Hello everyone. I was given a unique radio this past week, a Minerva Tropic Master model w117. Supposedly this radio was one of the few produced during war time, and apparently some were, but not this one. On the Rola 6 inch speaker edge is a date stamp of nov 8 1945. This would probably have been built in 1946, just after the war. However, it still may be a war radio. This radio belonged to my recently deceased brother, who was in turn given it by his father-in-law. His father-in-law was an infantry foot soldier during the Korean conflict. He brought this radio back with him. So it is quite possible that it would have still been available at the PX as it was called by the makers, a "Morale Booster" radio. Enough on its' background...
This thing is a tank! It is all metal construction, and tips the scale at 22 pounds. The chassis is hot on this set, and is suspended inside of the outer cabinet using four phenolic stand offs. there were three versions of this set, mine being the first run. In this run a series string for the heaters includes all tubes except the push-pull output 50L6's. They are wired in parallel. It wasn't a great system, but probably had a lot to do with parts scarcity in 1946. Anyway, this wiring insured that if one of the 50L6 failed it would take its' companion with it. The next version used two 25L6 wired in series so if one fails the other would just shut down. I may make this modification to the set.
I will be changing out caps and carbon, and may also add an in - line fuse, but will not do much more than a good cleaning to the cabinet itself. I prefer to keep it in its' "war hardened" condition. I welcome any and all information from my phorum phraternity. I found a usable schematic in Riders, but if anyone knows of a clearer copy or the original that came with it, please let me know. I'm sorry, but photographs will be coming shortly. Take care, and BE HEALTHY!
Gary
"Don't pity the dead, pity the living, above all, those living without love."
Professor Albus Dumbledore
Gary - Westland Michigan
When restoring the set, I strongly advise you to change the wiring of the off-on switch so that it switches the hot side of the line instead of the neutral side as designed (most AC/DC radios were designed this way, with the neutral having thew switch instead of the hot). Use a polarized AC plug on a new cord. The neutral lead goes to the wide blade of the plug.
Have fun with it - and remember to never touch the hot chassis when the set is in operation!
David - Thanks for the information. I had seen the Phorum thread, but not the second thread - very helpful.
Ron - Thanks for the advice. I had intended to polarize the set when I replaced the power cord. After switching the hot side, would I be able to install .01 Y2 line caps to eliminate the need for an attached external ground? I'm gonna pester a lot of you learning with this set! Please forgive me.
Mike - Warning heeded! 14 x 8 x 8.5 inch cabinet and 22 pounds!
Had an unexpected bonus so far. Inside the radio was a magnetic mechanical pencil, still filled with lead! No markings on it, and only about 4 inches in length.
The radio is painted grey crackle coat. There are a few areas with slight paint flaking. My plan was to gently wash the cabinet with soap and water, then leave it in the sun to dry. Once dry, I was considering giving it a coat of matte finish clear to re-fix the flaking paint. I'm not a finish expert either, so let me know if this is what should be done.
Take care, and BE HEALTHY! Gary
"Don't pity the dead, pity the living, above all, those living without love."
Professor Albus Dumbledore
Gary - Westland Michigan
OK Ron, time to enlighten me. It has both ANT and GND terminals on the isolated chassis. Doesn't that then need a longwire and ground for operation? I'm not about to challenge Kirk for electrocutions! Help me understand, please. And yes, I meant to say .01 X2 cap as a possible way to eliminate the need for an external ground. Looks like I need to know more about this unit. Take care and BE HEALTHY! Gary
"Don't pity the dead, pity the living, above all, those living without love."
Professor Albus Dumbledore
Gary - Westland Michigan
I have looked at the schematics of both the early and late versions of this radio. Nowhere does it actually show the connection to the ground terminals, although it does show "chassis ground" for many connections, and also a .005 mfd condenser used between the actual chassis and the cabinet. If you re-wire the power supply as Ron outlined, with the wide blade of the new plug going DIRECTLY to chassis ground, and the narrow blade to the switch and then to the proper place in the power supply, you should not have a problem even if you DO use a ground wire to the chassis, as long as the outlets in your house are properly wired. The wide blade is SUPPOSED to go to the "grounded" (neutral) white colored lead, and the narrower blade to the "hot" black colored lead. The white lead is supposed to be connected to ground at your breaker panel, as is the bare or green GROUNDING lead. To eliminate any potential problems, should someone ever plug the radio into a miswired outlet, I suggest simply using a .01 mfd, 600 V poly condenser between the ground screw terminal beside the antenna screw terminal, and the chassis, instead of a direct connection, should it be directly connected at present. This will appear as a direct connection at radio frequencies, but pretty much isolate the radio from a direct connection to ground at 60 cycles, saving any potential ugly damage. If it is presently a .001 mfd condenser, use a 600 V poly condenser of that value.
(This post was last modified: 07-10-2022, 12:13 AM by mikethedruid.)
Good luck with Gary, here is one well restored by my pal Joe Rebar, it got a very hefty price but they do not seem to come up often at least in New England. Yours will even look better!
My opinion is that no circuit modification is necessary to make this radio safe or "safer" than what it already is without introducing a certain amount of external risk.
Both the antenna and ground connections to the terminal strip are isolated with .01's. These can be replaced with the "X" 0.01mf cap at 400/600 volt. We know these fail open, safe.
There is at least one AC line to chassis cap, again the X does the job.
Though I have used the schematic provided, snipped and converted, look carefully at the power plug, it appears to me that one "blade" (lower) of that plug has a wider line width indicating a polarized plug. Reviewing the schematic and accepting the concept the wide blade exists, it appears that that line is the neutral line connection and takes an appropriate path. The bulk of the schematic can be reviewed in a previous post...
I would check the antenna input coils and the band switch for proper operation.
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
I printed that schematic before I made the heater string correction. The push - pull 50L6's are wired parallel, not in series as shown.
Take care and BE HEALTHY! Gary
"Don't pity the dead, pity the living, above all, those living without love."
Professor Albus Dumbledore
Gary - Westland Michigan
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