Gec Bc 4855 - UK Vintage Radio Repair and Restoration Discussion Forum

yestertech · 18th Jul 2008, 12:51 am

I have had a couple of the similar 4850 sets thro' the workshop , which have promptly been
snatched away !!
I may try and keep this one, as, unusually, all the RED print on the dial was intact !

References ( thus ) are to the "parts pile" in the second pic.

On powering up, there was a constant ferocious hum ; a sure sign that the large main smoother was shot. Once this was replaced ( top left ) things were quieter, but it still hummed like mad - I put in a much large value of cap for good measure, however this can lead to the U31 rectifier doing a self destruct, so a series resistor was added - this limits the charging current.
Some hum remained, but this is all too often the case in ACDC sets, as the Ht is only half wave rectified

There was some weak reception, which gradually improved as I worked my way through the dreaded wax capacitors ( top right )
Surprisingly, the IF response was spot on and needed no tweaking
As the reception improved, I decided to try and eradicate the rest of the hum. A H-K leak in the KT33C was suspected, but this proved not to be the case.
There was only one sure fire way to ensure a quiet background and that was to check the remaining smoothers. I do this with an ESR meter which checks the internal resistance of the cap. TO my surprise, all the readings were ZERO ( = no cap left, all dried up )
These were then all replaced ( bottom left of pic) which produced a wonderfully quiet set at last !

I could then concentrate on getting reception fully up to scratch. It was selective enough but still quiet.
I took out the ESR meter to measure the remaining electrolytics, which are in the audio circuits and can affect the gain. These don't normally give too much trouble, even if the values are a bit off.
Again, the ESR meter read zero, so out they came ! ( bottom centre )

On powering up again, the result was a good loud signal on all bands, with virtually no background hum
A far cry from the initial power up the day before.

There only remained the volume control to replace. This was either "on" or "off"
It has elaborate arrnagements to ensure the spindle is earthed, but via a cap to protect against potential shock shoudl the knob become detatched. A plastic spindle replacement rendered it all unnecessary.

As a final 'adjustment' I noticed that the dial bulbs suffered from a large switch on surge. With no thermistor, these would soon blow and as there is no bypass, render the set dead. I thought about a bypass resistance, but if the bulbs blew, then this resistor would fry !
I then hit on the solution. I fitted 2 No. 6v 0.5 Amp bulbs with large globes. These have thicker filaments and the surge was virtually nil. As they burn a bit more dimly, they will probably last ages !

So that was that - I left it on soak test whilst I did a few other repairs. It seems fine. The only problem seems to be the heat generated by the Barreter. This makes the metal filler section inserted to replace the damaged back panel red hot, so fingers are best kept away !!

Andy

Colin · 18th Jul 2008, 10:21 am

Nice restoration Andy, I really like the look of these GEC sets. I have a 4650 in the queue and I thing you've given me the nudge I needed to get it sorted out!

Well done!

howard · 18th Jul 2008, 10:41 am

Yes that is a good looking set, well done Andy

Howard

Nickthedentist · 18th Jul 2008, 6:16 pm

Beautiful set and sympathetically restored, Andy, excellent work again!

Tim · 18th Jul 2008, 8:27 pm

Nice resto Andy. What a cute little set!

yestertech · 19th Jul 2008, 9:08 am

many thanks for the comments folks - Yes these are great looking 'curvy' sets - which is why I keep "losing" them to fellow collectors !
Off to finish the 462A now...
A.

18th Jul 2008, 12:51 am	#1
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,131	Gec Bc 4855 I have had a couple of the similar 4850 sets thro' the workshop , which have promptly been snatched away !! I may try and keep this one, as, unusually, all the RED print on the dial was intact ! References ( thus ) are to the "parts pile" in the second pic. On powering up, there was a constant ferocious hum ; a sure sign that the large main smoother was shot. Once this was replaced ( top left ) things were quieter, but it still hummed like mad - I put in a much large value of cap for good measure, however this can lead to the U31 rectifier doing a self destruct, so a series resistor was added - this limits the charging current. Some hum remained, but this is all too often the case in ACDC sets, as the Ht is only half wave rectified There was some weak reception, which gradually improved as I worked my way through the dreaded wax capacitors ( top right ) Surprisingly, the IF response was spot on and needed no tweaking As the reception improved, I decided to try and eradicate the rest of the hum. A H-K leak in the KT33C was suspected, but this proved not to be the case. There was only one sure fire way to ensure a quiet background and that was to check the remaining smoothers. I do this with an ESR meter which checks the internal resistance of the cap. TO my surprise, all the readings were ZERO ( = no cap left, all dried up ) These were then all replaced ( bottom left of pic) which produced a wonderfully quiet set at last ! I could then concentrate on getting reception fully up to scratch. It was selective enough but still quiet. I took out the ESR meter to measure the remaining electrolytics, which are in the audio circuits and can affect the gain. These don't normally give too much trouble, even if the values are a bit off. Again, the ESR meter read zero, so out they came ! ( bottom centre ) On powering up again, the result was a good loud signal on all bands, with virtually no background hum A far cry from the initial power up the day before. There only remained the volume control to replace. This was either "on" or "off" It has elaborate arrnagements to ensure the spindle is earthed, but via a cap to protect against potential shock shoudl the knob become detatched. A plastic spindle replacement rendered it all unnecessary. As a final 'adjustment' I noticed that the dial bulbs suffered from a large switch on surge. With no thermistor, these would soon blow and as there is no bypass, render the set dead. I thought about a bypass resistance, but if the bulbs blew, then this resistor would fry ! I then hit on the solution. I fitted 2 No. 6v 0.5 Amp bulbs with large globes. These have thicker filaments and the surge was virtually nil. As they burn a bit more dimly, they will probably last ages ! So that was that - I left it on soak test whilst I did a few other repairs. It seems fine. The only problem seems to be the heat generated by the Barreter. This makes the metal filler section inserted to replace the damaged back panel red hot, so fingers are best kept away !! Andy Attached Thumbnails

18th Jul 2008, 10:21 am	#2
Colin Heptode Join Date: Jun 2004 Location: Ilkley, West Yorkshire, UK. Posts: 656	Re: Gec Bc 4855 Nice restoration Andy, I really like the look of these GEC sets. I have a 4650 in the queue and I thing you've given me the nudge I needed to get it sorted out! Well done! __________________ Colin Armstrong

18th Jul 2008, 10:41 am	#3
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Gec Bc 4855 Yes that is a good looking set, well done Andy Howard

18th Jul 2008, 6:16 pm	#4
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,846	Re: Gec Bc 4855 Beautiful set and sympathetically restored, Andy, excellent work again!

18th Jul 2008, 8:27 pm	#5
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Gec Bc 4855 Nice resto Andy. What a cute little set! __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

19th Jul 2008, 9:08 am	#6
yestertech Nonode Join Date: Dec 2002 Location: Coulsdon, Surrey, UK. Posts: 2,131	Re: Gec Bc 4855 many thanks for the comments folks - Yes these are great looking 'curvy' sets - which is why I keep "losing" them to fellow collectors ! Off to finish the 462A now... A.