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Author Topic: Lamp Fuse for Yaesu  (Read 2807 times)
WT8E
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Posts: 55




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« on: February 22, 2013, 11:54:21 AM »

Does anyone know what the RF lamp fuse is for a Yaesu FT 102 or where you might get one? It is pretty small and soldered on the antenna relay itself, not like the older 101's with an easy changable lamp in a socket on the back of the rig.

Tnx

WT8E
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K9YLI
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« Reply #1 on: February 23, 2013, 07:05:45 AM »

Mouser   or   digi-key

maybe    MCM   ..      replace  with a small axial lead, solder in  fuse.

lots of  options..
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AA4PB
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« Reply #2 on: February 23, 2013, 07:34:09 AM »

Here's some info: http://www.qsl.net/wy3a/Lamp_Fuse.htm

Note that this is the receiver input protection fuse which is in series with the antenna connection. If you use the wrong sized bulb then you may have reduced sensitivity and/or insufficient protection for the receiver front end.
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G3RZP
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« Reply #3 on: February 23, 2013, 12:50:28 PM »

It is also, I believe, a complete s*&^%%$£ to get at........

If you aren't running a separate rx antenna, it's not necessary. If you are into low band DXing with a separate rx antenna, then it is damn good idea! But even more so is a relay to isolate that antenna input on receive!
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AA4PB
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« Reply #4 on: February 23, 2013, 01:33:19 PM »

My understanding is that the bulb is in series with the normal receive antenna connection in order to protect the front end circuit from static discharges on the antenna. If you remove it then you have to replace it with a jumper wire or you will have no receive signal. But then you also won't have the intended protection for the receiver.
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G3RZP
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« Reply #5 on: February 24, 2013, 03:32:51 AM »

Don't know about static. I lost the one of the cascode front end FETs due to a close lightning strike, but not the bulb.

But static and lightning in the UK are a mere shadow of what you guys in the States have to put up with.
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WT8E
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Posts: 55




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« Reply #6 on: February 25, 2013, 07:07:23 AM »

Radio Shack has a 6 volt 100 Ma Incandescent bulb in a 2-pak for $2.19. It is not a grain of wheat lamp but I think there is room to use this bulb with some short leads soldered on to it.  Do you think this is an acceptable replacement?

I already have the finals out of the rig and have removed the board with the relay on it before. I agree it is a "Bear" but just takes some time.

One of the old FT 101's I have here has a #1815 12-14 volt lamp fuse in it rated at 200Ma. That is twice the volts and Ma as well as much larger.  It looks like something that you would use in a flashlight. Maybe the front end of the Ft102 is much more sensitive.
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AA4PB
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« Reply #7 on: February 25, 2013, 08:44:40 AM »

Radio Shack has a 6 volt 100 Ma Incandescent bulb in a 2-pak for $2.19. It is not a grain of wheat lamp but I think there is room to use this bulb with some short leads soldered on to it.  Do you think this is an acceptable replacement?

That should work okay.
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KE3WD
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« Reply #8 on: February 25, 2013, 10:16:21 AM »

Don't sweat the difference in mA value here, the bulb is not there to operate like a filament lamp, per se. 

That Radio Shack bulb should do the job just fine. 


73
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WT8E
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« Reply #9 on: February 25, 2013, 03:44:49 PM »

I went to R/S today to pick up a couple of the bulbs, 6 V/100 Ma.part number 272-1142.  They are called Midget E-5 Base lamp and have a screw in base. I asked if they have any sockets for that bulb, sorry, nothing that small. Does anyone know an application for these bulbs and where you might get a socket for them.  I checked Mouser and no luck there.

I am wondering if I could find a socket for the bulb and install it in the back chassis with short leads would that cause any kind of problem with the design of the radio front end. It would sure make it a lot easier to change the bulb with access to it from the rear of the chassis. Also easy to check for failure.

Just some thoughts since I am already about half way there already to try a mod like this.
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W8GP
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« Reply #10 on: February 26, 2013, 05:05:55 AM »

The radio shack lamp will offer no protection, how much current will it take to cause it to open like a fuse? Itwill be easier to just replace it with a jumper.
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AA4PB
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« Reply #11 on: February 26, 2013, 05:36:30 AM »

The original bulb is 8V at 100mA. The series bulb followed by back-to-back diodes has been used in many receiver designs over the years to protect the preamp from damage caused by excessive voltage applied to the antenna input. Unless the surge is **very** high, the bulb doesn't open like a fuse. It provides a series resistance to limit the current through the diodes. A bulb was used because its series resistance is low when it is cold, thus minimizing loss. If excessive current flows then the bulb lights and the series resistance increases.
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KE3WD
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« Reply #12 on: February 26, 2013, 05:50:42 AM »


Some call the use of the filament bulb like this, "negative resistance". 

Has nothing to do with how a fuse operates. 

Replacing the bulb with a jumper is not recommended for the health of the first stage RF receiving transistor. 

73
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