Rogers/Majestic 6R531

[AlNewman]

Hi,

So I have an old radio, one of the few I acquired that has the cabinet intact, and my Mom's birthday is coming up, I'd like to rebuild it and give it to her for her birthday(ish). 

I won't be doing anything other than stock, besides possibly adding an auxiliary in, but the original design incorporates battery cells to create bias, and I'm wondering if anybody could foresee any issues in using a cathode resistor as a bias reference instead? 

Thanks.

[AlNewman]

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[AlNewman]

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[AlNewman]

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[PRR]

Ignorant cathode bias will screw-up the detection and amplification.

Are the cells truly dead? Have you seen the trick to put a micro-drop of water in the cell? They may outlive you.

I've never used a '75, and the 6K7 is not on the tip of my tongue. I'm sure they can be cathode-biased but I'd want to find an example to steal. The 6K7 battery seems extra odd because a small cathode resistor was "good" for tubes on tuned circuits by reducing the change of parasitic capacitance with AVC.

Read Radiotron 3rd and 4th. They do not touch Mallory bias batts but do cover all other radio tricks.

You have the field coil speaker/filter situation figured out?

(Is there anything left on the AM band?)

[AlNewman]

So far I haven't done much, besides scabbing in a new cord.  Haven't even tried firing it up yet to see if the transformer is good.  Been busy with work, so haven't had heaps of spare time recently.  I do have the speaker, but I'll have to rewire the plug, apparently 80 year old wire gets pretty brittle.

I haven't seen a microdrop trick, but I did find a thread where somebody rejuvenated one by dropping it in boiling water, might be worth a shot.  From what I read there were 2 different varieties, a 1 volt and a 1.4 volt, of course there is no indication on the cell what voltage I have, but apparently people have used different varieties of new batteries as replacements as well.

A couple of the tubes have been replaced, I see a 6v6 in the output section, and a 6b6 as well, a 6a8 instead of a 6a7, one has no numbers left at all...   So I'll have to do some research as to whether or not those types are even proper replacements before I do much of anything.  I might be dead in the water before I even start.  I assume it was working, it has been serviced at least once so I'll take that as a good sign.

And of course we still have AM radio.  This is Canada, how else would we listen to hockey games, eh?

[AlNewman]

Well, I tried submersing the batteries in boiling water, and they seem to have recovered somewhat at least.  One holds .85 volts, and the other holds .89 volts.  I guess that should be enough to start replacing parts and see if I can get it to make noise.  I'll monitor them, and if they continue to hold charge, I'll seal them up again somehow.

The tubes seem to be comparable to those in the schematic, but are compatible with different sockets, so I'm thinking maybe this was an earlier version before the published schematic?  I'll check pinouts as I go, and see if anything blows up, that's always fun!

[AlNewman]

So I resoldered the socket leading to the field coil speaker, and I'm not getting any reading across the 2 main pins, reading red start and blue finish on the schematic.  Is that normal?  I get a reading of about 2.1k ohms across the other 2 connections on the plug.

Not really sure how these work, if it's supposed to be a continuous coil or not.  Maybe I'm wasting my time.

[PRR]

> not getting any reading across the 2 main pins, reading red start and blue finish on the schematic.  Is that normal?  I get a reading of about 2.1k ohms across the other 2 connections on the plug.

"Not getting a reading"? Tell us just what the display says. OV, or 0.2, or DAM, or??

But red/blu is just the transformer. Replace it with a Champ OT.

[AlNewman]

"Not getting a reading"? Tell us just what the display says. OV, or 0.2, or DAM, or??

But red/blu is just the transformer. Replace it with a Champ OT.

Yeah, reading open circuit.  The schematic was hard to read, so it didn't look like a regular coil in the drawing, so I was hoping against hope. 

I inserted a pin in the wires near the transformer and got continuity from the socket to the red, not the blue, so maybe just a bad wire.  I'll maybe try today to see if I can sort it out.  Could still be something simple.

