The PHILCO Phorum
Stromberg-Carlson 320-T - Printable Version

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Stromberg-Carlson 320-T - klondike98 - 04-15-2021

This was an antique/junk store find.  The sewing cabinet table contains a 320 Chassis and the combination is a 320-T manufactured around 1938.  There is a 1938 advertisement that describes it as a replica of a sewing table found in "Furniture Masterpieces of Duncan Phyfe" and made of Honduras swirl mahagony.

[Image: https://www.dropbox.com/s/m7f8rr49hbpa11r/Stromberg-Carlson320-T_01.jpg?raw=1]
[Image: https://www.dropbox.com/s/r41iawjmzc9o95e/Stromberg-Carlson320-T_02.jpg?raw=1]

The front has a nasty break that was "fixed" at one point and has failed again.  I'll also have to fashion up an escutcheon somehow since the original is crumbling. 
[Image: https://www.dropbox.com/s/z8c0quuj8dt31pe/Stromberg-Carlson320-T_04.jpg?raw=1]

The rest of the cabinet is in reasonably good shape and should refinish nicely.
[Image: https://www.dropbox.com/s/kcuwnqebs7dtfyb/Stromberg-Carlson320-T_03.jpg?raw=1]

The chassis was very dusty but also has had some work done to it in the past.
[Image: https://www.dropbox.com/s/kq9yvpcnnw0hijm/Stromberg-Carlson320-T_05.jpg?raw=1]<br />[Image: https://www.dropbox.com/s/s07v335j9ve70sd/Stromberg-Carlson320-T_06.jpg?raw=1]

This set uses a battery as a bias and a "replacement" Ray-O-Vac had been soldered in.  There is a white pincher like device that is also connected to the same place the battery is so I suppose that it was the holder for the original bias cell although I have never seen one before. 

[Image: https://www.dropbox.com/s/30tf4q15p7tny2i/Stromberg-Carlson320-T_07.jpg?raw=1]

At this stage I'm just cleaning off the years of grime and taking out the old dried up tuner and speaker mounting rubber.  I'm waiting for an order from Renovated Radios for some replacements.

[Image: https://www.dropbox.com/s/xa8z64g9hkck3j2/Stromberg-Carlson320-T_08.jpg?raw=1]

RE: Stromberg-Carlson 320-T - radiorich - 04-15-2021

Hello klondike98,
What great looking set !
Far as Chassis go it is in fairly nice shape and the cabinet looks nice too .
Sincerely Rich

RE: Stromberg-Carlson 320-T - OZ4 - 04-16-2021

Here's a bias battery for an RCA 28T set I worked on. I replaced it with a lithium button cell. 
   

RE: Stromberg-Carlson 320-T - Chas - 04-16-2021

I believe the bias battery can be eliminated by altering the cathode bias and adding a grid resistor of a high value. At the time of production, stable high value resistors were not available for that service. The battery was a relatively reliable alternative and will often out last the 5 year life of the radio as no current is drawn... chas

RE: Stromberg-Carlson 320-T - klondike98 - 04-16-2021

Thanks guys. Appreciate the photo of another bias cell holder OZ4! I'll probably also try a button Li battery and think about the bias mod Chas suggests if it all works when I'm done.

Stromberg-Carlson did a nice job on the schematic information with both an electrical schematic and a physical layout drawing!

RE: Stromberg-Carlson 320-T - mikethedruid - 04-16-2021

Just my two cents, you could also buy a single cell battery holder for an AA cell, and make any future changes simple. Once you rebuild the original electrolytic cans, and get rid of those big orange replacements under the chassis, there should be a whole lot more room. If you don't bother with re-stuffing the paper condensers, but just go with the yellow poly condensers as is, you'll have even more room. Should be able to find a place for an AA cell holder pretty easily.

RE: Stromberg-Carlson 320-T - klondike98 - 04-16-2021

Thanks Mike, good thought.

