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Author Topic: Help with SB 101  (Read 322 times)

K4NFX

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Help with SB 101
« on: December 12, 2021, 11:00:15 AM »

I just replaced a capacitor (12 Pf 500v 5%) that connects from the neutralizing veritable capacitor to the final tune capacitor. This capacitor had a broken lead where it connected to the capacitor. I hoped this would fix a problem I had with tuning (plate current would not dip it just in creased as power peaked 325 ma and no grid current on the meter). When I turned it on in receiving mode there is a hum in the audio and the driver tube is glowing violet, HV is about 800, no plat current or grid. I get a loud static pop from the speaker the HV drops to 775 and there is about 50 ma plate. Then there is a click and the voltage goes back to 800 and no plate. I have not set it to tune receive only.

Hope this makes sense I’m new at tube stuff and this gets out of my comfort zone.
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ZL1BBW

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Re: Help with SB 101
« Reply #1 on: December 12, 2021, 12:03:19 PM »

If the driver has gone a nice shade of violet, would suggest it has gone soft for some reason.  Do you have a spare to change it out?
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ex MN Radio Officer, Portishead Radio GKA, BT Radio Amateur Morse Tester.  Licensed as G3YCP ZL1DAB, now taken over my father (sk) call as ZL1BBW.

W9IQ

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Re: Help with SB 101
« Reply #2 on: December 12, 2021, 12:15:45 PM »

I just replaced a capacitor (12 Pf 500v 5%) that connects from the neutralizing veritable capacitor to the final tune capacitor.

Was the radio working otherwise prior to this capacitor failure due to a broken lead?

Where did you acquire the replacement capacitor? What part number? I suspect that the voltage/ESR rating is not sufficient and it is arcing internally or that you have a bad solder joint.

- Glenn W9IQ
« Last Edit: December 12, 2021, 12:20:04 PM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

K4NFX

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Re: Help with SB 101
« Reply #3 on: December 12, 2021, 12:50:02 PM »

ZL1BBW-I may have a tube but it would be 22 years old. I got some tubes with a Collins KWM-2 I bought 5 years ago, it hasn’t been lit up in 22 or 23 years. I will look but am not sure about using it if I have one.

Glenn- I bought this radio at a hamfest to learn on. There has always been a problem tuning.  The main issue was high plate current so I suspect the capacitor has been bad for a while with the lead broken the circuit from the final plate to the driver plat was open and I could never read grid current. Seems like something is shorting or overloading now.  I just haven’t learned enough about it yet to fully understand it.
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KD0REQ

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Re: Help with SB 101
« Reply #4 on: December 15, 2021, 02:05:36 PM »

drivers are eaten like peanuts in the HW/SB radios. hope you can test your spare. also check the socket voltages before you go active with a new good tube.  the 22K resistors in those Heathkits go way high due to age and humidity, and there is a 68K there that was also useless in the HW100 bench queen I've got. upsize the wattage rating on those, they are worked pretty hard.
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