Mini Scout

[238]

Have run this amp off & on far 10 plus years. The other nite it started to chatter when i would key. Was going to clean the relays, but only way you can that is remove the relay. You can turn the variable a little bit it will work okok, then do it again. Just asking about possible problems before i pull the relays out. Wondering about the 2.2K resister soldered across the legs of the variable? Jim/238

 

[Shockwave]

Have run this amp off & on far 10 plus years. The other nite it started to chatter when i would key. Was going to clean the relays, but only way you can that is remove the relay. You can turn the variable a little bit it will work okok, then do it again. View attachment 41489 Just asking about possible problems before i pull the relays out. Wondering about the 2.2K resister soldered across the legs of the variable? Jim/238

Sounds more like an issue with the keying circuit rather than the relay. I think this unit uses another small tube like the 6AQ5 to key the 110 volt DC relay. If it has that small tube, check or replace it.

 

[Tokin]

@nomadradio just posted a thread where he modified a tube type keying circuit on another amp. He may have some suggestions for this one. Pics please.

 

[238]

The early JB 12 & 150 had a keying tube. Do sure if a Mini Scout ever did. I ran it for a little while last nite never messed upped. Jim/238

 

[238]

I was wondering more about the resister that was soldered across the legs of the variable. I have seen caps soldered across, but not resisters.

 

[Shockwave]

I was wondering more about the resister that was soldered across the legs of the variable. I have seen caps soldered across, but not resisters.

That resistor is common on these grid driven amplifiers. It is across the coil winding feeding the grid and establishes the grid impedance. The only reason the problem changes with the variable is because that is changing the input VSWR too. When the match is poor, you're not getting enough drive into the keying circuit to hold the relay closed.

 

[Staybolt]

I had a super scout in the 1990s. Damn thing was mean and bled every tv around.

 

[238]

When i ran this amp before i used a Sonar 2340. Never had a problem. Running a MKll now. The resister is a 2.2K. You can tell it was changed the rest look original. Not sure what the original value was? Jim/238

 

[nomadradio]

The 2.2k resistor is called a "swamping" resistor. It serves to soak up unwanted RF feedback so the tube will not oscillate from the feedback. Some amount of the tube's output will leak back into its input. That resistor keeps it under control.

A SWR meter and coax jumper between the radio and the amplifier will reveal what's really going on when that capacitor is adjusted. The radio is not driving the antenna when the amplifier is keyed, it's driving the input coil and tubes. When the input capacitor is set for minimum SWR on the meter between it and the radio you should get a minimum of chatter. When that SWR is high, this reduces the sensitivity of the keying tube.

Keying tube could be getting weak. That would reduce its sensitivity to the radio's carrier.

One unadvertised problem with a tube-type keying circuit is that it never really switches the relay's coil current completely off. Just reduces it below the relay's "drop out" current when you unkey. The keying tube carries that "standby" current all the time it's powered up. Eventually it will wear out like any other tube even if you never key it.

Eventually.

See if setting that cap for minimum input-side SWR doesn't help.

73

 

[238]

My mini Scout does not have a keying tube. Also i have no sch on a mini Scout. As you can see the resister has been replaced all the others look original. So i am not sure what ohm resister came in it. The resister on the right, goes between the relay an the coil, reads 68 ohm should be 43. Thanks Jim/238

 

[238]

So the variable is not a true variable. You use that to match the radio to amp. I will put a SWR meter in between tonight an check it out. Jim/238

 

[Shockwave]

It's the same input circuit used on the JB-150. Original resistor was a 1.5k 2 watt. Raising it to 2.2k causes the grid impedance to be a little higher and a little more RF drive voltage to develop across that resistor. The difference here is small and would not impact the keying relay. Chances are the small value cap feeding RF into the keying transistor needs some more pf to hold the relay keyed, regardless of the position on the input cap.

 

[238]

l Had a problem with my Mini Scout keying up. Checked every thing out. finely just took it out of line. Talking on just the radio, Browning MKll after a while it did not key. When i pushed the spot it would start keying only done it a couple of times. What is the connection. Could it be the relay? The problem might have been the radio the hold time. Jim/238

 

[238]

Well i changed the 5763 & the 6GH8. The 6GH8 grid tested all the way over in the red. I put the scout back in line an every thing is working good. I did learn a lot about my mini scout. Thanks for all the help. Jim/238

 

[Shockwave]

The 6GH8A is the crystal oscillator and buffer for this radio. When it becomes weak, it may not start to oscillate on every channel. Things like hitting the spot button or changing the channel may get it to start but it still may not be stable, have weak signal or just drop out again while keyed. Sounds like you found the solution to both problems here.