Signal generator/Alignment question
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I am going to attempt my very first AM radio alignment on my 53-1750 console. Two questions:
1. The alignment instructions tell me to connect the "high side" of the signal generator to pin 6 of the 7A8 tube with a .1MFD capacitor in series with it. No problem. Then, it says to connect the "low side to B-".
"B-" means what??? A common ground in the circuit??
2. Alignment instructions tell me to connect a 1000 ohm per volt voltmeter across the voice coil for the output reading. My voltmeter is from Radioshack and has a digital readout. I don't know if it's a 1000 ohm/volt meter or not -- does it matter as long as the meter is reading volts going into the speaker? I don't want to blow anything up !!
Thanks folks,
Mike
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B- is most easily found at the black lead of the filter capacitor. Don't use the chassis ground, but do use an isolation transformer to plug the radio into if you have one.
DMM's are not the best for measuring a changing voltage, such as when you are aligning your radio. An analog meter is much better. In this case, the ohms per volt figure is not so critical, since you are looking for voltage peaks, not a particular voltage. If you don't have an analog meter, or, even better, a scope, you can probably do better by ear than with a DMM because of the latter's slow response curve.
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Yeah -- I've seen youtube videos of alignment where they just used the sound of the speaker and seems to work pretty well (for them). I'll give it a try. If you see your lights going dim later you'll know I'm working on it !!
Mike
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Hi Mike and welcome!
The B- can be different places in radios. Sometimes it's the chassis, other times it's a common point where the negaive leads go but not the chassis. Those are known as a floating ground. Check the diagram to make sure. ( I don't have it on front of me at the moment)
I just restored the chassis and phonograph of one of those models.
One note, the chassis is very tight and not much room to work in. I decided to gut the old filter cap can and install the new caps inside of the can. It would have been hard to install the new caps under that chassis. It looks totally stock, using the old can and cover, recrimping the can back into place.
Both of the IF cans I had to rebuild, because of the silver migration disease". I wasn't sure which on it was, so I redid both. If you have no symptoms, I'd leave them alone of course.
The connection to the speaker according to the instructions, is to 'see' exactly where the loudest peak of the signal is. Many of us don't use a meter and rely on the ear to get to the highest setting. I have a little meter just for that purpose, but I admit I use the ear about 50% of the time  .
It's just a little millamp meter with a diode on one of the terminals. Your digital meter is just about useless for this purpose.
If you have any other questions about the chassis, I'd be happy to help.
Oh, one last thing, those IF cans have to be adjusted from both the top and bottom holes.
Good luck!
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Thanks Gary -- yeah -- found in my schematic where there is atlteast one location for the B-. It is the common ground that hooks up to that big 4 in 1 capacitor - which I've already replaced with 4 separate capacitors. Interesting that on this model, the alignment instructions recommend only tapping into the voice coil, as opposed to other models where they recommend hooking a meter into other test points in the circuitry. You're right, it's a really tight circuit box on this model - not alot of room - but I did manage to replace all the caps. I didn't stuff that big electrolytic capacitor that sits on top with the tubes. I just replaced it and found room for it inside. The consensus seems to be that my Radio Shack digital meter is worthless, as you say, for this purpose, so I'll try to do it listening to the speaker output. I see that the IF cans have holes in the top and bottom and there's a hole in the circuit box frame to allow a screwdriver to pass through it into the bottom of the IF cans. Restoration of this thing has been a multi dimensional project, as you know, everything from woodworking, paint stripping, restaining and the electronics. The phonograph works great and I was able to find a new idler wheel and drive belt for it -- as well as the gold PHILCO decal . Thanks for your tips and help,
Cheers,
Mike
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Hi Mike, good deal on the recap. I do like to keep the old can in place if possibl. Even if thatmeans insalling the filers under the chassis.
One thing to watch for, those IF can slugs might be slightly stuck or tight. Don't put a lot of pressure on them a the start. Maybe just a little working back and forth lightly to break them loose from their resting place.
You also want to have a tool that will fit right into the slot. Don't use a screwdriver, first because it's metal and secondly the blade may be too thin. I made a couple of tuning wands out of both very hard plastic, and one out of hardwood.
IF you brak one of the ferrite slugs, that's bad news. You light have to disassemble the can, then install a new slug. or some have glued them together. Preventing brakage from the get go is best
The "ear method" of peaking will get you very close to the peak. Takes alittle concentration and no distractions. Keep your signal generator signal to the lowest possible, that will help.
Let us know how it goes!
Gary
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Agree with all. You can get an old fashioned VTVM for little money or an old fashioned 0-10 volt analog AC panel meter, either of which will do a bit better than by ear (at lest in my case.)
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One more question -- when the alignment instructions say to connect the "high side" of the signal generator lead to the #6 pin of the 7A8 tube and the "low side" to the B- -- what does that mean -- the high and low side? My signal generator has a coaxial cable connect to the Radio frequency output. The center of the cable wire has an alligator clip connected to it and the other thing would be the shielding of the coaxial cable seems to be a like a ground - is that the low side?? Thanks
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Yes, Center is High or positive, outer shielding is low or negitive
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