EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

McMurdo Silver / Jensen 18 inch speaker

So I finally found an 18" speaker for my Masterpiece V in Clifton cabinet.  The original speaker is most likely in Japan, courtesy of a greedy seller. 

The MP5 amplifier in my radio has a six-pin socket for the speaker cable, and the speaker I have has a cable with six-pin connector as well.  What concerns me are the measured field coil resistances - 3410 Ohms between pins 1-6, and 365 Ohms between pins 3-6, respectively.  Both coils are connected in series.

These resistances differ significantly from corresponding resistances for the 5-pin connector, Masterpiece VI speaker that are 8000 and 680 Ohms, according to what I found on web.  Am I missing something or were the field coils for MP5 and MP6 speakers different?

Any help will be greatly appreciated.

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Comment by Y2K Bruce on March 21, 2018 at 9:36am

I discovered that a Jensen tweeter from a Zenith 12A-58 console works fine plugged into the MP 5 woofer. The pins match up and the schematics are similar and mine has lots of tweeter sounds

Comment by J.N. Shetler on March 30, 2018 at 10:08pm

Jiri,  You have discovered an unfortunate issue with the later Masterpiece line. The field and shunt coil resistances are different between the V and VI. To further complicate matters, the late 6 production has another change wherein they moved the 2nd choke onto a bracket attached to the speaker pedestal. Anyhow, the values you give for your speaker are correct for a Masterpiece 5. The smaller resistance is the field, the larger one is the shunt winding. The shunt is between B+ and ground; it's the bleeder for the B+. The "swinging choke" input MS used requires a load to maintain regulation and keep voltage from really soaring when the power supply is energized at startup, the shunt is integral in this operation. On the V speakers the shunt winding is usually ok, on the VI speakers, it's often found to be open (undersized) and must be replaced with a resistor.

Justin

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