Heathkit AT-1 transmitter and the Keyall HV kit
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| The Keyall HV kit components |
http://wb9kzy.com/keyallhv.htm
Sometimes a kit producer will listen to customers or tries to :)
The Keyall HV kit came about because of a customer complaint about the Keyall kit. The original Keyall had 500 volt MOSFET transistors and used a International Rectifier PVI (Photo Voltaic Isolator) to drive the transistors. But a customer in Canada sent me a nastygram about the Keyall. He had used it with his Heathkit AT-1 transmitter and had blown out one of the transistors.
His main complaint: The name: Keyall was misleading because it couldn't key his transmitter.
Keyall is NOT going to literally key any transmitter ever made, for example the Alexanderson alternator in Sweden :) And I'm sure there are many other transmitters that it can't handle.
The Heathkit AT-1 is a cathode keyed tube rig from the 1950s, their first transmitter kit. The AT-1 has an oscillator and a power amplifier tube, both cathode keyed. The problem is the key-up voltage. A later Heathkit rig, the DX-35 can have up to 385 volts (!) on the key just after key-up and calming down to 230 volts. The key down current is 125 mA. A conventional J-38 type key can give a nasty jolt to an operator - that's what the Keyall and Keyall HV were meant to prevent.
BTW I have looked through many transmitter articles in ARRL handbooks and QST issues and have never seen the key-up voltage or key-down current ever mentioned.
Back to the customer's AT-1, the combination of the 385 volts on the key along with whatever inductive kick might have been seen was enough to overwhelm the 500 volt MOSFETs in the original Keyall.
So I tried buying some 1000 volt MOSFETs and soldered them into the Keyall board - it didn't work, the larger gate capacitance caused the problem. The original PVI5080 just didn't have the oomph.
The circuit was changed to use a Toshiba TLP591 PVI which had plenty of oomph. But the TLP591 didn't have the gate turn off circuit in the previous PVI. So I added a resistor to help discharge the gate voltage.
I then sent this Keyall HV prototype to my complaining customer - and never heard a thing. No response to emails.
I think I know why, the gate resistor may have altered the circuit enough so that it wouldn't key his AT-1. I eventually eliminated the resistor.
I haven't had any further complaints about transistors dying OR about the name: Keyall.
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm