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Author Topic: L-4B Problems  (Read 1237 times)
K3QYE
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Posts: 1




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« on: November 04, 2012, 05:38:16 PM »

I purchased the L-4b in 1976 second hand.  It has worked great.  I installed the Harbach Soft Start, Soft Key and the Replacement power supply modules.  It worked great until a month ago.  I came into the room and the soft-start was fried... (burnt)!  I replace it and now there is no power to the amp.  There is 22O coming through the Jones connector. The swithes are working.  There is power going into the T1 transformer. If I by pass the soft start black wire from pin one on the Jones connector to the power switch.  It seems to draw way to much power and arcs.  I am at a loss on this one. The transformer does not seem to be fried, but I am not sure the best way to check it without disconection the whole thing.  Any thoughts and suggestions will be greatly appreciated.
Thanks
Lloyd K3QYE
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W8JX
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Posts: 5898




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« Reply #1 on: November 04, 2012, 06:09:22 PM »

My first guess is a shorted tube. Remove tubes and then power it up and then replace tubes one at a time to find short
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W6LG
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Posts: 22




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« Reply #2 on: November 05, 2012, 09:48:21 AM »

Hi

I have restored 6 L4B's over the years. Some amps have done what you describe.  Here are some things you can check.  A shorted diode string(s) in the power supply.   Failure of some or all of the filter caps in the power supply.  A shorted doorknob or disc capacitor that has one side to HV and the other side connected to ground (chassis).  Doorknob capacitors can crack and that is not easily seen.  One of the tubes may have a plate to grid short.  That would cause HV to be grounded through the tube's grid.  Check the shorting bar or interlock at the back of the amp. It may be shorted to ground. There is a .82 ohm resistor in the power supply that will open up if too much current is drawn.  From what you have seen so far, that resistor should be open.  Check it to see if it is open and that there is no short to ground at that resistor.   

I would not use the rocker switches.  They are impossible to find since the manufacturer went out of business many years ago.

A quick word of caution, HV can kill you.  One dumb mistake can be your last.  So, you must take precautions.  If you want some help, I can help you via email.  Again, HV is to be respected.

73, Jim Heath W6LG       
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W8JX
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« Reply #3 on: November 05, 2012, 09:54:26 AM »

A quick word of caution, HV can kill you.  One dumb mistake can be your last.  So, you must take precautions.  If you want some help, I can help you via email.  Again, HV is to be respected.

Very wise advise.
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VR2AX
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Posts: 589




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« Reply #4 on: November 09, 2012, 04:25:30 AM »

The L-4/L-4B manual describes in clear terms how to disable the HV voltage.

It involves removing jumper(s) in the power supply. This leaves the filament transformer voltage still operative, provided the jumper in the main amplifier unit is left in place.

The advice given before is 101% accurate as regards compliance with HV safety. Single mistakes can and often do kill.

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