Decca Bradford - Dead Line Output Stage - UK Vintage Radio Repair and Restoration Discussion Forum

TV_Madness · 27th Nov 2009, 10:14 pm

Evening all...

It would seem that my Bradford has suffered from a fairly major fault in the Line Output stage.

I was quite happy, indulged in an evenings viewing when all of a sudden there was a fizzzzzzzz followed by that classic acrid stench that old tellys have a habbit of throwing out from time to time!

So.... Off comes the back.... Again!

I knew exactly what the fault was without even looking and sure as hell, I was right... Line Output Pentode flying lead burnt and had arced to earth against the shielding cage around the Pentode / Diode. I am aware of this being a reasonably common issue, but me being lazy, did nothing about it!

My initial thought was the worst... Open Circuit Line Output xformer but after a quick continuity test between the two top caps of the PL509/PY500, I was rewarded with a "beeeeep" atleast. Resistance checked out at about 3 Ohms.

The 500mA Fuse which protects the Line Stage had also blown nicely too.

A couple of questions.... Line output transformer and associated Valves. What are the chances of any permanent damage being inflicted on these components? Secondly, is there a suitable High Temp EHT Cable I can use / Repair tip to stop this happening in future?

All the best

Matt

Steve_P · 27th Nov 2009, 10:32 pm

Well, if the fuse has gone in time, they should be OK. Here's hoping.

Why not use the stuff they have on car coils - it sure does stand heat. Position it far away from any hot areas.

Prays for the LOPT....

Cheers,

Steve P.

DangerMan · 27th Nov 2009, 10:40 pm

Hi Matt,
I would use silicone insulated wire as I find it's better at high temperature than even high temperature PVC.
If you do try car spark plug wire, measure the resistance first as these days it's usually resistive in order to provide interference suppression.
TTFN
Pete

TV_Madness · 27th Nov 2009, 10:52 pm

Prayers for the LOPT Indeed!!!

I don't even have a spare Fuse to hand jsut to give it a quick test with the cage removed and the flying leads kept well away from the chassis! Knowing me I'll probably wake up in cold swear half way through the night thinking about it haha!

Silicone cable sounds like a good bet indeed!

Thanks

Matt

TV_Madness · 27th Nov 2009, 10:53 pm

*"In a cold sweat"

Mikey405 · 28th Nov 2009, 5:27 pm

I think I have a spare lopty if you need one Matt.

As other members know (to their great annoyance and my eternal shame) you'll have to keep pestering me to get it sent though...

From Mike.

murphyv310 · 28th Nov 2009, 6:01 pm

Hi.
It's unlikely that the lopt will have failed the fuse has done its business here.
I have some good quality plain copper HT cable that we use on the spark ignition on hot water pressure washers thats good for 30kv, PM me if you want some.
Good luck
Trevor

Hybrid tellies · 28th Nov 2009, 10:51 pm

It looks like your Bradford suffered the same fault as mine did back in the summer.After looking around for a suitable top cap lead replacement I think I am going to settle for some copper HT leads sold by a local auto parts shop. It looks robust but flexible enough to do the job and the shop sells it by the meter so it will only cost a pound to replace both top cap leads.
I think I was quick enough reaching for the on off switch to have prevented any further damage. Well I hope and pray that I was.

TV_Madness · 29th Nov 2009, 12:18 am

Thanks Mike and Trevor. I'm hoping the LOPT should (Touch wood) be ok. If it has been dealt a mortal blow, I know who I'll be nagging!!

I'll get a PM over to you Trevor I think, as that cable sounds like it could well do the job. I was thinking of installing a slightly longer than usual lead from the top caps down to the LOPT below, routing them behind the ally cage well away from the hot bottles, rather than inside it. The leads currently in place don't leave a lot of room for manoeuvre.

Thanks all

Matt

Hybrid tellies · 29th Nov 2009, 5:00 pm

Good idea that, the last leads that I replaced possibly nearly 20 years ago were routed behind the ally cage and only shorted out where the lead rested against the top of the cage. I am thinking of fitting some extra insulation at this point.

TV_Madness · 9th Dec 2009, 8:19 pm

Hi all

Trevor has kindly supplied me with a length of good quality HT cable. I will get on with repairing this as soon as I get a spare moment and post up the results!

All the best

Matt

TV_Madness · 13th Dec 2009, 1:41 pm

Hi All

I have finally managed to get round to replacing the flying leads from the two top caps, down to the LOPT.

I had also noticed that the Resistor R466 (1K8) Was not connected, as one end had come adrift from its solder tag?!?!?!

After soldering everything back up, I checked everything against the red book to ensure that all was in good order and fitted a new 500mA (Anti Surge) fuse.

After initial switch on, all seemed to be ok, a healthy line whistle, EHT and nice bright raster. I left the set running for about 3 mins with no problems at all. Just as I thought all was going well "Thud".... Blank screen. The 500mA fuse in the HT rail for the Line stage had blown again! I don't have another to hand so will have to order some, however It looks as though there is now another fault, possibly caused by the first.

Any ideas anyone?

