KB FB10 crackling - UK Vintage Radio Repair and Restoration Discussion Forum

vinrads · 23rd Dec 2013, 1:08 pm

Crackling even at low volume tried the valves, tried disconnecting HT from the wave change switch, tried the output TX. All to no avail. I was thinking the auto TX, but before I went down that path I had a good old tap round. Strange thing tapping the two AC feed resistors in parallel caused the noise to alter, so out they came. One read O/C whilst the other read about 3k ohm. A new wire wound cured the problem

Mick.

FERNSEH · 4th Apr 2014, 12:10 am

I was recently given this bright red FB10. It is a late production model employing an EZ80/6V4 as the HT rectifier and a 6BW6 output valve.
It was easy enough job to get it going, the usual capacitor replacement and realignment of the IFs and aerial and oscillator trimmers.

Not long after the restoration job was completed the set began to crackle. I noticed that the screen grid voltage of the 6BJ6 and 6BE6 was fluctuating slightly. The oscilloscope was hooked up to the oscillator grid of the frequency changer. When the set was crackling the sine wave amplitude varied considerably, the positive going tips of the sine wave voltage are clamped to ground. Switch off, remove the 6BE6 and measure the value of the 22Kohm grid resistor, the meter reading was 11Kohms. The resistor checked OK as did the grid coupling capacitor. So what is causing the leakage? After cleaning the tuning capacitor the leakage problem was solved. The oscillator voltage at GI of the 6BE6 is steady over the medium waveband @ >15volts peak to peak.
Realignment of the front end was needed to peak up performance at the HF end of the medium waveband.

DFWB.

23rd Dec 2013, 1:08 pm	#1
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,764	KB FB10 crackling Crackling even at low volume tried the valves, tried disconnecting HT from the wave change switch, tried the output TX. All to no avail. I was thinking the auto TX, but before I went down that path I had a good old tap round. Strange thing tapping the two AC feed resistors in parallel caused the noise to alter, so out they came. One read O/C whilst the other read about 3k ohm. A new wire wound cured the problem Mick.

4th Apr 2014, 12:10 am	#2
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: KB FB10 crackling I was recently given this bright red FB10. It is a late production model employing an EZ80/6V4 as the HT rectifier and a 6BW6 output valve. It was easy enough job to get it going, the usual capacitor replacement and realignment of the IFs and aerial and oscillator trimmers. Not long after the restoration job was completed the set began to crackle. I noticed that the screen grid voltage of the 6BJ6 and 6BE6 was fluctuating slightly. The oscilloscope was hooked up to the oscillator grid of the frequency changer. When the set was crackling the sine wave amplitude varied considerably, the positive going tips of the sine wave voltage are clamped to ground. Switch off, remove the 6BE6 and measure the value of the 22Kohm grid resistor, the meter reading was 11Kohms. The resistor checked OK as did the grid coupling capacitor. So what is causing the leakage? After cleaning the tuning capacitor the leakage problem was solved. The oscillator voltage at GI of the 6BE6 is steady over the medium waveband @ >15volts peak to peak. Realignment of the front end was needed to peak up performance at the HF end of the medium waveband. DFWB. Attached Thumbnails