I have 2 SB220 amplifiers. SB220 number one when I put some drive to it every now and then it sparks at tune capacitor. It will make a hum or spark and meter shows 2000 low and 3000 high . It does not spark or hum every time and works great other wise.... Any suggestions to problem????? The other one number 2 with same drive just works like it should but the right hand meter acts up some times and does not show any power on it but amp working. That same meter when it is working show 1850 volt on low and 2400 volt on high. I have not taken it apart to actually check voltage yet. What do i need to look for????? Thanks
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SB220 voltage and tune sparking.
- Thread starter Jimbo165
- Start date
I was using and testing a SB-220 that I repaired from a local CBer,
on 17 meters. It was sparking sometimes also, just like yours, so
I did the following to stop this problem, as in any of these
tube amplifiers: after tuning and peaking the Pi-network
output, turn the load capacitor to a lower capacitance, turn it
maybe 1/8 turn to open it more for less capacitance, and retune and
peak the tune capacitor. The tune capacitor will peak at a lower
capacitance also. Now there will be a lower impedance load
at the plates, the amplifier will have less gain. The high voltage
peaks will be less, hopefully enough to prevent the spark over.
on 17 meters. It was sparking sometimes also, just like yours, so
I did the following to stop this problem, as in any of these
tube amplifiers: after tuning and peaking the Pi-network
output, turn the load capacitor to a lower capacitance, turn it
maybe 1/8 turn to open it more for less capacitance, and retune and
peak the tune capacitor. The tune capacitor will peak at a lower
capacitance also. Now there will be a lower impedance load
at the plates, the amplifier will have less gain. The high voltage
peaks will be less, hopefully enough to prevent the spark over.
on your other SB-220, the problem could be in the switch
contacts for the metering circuits.
also, the band switch contacts go bad. For the local CB guy here,
we took out the band switch and just have a 27 MHz output circuit.
also, when Zener diode ZD1 is blown up, and either shorted out,
or blown open: just get about 7 or 8 high voltage 1 amp or more
rectifier diodes, and solder them all in series on a board and wire it up
for forward current flow in the cathode circuit. A replacement
Zener there is about $30. This has been done around here
on a local CBer's Kenwood TL-922 also. This seems to be
more "robust." As we all know, they just abuse their amps!
contacts for the metering circuits.
also, the band switch contacts go bad. For the local CB guy here,
we took out the band switch and just have a 27 MHz output circuit.
also, when Zener diode ZD1 is blown up, and either shorted out,
or blown open: just get about 7 or 8 high voltage 1 amp or more
rectifier diodes, and solder them all in series on a board and wire it up
for forward current flow in the cathode circuit. A replacement
Zener there is about $30. This has been done around here
on a local CBer's Kenwood TL-922 also. This seems to be
more "robust." As we all know, they just abuse their amps!
This is the solution to the problem. The problem is, where the arcing is taking place on the tune capacitor. That can start by simply having dust or debris in between the plates of the capacitor. Once the arc starts it will pit the plate and cause a prime location for the arc to continue, even after the dust is removed. Clean the tuner and try to smooth off any pits in the aluminum plates. The smoother and more mirror like you can make this finish, the less likely arcing is to occur. These variable capacitors can actually be completely disassembled, if you're having difficulty reaching a damaged plate. If you do that, be careful not to apply uneven pressure on the porcelain insulators when screwing it back together. They can crack if you're not cautious to how much torque and how even you screw them in. Also, pay attention to the small ball bearing on the back of the shaft. It can easily fall off during disassembly and get lost.
The voltage that's needed to strike an arc across the air gaps in the capacitor is higher when the plate surfaces are smooth. The spot where the arc occurred will now be disrupted and rough with pointy features on the surface.
The reason the tip of a lightning rod is sharpened is to make it break down and arc at a lower voltage than the objects around it. The rough surface on a capacitor plate where the arc occurred will now break down and arc over at a much-lower power level than when it was still nice and smooth.
Kinda like a snowball rolling downhill. Every time that capacitor arcs over, the wattage needed to make it arc the next time will be lower than last time.
73
The reason the tip of a lightning rod is sharpened is to make it break down and arc at a lower voltage than the objects around it. The rough surface on a capacitor plate where the arc occurred will now break down and arc over at a much-lower power level than when it was still nice and smooth.
Kinda like a snowball rolling downhill. Every time that capacitor arcs over, the wattage needed to make it arc the next time will be lower than last time.
73
Some of them. The capacitors that have the plates stacked onto a shaft with spacer rings can be disassembled. But Heathkit used capacitors that had the rotor and stator plates staked permanently in place in later production. Which version of each air-variable cap your amplifier has is a roll of the dice.
73
Replace the tune and load caps. Problem solved.
You have a 50 yead old amp that arced a cap or two. Consider yourself fortunate.
Rf parts has them.
You have a 50 yead old amp that arced a cap or two. Consider yourself fortunate.
Rf parts has them.
Easy enough to determine. If you have a brass nut compressing the plates on each end of the rotor shaft, that one can be disassembled. Otherwise, the only way to use this cap, may be to carefully rip out the defective plate with needle nose pliers, and probably say goodbye to low end 80 meter coverage.