Tazman1966 · 13th Dec 2009, 8:40 pm

Well...check for a very low resistance reading from the top cap of the PY500 to chassis. If short, check boost cap first also the PY itself. That's a start anyway.

Welsh Anorak · 15th Dec 2009, 12:21 am

Hi
Though it pains me to say it, I agree with Tas (only kidding!) Sounds very like an internal short in the PY500. I doubt it's anything more sinister - usually you get a moment or two before the fuse blows if the boost cap's shorted - easy to check as they don't usually heal themselves.
Glyn

TV_Madness · 15th Dec 2009, 9:24 pm

I have a spare PY500 (or two), so I'll try and change this.

I'm out of 500mA fuses at the mo, so can't do any more testing, until my delivery turns up.

Interestingly, I've just noticed that the original fuse that was fitted is 600mA (Any ideas Mikey!! hehe)

Boom · 15th Dec 2009, 9:41 pm

I haven't seen the Bradford circuit for years but you might well find that R466 is the screen grid feed resistor and that it has been deliberately disconnected by someone while fault finding in the LOP stage.

IIRC there was a resistor in the line driver stage on the bottom panel which MIGHT be around 33k (I said I haven't had a Bradford apart in yonks) which drifts in value and causes all sorts of oddball happenings in the LOP including the PL509 mysteriously melting. Maybe someone else can confirm this?

Dave

Hybrid tellies · 16th Dec 2009, 9:21 pm

If you have a short circuit from the top caps to chassis and its not the PY500A or usual boost cap problem the next suspect is the tuning cap which goes from the cathode of the PY500a to chassis. Check you have not inadvertantly damaged it when replacing the topcap lead, its a very fiddly part of the set to work on.

TV_Madness · 21st Dec 2009, 9:41 pm

Well... I have finally got round to replacing the Boost Diode and the HT fuse to the LOP Stage and am sitting here watching the set as I write this.

So far everthing seems to be going ok and haven't had any problems with it as yet (touch wood)!

I'll see how it is for the moment.

Have a good Christmas all!

Matt

27th Nov 2009, 10:14 pm	#1
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Decca Bradford - Dead Line Output Stage Evening all... It would seem that my Bradford has suffered from a fairly major fault in the Line Output stage. I was quite happy, indulged in an evenings viewing when all of a sudden there was a fizzzzzzzz followed by that classic acrid stench that old tellys have a habbit of throwing out from time to time! So.... Off comes the back.... Again! I knew exactly what the fault was without even looking and sure as hell, I was right... Line Output Pentode flying lead burnt and had arced to earth against the shielding cage around the Pentode / Diode. I am aware of this being a reasonably common issue, but me being lazy, did nothing about it! My initial thought was the worst... Open Circuit Line Output xformer but after a quick continuity test between the two top caps of the PL509/PY500, I was rewarded with a "beeeeep" atleast. Resistance checked out at about 3 Ohms. The 500mA Fuse which protects the Line Stage had also blown nicely too. A couple of questions.... Line output transformer and associated Valves. What are the chances of any permanent damage being inflicted on these components? Secondly, is there a suitable High Temp EHT Cable I can use / Repair tip to stop this happening in future? All the best Matt __________________ I don't want anymore junk in this house!

27th Nov 2009, 10:32 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Decca Bradford - Dead Line Output Stage Well, if the fuse has gone in time, they should be OK. Here's hoping. Why not use the stuff they have on car coils - it sure does stand heat. Position it far away from any hot areas. Prays for the LOPT.... 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...?

27th Nov 2009, 10:40 pm	#3
DangerMan Heptode Join Date: Jul 2009 Location: Stoke-on-Trent, Staffordshire, UK. Posts: 719	Re: Decca Bradford - Dead Line Output Stage Hi Matt, I would use silicone insulated wire as I find it's better at high temperature than even high temperature PVC. If you do try car spark plug wire, measure the resistance first as these days it's usually resistive in order to provide interference suppression. TTFN Pete

27th Nov 2009, 10:52 pm	#4
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage Prayers for the LOPT Indeed!!! I don't even have a spare Fuse to hand jsut to give it a quick test with the cage removed and the flying leads kept well away from the chassis! Knowing me I'll probably wake up in cold swear half way through the night thinking about it haha! Silicone cable sounds like a good bet indeed! Thanks Matt __________________ I don't want anymore junk in this house!

27th Nov 2009, 10:53 pm	#5
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage *"In a cold sweat" __________________ I don't want anymore junk in this house!

28th Nov 2009, 5:27 pm	#6
Mikey405 Octode Join Date: Jul 2006 Location: Solihull, West Midlands and Beaford, Devon Posts: 1,626	Re: Decca Bradford - Dead Line Output Stage I think I have a spare lopty if you need one Matt. As other members know (to their great annoyance and my eternal shame) you'll have to keep pestering me to get it sent though... From Mike. __________________ G7TRF

28th Nov 2009, 6:01 pm	#7
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	Re: Decca Bradford - Dead Line Output Stage Hi. It's unlikely that the lopt will have failed the fuse has done its business here. I have some good quality plain copper HT cable that we use on the spark ignition on hot water pressure washers thats good for 30kv, PM me if you want some. Good luck Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

28th Nov 2009, 10:51 pm	#8
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Decca Bradford - Dead Line Output Stage It looks like your Bradford suffered the same fault as mine did back in the summer.After looking around for a suitable top cap lead replacement I think I am going to settle for some copper HT leads sold by a local auto parts shop. It looks robust but flexible enough to do the job and the shop sells it by the meter so it will only cost a pound to replace both top cap leads. I think I was quick enough reaching for the on off switch to have prevented any further damage. Well I hope and pray that I was. __________________ Simon BVWS member Last edited by Hybrid tellies; 28th Nov 2009 at 11:00 pm.

