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

[James C. Garland]

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