Sobell T347 - UK Vintage Radio Repair and Restoration Discussion Forum

beltinge bore · 16th Apr 2014, 8:52 pm

Hello My first post for some time.
My knowledge and equipment is limited but I usually muddle through.
I acquired this old girl from Ebay for £3-70p to use as spares backup for my nice T347 that I managed to restore last year.It had a broken implosion guard and signs of very damp storage (rusty chassis/ mountings/cabinet damage etc) The fine tuner shaft had seized onto the channel selector shaft.The aquadag coating is very wrinkly & the earth spring had rusted through & the aerial socket was missing.
The frame section had some small hunts caps hanging in mid air & some odd looking replacements.The LOPT screening can was loose & the PL81 was missing.
Well I was looking for something to do so I had a go at it (as you do).
Fired up after usual checks etc .At about 180v strained line whistle & speaker crackle appeared .After a bit of fiddling & fettling with the Aurora I got weak sound & after changing caps in the line area(boost etc) a very weak low height scrambled raster.
Carried on De Hunting methodically I now have a very weak but synced picture with good linearity & good sound.
The EY51 PL81 & PY81 have been changed.The contrast control has no effect and to get the best picture the sensitivity control has to be set to minimum.The picture dose not balloon and fade when the brightness is advanced it just fades away.The left side of the picture has slight fold over.
My thoughts are low emission tube but the EHT measures only 7.5kv on my crude tester with the anode cap on or off the tube.
LOPT EHT overwind failure also seems a possibility.
HT seems ok measured at smoothers.A1 is about 400v.
Any advice would be most welcome.
Thanks for reading Regards Steve.

beltinge bore · 18th Apr 2014, 4:32 pm

No replies so back to my old Spreadbury books.Thanks for looking.
Regards Steve.

FERNSEH · 18th Apr 2014, 9:16 pm

At 7.5KV the EHT is too low for a 14" CRT. you should be looking for >11KV. Information for this set can be found in the 1956/57 Newnes Radio and Television servicing book. Models TS17, T346 and T347. The latter has an electrostatic focus CRT. Mullard AW36-21.
It could be that the LOPTX is faulty, but I do remember that this component was reliable when I used to service these sets more than forty years ago.
Check the screen feed resistor of the PL81, R62 is 8.2Koms, can change in value. C90 is the boost capacitor, 0.1mfd @1000volts, connects between the tapped width control and HT line.
Check the anode load resistors of the line oscillator, the triode section of an ECL80. To pin 1 are series resistors R56, 330Kohms and R57, 100Kohms. These high value resistors can go even higher.
HT voltage should be 190 to 200 volts.

DFWB.

beltinge bore · 18th Apr 2014, 9:32 pm

Many thanks for your help. Will report back.
Regards Steve.

beltinge bore · 19th Apr 2014, 7:10 pm

Had a go today.R62 PL81 screen feed resistor measured 9.72K (8.2K)
R56 450K (330k)
R57 122k (100k)Figures in brackets given in diagram.
Replaced R56 & R57 & tried another R62.a new ECL80 & PY82.No change .
HT is 198v. EHT measured with my "British neon EHT tester" is 8KV.Still have good sound & very weak raster.
Should my next job be to check the high value resistors R51,R52 etc?around V7
I still lean towards low emission tube but then to give anything with only 8KV to my simple mind it must be usable.
Many thanks Steve.

FERNSEH · 19th Apr 2014, 8:13 pm

Hi Steve,
check the heater voltage of the CRT, it should be around 6.3 volts, if it is less that, say 4 volts then the CRT might have a partial short circuit heater. Common problem with Mullard tubes.
Changing resistors R51 - R54 will not improve the EHT voltage but there might be an improvement in the sync separator performance.
Have you replaced C46? the coupling capacitor between the line oscillator and the grid of the PL81.

DFWB.

beltinge bore · 19th Apr 2014, 8:51 pm

Hello David (hope that,s the right name) Tube heater volts are 6.61.C46 was one of the first caps I changed.I have a spare AW 36-21 which is quite low emission but I think I will give it a try it,s not that difficult to change.
Thanks again Steve.

.

Freya · 19th Apr 2014, 9:45 pm

I have been scratching my head since your first post, still with no answers or anything helpful this time...