RE: Stromberg-Carlson 320-T - Arran - 04-16-2021

The watch batteries work but the polarity is the opposite of one of those vintage Mallory bias cells. With the style I run into in Rogers sets, the kind with the cylinder that fits the domed side, the cylinder is negative, and the spring contact is positive, so you have to rewire them to use the modern watch batteries which have the case as positive. I don't think the availability of one type of resistor or another had much to do with why bias cells were used, I've never seen a radio older then 1936 that had them, the newest set I have seen with them was a Canadian Marconi set from 1941. They claimed that it was to simplify the power supply, but since the usual method was to place a resistor, and cap, in series between the cathode of the tube, and ground the parts count would be negligible. I'm guessing that it was either to create a service item that would create extra business for repair shops (keep in mind that other then electrolytic caps the paper caps seldom needed replacement when these sets were under 10 years old), or that it was a scheme to avoid paying patent royalties. Weird that a Stromberg would use one, all the ones I have encountered do not, and Stromberg didn't care about patent royaties, they just paid them.
Ifg the set has all original caps I would restuff the paper ones, and the cans on top, there seems to be more then enough room for everything under that chassis.
Regards

Arran

RE: Stromberg-Carlson 320-T - klondike98 - 04-16-2021

Thx Arran. I was thinking that most of the resistors were no longer original and a few paper caps are replaced already so wasn’t thinking of stuffing the paper caps. Do those carbon resistors look like 1938 vintage to you?? If they are then I will go ahead and stuff the paper caps also.

RE: Stromberg-Carlson 320-T - Arran - 04-17-2021

I can't see most of the resistors, but the ones I can see look like replacement carbon comp ones by the way the leads are dressed, there are metal oxide ones that are brown, so they don't look out of place. I can't remember what style of resistors were used in the two 1937 models, but I don't think they had dogbone style ones . I usually restuff paper caps because I can't stand the way the yellow plastic ones look in a pre war radio, more so then keeping it all original, replacement resistors don't bother me as much unless they are bright blue. some were replacements it was probably the audio coupling caps like the one on the volume control, and the one between the plate of the 1st audio, and the grid of the power output tube. The most obvious replacements are the axial lead filter caps under the chassis electrically replacing the cans on top.
Regards
Arran

RE: Stromberg-Carlson 320-T - klondike98 - 04-17-2021

Thanks Arran.

RE: Stromberg-Carlson 320-T - klondike98 - 04-22-2021

A bit of progress on this chassis.  I've stuffed the 3 can electrolytic caps.  I have seen a few queries on restuffing these recently and in this particular case I took a little different approach than I've done before because of the style of cap.  These have the crimped end that holds the rubber insulation around the center aluminum lead (positive electrode).  The previous repair had clipped off the wiring lugs attached to the aluminum lead so it was necessary to find a way to connect new wires to these caps. 

[Image: https://www.dropbox.com/s/sav43w75y2vatok/Stromberg-Carlson320-T_09.jpg?raw=1]

I open these up by using a utility knife and rolling the blade along the can with a little bit of pressure.  Just keep rolling it with slight pressure and eventually it will cut through the can.  You do have to be gently so as not to push in the can but even that can be fixed.  I find it makes a cleaner cut than a hack saw blade.

[Image: https://www.dropbox.com/s/pkatx45ihsql5xp/Stromberg-Carlson320-T_10.jpg?raw=1]

Here's that center aluminum lead.  The rubber insulation around it was not in good shape and I did not want to trust it (actually the rubber in this picture was the best of the three and this electrode still had liquid in it!)  The aluminum lead will just pull out (you may have to clip off any remains of the wire lug that was on the lead) and the rubber all came out with a bit of digging and drilling.

[Image: https://www.dropbox.com/s/k6evn0h7re1rs33/Stromberg-Carlson320-T_11.jpg?raw=1]

[Image: https://www.dropbox.com/s/180b4x0e0rftn0i/Stromberg-Carlson320-T_12.jpg?raw=1]

I used plastic tubing from the hardware store to replace the rubber insulation.  From what I could find I think it will have sufficient insulation properties (we'll find out!).  I wound up only using the smaller diameter tubing.  It fit in the crimped space snuggly and when the bolt I used for the replacement electrode is inserted it gets a little tighter. In this set the negative goes to the chassis ground so I used some copper strap that will fit between the can and the chassis clamp to make the ground connection.

