Gec bt310 1959. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 17th Sep 2016, 6:19 pm

I've just started the restoration of this 14" GEC BT310 which was made in 1959. This set is electrically identical to the model BT1156. The only differences between the two models is that the later BT310 has a fake wood grain effect on the cabinet and the CRT mask is painted gold.
As the variac is not at present in the workshop the usual "just plug in" method was adopted for the initial tests. No bangs or flashes and it wasn't long before the valve heaters started to glow and in fact minutes later a very dull raster appeared on the screen although that didn't last very long.
Clicks could be heard when the channel selector knob was rotated so we know the sound circuits are working. Surprisingly, the turret tuner is loaded with only two sets of coils, channel 5 BBC and channel 8 for Tyne-Tees Television.
Resetting the Aurora for channel 5 resulted in having a very loud test tone from the loudspeaker, but still no picture.
The CRT electrode voltages were 100V on the grid 110 on the cathode and only 115V on the first anode, that's too low, should be >350volts.
Clipping off the 0.5mfd boost smoothing capacitor resulted in only a slight increase of first anode volts. Time to look for faults elsewhere in the circuits.
We did have a battle to remove the chassis from the cabinet, the chassis was stuck fast but eventually the chassis yielded to our brutal methods.
It was feared that the CRT might be low emission because it would seem a booster transformer might be fitted. However, there isn't one. There is evidence of some strange servicing practises having been carried out though. Just check out that twin cable connected to the tube base, it is about twelve inches long and simply reconnects back to the heater chain.
For a brief period a decent picture appeared and then disappeared again.
The U25 wired ended EHT rectifier valve has been badly soldered, all three wires have become disconnected. I'll resolder the valve and report my findings later today.

DFWB.

MonochromeMarc · 17th Sep 2016, 7:12 pm

Hi DFWB,

Is that GEC BT310 one of the pair from the Welsh job lot you bought a couple of years ago ?

Marc.

FERNSEH · 17th Sep 2016, 8:35 pm

Hi Marc,
This set was brought in today for restoration by a friend who lives in Sunderland. He is forum member and will be watching the progress of the restoration.
Since my initial post more favourable results. The U25 EHT rectifier has been resoldered so now the EHT has been restored. The EHT volts are rather low at 8KV, at least 12KV will be the correct figure for this set.
The main HT line voltage is low, being only 163V, so it possible the finned metal rectifier has high forward resistance.
The attachment shows some activity on the screen, the line TB oscillator is running on the low side of 10,125c/s.
Information for the BT1156 and BT310 can be found in the 1958/59 Radio and Television servicing book. These sets employ a 90 degree CRT, GEC type 7205A which is really a relabeled Mazda CME1402.
An earlier version of the BT1156 was the BT1155. This set employed a 14" 70 degree CRT. Early BT1155 sets were of completely hand wired construction.

Standby for more progress with the BT310.

DFWB.

DFWB.

MonochromeMarc · 17th Sep 2016, 9:24 pm

Hi DFWB,

So between us and your friend that makes at least 3 of the wee little beasties in circulation

Though I must admit my set could seriously do with a replacement CRT.

Marc.

FERNSEH · 17th Sep 2016, 10:22 pm

Hi Marc,
The CRT in my BT310 isn't all that good either.
The CRT in the set under discussion isn't bad at all, even with the low EHT voltage the picture is surprisingly bright albeit a bit defocussed. No doubt the focus will improve after the HT rectifier is replaced.

DFWB.

FERNSEH · 18th Sep 2016, 10:24 pm

The attachment shows the results of Saturday evening's work.
The only component in the frame timebase which needed replacement was the 1megohm height control.

DFWB.

MonochromeMarc · 18th Sep 2016, 10:48 pm

Hi DFWB,

That's not looking too bad considering the minimal parts count so far.

Have you done the HT rectifier yet ?

Marc.

FERNSEH · 18th Sep 2016, 11:07 pm

Hi Marc,
The HT rectifier will be replaced tomorrow. Also, as an attempt to increase the EHT volts the line drive waveform to the grid of the line output valve will checked. The anode load resistor of the line oscillator valve can change in value.

DFWB.

Focus Diode · 19th Sep 2016, 8:14 am

I was present at the first switch on. It arrived fitted with a 2-pin Ediswan plug suggesting it hadn't been used for years. This of course had to be changed. There was also a curious twin flex and two pin in line socket emerging from the back. First impressions suggested this may have been connected to the mains to allow a table lamp to be connected. In those days power points were few and far between.

It turned out to be connected to the speaker, presumably to allow connecting to a tape recorder. Quite a lethal combination seeing the set's plug was reversible!

Seems very common for the older BI/III models to only have the local coils fitted. I remember a GEC in Devon fitted only with ch2 BBC & 9 Westward (Stockland Hill) coils for example.

Very nice compact and well made set, perfect for the small living room, even now!

Results are looking good already.