David is kind of clever in these areas

beltinge bore · 20th Apr 2014, 8:59 pm

Fitted spare tube (known low emission but worth a try) Better picture but still not great.EHT falls to 8KV after a couple of minutes (starts at about 10KV).LOPT overwind would seem the likely cause IMHO.The set has been very damp in storage.
Many thanks & regards Steve.

mark pirate · 20th Apr 2014, 9:10 pm

I would remove The LOPT and put it in the airing cupboard for a few weeks, or put it in in the oven on low heat for a couple of hours.

It does take a fair while for the damp to be drawn out of the windings, but it should work fine when dried out properly.

Mark

julie_m · 21st Apr 2014, 2:07 am

Or if you haven't got an airing cupboard (quite likely in a modern plumbing setup with a combination boiler) but have got a dehumidifier (not all that uncommon nowadays; better insulation can lead to condensation problems, if moisture-laden air cannot escape), you could prop the LOPT in the stream of dry air blowing out of the dehumidifier.

beltinge bore · 21st Apr 2014, 9:06 am

Thanks for the advice chaps.Will try the airing cupboard method .
I only bought it as a spares donor as the vendor was going to take it to the tip if not sold.Cabinet is a bit rough but these little tellys have happy memories for me.
Regards Steve.

Heatercathodeshort · 21st Apr 2014, 7:22 pm

Very reliable LOPT on these receivers as David says, I have never replaced one and there were large quantities of these around when I was in my youth.
Could be damp as suggested. Might be worth passing a DC current from a stabilized power supply [around 12v?]through the primary windings. Around 1 amp should warm it up but wil need to be monitored for the first hour. I have never tried this with the overwind but a higher voltage would be required. J.

beltinge bore · 22nd Apr 2014, 8:52 am

Thanks John. The LOPT is now in the airing cupboard.It looks in VGC & I'm not convinced damp is the trouble but I can't think of any other reasons for low EHT that David has not covered.
Clutching at straws but one tag on the LOPT had a very dry looking joint.
Thanks again.Regards Steve.

FERNSEH · 22nd Apr 2014, 9:33 am

Hi Steve,
A good indication of the line drive is the negative volts measured at the control grid of the PL81. The line drive waveform should be at least 100 volts peak to peak. The PL81 is biased by grid current. It follows if the drive is correct then the negative voltage at G1 of the PL81 should be more than -40 volts. An under driven line output valve will glow red hot.
If an oscilloscope is available check the waveform at G1 of the PL81. With the scope set to DC input, the waveform should be negative going, that is, the peak of the waveform to ground potential.

DFWB.

FERNSEH · 22nd Apr 2014, 12:14 pm

Simplified circuit of the line timebase.

DFWB.

Freya · 22nd Apr 2014, 1:09 pm

Is switch SW.2 for width settings ?

FERNSEH · 22nd Apr 2014, 1:28 pm

The transportable model TPS147 employs a similar line timebase circuit and SW2 is indicated as the width control.

DFWB.

beltinge bore · 23rd Apr 2014, 9:24 am

Sadly a scope is beyond me .One of my many jobs in the past involved selling tools to amateur car repairers .I knew that 50% of hub extractors would be returned with the words "it's rubbish it's broken" .In other words sadly I know my limitations but still enjoy this.I haven't wrecked anything yet(i hope!).
Old strowger telephone eqiupment is my area of knowledge but 30 years out of date & I never did like telephones!Age makes learning more difficult in my case!Still onward & upward.
Thanks & Kind Regards Steve.

beltinge bore · 11th Jul 2014, 9:02 pm

Hello
Retrieved LOPT from the airing cupboard after 10 weeks in a plastic bag with silica gel & re-fitted.EHT is now 11kv steady.
I put the tube on the Muter (thanks Stephen/Freya for help wiring an adapter) It tested low emission so i gave it a quick re-gen burst emission came up nicely(for how long?).
Now i have a good picture ! Looks like damp was the cause.Cabinet re-furb now in progress.
I have another LOPT from a GEC BT 302 "drying out" as well so here's hoping.
The contrast control seems to have little effect but a combination of brightness & sensitivity gives a very nice picture.Will try a new 500k pot.
Thanks for reading Regards Steve.

