Minerva Tropic Master

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Minerva Tropic Master

Post by Wildcat445 on Tue Aug 11, 2015 1:31 pm

I am cleaning off my bench. I finished a Hallicrafters SX-110, and this radio chassis was next. I say chassis since that is all I have. I decided to see if this thing would work, or if it belonged in the parts pile. I have repaired many of my parts sets, so parts would not be a bad thing.

I believe the correct model nomenclature for this set to be W-110. There are at least three versions of this set that were produced, according to Riders. An early model with P-P 50L6 tubes, a late model with two 35Z5 rectifiers and P-P 50A5's, then this one. It is essentially the early version, with 25Z6 rectifier, but with P-P 25L6 output tubes. I initially questioned the 25L6 tubes, but, under the dirt on the chassis, it is stamped in ink that the 25L6's are correct for this particular chassis. It appears to have been equipped from the factory with Ken-Rad tubes. Most of the metal tubes are original Ken-Rad and are dated sometime in 1941. The rectifier and both output tubes are replacements, all different brands, GE, RCA and Westinghouse. I had Ken-Rad tubes to replace these three, but not correctly date coded. The pilot lamp has been disconnected, so I have to fix that. I recapped the entire set and fixed some hacked wiring and poor previous repair. The chassis is still fairly nasty, but I wanted to find out its operating condition before I got concerned with a lot of cleaning. This is an eight tube chassis with a tuned RF section, and P-P audio, but with heater string power supply.

I recapped the chassis and that went fairly straightforward. The odd thing is the filter caps. They are wired with the first cap, a 40uf at 250 volts wired with positive to the rectifier cathode and negative to common negative, one side of the power switch. The cap following the choke, a 40 uf at 150 volts, is wired with positive to B+ and negative to the chassis. This is a fairly uncommon arrangement, so I checked with the schematic to make certain of that being correct. When I assured myself that this was correct, I replaced the filter caps then performed my typical "safety test." This is a little "CYA" procedure my stepdad insisted upon whenever a filter cap was changed, or when an instrument of unknown condition was put on the bench. This test measures the resistance that the rectifier "sees" in the circuit. On transformer sets, I like to see at least 250K ohms from rectifier cathode to common negative, usually the chassis. On a transformerless power supply chassis, I like to see a minimum of 50K ohms from rectifier cathode to common negative, usually one side of the power switch, before being considered safe to apply power. This particular chassis showed the filter caps to be dead shorted. Essentially zero ohms. I broke the B+ circuit and measured each leg. Everything appeared okay. So I connected the set to my dimbulb, Variac, and isolation transformer and slowly powered the chassis up. The 60 watt dimbulb was fairly bright, so I switched to the 100 watt bulb. The radio started working weakly at about 80 volts. I slowly applied full power and the radio still worked weakly. I touched up the alignment, which brought the chassis to life. This thing is a good performer. Not quite as sensitive as the Hallicrafters SX-110, but certainly no slouch. It has a five-inch speaker and a tone control, along with P-P audio, so it sounds good. I have no idea why those filter caps showed the resistance they did. I checked with two VTVM.s, an analog DVM and a digital and they all showed the same resistance.

I am going to do some cleaning and figure out how to kluge a cabinet of some kind for it. This thing will make a good bench or kick-around set. Certainly too good for parts or to just toss. I like fooling with old junk that has little interest to anybody and seeing it come out well like this one did.

WC

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Re: Minerva Tropic Master

Post by 75X11 on Tue Aug 11, 2015 5:40 pm

I remember about 20-25 years ago there was an in depth article someone had written in one of the monthly electronics magazines about a restoration of one of these units. It had a metal housing along the lines of a square tool box. the only thing special about it was the speaker cutout.
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Re: Minerva Tropic Master

Post by Wildcat445 on Tue Aug 11, 2015 5:55 pm

That is the one. I have "engineered" a cabinet, from an old wooden box I have in the shop. This one will be a good one to hot rod. New dial covers are available (shock, shock!) The thing works fine. I can't throw things away if I can think of a use for it.

WC

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Re: Minerva Tropic Master

Post by Guest on Tue Aug 11, 2015 8:35 pm

I've always like a well working AM radio (metal chassis and octal tubes) with no cabinet sitting on the bench, playing while I worked on whatever. WC, your radio sounds perfect for this! Very Happy

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Re: Minerva Tropic Master

Post by willy3486 on Tue Aug 11, 2015 10:08 pm

I want to fix a radio one day that plays great but the cabinet is shot. I want to make a cabinet out of pexiglas and use it that way. So it fairly safe from wandering fingers but you can see the inner workings.

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Re: Minerva Tropic Master

Post by 75X11 on Tue Aug 11, 2015 10:43 pm

Bet that would look nice in a darkened room.
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Re: Minerva Tropic Master

Post by Wildcat445 on Wed Aug 12, 2015 8:37 am

Leaving the chassis out of the cabinet and using it like that was a thought that crossed my mind. If my bench was neater, that might work. I make too much of a mess on my bench. Something would be placed somewhere "hot" and might cause a problem like that. This chassis has five metal tubes, so it is not really pretty when it runs. Pin #1 on the metal tubes are all grounded, so I might try glass tubes and see if that introduces some type of hum or squeal. It was fun to "rescue" this thing regardless of what its final position might be.

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