Posts: 64
Threads: 10
Joined: Dec 2010
City: Baltimore, MD
Hello, I'm new to this forum and can't wait to get started. This radio cabinet is in very good condition. When I received the radio, it just hums lowly, and the volume control does not change this. After changing caps and a few resistors, the set still hums lowly. Now, I checked all the tubes with a nice tester. All are good. One of the 7b7 tubes seems to stay cold after the others heat. I assume I am at the stage where I will be taking voltage readings on all the tubes. Never had to do that before, and it's tight in there. I assume to ground multi-tester to ground end of electrolytic caps. Must the tubes be in to take all the voltage readings?? I have the diagrams of each tube. It IS tight in there. Any suggestions? Much appreciated and glad to be aboard.
Posts: 7,294
Threads: 268
Joined: Dec 2009
City: Roslyn Pa
Hi and Welcome!
You may want to check the diagram to see where they measure the voltages from sometimes it's form B- and on some sets it's from chassis.
http://www.nostalgiaair.org/PagesByModel...013514.pdf
The filaments of the tubes are in series so if one is bad or unplugged none of the filaments will light. Must measure voltages all the tubes in place.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 271
Threads: 12
Joined: Nov 2005
City: Nashville, TN
The 7B7 is a Loktal tube. They have been know to have oxidized pins and sometimes don't make good contact.
Find the dimple on the tube base, then rock it in that direction toward you. It should come right out. Clean off the pins and reinstall.
As Terry says, the tube filaments are in series, so If the other tubes are lit, we can assume your filaments are ok. However one of the pins that carries the B+ voltage might not be making contact. For good measure, I'd clean all the pins.
Yes, you are going to have to have all the tubes installed for voltage measurement.
You can see on the diagram what and where to measure the voltages. Start just after the rectifier tube, measuring the B- and the positive lead of each 20uf cap. As far as I can see, the B- or negative side of the caps is a floating ground, not the chassis.
Posts: 7,294
Threads: 268
Joined: Dec 2009
City: Roslyn Pa
Since you mentioned you are a newbie I'll mention that when you work on this set ALWAYS plug it in and measure the voltage from the chassis to ground (like a water pipe or the ground at the outlet). If plug it in one way you will see 120vac turn the plug over and plug it and you will measure a very low voltage. You want to have it plugged in with Minimum voltage between the chassis and ground. This will save you from getting a nasty shock or damaging test equipment.
Terry
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
Posts: 271
Threads: 12
Joined: Nov 2005
City: Nashville, TN
Hi,
I'd suggest that any piece of radio equipment that is going to be connected to test equipment, be ran through an
isolation transformer. Especially non-transformer sets.
Years back I had a hot chassis I was working on , and when I attched the ground lead to the chassis, a big spark 'happened'. Lesson learned. Good thing I was using the 'one hand in the pocket' while touching a live chassis method.
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