RESISTORS

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RESISTORS

Post by RJP0614 on Thu Mar 28, 2013 8:13 pm

I am rebuilding an old Zenith radio with the infamous eye. I replaced all caps and all resistors except for three. All the resistors I replaced have have three color bands with that usual brown carbon base color. Three of them are black with only two color bands which are brown and green, in that order. All three give different readings on the multimeter, so I can't tell which one is good. I've looked for a while now and can't find any info on these resistors on the net.
Is the black base color considered the third color? None of them appear to be charred.
The other thing the radio does, is it works great now that I replaced most of the parts until I connect the eye tube then the radio gets louder then goes silent immediately. The eye tube lights up brightly but thats all it does.
Any ideas out there?

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Re: RESISTORS

Post by Motorola man on Thu Mar 28, 2013 9:18 pm

The body of the resistor is the first color band, followed by the band closest to the end, and then the multiplier band. There's usually a 1 meg resistor inside the eye tube socket, make sure that you replace that if you haven't already.
Dave
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Re: RESISTORS

Post by RJP0614 on Thu Mar 28, 2013 11:12 pm

So my guess was correct just backwards. The resistor is black, green, brown... in that order
I did replace the 1meg resistor. I also placed it outside the socket because it was such a pain tring to put it back in again. Not much room.
Thank you Dave

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Re: RESISTORS

Post by jtauser on Fri Mar 29, 2013 2:35 am

Google "dogbone resistor color codes" for more information than you ever wanted on these.

The eye tube is a visual indicator of the AVC (automatic volume control) voltage; it acts as a visual voltmeter. The problem you are describing indicates that when you connect the tube it is affecting the control voltage rather than merely indicating it and turning the volume down for you.

What is the model number of the Zenith you are working on? Do you have the schematic?

Joe T.
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Re: RESISTORS

Post by Ken g on Fri Mar 29, 2013 4:45 am

Did you replace all the resistors?


Last edited by Ken g on Wed Apr 10, 2013 9:08 pm; edited 1 time in total

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Re: RESISTORS

Post by RJP0614 on Fri Mar 29, 2013 8:37 am

Hi Ken g
I only replaced the ones that were way out of tolerance and or visually burned or broken. The three that are suspect have three different readings even though they are marked identical.
I get what you're saying
Thanks

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Re: RESISTORS

Post by RJP0614 on Fri Mar 29, 2013 8:58 am

Hi jtauser
The resistors are the old carbon comps.
The model is a Zenith 7s363. I have a schematic but it is very fuzzy and tiny to really see the literature. That's why I tested each resistor one at a time. I also went to a web site that calculates them via band color.
I did the same to another zenith that wouldn't power up at all. I replaced all the caps except the electrolytic "1st mistake" then one by one the resistors. 90% of which did not match the schematic. Most crumbled when touched. That radio was a major disaster. Noe it powers up but emmits a loud hum. The resistor that was burned was connected to the electrolytic. That's where I stopped until I get a replacement.
By the way, I use surgical gloves when working on these babies. One time I forgot to unplug the unit abd got a jolt just by touching the chasis. I now have a surge protector in the wall just in case and a dimwatt tester as well. " 2nd mistake"
Thanks

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Re: RESISTORS

Post by RJP0614 on Fri Mar 29, 2013 9:18 am

Hi again jtauser
I forgot to mention you are correct. The stations come in strong without the eye, with the eye the volume drops drasticly. I can still hear the staions but only faintly.

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Re: RESISTORS

Post by RJP0614 on Fri Mar 29, 2013 11:01 pm

Hey Ken
Trust me I didn't want to get that involved but I did. Yes I did it the hard way and tested each part I installed and it actually gets stations in better. But the eye tube still makes the volume disappear. The only movement I get out of the eye is when I changed broadcast bands.
I could just leave the radio as is since it plays nicely without the eye but I want it all to work.

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Re: RESISTORS

Post by Dr. Radio on Sat Mar 30, 2013 12:07 am

Have you tried another eye tube?

Is there a possibility this radio has been worked on before and there is a miss-wire in the eye tube related circuit?
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Re: RESISTORS

Post by Dr. Radio on Sat Mar 30, 2013 12:13 am

RJP0614 wrote:
By the way, I use surgical gloves when working on these babies. One time I forgot to unplug the unit abd got a jolt just by touching the chasis. I now have a surge protector in the wall just in case and a dimwatt tester as well. " 2nd mistake"
Thanks

Sounds like your bench should include an isolation transformer. I explain this "situation" in my step-by-step rebuild thread in the electrical restoration section.
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Re: RESISTORS

Post by GaryRabbitt on Sat Mar 30, 2013 3:15 am

The Surge protector will do nothing at all to protect you from a shock.
Isoation trans will keep you isolated from the line, but you still can get zapped from voltage on the chassis itself.
No need for surgical gloves, it gives a false sense of security. When working with a live chassis, best to keep the free hand in your pocket, and not resting on anything metal especially the chassis.
Dr. Radio's rebuild thread will help you out.
As for replacing all thos resistors, you should measure them before deciding to replace. Most of the time you don't have to raise one end to measure.
You have carbon comp resistors and not the Dogbone type?
Black will never be the first bandon a standard carbon resistor.
Dogbones are read by colors- Body/End/Dot.

Replacing too many parts unnecessarily could result in a wiring mistake. It happens.
I can't say for sure about the signal drop when the eye tube is plugged in , but maybe it has to do with the AVC voltage to the tube, or the AVC circuit itself.
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Re: RESISTORS

Post by RJP0614 on Sat Mar 30, 2013 8:27 am

Hey Dr. Radio
The radio actually works better than it did. Before I put in the new parts it would get staions but I would have to hold the antenna wire to get better reception. Now it gets more stations and stronger signals.
You may be right ...it might be the tube itself. I was thinking the same thing after playing the radio last night. The original tube was dead...I bought another tube from a vender for another radio and he threw the eye tube in for free. He said it might be week but it still works. As stated earlier the only movement in the eye is when I switch bands.
The radio might have been worked on before. The eye tube socket was wrapped in black electrical tape, when I unwrapped it I found it was shattered. Pins 2 and 4 were connected via a 1mg resistor. 1 and 6 were the larger of the pins for the heater. Pin 5 , according to the tube schematic went to ground and that left pin 3 to go to avc. I hope I got them correct. I'm going to triple check the connections.
Thank you

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Re: RESISTORS

Post by jtauser on Sat Mar 30, 2013 6:47 pm

Here's the schematic for the 7S363 from my Rider's cd-
http://www.jtauser.com/radios/Zenith_7S363.pdf

It's not a very good scan but it may be better than what you have.

Here's the data on the 6U5 eye tube-
http://www.r-type.org/pdfs/6u5.pdf

Pin 4 should be connected to B+ and the resistor, and the other end of the resistor should go to pin 2. Check this first.

The tube's grid on pin 3 is connected to the radio's AVC line and controls the eye opening. It needs to be between 0 and -22V to control the eye. I would measure the voltage on pin 3 without the tube in place and then see what happens when you plug in the tube. It is very possible that you have a short in the tube and the voltage is going positive, which would shut down the 6A8 converter.

My rule of thumb for replacing resistors is if they're within 20% of the schematic value I leave them alone. They almost always drift up.

Joe T.
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