[BC] Continental 315R1 troubles

[John Lyles]

Bill
Please help me understand what happens here. You hit the low power button (I assume that low power setting is set at 1 kW or less?), and it immediately pegs the HV meter?  What is the normal value of HV for low power setting, from your logbook? If you remove (mute) the PDM drive (kill the oscillator, for example), so that you have no (-) plate voltage on the final tube cathode, the HV meter still pegs? Is there anyway that the primary taps got swithed, to the lower primary voltage taps? This would in effect boost the HV for a given line voltage that is incorrectly tapped. I agree with the other advice, first check the HV meter multiplier string. It could be cooked or have a few out of tolerence resistors now. There should be a string of them on a PC board I believe. Take that board, disconnect the hot end, and check each resistor with a DMM. Should be within the tolerence band value of the resistors. If that checks, then take that meter, and calculate what sort of current sh!
 ould give
full scale for 15 kV applied to the series multiplier string. Sorry, my schematic is out in a drawer, but you should be able to do Ohms law, I=15kV/R. 

Now, you can put a simple power supply, say a 5 V power supply, or 1.5 volt battery, with a series resistor, to inject similar current into the meter. This is crude, but it will at least tell you if your meter isn't lying. 

If the metering all checks out, then, as someone suggested, you could have an open phase on the AC line. Especially odd that the AC phase voltage meter took a hit (I think thats what you said). 

A shorted plate transformer wouldn't cause the hv to rise higher like this, would it? I would think it would kick out the plate breaker when that happens. 

Finally, when you bring up the PDM again, kill audio and run only carrier. This should limit the 'cathode' voltage, which is called plate voltage in that rig, to that required for 5 kW, not with the peaks of modulation. Does the sizzling sound still occur? 

Its also possible, that you have a multiple failure scenario. This is quite common in high power electronics, one thing takes out another, etc. The damper diode on the output of your PDM filter has me worried too. If it were to pop (and fail open, not shorted as the one on my 314R1 was), the PDM filter inductors could make excessive voltage on the output to the cathode of the final tube. This would also show up at low power as the duty factor of the PDM is low, but you still have some big switching transients with the inductive kick that need that damper. You can check the diode with a meter, but it is HV rated so it needs to have the many diode junctions forward biased with a small power supply. I usually set a small bench supply in current limit, say 1 Amp with the leads shorted, then apply it to the diodes, and slowly turn V up. Somewhere between 20 and 40 VDC you will begin to draw current. Don't hold it on too long as you could overdissipate a diode if not careful. Also,
applying an potential in the reverse direction, should not conduct at all. However, if the damper were shorted, you would be popping off the plate breaker and the plate overload fast. 

One more thing, if it is really arcing from the plate of the final tube through the neutralization capacitor, then wouldn't that be an RF arc? As mentioned here, the plate has no DC voltage on it. Try killing the PDM first, and get the HV to run alone without pegged meter. Then work on the RF problem. Is it possible that that thing got mistuned and is somehow oscillating? I think it has a 3rd harmonic resonator network or two around the final; did they get altered? 

Sorry for all the questions, but they need to be answered to troubleshoot this thing wisely. Good luck. I remember that Bill Blatz up at WDEL in Wilmington, DE may still be running one, or just retired it. You might give him a call and see if he's seen these symptoms. 

Did you take a lightning hit recently, when Columbia finally got some rain? My family is from Winnsboro, up the road. 
Good luck, and keep the fingers out of that box when its powered, cause it will kill ya fast. That 14 kV can also arc quite a distance when conditions are right. 

John Lyles
New Mexico

> now meter information,,, as soon as I hit low power button the high voltage  
> meter pegs ( max on meter is 15kw volts), the transmitter should go no higher  
> than 14kw volts.  the plate voltage starts to come up but arcing occurs  
> before the meter reaches it's place on the scale,  the plate current  comes up and 
> arcing occurs when as the meter reaches it's point.
> I feel the plate transformer has a short causing the extreme arcing on the  
> pa tube...
>  
> ok all engineers what are your thoughts??