GEC 1748 EHT (or lack thereof) - UK Vintage Radio Repair and Restoration Discussion Forum

stuie319 · 28th Feb 2007, 9:00 pm

Hi folks,

Can anyone tell me the number of the EHT rectifier in this set? The number has rubbed of the one in mine and i fear it needs a new 'un. Basically theres no EHT, while the line osc is very much alive. The line output valve on this set was cracked at the base, and although the getter hadn't whitened there was no activity at the anode. Two new valves later (the first arrived with the heater o/c) and I can draw a healthy spark from the cap, and another (albeit less healthy looking) from the output of the EHT overwind on the loptx, but beggar all from the output of the rectifier. While peering at it to try and establish whether the heaters were alight i could see theres some small amount of internal arcing going on in it, thus i can't tell if i've really got heaters or not (its pretty much half encapsulated). I'm minded to change the reccy first of all, while hoping the lopty isnt stuffed! the overwind seems to be running cool....

Any thoughts greatly appreciated as ever

Regards,
Stu

mickjjo · 28th Feb 2007, 9:08 pm

Hi, According to the 1956/7 R+TV servicing book, The EHT rectifier in the GEC BT1748 is a U45.

.

Regards, Mick.

Steve_P · 28th Feb 2007, 9:11 pm

What are the other two valves in there, and is it a B9A, Octal or wire ended EHT rectifier. When about is the set?

Then I should be able to work out the EHT Rectifier for you. Unrectified EHT should jump about an inch or so, and a cool EHT overwind is either OK or open circuit.

Cheers,

Steve P

stuie319 · 28th Feb 2007, 9:23 pm

Thanks Mick and Steve,

The set is dated 1956 or thereabouts, the rectifier is wire ended.
Whats worrying is the unrectified EHT seems quite weak, certainly not enough to jump an inch, whereas the spark from the anode of the N339 line output is much healthier. The fact that its there suggests the overwind can't be o/c, worst case scenario is that its got shorted turns of course.

Regards
Stu

stuie319 · 28th Feb 2007, 9:26 pm

Steve
You asked about the other two valves, I forgot to add the boost diode is U329

Stu

Steve_P · 28th Feb 2007, 9:41 pm

Got the set up now. It is a U45 by the way! Get a new one, put it in, and see what happens....

There's not a lot else it can be apart from the LOPT, as the Line Drive sounds OK. Just check that it's on the right frequency if you can, you can hear 10 kHz or there abouts.

You could try swapping C55 and L23 I suppose. L23 will probably be OK, and C55 could cause a problem.

Cheers,

Steve P

stuie319 · 28th Feb 2007, 9:52 pm

Disconnected the U45 anode, and can now draw about an inch from the output of the lopt winding, this is encouraging, ok-its off load, but I feel more hopeful that the lopt is servicable. I'll go ordering a new diode.

Thanks chaps!

Regards
Stu

Danny · 28th Feb 2007, 11:15 pm

Hi
I hope I'm wrong, but try just disconnecting the anode cap from the crt to see if the rectified eht improves.
If it does, then your tube is soft.
(Not uncommon on gec sets).
Danny

stuie319 · 1st Mar 2007, 12:54 am

Danny,

I hadn't thought of that. OK, I had thought of it and said naaaaaa; I'm not that unlucky....Then I thought of it again. You could have been absolutely right. Which would be worse?
Duff CRT...could be replaced and modified, but well outside my experience of valve kit.
Duff loptx? easy to change but where's the new one?
I took the CRT anode cap off and ran it up.
Lots of big arcy sparky plasma effect stuff on the reccy anode, great fun; nowt on the cathode.
It's gotta be worth a new rectifier, the loptx is lively enough and the tube wasnt decking the EHT, for now at least,

let's live in hope

Regards
Stu

Danny · 1st Mar 2007, 10:38 pm

It does sound like the rec is soft.
If you haven't got a rec you could use a stick rectifier to check the set. Danny

stuie319 · 16th Mar 2007, 2:24 pm

Hi All,

I aquired a new U45, but only just got around to fitting it, my 'workshop/spare room/ office' being out of commission after a small decorating job turned up loads of hidden damp and has become a major episode

The good news is, a nice bright raster appeared, albeit with frame collapse. So thus far the CRT is servicable, and the loptx lives

.
Now is the time to aquire the circuit, but its encouraging so far.

Regards
Stu

Steve_P · 16th Mar 2007, 3:50 pm

Great stuff! Right - Frame stages are V16 and V17. For the record,

N329 = PL82
LN309 = PCL83

The PCL83 tended to be a bit unreliable in my experience.

