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Palomar 300a key up and pre amp problems
- Thread starter dozerman
- Start date
Got tired of paying 30 bucks or more for R50 relays. I don't usually talk about design ideas before I have pics of them, but we'll be selling an adapter pc board that goes into the place of that relay, and takes a DIP type relay that you can still buy from multiple sources. For a lot less.
If it works.
The pc boards haven't come back yet, but I'll post pics when we have put one or two into a 300A to see that it works as intended.
73
If it works.
The pc boards haven't come back yet, but I'll post pics when we have put one or two into a 300A to see that it works as intended.
73
I will be first in line to buy one. I'm still having issues with mine. After ordering the correct transistors for the key up and pre amp, my recieve is all or nothing. Meaning, pre amp operates as it should, but with it off, the rx is so diminished I have to run the preamp. Thinking about just sticking with my trashy Maco's until I get my feet wetter with tube amps. Don't wanna mess this 300a up. It's cherry.
Shoot me a PM with your snail-mail addy. So long as I'm still in "Guinea-Pig" mode on this, it will be a smokin' cheap deal.
73
73
I meant to add that my rx was diminished because of some bad solder joints under those relays you were talking about @nomadradio. I touched those up and have rx with or without the pre amp again. And how do you send pm's here?? I've looked and don't see how to do it other than a conversation @nomadradio
I know this thread is old but I have a problem with the same amp. When it gets warmed up, generally toward the end of tuning the relays begin to chatter and sometimes stay stuck chattering even after the mic is unkeyed. I have checked every component on the relay board with no luck. All the filter caps were replaced along with the voltage doubler diodes. Could it be bad relays? Even the hv relay does it from time to time. Oscillation perhaps? I'm by far no radio technician but I know a few basics. Any help would be appreciated, tia
Probably not the fault of relays. Most likely it's oscillating.
Sounds like maybe missing parasitic chokes. A lot of these were built with a black ferrite bead slid over a bare wire leading to the plate-cap clip. The bead would get hot, crack and fall off. Usually around the first time the amplifier got pushed a little too hard. Seems like all of them used this on the four final tubes.
Unless someone with sharp eyes found the fragments loose in the bottom of the amp there is no trace they were ever installed. Just four bare wires, one to each plate cap.
A two-Watt resistor with four turns of wire around it is what we install on each plate-cap clip in place of the bare wire. to make this model stable. The resistance value is not critical, between 47 and 100 ohms more or less. Just so long as all four have the same resistance value.
The two driver tubes were sometimes found with a resistor-type choke from the factory. Others would have the ferrite bead on each driver tube. It was a lot less common to see these fail on the driver stage. Those two tubes don't work as hard, and won't get the beads as hot.
There are other things that can make this amplifier unstable. Tube sockets mounted on circuit boards depend on the mount screws for a ground connection. If plastic spacers over the screws get hot and shrink, the ground connection comes loose.
Internal coax connections should have the shield grounded. If someone did a sloppy job replacing old coax inside the amp, that's a way to create instability.
Sounds like maybe missing parasitic chokes. A lot of these were built with a black ferrite bead slid over a bare wire leading to the plate-cap clip. The bead would get hot, crack and fall off. Usually around the first time the amplifier got pushed a little too hard. Seems like all of them used this on the four final tubes.
Unless someone with sharp eyes found the fragments loose in the bottom of the amp there is no trace they were ever installed. Just four bare wires, one to each plate cap.
A two-Watt resistor with four turns of wire around it is what we install on each plate-cap clip in place of the bare wire. to make this model stable. The resistance value is not critical, between 47 and 100 ohms more or less. Just so long as all four have the same resistance value.
The two driver tubes were sometimes found with a resistor-type choke from the factory. Others would have the ferrite bead on each driver tube. It was a lot less common to see these fail on the driver stage. Those two tubes don't work as hard, and won't get the beads as hot.
There are other things that can make this amplifier unstable. Tube sockets mounted on circuit boards depend on the mount screws for a ground connection. If plastic spacers over the screws get hot and shrink, the ground connection comes loose.
Internal coax connections should have the shield grounded. If someone did a sloppy job replacing old coax inside the amp, that's a way to create instability.
Thanks Nomad, I'll give that a try. This amp has had many hands in it before I acquired it. The wires for the cap clip look to have been changed, they aren't bare. Red vinyl coated, no beads. I will install the chokes and make sure socket boards have proper ground and report back the results.
Installed the chokes and now for some reason the high voltage relay won't close when the amp is keyed. Ran out of time last night, will look at it again this weekend but caps and high voltage diodes are good.
Might want to make sure the 1000uf 25 Volt filter cap that's on the 12-Volt DC relay supply has not gone bad. If it's original it should have by now.
Other possibility is that the "keying" relay is not closing the circuit that activates the HV relay.
Does yours have the plug-in board with 3 relays, or the wired-in relay board with one plug-in relay?
73
Other possibility is that the "keying" relay is not closing the circuit that activates the HV relay.
Does yours have the plug-in board with 3 relays, or the wired-in relay board with one plug-in relay?
73
It has the wired in board with one plug in relay and one of the square soldered in relays, although the square relay had no cover when I got the amp. Caps and diodes were just replaced but I guess there is a possibility one of them has gone out.
The flat relay closest to the plug-in relay is the receiver preamp. And if someone removed it a short wire jumper can take its place. The flat relay farthest from the plug-in socket is the only one that's activated by the radio's carrier. The contacts in that one activate the plug-in relay and the HV relay.
The flat relays were supposed to be sealed, but the glue holding the top cover will degrade and let go. Or it could be that someone removed it to clean the contact points.
If you hear a "click" when keying the radio, but the mechanism inside the plug-in relay doesn't activate, that flat relay is probably the problem.
I would also have a close look at the plug-in relay under a bright light. The clear plastic cover should not have any smudges on the inside that are near the contact points. Likewise, have a look at the color of the exposed metal on the contact lever. If one side is darker than the other, this is a high-mileage relay. The metal will darken over time from heat. Only the output side gets this stress, so the input side lever will show the original color.
Have a look at those details. Let us know what you find.
73
The flat relays were supposed to be sealed, but the glue holding the top cover will degrade and let go. Or it could be that someone removed it to clean the contact points.
If you hear a "click" when keying the radio, but the mechanism inside the plug-in relay doesn't activate, that flat relay is probably the problem.
I would also have a close look at the plug-in relay under a bright light. The clear plastic cover should not have any smudges on the inside that are near the contact points. Likewise, have a look at the color of the exposed metal on the contact lever. If one side is darker than the other, this is a high-mileage relay. The metal will darken over time from heat. Only the output side gets this stress, so the input side lever will show the original color.
Have a look at those details. Let us know what you find.
73