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3TF7




 
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Author Topic: 3TF7  (Read 462 times)
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Joe Connor
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« on: August 17, 2018, 06:41:05 PM »

I have a question based on Skip's current thread on the R-390A, and I don't want to hijack his thread.

I have a St. Julien's Creek survivor that I've had for about 10 years. I replaced the filter caps, the killer cap and the other ones Chuck Rippel suggested should be replaced. It worked well until about five years ago, when it crapped out. There is no filament voltage on the crystal oscillator tube. (I seem to remember a small bit of smoke when it went out, but I'm don't recall for sure). Since then, the R-390A has been on my one-of-these-days pile of projects. From Skip's thread, it appears a dead 3TF7 can cause loss of filament voltage to the crystal oscillator. Here are my questions:

1. How can I test an 3TF7 to see if it's dead? I have a spare one, but I don't want to mess with it unless necessary. Can I ohm it out? If so, what pins?

2. Is a resistor-in-lieu-of-3TF7 an acceptable solution? If so, what value? I've seen 42 ohm/5W suggested in some articles, but the closest Mouser stocks is 43 ohms/5W. Good enough?
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WD8KDG
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« Reply #1 on: August 17, 2018, 08:20:42 PM »

I have a question based on Skip's current thread on the R-390A, and I don't want to hijack his thread.

I have a St. Julien's Creek survivor that I've had for about 10 years. I replaced the filter caps, the killer cap and the other ones Chuck Rippel suggested should be replaced. It worked well until about five years ago, when it crapped out. There is no filament voltage on the crystal oscillator tube. (I seem to remember a small bit of smoke when it went out, but I'm don't recall for sure). Since then, the R-390A has been on my one-of-these-days pile of projects. From Skip's thread, it appears a dead 3TF7 can cause loss of filament voltage to the crystal oscillator. Here are my questions:

1. How can I test an 3TF7 to see if it's dead? I have a spare one, but I don't want to mess with it unless necessary. Can I ohm it out? If so, what pins?

2. Is a resistor-in-lieu-of-3TF7 an acceptable solution? If so, what value? I've seen 42 ohm/5W suggested in some articles, but the closest Mouser stocks is 43 ohms/5W. Good enough?


Issue #2: In a R-390/A the 3TF7 tube isn't really needed. Now if your RX is in the back of a duce & a half, towing the generator over hill, dale, and thru mud holes; then consider it might be needed. If not under these conditions save the cost of a 3TF7 and buy good whiskey Grin.

Jumper pins 2 & 7 on the tube socket (3TF7) and replace both 6BA6 tubes with 12BA6 tubes.


I saved my $$$ for the whiskey.


Craig
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Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
Joe Connor
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« Reply #2 on: August 17, 2018, 09:21:18 PM »

Thanks, Craig. Sounds like a great idea (on both the 12BA6s and the whiskey).
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WD8KDG
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« Reply #3 on: August 18, 2018, 08:40:03 AM »

Should of added: Replace the PTO & BFO 6BA6's with 12BA6's.
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Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
Joe Connor
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« Reply #4 on: August 18, 2018, 07:11:40 PM »

My 3FT7 is fine. It lights and gets very warm. Any other ideas why the crystal oscillator tube has no filament voltage? The tube (5654) lights and test good in my tube tester. I don't want to pull the crystal-oscillator module unless I have to do, but if I do, I would like an idea of probable/possible failed components before I go in there.

Any ideas/suggestion would be greatly appreciated.
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WD8KDG
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« Reply #5 on: August 18, 2018, 08:54:28 PM »

My 3FT7 is fine. It lights and gets very warm. Any other ideas why the crystal oscillator tube has no filament voltage? The tube (5654) lights and test good in my tube tester. I don't want to pull the crystal-oscillator module unless I have to do, but if I do, I would like an idea of probable/possible failed components before I go in there.

Any ideas/suggestion would be greatly appreciated.

Y2K R3 to the rescue. If'n you don't have a copy, get one. Chapter 5, page 38, figure 5-12 (Filament and Oven Circuits) Easy Peasy, anyone can follow the filament voltage from the tube socket, pins 3&4.....to J410, thru P110 (B), to P111, to J811, to the secondary of T801. Which is 6.3VAC @ 8amp.

Craig,
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Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
Joe Connor
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« Reply #6 on: August 18, 2018, 10:03:24 PM »

Thanks, Craig. It's the crystal oscillator module. The plug going into that module  (Pin B on P-110) has 6.3V.

I guess I'll have to remove that module. I have a spare module for a replacement. From reading the manual, the procedure for removing that module seems quite complicated. Yuck.
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WD8KDG
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« Reply #7 on: August 18, 2018, 10:40:23 PM »

Thanks, Craig. It's the crystal oscillator module. The plug going into that module  (Pin B on P-110) has 6.3V.

I guess I'll have to remove that module. I have a spare module for a replacement. From reading the manual, the procedure for removing that module seems quite complicated. Yuck.

Playing with either the RF deck or Crystal oscillator module can be a pain in the back side. In R3 there is a section on how to disassemble and reassemble the gear train. Keep this handy! Haven't been in either deck for a while, but the switch inside the crystal deck has to be timed correctly. I think it wasn't that bad and my "Man Card" was valid after repairs. There seems to be a hole in the back of the RX where a long screw driver can be inserted to twist the shaft of the switch. The only other thing that comes to mind (and I ain't gonna take the top cover off of my R-390/A's) drop the front panel, remove the IF deck; there is something else in the way of moving the crystal deck. And then there are those little springie things along the left edge of the RF deck.


Use the $30 not spent on a 3TF7 for a good bottle of whiskey, this helps also in repairs.


Good Luck,

Craig
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Ham radio is now like the surprise in a box of "Cracker-Jacks". There is a new source of RFI every day.
Joe Connor
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« Reply #8 on: August 18, 2018, 10:47:37 PM »

Use the $30 not spent on a 3TF7 for a good bottle of whiskey, this helps also in repairs.

Before or after?

It will have to wait until I clear a few receivers off my repair bench. I am sure I will be back with more questions.
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