For the circuit, and loads of others, try Duke Nukem's Vintage TV Service Data CD-Rom. Available thruogh his site : www.thevalvepage.com

Cheers,

Steve P

28th Feb 2007, 9:00 pm	#1
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	GEC 1748 EHT (or lack thereof) Hi folks, Can anyone tell me the number of the EHT rectifier in this set? The number has rubbed of the one in mine and i fear it needs a new 'un. Basically theres no EHT, while the line osc is very much alive. The line output valve on this set was cracked at the base, and although the getter hadn't whitened there was no activity at the anode. Two new valves later (the first arrived with the heater o/c) and I can draw a healthy spark from the cap, and another (albeit less healthy looking) from the output of the EHT overwind on the loptx, but beggar all from the output of the rectifier. While peering at it to try and establish whether the heaters were alight i could see theres some small amount of internal arcing going on in it, thus i can't tell if i've really got heaters or not (its pretty much half encapsulated). I'm minded to change the reccy first of all, while hoping the lopty isnt stuffed! the overwind seems to be running cool.... Any thoughts greatly appreciated as ever Regards, Stu

28th Feb 2007, 9:08 pm	#2
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: GEC 1748 EHT (or lack thereof) Hi, According to the 1956/7 R+TV servicing book, The EHT rectifier in the GEC BT1748 is a U45. . Regards, Mick.

28th Feb 2007, 9:11 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: GEC 1748 EHT (or lack thereof) What are the other two valves in there, and is it a B9A, Octal or wire ended EHT rectifier. When about is the set? Then I should be able to work out the EHT Rectifier for you. Unrectified EHT should jump about an inch or so, and a cool EHT overwind is either OK or open circuit. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Feb 2007, 9:23 pm	#4
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: GEC 1748 EHT (or lack thereof) Thanks Mick and Steve, The set is dated 1956 or thereabouts, the rectifier is wire ended. Whats worrying is the unrectified EHT seems quite weak, certainly not enough to jump an inch, whereas the spark from the anode of the N339 line output is much healthier. The fact that its there suggests the overwind can't be o/c, worst case scenario is that its got shorted turns of course. Regards Stu

28th Feb 2007, 9:26 pm	#5
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: GEC 1748 EHT (or lack thereof) Steve You asked about the other two valves, I forgot to add the boost diode is U329 Stu

28th Feb 2007, 9:41 pm	#6
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: GEC 1748 EHT (or lack thereof) Got the set up now. It is a U45 by the way! Get a new one, put it in, and see what happens.... There's not a lot else it can be apart from the LOPT, as the Line Drive sounds OK. Just check that it's on the right frequency if you can, you can hear 10 kHz or there abouts. You could try swapping C55 and L23 I suppose. L23 will probably be OK, and C55 could cause a problem. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Feb 2007, 9:52 pm	#7
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: GEC 1748 EHT (or lack thereof) Disconnected the U45 anode, and can now draw about an inch from the output of the lopt winding, this is encouraging, ok-its off load, but I feel more hopeful that the lopt is servicable. I'll go ordering a new diode. Thanks chaps! Regards Stu

28th Feb 2007, 11:15 pm	#8
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: GEC 1748 EHT (or lack thereof) Hi I hope I'm wrong, but try just disconnecting the anode cap from the crt to see if the rectified eht improves. If it does, then your tube is soft. (Not uncommon on gec sets). Danny Last edited by Mike Phelan; 20th Nov 2007 at 9:24 am. Reason: Legibility

1st Mar 2007, 12:54 am	#9
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: GEC 1748 EHT (or lack thereof) Danny, I hadn't thought of that. OK, I had thought of it and said naaaaaa; I'm not that unlucky....Then I thought of it again. You could have been absolutely right. Which would be worse? Duff CRT...could be replaced and modified, but well outside my experience of valve kit. Duff loptx? easy to change but where's the new one? I took the CRT anode cap off and ran it up. Lots of big arcy sparky plasma effect stuff on the reccy anode, great fun; nowt on the cathode. It's gotta be worth a new rectifier, the loptx is lively enough and the tube wasnt decking the EHT, for now at least, let's live in hope Regards Stu Last edited by Mike Phelan; 20th Nov 2007 at 9:27 am. Reason: Legibility

1st Mar 2007, 10:38 pm	#10
Danny Heptode Join Date: Jul 2003 Location: North Walsham, Norfolk, UK. Posts: 516	Re: GEC 1748 EHT (or lack thereof) It does sound like the rec is soft. If you haven't got a rec you could use a stick rectifier to check the set. Danny Last edited by Mike Phelan; 20th Nov 2007 at 9:28 am. Reason: Legibility

16th Mar 2007, 2:24 pm	#11
stuie319 Pentode Join Date: Jan 2007 Location: Stockport, Greater Manchester, UK. Posts: 229	Re: GEC 1748 EHT (or lack thereof) Hi All, I aquired a new U45, but only just got around to fitting it, my 'workshop/spare room/ office' being out of commission after a small decorating job turned up loads of hidden damp and has become a major episode The good news is, a nice bright raster appeared, albeit with frame collapse. So thus far the CRT is servicable, and the loptx lives . Now is the time to aquire the circuit, but its encouraging so far. Regards Stu

16th Mar 2007, 3:50 pm	#12
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: GEC 1748 EHT (or lack thereof) Great stuff! Right - Frame stages are V16 and V17. For the record, N329 = PL82 LN309 = PCL83 The PCL83 tended to be a bit unreliable in my experience. For the circuit, and loads of others, try Duke Nukem's Vintage TV Service Data CD-Rom. Available thruogh his site : www.thevalvepage.com Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?