General Electric E-91
There was a discussion on the Antique Radio Forum about GE's Colorama radios in early 2004. This sparked an interest in these sets. I happened across this set that was located in Nebraska on eBay. It wasn't in the best of shape as can be seen in the photos from the auction below. I won the auction and am the proud owner of a GE Colorama radio.
Colorama was a method of visual tuning used by General Electric for one year: 1936. The heart of the system is a saturable-core reactor. The reactor is driven by a 6C5 triode with its grid connected to the AVC line. At zero signal, the grid of the 6C5 is less negative (about -6V). A maximum amount of current flows through the 6C5 and the primary of the reactor, decreasing the inductance of the secondary winding. The decreased inductance allows more of the current to be passed through the secondary than through the green lights, making the red lights brighter.
As a station is tuned in, the grid of the 6C5 goes even more negative (about -28V) cutting off the tube and nearly zero current flows through the primary winding. The inductance of the secondary increases, shunting current through the green lights making them brighter.
Here is the
dial light assembly.
Other manufacturers had previously used saturable core reactors as tuning indicators. One of them was Majestic who used it in sets such as the model 320. That set used the plate current of the IF amplifier as the control current to brighten and dim the dial light. The 320 dates from 1933 so the use of saturable reactors was not a new idea by the time of the colorama sets, it was just a slightly different use.
Back to the restoration. I pulled the chassis out of the cabinet and found a hole where a filter capacitor once was mounted and an adjustable core mounted on the top of the chassis. Flipping the chassis over showed that one of the antenna coils was missing and discoloration on the shielding around where it should be indicates that it had gotten fried.
The reactor is housed in the black rectangular box on the left side of the chassis in the above photo.
This set was built in an era when manufactuers were beginning to use rubber insulated wire. This set is full of it. The problem is that rubber does not hold up very well over 65 years and becomes dry and brittle. The best remedy is to replace it all but as long as the wire is not disturbed the insulation will remain intact. I decided to leave it alone and just do a typical restoration.
I brought the chassis up slowly on a variac and monitored the current. I got to about 60V on the input and the current shot up to about 2A. That would indicate a power consumption of 120W at half the input voltage when the total power consumption should be 105W at full voltage. I turned the set off and decided not to power it up again until I recapped it.
I first wanted to clean the chassis up so I removed the tuning capacitor and the IF transformer cans. I cleaned the chassis with hot soapy water and a scotchbrite pad. Later I would clean and relube the vernier drive on the tuning capacitor.
Now the recapping can commence. I found that the adjustable coil on top of the chassis was a replacement for the missing antenna coil under the chassis. I posted a request for an original replacement coil on the Antique Radio Forum and a member happened to have one. In the meantime I recapped the chassis and checked the values of all resistors. I installed the new antenna coil and fired up the set. It worked!
The dial lamps are #40 types that have been colored red and green. A couple of the red lights were burned out and I like to replace the dial lamps in a radio with new ones anyway. I bought some glass stain from a local craft store and painted the new lamps with it. Four were painted red and three were painted green.
The power cord was replaced, I added an inline fuse, the set was aligned, the cabinet refinished and the whole thing put back together. This set has nine tubes (6K7 R.F., 6A8 Osc./Mixer, 6K7 I.F., 6K7 I.F., 6H6 detector, 6F5 1st audio, 6C5 Colorama, 6L6 audio out, 5Z4 rectifier) and the performance is very good. The Colorama feature does not give a very sharp indication of tuning like a magic eye does but it is still a pretty cool feature.
After restoring the set and setting it on my entertainment center for display, I noticed a slight static noise in the audio. I pulled the chassis again thinking that a mica capacitor might be going bad. I jumpered a new mica in place of the one that shunts RF from the 1st audio amp plate to ground with no change in the results. After poking around some more I disconnected the candohm cathode bias resistor from this tube and replaced it with a separate carbon unit. This decreased the amount of static. I deduced that the problem was stemming from the candohm unit having intermittent connections. I rebuilt the candohm using carbon and wirewound power resistors. The static sound was eliminated.
I still need to find an electrolytic can to fill the hole on the
rear of
the chassis.
I finally found some advertisements for the Colorama feature. One
ad
comes from Collier's magazine and the other from the Saturday Evening
Post
of September 26, 1936. These ads mostly describe features that were
found
on the higher-end models and used in conjunction with Colorama.
Click
on the images above to see the full sized scan at 150dpi. These
files
are pretty large (~450k).
I also bought a digital camera so keep an eye out for some video of
the
Colorama set in action.
It has been brought to my attention that a certain website that has
the Rider Perpetual Trouble Shooters manuals for download has missed
the pages that contain the schematic for this set. So, I have
scanned the pages from my Rider manual and have posted them in PDF
format: GE E-91 Schematic.
This site created and maintained by Sean Barton.
Last updated 3/20/2009.
