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Old 4th Dec 2017, 10:14 am   #1
music-centre
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Default GEC BC 4750 Fault

Hello All,
I am getting a strange fault on this GEC BC4750 that I wondered if anyone else had experienced.
I changed the faulty HT Smoothing and output valve grid coupling caps and powered it up, even without an aerial it was getting the strongest MW Stations quite well - this is going to be an easy one I thought (mistake)

The weird thing is connecting up the usual 15ft of wire that I use actually makes reception worse, or at best on some stations a very slight improvement, on many it drops the volume level.

I noticed by accident that a finger, or the length of wire, touched on the SW Aerial trimmer (C26 on trader sheet 919 ) makes reception quite good on MW with plenty of volume.
I thought possibly switching problems so have cleaned the wave change switches with no improvement.
I think I had something similar with a GEC BC4850 years ago and never found out what it was!
Cheers,
Steve.
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Old 4th Dec 2017, 11:30 am   #2
Silicon
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Default Re: GEC BC 4750 Fault

It could be a high resistance connection or short on one of the trimmer capacitors.

Try moving the adjustment screw and then return it to the original position.
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Old 4th Dec 2017, 11:31 am   #3
Radio Tech
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Default Re: GEC BC 4750 Fault

Hi Steve

it might be worth checking to see if any of the wafer switches have defective contacts and some of the coupling and de-coupling capacitors. I personally have not come across this with sets but there it is.

Best wishes

Ken
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Old 4th Dec 2017, 12:05 pm   #4
ukcol
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Default Re: GEC BC 4750 Fault

Quote:
Originally Posted by Radio Tech View Post
....and some of the coupling and de-coupling capacitors.
Yes, change ALL the paper dielectric capacitors. They will all be leaky to a greater or lesser degree. Leaky decouples can alter the DC conditions on valves causing various symptoms and the AGC function can also be defeated by leaky capacitors.

I always change all the paper capacitors in a set and they always test leaky at their working voltage.
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