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Old 06-10-2021, 05:49 PM   #1
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Default ECH81/6AJ8 Vacuum Tube Question

Recently I acquired an Eico HFT 92 AM/FM radio made back in 1958. The unit has had some decent modifications done such as replacing the rectifier tube with silicon diodes and replacing the main filter capacitor with electrolytic capacitors. When I powered it up FM wouldn't tune past 105Mhz. I checked the FM tuner front end assembly and found a capacitor not soldered to the coil from the Plate B+ to and ground on the other side of the coil. Some calibration was needed on the trimmer capacitor on the tuner. My question is AM is extremely weak. When I tap on the tube I get all kinds of snapping and popping noises on AM. Also the magic eye doesn't peak well when centered. I have a cool Seco model 1100 tube tester and tested the 6AJ8 in the correct socket and set the selector to "B" and the load to"10%" I let the tube warm up for a minute and the meter doesn't move out of the replace range. When I hit the test button it even gets worse and shows about 38%. This tube also shows a second test and that test just stays in the replace area with no movement at all when testing. It's my understanding radio tube failure is very rare. Any suggestions? Should I check all the DC voltages on the pins while the unit is running? Should I observe the wave forms from the AM tuner with an oscilloscope? All the other tubes test is the good range so I'm assuming my tube tester works. I am attaching a block of the schematic below.
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File Type: jpg EICO HFT-92.JPG (46.0 KB, 8 views)
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Old 06-10-2021, 06:36 PM   #2
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Default Re: ECH81/6AJ8 Vacuum Tube Question

It looks like the tube is used for both FM and AM I suspect the tube is ok but might test low in the tube tester. What does it use for an AM antenna? It might require an external AM antenna.
If you tap the tube when it's in the tester, does the short light flash? or the needle jump?
One thing to note is when the rectifier tube is replaced with solid state diodes the B+ will be a lot higher as there is a higher voltage drop across the tube than the new diodes.

Last edited by R_J; 06-10-2021 at 06:40 PM..
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Old 06-10-2021, 07:40 PM   #3
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Default Re: ECH81/6AJ8 Vacuum Tube Question

The Seco 1100 is an emissivity only tester, so it doesn't do very much for gain. So it might not completely test the tube. So it still could be weak. That ECH81/6AJ8 is not completely used in FM mode, it appears to be just another IF amplifier stage in FM mode, but in AM it's a lot of stuff.

Kind of surprised. This is like the first radio I saw with something not a 12BE6 or 12SA7 as the AM first stage tube...
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Old 06-11-2021, 11:03 AM   #4
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Default Re: ECH81/6AJ8 Vacuum Tube Question

It's my understanding radio tube failure is very rare.
Actually, tube failures were somewhat common back in the 50s (and before) and into the 60s. Among other things, the heat needed for the tube to operate is also a long-term self-destruction mechanism. Having worked for a company that produces travelling wave tubes and uses them in its microwave power modules, I saw quite a few RMAs whose failure was an open or high resistance heater.
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