Conventional wisdom in electronics troubleshooting is that moving parts tend to fail first. Relays that old are a likely failure, but what we see more often is for the preamp relay to interrupt the receive signal with the preamp turned off. Customer will commonly report his linear's relay is "sticking", because he has to click the mike button a time or two to get his receive signal back. More likely the relay is operating okay, but the contact points in the standby side of the preamp relay are cutting out.
But that problem is only when the preamp turned off. Biggest enemy of the preamp is the "SSB" switch. When you key the mike with the preamp turned on, it takes the relay a short time to respond and change over to transmit side. During this short interval, your radio's four Watts is being crammed up the backside of the preamp circuit. The tiny preamp transistor is typically protected by a diode that will safely 'catch' that burst of transmit power so long as it doesn't last too long.
Problem with that sideband switch is that it works too well. Not only does it make the relay hesitate when you unkey, but also makes it hesitate when you key the mike. That stretches out how long that burst of 'upstream' transmit power pumps into the preamp circuit. Frequently long enough to pop the preamp transistor, the surge diode or both.
The relay may very well be causing trouble. And the preamp transistor may be popped also.
Or both.
73
But that problem is only when the preamp turned off. Biggest enemy of the preamp is the "SSB" switch. When you key the mike with the preamp turned on, it takes the relay a short time to respond and change over to transmit side. During this short interval, your radio's four Watts is being crammed up the backside of the preamp circuit. The tiny preamp transistor is typically protected by a diode that will safely 'catch' that burst of transmit power so long as it doesn't last too long.
Problem with that sideband switch is that it works too well. Not only does it make the relay hesitate when you unkey, but also makes it hesitate when you key the mike. That stretches out how long that burst of 'upstream' transmit power pumps into the preamp circuit. Frequently long enough to pop the preamp transistor, the surge diode or both.
The relay may very well be causing trouble. And the preamp transistor may be popped also.
Or both.
73
