[Drake] L-7 vs. L7-PS failure

[kb6oks00]

Torrey
The correct resistor can be ordered from Newark electronics.  I did a
complete rebuild of my pwr.supply, get a few there cheap.  

Lynn KB6OKS

---- Original Message ----
From: 4cx250b@muohio.edu
To: torreym@mchsi.com, drake@mailman.qth.net
Subject: Re: [Drake] L-7 vs. L7-PS failure
Date: Thu, 13 Nov 2003 21:48:30 -0500

>At 08:46 PM 11/13/2003, you wrote:
>>Good evening all.
>>
>>As I was tuning up my L-7 amp this evening there was a very loud 
>bang and 
>>a visible flash behind the plate voltage meter on the left.  
>Foolish or 
>>not I reset the power supply circuit breakers and tried again with 
>low RF 
>>drive.  Same story again.  I took the top of the L-7 and could see 
>no 
>>evidence of arcing or burned components.  Final tubes and attached 
>>resistors look normal.  In short, everything looks normal topside.  
>I have 
>>not removed the bottom panels to look.
>>
>>The L7-PS power supply reveals charring and a burn-through of the 
>R12 
>>resistor (2 watt 0.82 ohm IRC type BWH).  The charring put a big 
>black 
>>mark on the adjacent large tubular capacitor, but the capacitor 
>does not 
>>look damaged and all the large resistors across these capacitors 
>appear intact.
>>
>>The R12 resistor appears to function as a protective fuse in the 
>high 
>>voltage output line.  Am I thinking correctly?  If so, where could 
>such a 
>>impressive short circuit have occurred in the linear amplifier 
>without any 
>>evidence of damage topside?  I may have to send both units out for 
>repair, 
>>but I certainly would like to avoid it...given all the weight, 3 
>boxes, etc.
>
>Hi Torrey,
>Yes, the 0.82 ohm resistor is a fuse, intended to explode when the 
>HV is 
>shorted. The explosion sounds like a gunshot and sends resistor 
>fragments 
>flying, but it quenches the arc and interrupts the circuit. Any 
>WIREWOUND 
>resistor of similar value (e.g., 0.81-1.5 ohm) and wattage will 
>work. Don't 
>use a composition (Carbon) or metal film resistor. It must be 
>wirewound.
>
>It is hard to say what has caused the short circuit. An internal arc 
>in a 
>tube could be the culprit. You can check this by pulling the tube 
>plate 
>caps and seeing if the problem disappears. Also, check the 
>protective diode 
>that clamps the B-to chassis ground, as it is probably shorted also. 
>It 
>tends to fail when the HV shorts, but isn't the cause of the short. 
>The 
>amplifier will work even with a bad diode, but your grid current 
>readings 
>will be wrong.
>
>'Check the HV crowbar interlock switch, to make sure it hasn't 
>somehow 
>shorted the power supply. Keep your fingers crossed and hope this is 
>the 
>problem, because that would be an easy fix.
>
>Your problem won't be a shorted tuning or load capacitor, since the 
>short 
>is present with no drive applied. In principle, you could have a 
>parasitic 
>oscillation, but that is unlikely if the parasitic resistors in the 
>plate 
>leads of the tubes are okay.
>
>You should also make sure you don't have catastrophic failure of  
>your HV 
>capacitor bank, because that would cause your symptom. Check also, 
>to make 
>sure your HV recitifier diodes aren't shorted.
>
>I'd start troubleshooting by disconnecting the filter bank and 
>transformer 
>secondary, and then use an ohmmeter to find the short.   I'm 
>guessing it 
>will be pretty easy to find. Don't worry about the charred mark on 
>the 
>capacitor next to the resistor. That isn't causing your problem.
>
>Good luck!
>
>73,
>
>Jim Garland W8ZR
>
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