[BC] Continental 314R1

[John Lyles]

Steve

Could you be specific about which meters peg. Do you mean the plate voltage, the plate current, or the test meter (and if so, what function is it set on?). Also, when they peg, do they just go fullscale momentarily and then the transmitter kicks off, overload on power supply? Or does it trip the HV breaker too? 

What do you mean by short period? One minute, one second?
How are you shutting off the bias PS? Do you remove the Bias fuse, or just turn off the LV (filament goes off also?). 

If you mean it will not stay on high power even momentarily, it could very well be a component in the high voltage network, from the switch tube over to the PA tube cathodes. This includes the three big mica capacitors to chassis, in the PDM filter, the big diode CR1 that goes back to common or the other diode CR21 that goes to the -8500 volt supply. In high power, the resting duty factor (without modulation) is about 40%, so it gives about -3000 volts to the RF tube cathodes. This makes about a kW of RF power, as it should. In low power mode, the duty factor is reduced even more, to give a lower voltage. So, yes, in high power, the DC voltage out of the PDM filter is higher. Since it does try to work in low power,and the PDM switch tube is basically switching between zero and -8500 volts, that eliminates the first components in the low pass filter, as they would be breaking down in either power setting. You're probably correct on the diode too, as it would be getting full s!
 wing even
at lower power, from where it is in the circult. But on the output of the filter, the mica cap to ground (under the RF tube sockets) or anything around those tubes (including those two 3-500Z tubes themselves) is suspect to be failing at higher voltage. Even the filament transformer for the RF tubes, and the white wiring could be breakding down at higher voltage. If you had a hi potter, you could test components here. As you probably know, it ain't easy to troubleshoot a 314R1 when it is running, due to everything under the tube sockets being at very high voltage. Can't easily stick a scope probe in there when RF is on. 


CR1 is there to provide a path for the counter EMF created in the first inductor in the PDM low pass filter. When the switch tube turns off that inductor current flows back through CR1. You can read a more detailed description of this portion of the circuit on my website: http://jtml.info/314R1/314R1.html, 
click "How the Power Rock Modulates" at the bottom for a four page paper I wrote about this, the real meat of the Power Rock rigs. 

I guess the switch tube, if it were sticking on or being driven on too much of the time (at high DF beyond what resting carrier would need), could be raising the HV to the PDM filter and the final tubes, if the switch mode car was broken. Usually the failure mode is zero output, not fully on like that. 

Check the transistors (big TO3 package) on the Switchmod car with an ohmeter, that they are not shorted. The later MOSFET version is different, no more 0.6 V across base to emitter. 

Normally, you cannot get PDM information out of the light pipe unless you cheat the carrier interlock ( I forget how. but clip leads work). You can look at the switchmod card output on a scope where it drives the light pipe transmitter, without HV on. It ain't easy to troubleshoot that card. Some folks have a spare card, and have Continental go over the bad one and retest it in their fixture. 

Good luck, you will eventually find the culprit. 

John Lyles
New Mexico


> Message
> Date: Fri, 9 May 2008 12:20:22 -0400
> From: "Steve" <avcradio at roadrunner.com>
> Subject: [BC] Continental 314R-1 Trouble
>
> Having some trouble with this transmitter, It will run for a short period of
> time at low power, put it at high power and it trips off after all the
> meters peg. I assumed
> 
> the switch tube was going bad and I changed it, same problem. Bias voltage
> was a little low so I replaced some of the caps that have been drying out
> and the voltage
> 
> is normal +115, -115 turn to low power good so far then high power and same
> thing trips off, I have found out that if I would shut off the bias supply
> for a second or two
> 
> and back on it will run for a while at low power, but not at high power.
> Would there be something wrong with the switchmod card ? Also Continental is
> telling me
> 
> that the clamp diode could be breaking down but I disagree since when I
> remove bias voltage briefly it will work. Does anyone have any ideas?