SB 220 Went Bang, No Smoke

[FlatlandBusa]

I was running my SB 220 on 20 meters last night for 3 hours straight, approximately 15 minutes after the last keyup an arcing noise was heard and the circuit breakers opened. The short is still there with the tubes pulled.

Nothing is visibly burnt and there is no burnt electronic smell (even had the wife come take a sniff).

I'm leaning towards the plate transformer or capacitor bank, going to look at the wiring diagram tonight and see if I can figure out where to unhook the high voltage to isolate the issue to the transformer.

The SB 220 has the Harbach diode board and capacitor bank already installed.

 

[BJ radionut]

The HV VAC from transformer secondary is the Red/(YEL) should be going to center of Cap board (screw terminal?) RED wire to diode board...Remove those two wires and you have the HVAC lines!!!
You need a GOOD voltmeter capable of reading at least 2000 volts AC/DC...Best quality leads...gloves/googles etc...Thats like 1,500 (1200-1300) VOLTS AC when energized.
If you have never done this type of work before, I would suggest you don't.
Be Safe

 

[BJ radionut]

https://harbachelectronics.com/wp-content/uploads/2020/05/ES4SB220-v1.0.pdf

https://harbachelectronics.com/wp-content/uploads/2023/06/RM-220-v3.3.pdf

 

[Shadetree Mechanic]

With the power unplugged, you should be able to check the resistance of the transformer primary and secondary. It should be approximately 10 to 1 step up. Also check resistance from primary to secondary, it should have no continuity.

 

[linearone]

The HV VAC from transformer secondary is the Red/(YEL) should be going to center of Cap board (screw terminal?) RED wire to diode board...Remove those two wires and you have the HVAC lines!!!
You need a GOOD voltmeter capable of reading at least 2000 volts AC/DC...Best quality leads...gloves/googles etc...Thats like 1,500 (1200-1300) VOLTS AC when energized.
If you have never done this type of work before, I would suggest you don't.
Be Safe

More like 800-900vac

 

[FlatlandBusa]

I think I found my issue. The plate transformer yellow/red wire to the red wire is only 10 ohms. Also tested the high voltage capacitors with my LCR 45 and one is waaay out of range compared to the others.

Thank you for the replies that got me pointed in a good direction.

 

[linearone]

I think I found my issue. The plate transformer yellow/red wire to the red wire is only 10 ohms. Also tested the high voltage capacitors with my LCR 45 and one is waaay out of range compared to the others.

Thank you for the replies that got me pointed in a good direction.

I would bet a close inspection of the caps once removed from the pcb would show a tattletale black mark somewhere.
Disconnect the AC leads and secure them out of the way. Maybe with some electrical tape on them. Turn the supply on and you shouldn't have any issues if the caps are the issue. If you still get a short the transformer is bad

 

[linearone]

I would bet a close inspection of the caps once removed from the pcb would show a tattletale black mark somewhere.
Disconnect the AC leads and secure them out of the way. Maybe with some electrical tape on them. Turn the supply on and you shouldn't have any issues if the caps are the issue. If you still get a short the transformer is bad

Good 3m tape is rated for 600v per layer, a few good layers on each end will insulate it.

 

[FlatlandBusa]

The short goes away with the HV leads unhooked. One of the HV capacitors is shorted. Measuring it with the DVOM in the ohms position, it stays at 3.6 ohms, the rest of the capacitors start at a low resistance then go open as the DVOM charges them. This was the upgraded capacitor bank from Harbach. https://harbachelectronics.com/shop/heathkit-sb220-sb221/fb-220-replacement-filter-capacitor-block/ Is there a better capacitor upgrade for the SB 220? This is my first tube type linear so the learning curve is steep, I think I jesus sticked the high voltage circuit about 10 times before touching anything.

 

[linearone]

The short goes away with the HV leads unhooked. One of the HV capacitors is shorted. Measuring it with the DVOM in the ohms position, it stays at 3.6 ohms, the rest of the capacitors start at a low resistance then go open as the DVOM charges them. This was the upgraded capacitor bank from Harbach. https://harbachelectronics.com/shop/heathkit-sb220-sb221/fb-220-replacement-filter-capacitor-block/ Is there a better capacitor upgrade for the SB 220? This is my first tube type linear so the learning curve is steep, I think I jesus sticked the high voltage circuit about 10 times before touching anything.

Replace the caps and bleeders. If im not mistaken they are either 180 or 200uf 450v or 500v caps. Use 100kohm 5w resistors for bleeders.

 

[FlatlandBusa]

Replace the caps and bleeders. If im not mistaken they are either 180 or 200uf 450v or 500v caps. Use 100kohm 5w resistors for bleeders.

Thanks for the reply. The HV capacitors installed were already the 210uf 450 volt US made ones from Harbach. Pretty disappointed that one cap already shorted with less than 10 hours of talk time. Hope the failed cap was an anomaly.

 

[linearone]

Thanks for the reply. The HV capacitors installed were already the 210uf 450 volt US made ones from Harbach. Pretty disappointed that one cap already shorted with less than 10 hours of talk time. Hope the failed cap was an anomaly.

Id call jeff and see if hed warranty it. He likely will, hes a reasonable guy.

 

[FlatlandBusa]

To update this: this sb 220 is running good again. A new HV capacitor bank and board is what it took. Thankfully the meters didn't get hurt when it went bang. All of the rectifying diodes also tested good. 70 watts drive makes 1500+ out on the lower bands.
Thank you everyone who replied and got me heading in the correct direction.

 

[nomadradio]

Rock on! Good to hear you got it going.

I suggest the wisdom of protection diodes on the meters.

A parallel pair of rectifier diodes are twisted together, each pointing the opposite direction. This parallel pair is connected to the two terminals on each meter.

The diodes will hopefully absorb fault currents if one grid wire in a tube comes loose and touches the cathode, or an arc to ground occurs inside the HV transformer around the 45-year mark.

There are no cheap choices to replace a blown meter. Rectifier diodes in comparison are incredibly cheap. That amplifier's grid meter is vulnerable to either of those faults. With any luck it will be a long time before either one happens.

73

 

[FlatlandBusa]

Rock on! Good to hear you got it going.

I suggest the wisdom of protection diodes on the meters.

A parallel pair of rectifier diodes are twisted together, each pointing the opposite direction. This parallel pair is connected to the two terminals on each meter.

The diodes will hopefully absorb fault currents if one grid wire in a tube comes loose and touches the cathode, or an arc to ground occurs inside the HV transformer around the 45-year mark.

There are no cheap choices to replace a blown meter. Rectifier diodes in comparison are incredibly cheap. That amplifier's grid meter is vulnerable to either of those faults. With any luck it will be a long time before either one happens.

73

Thank for this information! I'm pretty sure the meters have the diodes already on them, I'll double check the next time I have the covers off.