 I probably have another transformer that is similar to the champ I can try, I didn't realize it would be the same winding ratio as a regular output transformer.  May have to mount it to the chassis somewhere.  If all else fails I have other speakers I could replace it with.  The good news is the batteries are holding a charge.

Edit:I suppose the way the transformer is wired from grid to plate of the output tube maybe a regular speaker setup won't work?

[sluckey]

Edit:I suppose the way the transformer is wired from grid to plate of the output tube maybe a regular speaker setup won't work?

There is no transformer connection between grid and plate. What are you talking about?

[AlNewman]

There is no transformer connection between grid and plate. What are you talking about?
[/quote]

From what I see, looking at the socket wiring on the schematic, the primary is bridged on both sides.  One side feeds the plate of the 6f6, the other feeds not only the grid of the 6f6, but also the screen and all the other plates in the amp.  Maybe cheap filtering during wartime?  Maybe I'm missing something...do you see another way the plates could get dc voltage?

Edit:  actually you're right, the grid ties into the secondary.  And the plates are fed from the socket, just the plate of the 6f6 is fed through the transformer....Unless I'm still reading it wrong....

Luckily though, it may not be an issue.  I have a transformer that will drop right in, don't even have to change the housing.  Not sure if it will work though, but it's worth a shot.  It has a 48:1 winding ratio, 2300 ohm impedance ratio.  Think it'll work, or will I just get the magic smoke?

[AlNewman]

Ok, I'm starting to understand this field coil better, I feel better about the transformer working.  I tested continuity between the field coil without the transformer wired in and still have continuity, so that's good.

[sluckey]

Ok, I'm starting to understand this field coil better

I don't think you do. Let's see if this helps.

Referring to the schematic... The top side of the FC connects to ground. The bottom side of the FC connects to the PT high voltage winding CENTER TAP. This arrangement creates a negative voltage at the CENTER TAP and is known as "back biasing". It was an easy way to obtain negative bias voltage back in a time when silicon diodes did not exist. R14 and R15 make a voltage divider connected from the CENTER TAP to ground. The negative voltage present at the junction of R14/R15 connects to the grid of the 6F6 through grid leak resistor R13.

Here's a simplified schematic of the field coil and back bias circuit. Connectors eliminated for clarity.

[scstill]

I have rejuvenated old bias battery cells by dropping it in very hot water
The water replaces the air and the cell works again.
When the bubbles stop the process is good dry it off and measure a voltage close to what it should be.
Can also replace with a modern battery, but what's the fun in that :-)

Here is the ARF post where I learned about this
https://antiqueradios.com/forums/viewtopic.php?f=19&t=391764

Alot of great radio knowledge at ARF

Here is your radio discussed in the museum
https://www.radiomuseum.org/r/rogerstube_rogers_eleven_5211_5_ch_6.html

Sorry if these ideas were already discussed here....

[AlNewman]

Ok, I'm starting to understand this field coil better

I don't think you do. Let's see if this helps.

Here's a simplified schematic of the field coil and back bias circuit. Connectors eliminated for clarity.

That did help actually, thanks.

Here it is connectors included.  I hope that transformer has enough jam to push that massive speaker, because I don't wanna replace it again.

[AlNewman]

I have rejuvenated old bias battery cells by dropping it in very hot water
The water replaces the air and the cell works again.
When the bubbles stop the process is good dry it off and measure a voltage close to what it should be.
Can also replace with a modern battery, but what's the fun in that :-)

Here is the ARF post where I learned about this
https://antiqueradios.com/forums/viewtopic.php?f=19&t=391764

Alot of great radio knowledge at ARF

Here is your radio discussed in the museum
https://www.radiomuseum.org/r/rogerstube_rogers_eleven_5211_5_ch_6.html

Sorry if these ideas were already discussed here....

Cool, thanks.  I'll check out that thread.  I got mine reading 0.9 volts now, hopefully they hold.  Did you seal up the edges after as well?

