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75a4 rectifier problem




 
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KB5MD
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« on: March 17, 2017, 05:12:36 PM »

I have a Collins 75A4 receiver that starts to loose volume and sensitivity after a while.  The problem is the 5Y3 rectifier starts to have a larger voltage drop across it than normal.
I can replace the 5Y4 and everything returns to normal.  This has happened twice in the last  year.  Does anyone else have this problem?
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N8ETQ
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Mort


« Reply #1 on: March 17, 2017, 09:26:22 PM »




             cHECK ur line voltage.. Spounds like
hes cooking to me,,
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WD4DMZ
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« Reply #2 on: March 18, 2017, 11:13:18 PM »

Check the electrolytic caps in the HV power supply. I had an HQ150 that ate two 5U4s because a bad cap was leaking current so bad. Swapped them all out and it has been fine since.

Also had a shorted power xformer in an SX42 that did a similar thing. The total AC voltage across the HV leads was ok but it was 600v to center to 100v. The VR tube did a heroic job keeping the radio operational before I noticed the VR tube strobing! Radio survived, xformer and rectifier tube did not. The VR tube also survived somehow.

So, check the voltages across the caps and xformer HV secondary.

Rich
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WD5JKO
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WD5JKO


« Reply #3 on: March 19, 2017, 12:44:02 AM »


Several good answers thus far.

Tubes like a 5Y3 have been out of production for a really long time. We tend to recycle used ones, and occasionally find a NOS tube.

How long does a 5Y3 last when operated within ratings? I would think the tube life would be longer with a choke input filter, and the life would be shorter with a capacitor input filter. Sometimes when we re-cap a receiver, we UP the value in MFD. In the case of a capacitor input filter, boosting the capacity of the first cap will hasten the demise of a tube like a 5Y3. The "Hot Switching Transient Plate Current" increases with increased first section filter capacitor selection. RCA recommends adding a series resistance if the value is above 20 uf for one condition, and 10 uf for another condition.

All that said, we are often starting with a tube that is already 50 plus years old.

If the B+ draw is reasonable, I'd hammer a bad 5Y3, stick two 1n4007's per leg in there, and add a series resistor. As a starting point try something like 220 ohms 10 watts, and see where the B+ comes out.

Jim
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N8ETQ
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Mort


« Reply #4 on: March 19, 2017, 09:55:49 AM »



    There is also a 6AL5 bias rect. in there.
You didn't mention distortion but that can be
affected by excessive LV too.

     If your going to ditch the 5Y3 you could
always use the 5.0 volt winding to buck the
AC input to the power xfmr as well.

     Specs say he's good with 105 to 125 VAC
input.  Bulk DC should be around 170.

     As has been pointed out, You should find
the AC input voltage that will supply 6.0 to 6.3
VAC to the tube fil.  Since the turns ratio
of the fil. winding is "Fixed", and assuming he
makes 6.3 volt fil @ 125VAC input he will make
5.89 VAC fil. @ 105 VAC input.


     I can't remember the last time I had to
swap a tube in any of my Rigs.  Did loose a 2A3
in my 610E about 15 years ago due to an internal
ARC.

GL

/Dan


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WBear2GCR
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Brrrr- it's cold in the shack! Fire up the BIG RIG


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« Reply #5 on: March 19, 2017, 09:40:11 PM »


A reasonably effective trick to drop filament voltage, but not HV/B+ is to use a string
of diodes, each in the opposite direction. The idea is that the forward voltage drop is typically
~0.6v, so if you were say at 6.9vac, a single bi-directional diode drop would bring you back to
a nominal 6.3vac... etc... (obviously one does not need a "string" to do one diode drop, just two)

                              _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
W3RSW
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Rick & "Roosevelt"


« Reply #6 on: March 21, 2017, 04:12:38 PM »

i like the idea of using the 5 volt winding as a primary buck too in this type of equipment  provided the 2 or 3 amps is enough to buck the entire sets current draw and if nothing else uses the 5 volt winding.  120 to 125 primary is also probably a little hot for the HV so this cools the whole set.  The 6aq5, 6v6's , etc. AF output tube types will thank you.
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RICK  *W3RSW*
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