16th Apr 2014, 8:52 pm	#1
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Sobell T347 Hello My first post for some time. My knowledge and equipment is limited but I usually muddle through. I acquired this old girl from Ebay for £3-70p to use as spares backup for my nice T347 that I managed to restore last year.It had a broken implosion guard and signs of very damp storage (rusty chassis/ mountings/cabinet damage etc) The fine tuner shaft had seized onto the channel selector shaft.The aquadag coating is very wrinkly & the earth spring had rusted through & the aerial socket was missing. The frame section had some small hunts caps hanging in mid air & some odd looking replacements.The LOPT screening can was loose & the PL81 was missing. Well I was looking for something to do so I had a go at it (as you do). Fired up after usual checks etc .At about 180v strained line whistle & speaker crackle appeared .After a bit of fiddling & fettling with the Aurora I got weak sound & after changing caps in the line area(boost etc) a very weak low height scrambled raster. Carried on De Hunting methodically I now have a very weak but synced picture with good linearity & good sound. The EY51 PL81 & PY81 have been changed.The contrast control has no effect and to get the best picture the sensitivity control has to be set to minimum.The picture dose not balloon and fade when the brightness is advanced it just fades away.The left side of the picture has slight fold over. My thoughts are low emission tube but the EHT measures only 7.5kv on my crude tester with the anode cap on or off the tube. LOPT EHT overwind failure also seems a possibility. HT seems ok measured at smoothers.A1 is about 400v. Any advice would be most welcome. Thanks for reading Regards Steve.

18th Apr 2014, 4:32 pm	#2
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell t347 No replies so back to my old Spreadbury books.Thanks for looking. Regards Steve. Last edited by beltinge bore; 18th Apr 2014 at 4:46 pm.

18th Apr 2014, 9:16 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sobell t347 At 7.5KV the EHT is too low for a 14" CRT. you should be looking for >11KV. Information for this set can be found in the 1956/57 Newnes Radio and Television servicing book. Models TS17, T346 and T347. The latter has an electrostatic focus CRT. Mullard AW36-21. It could be that the LOPTX is faulty, but I do remember that this component was reliable when I used to service these sets more than forty years ago. Check the screen feed resistor of the PL81, R62 is 8.2Koms, can change in value. C90 is the boost capacitor, 0.1mfd @1000volts, connects between the tapped width control and HT line. Check the anode load resistors of the line oscillator, the triode section of an ECL80. To pin 1 are series resistors R56, 330Kohms and R57, 100Kohms. These high value resistors can go even higher. HT voltage should be 190 to 200 volts. DFWB.

18th Apr 2014, 9:32 pm	#4
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell t347 Many thanks for your help. Will report back. Regards Steve.

19th Apr 2014, 7:10 pm	#5
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Had a go today.R62 PL81 screen feed resistor measured 9.72K (8.2K) R56 450K (330k) R57 122k (100k)Figures in brackets given in diagram. Replaced R56 & R57 & tried another R62.a new ECL80 & PY82.No change . HT is 198v. EHT measured with my "British neon EHT tester" is 8KV.Still have good sound & very weak raster. Should my next job be to check the high value resistors R51,R52 etc?around V7 I still lean towards low emission tube but then to give anything with only 8KV to my simple mind it must be usable. Many thanks Steve.

19th Apr 2014, 8:13 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sobell T347 Hi Steve, check the heater voltage of the CRT, it should be around 6.3 volts, if it is less that, say 4 volts then the CRT might have a partial short circuit heater. Common problem with Mullard tubes. Changing resistors R51 - R54 will not improve the EHT voltage but there might be an improvement in the sync separator performance. Have you replaced C46? the coupling capacitor between the line oscillator and the grid of the PL81. DFWB.

19th Apr 2014, 8:51 pm	#7
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Hello David (hope that,s the right name) Tube heater volts are 6.61.C46 was one of the first caps I changed.I have a spare AW 36-21 which is quite low emission but I think I will give it a try it,s not that difficult to change. Thanks again Steve. .

19th Apr 2014, 9:45 pm	#8
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Sobell T347 I have been scratching my head since your first post, still with no answers or anything helpful this time... David is kind of clever in these areas __________________ Stephen _________"It`s only an old telly" ___

20th Apr 2014, 8:59 pm	#9
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Fitted spare tube (known low emission but worth a try) Better picture but still not great.EHT falls to 8KV after a couple of minutes (starts at about 10KV).LOPT overwind would seem the likely cause IMHO.The set has been very damp in storage. Many thanks & regards Steve.