[scstill]

I did not seal the edges.
I suppose we might need to rejuvenate every so often

I also restore old vintage radios.
Started out buying them to make into guitar amps
But it was to hard to destroy the beauty (including the circuit) of the radio
So I restored a bunch (car, desktop and farm/battery); Learned a lot at ARF
There are still a bunch made into guitar amps though
At least that way they get some use. The restored AM radios mostly sit on the shelf

[AlNewman]

I did not seal the edges.
I suppose we might need to rejuvenate every so often

I also restore old vintage radios.
Started out buying them to make into guitar amps
But it was to hard to destroy the beauty (including the circuit) of the radio
So I restored a bunch (car, desktop and farm/battery); Learned a lot at ARF
There are still a bunch made into guitar amps though
At least that way they get some use. The restored AM radios mostly sit on the shelf

I wouldn't really call this a restore, more of a rebuild and that's if I'm lucky.  I have lots of amps to convert to guitar amps, this is one that I had all the pieces to return to original working order.  I have one more, a Philco console with a turntable that I will do at some point as well.  It's in pretty good shape too.  Same story, field coil speaker...it's a push pull 6k6 though, and all loctal tubes.  Should be interesting.  I'll see if I can get that one working, then try and get a few bucks out of it.  Save myself a trip to the dump.

I'm learning a lot more about different circuits this way too, which is really where the fun is.

[PRR]

...Can also replace with a modern battery...

I don't know of any modern battery with a hope of lasting 30 years, as these did.

I guess there are smoke detectors with 10 year battery but that may be too much of everything (size, voltage, cost).

[scstill]

More on the bias cell. Sounds like they used these cells to save $$ on the cathode resistor and Bypass Cap.
https://antiquewireless.org/wp-content/uploads/09-the_mallory_bias_cell.pdf

ARF link on my radio had suggested that a button or hearing aid battery could also work (no longevity guarantees). Same link suggested that replacing the grid resistor and bias cell with a 4.7Mohm to ground would also work. https://antiqueradios.com/forums/viewtopic.php?f=19&t=391764

[PRR]

Biasing the amplifier stage is not a big deal.

The detector bias is new to me. It may work just shorted-out if you don't need to be the best radio in the showroom.

FWIW, my HA batts last a week. True, they power a computer unimaginable when that radio was new. But they are "air zinc" which means they get half their fuel from the air (so, not weighting my ear) and will oxidize-out whether loaded or not. Your radio is bigger than my ear, you can use a more stable 2-part chemistry.

"Lithium" seems to be the ting in smoke-alarms, EZ-Pass transponders, and other long-life gizmos. Of course some of these are just too big. And "lithium" covers many types, and all quality levels.

Solar cells? 0.6V. "Plays better in bright light."

[scstill]

BTW - you probably already know this, but all of the paper caps need to be replaced. Especially filter caps.
and I have not been successful finding any shortwave signals, it seems that not to many use it anymore.

[AlNewman]

Yes, I started in the power end replacing all resistors and caps.  Then I fired it up and got noise, just no radio.  I continued on, and replaced pretty much everything except the mica caps, and have been listening to AM news and weather for the last bit. 

Now I have to figure out how to tune it. 

Luckily, somebody has already installed an aux input, so I might be ahead of the game.  One of my batteries has all but died, and my 6v6 is running at about 90% at idle...  But I'll have to figure that all out.  Maybe I'm good now, I haven't checked voltages in a bit.  Maybe this new transformer doesn't have enough load. 

[AlNewman]

So my batteries didn't make it, I decided to jump them with 4.7 meg resistors.  It sounds ok, but a little bassy, which I suppose could be expected.  So I think I'll tweak C21 and C22 to make the output more mid focused, and I also have to align the amp with the faceplate, so I need a couple fender washers to roll it back a little bit, but I've been listening to cbc on and off for the last few days, and nothing's blown up yet.  It appears that mid-30's technology still holds true.