Motorola 77XM AM/FM Radio found today

[jr_tech]

Originally Posted by wa2ise

First time I've seen a radio make use of the AM pentagrid converter 12BE6 tube as an FM IF stage. I figured that that would be possible, and sensible, to put that tube to work when the radio is in FM mode. Don't know why this wasn't done more often. You'd need a switch contact that disables the AM local oscillator. Just short the cathode portion of the osc coil.

And then they skipped over V-4, which *could* have been used as an AM IF.

Do you know why they designate V-1 as first *and* second FM converter? It doesn't look like a dual conversion circuit.

jr

[Captainclock]

Originally Posted by jr_tech

And then they skipped over V-4, which *could* have been used as an AM IF.

Do you know why they designate V-1 as first *and* second FM converter? It doesn't look like a dual conversion circuit.

jr

I don't know but I do know that I have my power cord for the radio now which I had found an old computer monitor power cord (the flat kind) at Goodwill today in a basket of miscellaneous cables so I got it home lopped off the ends and got a plug end put onto the one end (leaving the ground cable cut off just shy of where it enters the plug end) and now all I need to do is get a small rubber grommet to hold the wire into the chassis (they had a couple of pieces of cardboard with holes drilled into them in different spots with two rivets holding it in place originally) and wire in the new cord and I'll be all set as far as the cord goes.

Now as far as the capacitors go, will I necessarily need to replace the old blown out death cap with a new one? Couldn't I just cut out the old death cap and just leave it out of the circuit?

[wa2ise]

Originally Posted by jr_tech

I Use these LED bulbs on my vintage night-lights and radios that require a line volrage bulb. They work fine for me and run very cool (only .36 watts!).

You could try LED bulbs in your radio, but they may use miniature switching power supplies and may RFI the radio.

Someone may have used a bulb stolen out of a Xmas light set to replace a burned out original bulb. Yellow being the best match for the dial.

[jr_tech]

Originally Posted by wa2ise

You could try LED bulbs in your radio, but they may use miniature switching power supplies and may RFI the radio.

Scope probe held near the bulb just picks up 60 hz and a tiny bit of modulated carrier from a nearby AM radio station... no switching hash.

jr

[davet753]

Don't use anything except the bulb it was designed to use (or something close to it). On sets with series strung tube filaments, the bulb is a vital part of the circuit that, when blown or removed, will reduce the life of the tubes (especially the rectifier).

When you first turn on the radio the bulb glows brightly, quickly dims, and gets back brighter once the tubes are all warmed up. If you remove the bulb, the tube filaments get all that power at turn-on. A 50 cent bulb is a lot cheaper than a rectifier tube.

[jr_tech]

Originally Posted by davet753

Don't use anything except the bulb it was designed to use (or something close to it). On sets with series strung tube filaments, the bulb is a vital part of the circuit that, when blown or removed, will reduce the life of the tubes (especially the rectifier).

When you first turn on the radio the bulb glows brightly, quickly dims, and gets back brighter once the tubes are all warmed up. If you remove the bulb, the tube filaments get all that power at turn-on. A 50 cent bulb is a lot cheaper than a rectifier tube.

Usually very good advice for a series strung set. but in this case, the bulb is a 120 volt "night-light" sized bulb connected directly to the AC line after the line switch. Almost any candelabra base bulb (including a C-7 xmas bulb) may be substituted. The set does not use a rectifier tube, it has a Selenium rectifier.

jr

[Captainclock]

Originally Posted by jr_tech

Usually very good advice for a series strung set. but in this case, the bulb is a 120 volt "night-light" sized bulb connected directly to the AC line after the line switch. Almost any candelabra base bulb (including a C-7 xmas bulb) may be substituted. The set does not use a rectifier tube, it has a Selenium rectifier.

jr

That's exactly what I was going to say, but you beat me to it! Anyways, I think I'll definitely see about getting some C-7 style Christmas lightbulbs of varous colors to try out in the radio. I'll also see about that LED C-7 Style bulb you linked to earlier in the thread.