Zenith G730 AFC questions etc.

[gyrene]

While searching for a schematic for my Zenith G730 radio I came across a few posts in this forum.
Maybe I landed in the right place?
My Zenith is in really good condition,I think.
I replaced the capacitors already but I think the AFC is not working as well as it should.
The schematic I have is almost unreadable.The print is way too small and even under a magnifiying glass I can't read it.
Could anyone link me to a readable schematic?

Also I saw a youtube movie about putting a dial lamp in there.Very cool idea.
Can anyone guide me in how to do that and the AFC upgrade without getting shocked to death?Some reading about tube radios has me scared out of my wits to get shocked and killed while working on these tube radios.One hand in the pocket and use a wood stick to probe parts.Anything else about saftey I should know? Tell it to me like I was nine years old.Don't assume I know what an antgc14av is.

I did not replace the diode because I don't know what symptoms a bad one would display.Can anyone help on that issue?Should it be replaced symptoms or not?

The smaller speaker is no longer hooked up.Can I replace it with a modern tweeter with no ill effects?
The radio when hooked up with an iPod works very well.No static at all.The one working speaker is fine.Wife loves it that way.
Thank you all in advance.

[gyrene]

out of cabinet

[Reece]

Looks like somebody replaced the selenium rectifier. It would normally be mounted on that bracket on top of the chassis to the right of the speaker output transformer. The diode that replaced it is just a little guy, looks like a resistor but is usually black with a band on one end only. They may have also replaced the electrolytics when they did that or perhaps you did.

The tweeter in that radio is electrostatic: a PM tweeter won't work in that circuit. It's possible to rebuild electrostatic tweeters if it doesn't work. You could just leave it out, and tack a piece of hard board on the inside across where it went, to close off the hole (for better main speaker response). I'm guessing this has a 4 x 6" oval speaker? The tweeter in mine worked OK. Yours is similar to a Zenith I restored:

http://antiqueradios.com/forums/viewtopic.php?t=120245

If you Google for "rebuild Zenith electrostatic tweeter" you'll find a whole slew of articles on the subject.

Normally you tune with AFC off, then switch to AFC and hope that it grabs and keeps the station in tune. After twenty minutes or so the set ought to be warmed up enough not to drift much.

As to shock hazard: respect, but don't fear. Knowledge and attention are your lifesavers. Don't do things that might put a shock across your heart, e.g., two hands into the guts of the radio while testing. Or standing in bare or sock feet on a concrete floor while working on a live set. One side (white wire, neutral) of the power line is connected to ground. If you take a light bulb in a socket, connect one wire from the socket to power line black ("hot"), and connect the other wire from the socket to a ground like a rod stuck in the ground, the bulb will light. Same thing happens to people who go from "hot" to ground, they "light" too. Rubber mat, rubber soled shoes, if on concrete. One guy got a shock working on a radio while barefoot on his patio! Well, I guess! Common sense should prevail.

[DaveWM]

if switching to AFC pulls the tuning off then there is a problem with the alignment of the discriminator transformer, and its not something you want to mess with.If the AFC does nothing then the variactor diode could be bad, but I dont think I have ever seen that. I don't think the variactor afc zeniths are as strong on pull in as the older reactance tube zenith like the 835 series.

[gyrene]

"...The diode that replaced it is just a little guy, looks like a resistor but is usually black with a band on one end only."

And it has to be there,correct? or the machine will not work,right?

The AFC seems to make almost no difference.Lets say that if this radio were still in the development stages at Zenith and I came in with this AFC idea and activated it to show it off to all the other design engineers...it would not impress to much.
Can it be "weak"? or does it work or not work?

[DaveWM]

the AFC should help some, but its not that strong on the variactor equipped sets. De tune it just a bit in reg FM mode, then switch to AFC, does it come in a bit better? if so then that's prob about as good as its going to get. Again, I don't know if its possible, but the variactor diode may be going bad, but I don't recall seeing that. Its not just any old diode its a variactor diode.

[Reece]

The silicon diode that replaced the selenium rectifier has to be there, but it can hide, it's so little. Probably under the chassis. In the post that I referenced I circled where I put mine, but the last repairman on yours could have put it anywhere underneath the chassis. Here's a picture of one with its schematic symbol. The end with the line is the cathode and that end goes towards the B+ plate voltage of the radio.

http://t2.gstatic.com/images?q=tbn:A...uiSGgwrjC6NHWu