[Image: https://www.dropbox.com/s/mdhcl537rlbvp6j/Stromberg-Carlson320-T_13.jpg?raw=1]

[Image: https://www.dropbox.com/s/tpjqvw5o2dttb0h/Stromberg-Carlson320-T_14.jpg?raw=1]
[Image: https://www.dropbox.com/s/h7xj9lmxqsndtd7/Stromberg-Carlson320-T_15.jpg?raw=1]

I wrapped the replacement cap and bolt with electrical tape to help keep things insulated and add a litte support.  A thin plastic sign from the hardware store, cut into a strip and then formed into a cylinder also helps keep the top and bottom of the aluminum can together.  I could have epoxied both ends of the plastic but elected to only epoxy the bottom end.  Once the can is in the chassis clamps it seems to be pretty secure.

[Image: https://www.dropbox.com/s/7ln76gk8cy7vabo/Stromberg-Carlson320-T_16.jpg?raw=1]

[Image: https://www.dropbox.com/s/t3qkxl9e856x8k8/Stromberg-Carlson320-T_17.jpg?raw=1]

RE: Stromberg-Carlson 320-T - Eliot Ness - 04-23-2021

Nice work Bob Icon_thumbup

The clamps cover your incisions well. It's always interesting to see a slightly different approach to re-stuffing.

RE: Stromberg-Carlson 320-T - klondike98 - 04-25-2021

A little more movement on this one...basically the right half or so of the underside of the chassis recapped.  Most of the resistors were also well out of spec and replaced.

BEFORE and AFTER

[Image: https://www.dropbox.com/s/7nb327d9n33b44y/Stromberg-Carlson320-T_19.jpg?raw=1] [Image: https://www.dropbox.com/s/silepzccir9r6i7/Stromberg-Carlson320-T_18.jpg?raw=1]

RE: Stromberg-Carlson 320-T - klondike98 - 05-02-2021

I could use some advice here.  I am recapping a section where the previous repair person made some changes to the circuit.  I believe the reason they did so is because the OFF-ON-TONE three position switch is defective.  The OFF-ON power section of the switch works OK but one of the switches piggy backed on the same shaft but related to the TONE function does not seem to be working (I have not yet disassembled the switch to see if I can fix it).  In going from OFF to the ON position I believe the tone switch portion should engage the ground connection.  Then when moving from the ON to the TONE position the switch engages so the green and blue wires are connected.  In the modified circuit the switch lug that was connected to ground has been disconnected completely so that in the ON position C27 has no connection (checking this portion of the switch with an ohm meter it does indeed fail to engage properly so they unsoldered it from ground).  In the TONE position there is now a connection through a 310K resistor to the plate of the  6V6 tube instead of connecting to one of the grids in the 6V6. Here's an original schematic and a schematic of the modified circuit.  I'm assuming it actually worked with this set up at some point...does it make sense?  I could recap and leave it as is or try to repair the switch.  Thoughts???

[Image: https://www.dropbox.com/s/30su0tyux6gwhcd/Stromberg-Carlson%20320T%20-Original.png?raw=1][Image: https://www.dropbox.com/s/bmvr8s0sifgblca/Stromberg-Carlson%20320T%20AS%20RECIEVED.png?raw=1]

Stromberg-Carlson also has a wiring diagram which has been very helpful.  I've also done and Original and a Modified version.

[Image: https://www.dropbox.com/s/ego0c7h2fp6yr33/Stromberg-Carlson%20320T%20ORIGINALWiring.png?raw=1][Image: https://www.dropbox.com/s/talitim81w7cskd/Stromberg-Carlson%20320T%20AS%20RECEIVED%20wiring.png?raw=1]