Cheers
Brian

FERNSEH · 19th Sep 2016, 11:58 am

The line drive waveform present at the grid of the line output valve is correct being the same amplitude and shape as shown in the service manual, -100V P - P.

DFWB.

FERNSEH · 19th Sep 2016, 8:01 pm

The performance of the set has been improved by replacing the original finned metal rectifier with a silicon diode. The HT voltage is now 205V, up by 40 volts. The EHT is now 11KV, 2KV below the figure quoted in the service manual.
The new rectifier diode along with the surge protection components are mounted on a bracket and is similar in the method of construction of the assembly that was made by Rediffusion to replace the RM4 type of metal rectifier employed in many of the companies' sets.
The attachment shows the original HT rectifier and the replacement assembly.
Components used are a BY127 diode, a 22ohm 7 watt resistor, a 0.1microfarad capacitor and a 0.001 microfarad capacitor.

DFWB.

Focus Diode · 20th Sep 2016, 3:05 am

That's a very neat job indeed. The metal used should act as an excellent heat shield.

I'm not familiar with adding capacitors, presumably these act as protection devices?

Cheers
Brian

Hunts smoothing bomb · 20th Sep 2016, 12:44 pm

Very neat David, maybe the caps snub any spikes from the fast switching rectifier Brian?

Cheers

FERNSEH · 20th Sep 2016, 4:22 pm

Hi Brian, Hi Lee,
Those capacitors might be a case of belt and braces, but the set designers were not taking any chances with the then new silicon diodes failing because of spikes on the mains supply. Looking though the 1963/64 R & T servicing book we find Pye group dual standard receivers employed two capacitors to protect the silicon diode from transient spikes. Also Rediffusion in the Mark 8 cable receiver, in fact in the Mark 8 the silicon HT rectifier was connected into the circuit by using screw terminals, easy replacement without any soldering.
Some makers did not fit a small value capacitor across the diode but nevertheless employed a 0.1mfd capacitor between the anode of the diode and ground.
Even into the early sixties many set designers still preferred to use valves for the HT rectification, for example certain Sobell and Ferguson models.

DFWB.

17th Sep 2016, 6:19 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Gec bt310 1959. I've just started the restoration of this 14" GEC BT310 which was made in 1959. This set is electrically identical to the model BT1156. The only differences between the two models is that the later BT310 has a fake wood grain effect on the cabinet and the CRT mask is painted gold. As the variac is not at present in the workshop the usual "just plug in" method was adopted for the initial tests. No bangs or flashes and it wasn't long before the valve heaters started to glow and in fact minutes later a very dull raster appeared on the screen although that didn't last very long. Clicks could be heard when the channel selector knob was rotated so we know the sound circuits are working. Surprisingly, the turret tuner is loaded with only two sets of coils, channel 5 BBC and channel 8 for Tyne-Tees Television. Resetting the Aurora for channel 5 resulted in having a very loud test tone from the loudspeaker, but still no picture. The CRT electrode voltages were 100V on the grid 110 on the cathode and only 115V on the first anode, that's too low, should be >350volts. Clipping off the 0.5mfd boost smoothing capacitor resulted in only a slight increase of first anode volts. Time to look for faults elsewhere in the circuits. We did have a battle to remove the chassis from the cabinet, the chassis was stuck fast but eventually the chassis yielded to our brutal methods. It was feared that the CRT might be low emission because it would seem a booster transformer might be fitted. However, there isn't one. There is evidence of some strange servicing practises having been carried out though. Just check out that twin cable connected to the tube base, it is about twelve inches long and simply reconnects back to the heater chain. For a brief period a decent picture appeared and then disappeared again. The U25 wired ended EHT rectifier valve has been badly soldered, all three wires have become disconnected. I'll resolder the valve and report my findings later today. DFWB. Attached Thumbnails Last edited by FERNSEH; 17th Sep 2016 at 6:33 pm.

17th Sep 2016, 7:12 pm	#2
MonochromeMarc Pentode Join Date: Jan 2015 Location: Wakefield, West Yorkshire, UK. Posts: 151	Re: Gec bt310 1959. Hi DFWB, Is that GEC BT310 one of the pair from the Welsh job lot you bought a couple of years ago ? Marc.

17th Sep 2016, 8:35 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. Hi Marc, This set was brought in today for restoration by a friend who lives in Sunderland. He is forum member and will be watching the progress of the restoration. Since my initial post more favourable results. The U25 EHT rectifier has been resoldered so now the EHT has been restored. The EHT volts are rather low at 8KV, at least 12KV will be the correct figure for this set. The main HT line voltage is low, being only 163V, so it possible the finned metal rectifier has high forward resistance. The attachment shows some activity on the screen, the line TB oscillator is running on the low side of 10,125c/s. Information for the BT1156 and BT310 can be found in the 1958/59 Radio and Television servicing book. These sets employ a 90 degree CRT, GEC type 7205A which is really a relabeled Mazda CME1402. An earlier version of the BT1156 was the BT1155. This set employed a 14" 70 degree CRT. Early BT1155 sets were of completely hand wired construction. Standby for more progress with the BT310. DFWB. DFWB. Attached Thumbnails

17th Sep 2016, 9:24 pm	#4
MonochromeMarc Pentode Join Date: Jan 2015 Location: Wakefield, West Yorkshire, UK. Posts: 151	Re: Gec bt310 1959. Hi DFWB, So between us and your friend that makes at least 3 of the wee little beasties in circulation Though I must admit my set could seriously do with a replacement CRT. Marc.

