Zenith K731

[YamahaFreak]

My Gramps sent me this one for my birthday. It's a neat radio, and works well. However, the electrostatic tweeter is having problems. It sounds very quiet in comparison to the cone fullrange, and sometimes there is only a faint buzz. I think maybe the screens are shorting? I'm not quite sure how to access the panel, as the front grille is non-removable, and the plastic back of the ES panel is stapled in. Also, if I try this maintenance while the radio is playing, will the panel bite me? Furthermore, is this even the problem I think it is, and can I fix it at all? Thanks for your help.

[Tube Radio]

I have had three K-731 radios. I have yet to find a fix for the electrostatic tweeter. Your best bet is to find a small high impedance horn tweeter and put that in there. I've been inside one of those electrostatic tweeters and there's no real fixing to them as what's not riveted is glued and the plastic is quite fragile also. If you havn't already replaced the selenium rectifier do so. Take a voltage measurement where the 22 ohm resistor from the rectifier connects to the first filter cap. Then replace the selenium with a diode and note the increase in voltage. Now comes the fun part. Try different resistors until the voltage at the first filter cap is close (+/- 2-3 volts) to what you measured with the selenium rectifier in place. Don't replace any components in the shielded box where the FM tuning slugs are unless you have a problem with FM operation as parts placement is critical in that area. Even using the radio without that cover soldered in place affects the operation of the FM circuitry. What chassis do you have? The ceramic capacitors will be ok. All resistors, paper caps and electrolytic caps should also be replaced.

[YamahaFreak]

Originally Posted by Tube Radio

I have had three K-731 radios. I have yet to find a fix for the electrostatic tweeter. Your best bet is to find a small high impedance horn tweeter and put that in there. I've been inside one of those electrostatic tweeters and there's no real fixing to them as what's not riveted is glued and the plastic is quite fragile also. If you havn't already replaced the selenium rectifier do so. Take a voltage measurement where the 22 ohm resistor from the rectifier connects to the first filter cap. Then replace the selenium with a diode and note the increase in voltage. Now comes the fun part. Try different resistors until the voltage at the first filter cap is close (+/- 2-3 volts) to what you measured with the selenium rectifier in place. Don't replace any components in the shielded box where the FM tuning slugs are unless you have a problem with FM operation as parts placement is critical in that area. Even using the radio without that cover soldered in place affects the operation of the FM circuitry. What chassis do you have? The ceramic capacitors will be ok. All resistors, paper caps and electrolytic caps should also be replaced.

I was worried about dropping in a regular type speaker because of the difference in impedance for the ES panel. What would be a good rating for the new driver?

[Tom Bavis]

You can't wire a regular tweeter to the ES terminals - there's DC bias there too... wire your tweeter across the main speaker with a series cap - maybe 3-8 uF, depending on the crossover frequency you want.

[YamahaFreak]

Originally Posted by Tom Bavis

You can't wire a regular tweeter to the ES terminals - there's DC bias there too... wire your tweeter across the main speaker with a series cap - maybe 3-8 uF, depending on the crossover frequency you want.

That makes sense.

[Tube Radio]

There is a resistor that supplies voltage to the tweeter. Remove that resistor (should be on terminal strip IIRC connected between one of the terminals that the tweeter connects to and B+) and DC bias goes away. Radio Shack sold a few years back a horn tweeter silver in color that was high impedance. I forgot the impedance value as I no longer have the tweeter, but it did work. You could also get another transformer and connect it to where the electrostatic tweeter connected and be able to use a normal tweeter. I've tried connecting a tweeter across the woofer and IIRC it didn't work very good. IMHO for the amount of available bass boost you would think that they would have either used a tube with greater output power or push-pull audio as the radio runs out of headroom pretty quick when playing most modern music. I wonder how hard it would be to convert one of those radios for use just as a tuner usiung a transformer of course?