[BC] 314R1 trouble

[John Lyles]

It might be that your switch tube card has gone south, is sticking on, instead of sitting at 40% Duty Factor without modulation (for full power 1 kW). You can look at the optical transmitter on the big mother circuit board that folds down in front, while it is running. Be very careful of the 220 VAC on the edges for the big filament potentiometers. Put a scope on the pins of the part that drives the fiber optic, and see what the pulse looks like. It should be 70 KHz repetition rate, with slightly less than a square wave (less than 50% on time). I cannot remember if it is inverted or normal. Try this at low power if you cannot get 1 kW to stay on, and it will be even lower duty factor. It this test proves OK, then you should remove the switch tube driver card carefully and remove all the big TO3 transistors and check them with an ohmmeter or transistor checker. While you are at it, look at all components, esp the big resistors on the board, for overheating signs. If yours has!
  the
original Collins-design bipolar HV transistors, they test differently from the later Continental MOSFET design board. Richard Garrett at CEC can help you there. IF you can find a spare board, it would be beneficial to try it. Search around and maybe you can buy one off some parts unit. That board is one of the weaknesses of the design, in that if it fails, it is very dangerous and difficult to troubleshoot working. CEC has a test load that they can test it into (110 ohms I think) that simulates the grid current of a 3-500Z triode, so that you can test it outside of the transmitter, with the proper power supplies of course. 

I wouldn't think that a bad clamper/damper diode would allow the RF power to rise suddenly, the plate voltage must be really cranking up sometimes. This again points to the switch modulator function. One more thing, though, the steady high DC voltage may have eaten some of the following components, such as the diode or capacitors, as I alluded to earlier, but this is sounding less probable. And yes, loosing bias for the switch tube would indeed cause it to stick on, and create great HV and trip overloads. If this happens, your switch tube will glow very brightly, either purple around the glass and super yellow/red in the plate. This won't last long. 

Again, a warning. That transmitter is one of the more dangerous to troubleshoot due to the presence of hot deck modulator and final tube. Cathode is not at ground, but at -HV. It is potentially deadly to jumper interlocks and reach in or stick a meter or scope into the grid compartment under the tube deck. At least one person has been electrocuted working on a Power Rock, to my knowledge. Please be very careful, and if you are unsure about this, refer to Continental for help! 

John Lyles
314R1 owner

Date: Sun, 11 May 2008 10:17:12 -0400
From: "Steve" <avcradio at roadrunner.com>
Subject: [BC] Re: Continental 314R-1 Trouble
To: <broadcast at radiolists.net>
Message-ID: <004e01c8b371$b5489b90$2101a8c0 at engineering>
Content-Type: text/plain; charset="us-ascii"

To answer some of the questions about my problem, earlier someone asked what
meters were pegging, Plate voltage and current, as well as a inline amp
meter that I put on the output.

The modulation was fine when it would work for the short period of time it
does, also I don't think it is heat related a few times it would run for a
few minuets to almost an hour before it would

overload and trip off. Now when I say it trips off, it overloads then trips
off.
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