selector unit with a socket for an additional 484/485 tube.  The RF Amplifier unit is the same as that found in most models (ie. 931 type).

 

Download schematics for the 750 AC

Download schematics for the 750 DC

 

 

 

  Photo courtesy of  Jeff Hed.

870

The model 870 (manufactured 1930-1931) apparently came in an AC version only.  It uses the same chassis as Models 574 and 111-A. The 870 is a 13 tube radio. The RF Amplifier unit is the same as that found in most models (ie. 931 type).   The tuning selector unit is of the type which houses an additional 484 tube.  This particular tuning selector was also used in the majority of the other Equasonne models (103, 111-A, 235, 301-A, 564, 570, 574, 578, 600, 610, 620, 737, 740 and 750).  Models 111-A, 574 and 870 differ from other Equasonne models in the use of an additional audio connector unit which houses an input audio transformer and a pair of 226 tubes.  This unit provides an extra stage of audio frequency amplification.

 

Photos courtesy of Russ Peters

Download schematics

 

930

The model 930 (manufactured 1929) is similar to the model 931AC which replaced it.  The electronics of the 2 models are very similar and differ mainly in the construction of the filter circuit in the power converter unit, the 931 taking advantage of larger value primitive wet electrolytics that had become available, which greatly simplify and probably improved the circuit.  The 930 and probably the model 89A have virtually the same RF amplifier and selector units as many other models, but there are some minor differences.

 Sparton 930 with doors open.  Photo courtesy of Mark Palmquist.

Download schematics

For the 930

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Undocumented Extra Amplification Stage for Model 931

Some, but not all model 931 Equasonnes feature an extra undocumented chassis that provides an extra stage of amplification. This small chassis contains a single tube, and hangs upside down under the main shelf like a bat. I located and restored one of these units and installed it in my 931 Equasonne. It provides a noticeable improvement in performance. I have never come across any service literature pertaining to this unit. It was either an extra upgrade feature, or was possibly added to late production sets in an attempt to upgrade their performance. With the help of another radio collector who also has a 931 with the same unit I have made estimates of the original values of the parts and created a schematic to aid in restoration of this unit. The major issue with the unit will be the large tin box under the chassis that contains a dual section capacitor.

Download schematics for the undocumented mystery amplifier

931

The model 931 (manufactured 1929-1930) is very similar to the model 930 which it replaced.  The cabinets differ a bit, the 931 is more ornate.  The electronics of the 2 models are very similar and differ mainly in the construction of the filter circuit in the power converter unit, the 931 taking advantage of larger value primitive wet electrolytics that had become available  There are also some minor differences in the RF amplifier unit circuit and selector units.  The RF Amplifier unit of the 931 is identical with that found in virtually all models, AC and DC.  The selector unit of the 931 is identical to that of Models 301, 591, and 593 (Both AC and DC models).

The AC model of the 931 is designed for use with an alternating current power supply.  A terminal strip beside the power transformer allows you to adjust the set to operate on one of three possible AC line voltage ranges (100-110 Volts, 110-120 Volts, or 120-130 Volts).

A 931 Equasonne, photo courtesy of Clay Turner.

The DC model was designed for use in areas where household power was supplied as direct current, presumably no longer the case anywhere in the Continental US.  The power converter unit of the DC model is completely different from that of the AC model (obviously), but in all other respects, the two versions of the 931 are the same.  The DC power converter does not require a rectifier tube, or a filter capacitor section, as it has no need to convert AC volts to DC.  The DC model still contains a pair of 182 power output tubes, but does not feature a phono jack as does the AC Model.  Thus the power supply of the DC model can easily be distinguished from that of the AC model as it contains only 2 tubes, while that of the AC model contains 3.

As far as I can tell, the chassis of the 931 DC is identical to that of the 301 DC.  The two models differed only in the nature of the cabinet in which they were housed.

Ever wonder how a Sparton Equasonne radio found its way from the factory to its new home?  Photograph of packing crate was kindly provided by Sharon Mayor. 

Download schematics for the 931 AC

Download schematics for the 931 DC

 

 


 

 

 The cabinet of my 931 Equasonne, before and after restoration.

 

 

Undocumented Extra Amplification Stage

Some, but not all model 931 Equasonnes (and model 593), feature an extra undocumented chassis that provides an extra stage of amplification. This small chassis contains a single tube, and hangs upside down under the main shelf like a bat. I located and restored one of these units recently and installed it in my 931 Equasonne. It provides a noticeable improvement in performance. I have never come across any service literature pertaining to this unit. It was either an extra upgrade feature, or was possibly added to late production sets in an attempt to improve their performance. With the help of another radio collector who also has a 931 with the same unit I have made estimates of the original values of the parts and created a schematic to aid in restoration of this unit. The major issue with the unit will be the large tin box under the chassis that contains a dual section capacitor.

Download schematics for the undocumented mystery amplifier