29th Nov 2009, 12:18 am	#9
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage Thanks Mike and Trevor. I'm hoping the LOPT should (Touch wood) be ok. If it has been dealt a mortal blow, I know who I'll be nagging!! I'll get a PM over to you Trevor I think, as that cable sounds like it could well do the job. I was thinking of installing a slightly longer than usual lead from the top caps down to the LOPT below, routing them behind the ally cage well away from the hot bottles, rather than inside it. The leads currently in place don't leave a lot of room for manoeuvre. Thanks all Matt __________________ I don't want anymore junk in this house!

29th Nov 2009, 5:00 pm	#10
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Decca Bradford - Dead Line Output Stage Good idea that, the last leads that I replaced possibly nearly 20 years ago were routed behind the ally cage and only shorted out where the lead rested against the top of the cage. I am thinking of fitting some extra insulation at this point. __________________ Simon BVWS member

9th Dec 2009, 8:19 pm	#11
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage Hi all Trevor has kindly supplied me with a length of good quality HT cable. I will get on with repairing this as soon as I get a spare moment and post up the results! All the best Matt __________________ I don't want anymore junk in this house!

13th Dec 2009, 1:41 pm	#12
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage Hi All I have finally managed to get round to replacing the flying leads from the two top caps, down to the LOPT. I had also noticed that the Resistor R466 (1K8) Was not connected, as one end had come adrift from its solder tag?!?!?! After soldering everything back up, I checked everything against the red book to ensure that all was in good order and fitted a new 500mA (Anti Surge) fuse. After initial switch on, all seemed to be ok, a healthy line whistle, EHT and nice bright raster. I left the set running for about 3 mins with no problems at all. Just as I thought all was going well "Thud".... Blank screen. The 500mA fuse in the HT rail for the Line stage had blown again! I don't have another to hand so will have to order some, however It looks as though there is now another fault, possibly caused by the first. Any ideas anyone? __________________ I don't want anymore junk in this house!

13th Dec 2009, 8:40 pm	#13
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Decca Bradford - Dead Line Output Stage Well...check for a very low resistance reading from the top cap of the PY500 to chassis. If short, check boost cap first also the PY itself. That's a start anyway. __________________ All the very best, Tas

15th Dec 2009, 12:21 am	#14
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,916	Re: Decca Bradford - Dead Line Output Stage Hi Though it pains me to say it, I agree with Tas (only kidding!) Sounds very like an internal short in the PY500. I doubt it's anything more sinister - usually you get a moment or two before the fuse blows if the boost cap's shorted - easy to check as they don't usually heal themselves. Glyn

15th Dec 2009, 9:24 pm	#15
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage I have a spare PY500 (or two), so I'll try and change this. I'm out of 500mA fuses at the mo, so can't do any more testing, until my delivery turns up. Interestingly, I've just noticed that the original fuse that was fitted is 600mA (Any ideas Mikey!! hehe) __________________ I don't want anymore junk in this house!

15th Dec 2009, 9:41 pm	#16
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Decca Bradford - Dead Line Output Stage I haven't seen the Bradford circuit for years but you might well find that R466 is the screen grid feed resistor and that it has been deliberately disconnected by someone while fault finding in the LOP stage. IIRC there was a resistor in the line driver stage on the bottom panel which MIGHT be around 33k (I said I haven't had a Bradford apart in yonks) which drifts in value and causes all sorts of oddball happenings in the LOP including the PL509 mysteriously melting. Maybe someone else can confirm this? Dave

16th Dec 2009, 9:21 pm	#17
Hybrid tellies Nonode Join Date: Jan 2004 Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area. Posts: 2,965	Re: Decca Bradford - Dead Line Output Stage If you have a short circuit from the top caps to chassis and its not the PY500A or usual boost cap problem the next suspect is the tuning cap which goes from the cathode of the PY500a to chassis. Check you have not inadvertantly damaged it when replacing the topcap lead, its a very fiddly part of the set to work on. __________________ Simon BVWS member

21st Dec 2009, 9:41 pm	#18
TV_Madness Pentode Join Date: Jan 2008 Location: Titchfield Common, Hampshire, UK. Posts: 245	Re: Decca Bradford - Dead Line Output Stage Well... I have finally got round to replacing the Boost Diode and the HT fuse to the LOP Stage and am sitting here watching the set as I write this. So far everthing seems to be going ok and haven't had any problems with it as yet (touch wood)! I'll see how it is for the moment. Have a good Christmas all! Matt __________________ I don't want anymore junk in this house!