17th Sep 2016, 10:22 pm	#5
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. Hi Marc, The CRT in my BT310 isn't all that good either. The CRT in the set under discussion isn't bad at all, even with the low EHT voltage the picture is surprisingly bright albeit a bit defocussed. No doubt the focus will improve after the HT rectifier is replaced. DFWB.

18th Sep 2016, 10:24 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. The attachment shows the results of Saturday evening's work. The only component in the frame timebase which needed replacement was the 1megohm height control. DFWB. Attached Thumbnails

18th Sep 2016, 10:48 pm	#7
MonochromeMarc Pentode Join Date: Jan 2015 Location: Wakefield, West Yorkshire, UK. Posts: 151	Re: Gec bt310 1959. Hi DFWB, That's not looking too bad considering the minimal parts count so far. Have you done the HT rectifier yet ? Marc.

18th Sep 2016, 11:07 pm	#8
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. Hi Marc, The HT rectifier will be replaced tomorrow. Also, as an attempt to increase the EHT volts the line drive waveform to the grid of the line output valve will checked. The anode load resistor of the line oscillator valve can change in value. DFWB.

19th Sep 2016, 8:14 am	#9
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: Gec bt310 1959. I was present at the first switch on. It arrived fitted with a 2-pin Ediswan plug suggesting it hadn't been used for years. This of course had to be changed. There was also a curious twin flex and two pin in line socket emerging from the back. First impressions suggested this may have been connected to the mains to allow a table lamp to be connected. In those days power points were few and far between. It turned out to be connected to the speaker, presumably to allow connecting to a tape recorder. Quite a lethal combination seeing the set's plug was reversible! Seems very common for the older BI/III models to only have the local coils fitted. I remember a GEC in Devon fitted only with ch2 BBC & 9 Westward (Stockland Hill) coils for example. Very nice compact and well made set, perfect for the small living room, even now! Results are looking good already. Cheers Brian Last edited by Focus Diode; 19th Sep 2016 at 8:23 am.

19th Sep 2016, 11:58 am	#10
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. The line drive waveform present at the grid of the line output valve is correct being the same amplitude and shape as shown in the service manual, -100V P - P. DFWB.

19th Sep 2016, 8:01 pm	#11
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. The performance of the set has been improved by replacing the original finned metal rectifier with a silicon diode. The HT voltage is now 205V, up by 40 volts. The EHT is now 11KV, 2KV below the figure quoted in the service manual. The new rectifier diode along with the surge protection components are mounted on a bracket and is similar in the method of construction of the assembly that was made by Rediffusion to replace the RM4 type of metal rectifier employed in many of the companies' sets. The attachment shows the original HT rectifier and the replacement assembly. Components used are a BY127 diode, a 22ohm 7 watt resistor, a 0.1microfarad capacitor and a 0.001 microfarad capacitor. DFWB. Attached Thumbnails Last edited by FERNSEH; 19th Sep 2016 at 8:27 pm.

20th Sep 2016, 3:05 am	#12
Focus Diode Octode Join Date: Jan 2006 Location: Borough of Gateshead, UK. Posts: 1,420	Re: Gec bt310 1959. That's a very neat job indeed. The metal used should act as an excellent heat shield. I'm not familiar with adding capacitors, presumably these act as protection devices? Cheers Brian

20th Sep 2016, 12:44 pm	#13
Hunts smoothing bomb Octode Join Date: Oct 2005 Location: Wimborne, Dorset, UK. Posts: 1,407	Re: Gec bt310 1959. Very neat David, maybe the caps snub any spikes from the fast switching rectifier Brian? Cheers __________________ Lee

20th Sep 2016, 4:22 pm	#14
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Gec bt310 1959. Hi Brian, Hi Lee, Those capacitors might be a case of belt and braces, but the set designers were not taking any chances with the then new silicon diodes failing because of spikes on the mains supply. Looking though the 1963/64 R & T servicing book we find Pye group dual standard receivers employed two capacitors to protect the silicon diode from transient spikes. Also Rediffusion in the Mark 8 cable receiver, in fact in the Mark 8 the silicon HT rectifier was connected into the circuit by using screw terminals, easy replacement without any soldering. Some makers did not fit a small value capacitor across the diode but nevertheless employed a 0.1mfd capacitor between the anode of the diode and ground. Even into the early sixties many set designers still preferred to use valves for the HT rectification, for example certain Sobell and Ferguson models. DFWB.