Scott 800B - Tweeter

[José Bustos]

Hello everybody:

I have a problem with the speaker of my 800B.  The tweeter of my Scott Articulated Coaxial Loudspeaker made by Jensen is cut.

I will really appreciate some member can tell me what is the DC Resistance value and the Impedance of the coil of the tweeter.

Thank you very much and kind regards,

José Bustos

[David C. Poland]

Be aware - tweeter does not play on AM Broadcast nor on shortwave. Radio is wired to activate tweeter on FM. Review the operating instructions for required control settings.

If does not operate on FM, verify tweeter has continuity, then trace the tweeter circuit to locate the problem. I don't know the tweeter impedance.

[José Bustos]

Hi David:

Unfortunatelly the tweeter is cutted. I measured it and it is visible the wire open in different places in the coil (corrosion).

I need to know the DC Resistance value in order to rebuild it.

Thank you and regards,

José

[David C. Poland]

Your black speaker looks different from the darker blue Jensen coax I am familiar with for the 800B.
Documentation says the tweeter was a PM speaker. Diagram indicates a field coil only for the big speaker at 675 ohms field and a 9000 ohm field shunt. .
Perhaps your black speaker is a replacement. If your black speaker has only the two fields in total, perhaps your tweeter should be one of the two values above.
Maybe someone else has additional information.

[Norman S Braithwaite]

Dave: Jose's speaker is the scarce optional Jensen series HNP dual coaxial speaker with internal horn tweeter, not the usual JNP series with a coaxial mounted cone tweeter.

Norman

[José Bustos]

Dave:

Yes, it is. Unfortunatelly the coil is bad.

I believe the rewinding will not be a hard job. I will try by myself but I need to know the DC resistance.

Kind regards,

José

[David C. Poland]

I expect it is either 675 or 9000 ohms in order to be compatible with the requirements of the 800-B. Carefully compare your 800B with the documentation circuit diagram to be sure. If your radio seems to conform to documentation and the good field is, say, 675, then the damaged field coil ought to be 9000 ohms.

II would approach the task thus. Measure the gage of the wire used in the bad coil. Take a sample and use a match to burn off the varnish insulation to obtain a good measurement of the wire gage that was used. Locate a table of wire gage weight per 100 or 1000 feet to help estimate the amount of magnet wire needed. Unwind the entire bad coil and then measure the weight of just that mass of wire as a reference point. Also weigh the empty coil form. Begin winding the new wire of same gage on the form. Periodically take a resistance reading of the wire on the form - sand off a bit of the varnish, measure resistance, varnish or shellac that spot and continue winding. As a cross check, when you get close to the target resistance, weigh the coil (subtract the form weight) and compare with the weight of the old wire - should be close.
Good Luck.

[José Bustos]

David:

For the quantity of wire of my coil I believe could be 675 ohms. Your tips will be very useful to me.

By the way, Why 675 ohms?

Thank you very much,

José

[David C. Poland]

Jose,

Don't guess. Just look carefully at the circuit diagram, in particular the portion for the speaker. Trace out your speaker wiring to the speaker plug in order to determine which pins on the plug connect to the bad field coil. That should tell you which coil is bad and hence the value for the field coil you rewind. The resistance of the field coil is determined by both the gage (wire diameter) and the total length of the wire. Don't guess on the wire gage, measure it so you can duplicate the field coil.. Also, the new coil you make still has to fit the available space.

The two field coils do different jobs in the radio. The 675 ohm field coil you asked about was specified by the engineer who designed the radio power supply. That 675 ohm coil achieves the desired voltage drop in the high voltage circuit and helps filter the high voltage to eliminate 60 cycle hum, and also powers the magnetic field for the speaker.

[José Bustos]

David:

It seems there is an misunderstanding.  The coil that is cutted is not the field coil, is the móvil coil of the tweeter.  My speaker has permanent magnet.

I am trying to know the DC resistance for the small móvil coil of the tweeter in order to rebuild it.

However all your tips are very useful for my proposal.

Thank you very much and kind regards,

José

[Norman S Braithwaite]

Jose:

I have one of these optional speakers in my 800B.  I will measure the tweeter voice coil DC resistance and report back after attending a meeting this morning.

Norman

[Norman S Braithwaite]

6-ohms DC. The nominal voice coil impedance would be 8-ohms.
Norman

[José Bustos]

Norman:

Excellent!!  Great help. I will get my head down on this.

I will let you the advances.

All the best and kind regards,

José