20th Apr 2014, 9:10 pm	#10
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Sobell T347 I would remove The LOPT and put it in the airing cupboard for a few weeks, or put it in in the oven on low heat for a couple of hours. It does take a fair while for the damp to be drawn out of the windings, but it should work fine when dried out properly. Mark

21st Apr 2014, 2:07 am	#11
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Sobell T347 Or if you haven't got an airing cupboard (quite likely in a modern plumbing setup with a combination boiler) but have got a dehumidifier (not all that uncommon nowadays; better insulation can lead to condensation problems, if moisture-laden air cannot escape), you could prop the LOPT in the stream of dry air blowing out of the dehumidifier. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

21st Apr 2014, 9:06 am	#12
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Thanks for the advice chaps.Will try the airing cupboard method . I only bought it as a spares donor as the vendor was going to take it to the tip if not sold.Cabinet is a bit rough but these little tellys have happy memories for me. Regards Steve.

21st Apr 2014, 7:22 pm	#13
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Sobell T347 Very reliable LOPT on these receivers as David says, I have never replaced one and there were large quantities of these around when I was in my youth. Could be damp as suggested. Might be worth passing a DC current from a stabilized power supply [around 12v?]through the primary windings. Around 1 amp should warm it up but wil need to be monitored for the first hour. I have never tried this with the overwind but a higher voltage would be required. J.

22nd Apr 2014, 8:52 am	#14
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Thanks John. The LOPT is now in the airing cupboard.It looks in VGC & I'm not convinced damp is the trouble but I can't think of any other reasons for low EHT that David has not covered. Clutching at straws but one tag on the LOPT had a very dry looking joint. Thanks again.Regards Steve.

22nd Apr 2014, 9:33 am	#15
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sobell T347 Hi Steve, A good indication of the line drive is the negative volts measured at the control grid of the PL81. The line drive waveform should be at least 100 volts peak to peak. The PL81 is biased by grid current. It follows if the drive is correct then the negative voltage at G1 of the PL81 should be more than -40 volts. An under driven line output valve will glow red hot. If an oscilloscope is available check the waveform at G1 of the PL81. With the scope set to DC input, the waveform should be negative going, that is, the peak of the waveform to ground potential. DFWB.

22nd Apr 2014, 12:14 pm	#16
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sobell T347 Simplified circuit of the line timebase. DFWB. Attached Thumbnails

22nd Apr 2014, 1:09 pm	#17
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Sobell T347 Is switch SW.2 for width settings ? __________________ Stephen _________"It`s only an old telly" ___

22nd Apr 2014, 1:28 pm	#18
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Sobell T347 The transportable model TPS147 employs a similar line timebase circuit and SW2 is indicated as the width control. DFWB.

23rd Apr 2014, 9:24 am	#19
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Sadly a scope is beyond me .One of my many jobs in the past involved selling tools to amateur car repairers .I knew that 50% of hub extractors would be returned with the words "it's rubbish it's broken" .In other words sadly I know my limitations but still enjoy this.I haven't wrecked anything yet(i hope!). Old strowger telephone eqiupment is my area of knowledge but 30 years out of date & I never did like telephones!Age makes learning more difficult in my case!Still onward & upward. Thanks & Kind Regards Steve.

11th Jul 2014, 9:02 pm	#20
beltinge bore Pentode Join Date: Dec 2012 Location: Herne Bay, Kent, UK. Posts: 233	Re: Sobell T347 Hello Retrieved LOPT from the airing cupboard after 10 weeks in a plastic bag with silica gel & re-fitted.EHT is now 11kv steady. I put the tube on the Muter (thanks Stephen/Freya for help wiring an adapter) It tested low emission so i gave it a quick re-gen burst emission came up nicely(for how long?). Now i have a good picture ! Looks like damp was the cause.Cabinet re-furb now in progress. I have another LOPT from a GEC BT 302 "drying out" as well so here's hoping. The contrast control seems to have little effect but a combination of brightness & sensitivity gives a very nice picture.Will try a new 500k pot. Thanks for reading Regards Steve.