Hello everyone,
I just finished Electrically restoring my Mitchell 1260 Lumitone radio lamp .
First off the start the set works well even without alighment done yet .
But I did notice there no real midrange lots highs thou what I am thinking is to change some coupling cap values and maybe a a capacitor or resistor to make this set less tiny .
Here is the Audio Section any suggestions far as maybe a RC circuit or maybe a LC circuit .
Far as this set I know that poor little speaker is not much .
So capacitor and resistor values are as follows
C4- 005 ufd
C6 ,C7 - .01ufd
R6 - 6.8 megs ohms
R7 - 220k ohms
R8 - 470k ohms
I would check the speaker or hook a different one to the set. It could be that the cone can't move freely. The values of the caps/resistor look pretty average. If you want to lower the highs increase the value of C-7 to .015 - .02 @630v. You can go larger if you like but it will start to sound muffled.
GL
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Years ago I bought a vintage no-name speaker substution box. I didn't use it much but when it helped me determine the slightly distorted sound from a Zenith console was the speaker it was well worth the $20 or so that I paid for it.
Today I use a different brand, but in addition to troubleshooting bad sound it lets me use the more convenient sub box when a console chassis is on the bench instead of pulling the original speaker and risking poking a finger in the cone.
John KK4ZLF
Lexington, KY
"illegitimis non carborundum"
Hello john,
Yes currently I have speaker from a surround system that I have used as a sub a few times I might try that one.
You can even see wjhere this speaker was missing part of it's cardboard surround and I used some ceral box cardboard to build up to the right thickness ,
Sincerely Richard
(This post was last modified: 12-26-2022, 08:29 AM by radiorich.)
Richard, if you determine it's a speaker problem I've had luck with spraying the cone lightly with water then lay it face down to dry. It allows the voice coil to center itself. If that works then I put a coat of contact cement on the outer edge of the cone where it flexes. The cement will dry flexible and bond the paper fibers. Result is a little more bass response. Give it shot, nothing to lose. BTW, anyone know why the tubes are upside-down?
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
I looked at several other radios with 4" or 5" speakers that have "good tone". (The 2 RCAs are certified by their Chief Engineer to have the tone of the "Golden Throat")
The variations in the coupling caps between the volume control and 12SQ7 Grid (C4 on your schematic) and between the 12SQ7 plate and 50L6 grid (C6 on your schematic) vary greatly among different brands of AA5 radios. This may be due to voltages on the tubes, Grid leak resistors, cathode bias, etc. Following is a list of cap and resistor values translated to your schematic: (edited, apparently, the spacing does not translate, image of table attached)
C4 C6 C7 R6 R7 R8 R9
Your Mitchell 0.005 0.01 0.01 6.8 Meg 220K 480K ?
Emerson 330 0.002 0.024 0.024# 15 Meg 500K 500K 140
RCA 8X541 0.02 0.002 0.05* 5.7Meg 220K 470K 150
RCA 8X71 AM FM 0.01 0.001 0.02* 10 Meg 680K 820K 150
What does seem to happen is that if the coupling cap from the volume control to the 1st AF is high, the coupling cap between the 1st AF and Output Tube is low and visa versa.
* These radios have an output transformer with a tap for B+ to the other tubes. The Output Bypass Cap is
between the 50L6 Plate and the tap, not between the plate and output stage B+
# This seems a little high.
Do also check or replace C3 and C5. These are usually 100 to 250 pF and are used to filter remaining RF in the audio signal.
3 Mods I think may help:
Up C6 to 0.02
Add a 25uF 50V Cap parallel to R9 (the cathode resistor). The RCA 8X71 has it.
If there is room for a higher compliance speaker with a modern magnet, replace it. I know that this may be difficult.
As far as the really unclear upside down tubes, several manufacturers did this in the 1940s. I have seen this on Crosley and even some RCA schematics (for RCA built units).
I agree with Terry, John and Rod who recommend checking out the speaker. I will hazard a guess that most of these were around 4 Ohm. What diameter is the speaker? Is there decent bass response? you should see the speaker cone move with bass notes or deep male voice. If you press dead center of the speaker, it should move relatively freely. Compare to other 4" radio speakers. Sub the speaker or test the speaker on a known good radio.
Good luck with this unique unit.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
(This post was last modified: 12-28-2022, 12:12 AM by MrFixr55.)
Another major consideration is the size or or the inductance of the output transformer. When manufacturers design inexpensive sets small speakers, output transformers, filter capacitors and coupling capacitors all go hand in hand. All of these parts can be used the limit the low frequency response thus making a set smaller and less expensive. The less lower frequency response the more 60cy hum can be on the hv but it's less less noticeable $$ savings for the manufacturers.
Another trick you can try to add some bass boost is to connect a small hv mica cap from the plate of the 12SQ7 to 50L6. Something around 25 to 100mmfd for starters. I've just read about and haven't tried it myself. I would suspect if the capacitance is too high it would break into oscillation.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Crosley did a similar trick in the 5C2 (and likely others) ca 1834, and really did boost the bass. The difference is tha the 1st AF was a pentode. The screen B+ was taken from the audio output plate and the higher frequencies were bypassed to ground by a capacitor, therefore supplying positive bass feedback.
+1 on transformer size, but I have a Motorola with a rather small output xformer but the tone is still rather good. the Mitchell has a 60uF input filter, therefore they were serious about filtering the DC supply.
Start with the speaker.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
One easy way to improve the low range, and probably the mid range to, is to install a cathode bypass capacitor in parallel with the cathode resistor of the power output tube, something around 22-33 uf 25vdc, but value isn't that critical. I've often done this with sets that exclude them and it really boosts the performance.
Regards
Arran
(This post was last modified: 01-05-2023, 07:02 PM by Arran.)
Hello everyone,
first all happy new years and I had been away from the forum with family so this one first times to read this post in a while and Rob yes I have done just that in the past with the cement but not with this set .
MrFixr55 Wow thank you for all the great info and yes changing the capacitors and resistor to match the RCA 8X541 seems like a plan also the idea Add a 25uF 50V Cap parallel to R9 and Terry the idea to add Something around 25 to 100mmfd for starters.
Far as these sets I know the company was building the chassis as cheap as they could .
Happy New year to U also, Richard. When it comes to AA5s, I think that Everyone built them as cheaply as possible. Throughout the history of electronics, steps were taken to reduce the steps required to manufacture radios. The printed circuit board is a perfect example. eliminating power transformers, then remove the filament dropping resistor or cordohm was another. Even before printed circuit boards, reducing the bends required for a chassis was another step. So was the multiple capacitor "can".
Of course, the real revolution was the monolithic integrated circuit. Amazing to look at the parts count for the 30 tube RCA 630, then to some of the last USA built RCA color TVs with about 6 VLSI ICs, a flyback, a few transistors, controls, the CRT, a speaker and little else.
Unfortunately, the next step in lowering prices for electronic equipment was to offshore the entire electronics industry! We will live to regret this.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
I have a Lumitone waiting to be restored, but it is the Canadian version...built under license by Brand and Millen in Longview, Ontario. Now you have me wondering what (if any) build differences there are in the chassis. I'll have to bring it onto the bench.
Hello mr Fixr,
I agree AA5's were made a price point and also with this set they did not have a lot room to work with I think a better speaker at least good cabinet I notice some sets are better designed to seal off the front of the speaker from that back .
Well Brad I hope you do post your lamp when you get ready to restore it since my good weather is here I can paint this set since electrialy it is restored and working .
