Masthead Amp Fire. SERIOUS PROBLEM! - Technical

wideboyal · 07-03-2010, 21:44

Had a call out last week to a customer who had lost all terrestrial and Satellite signals. When I got onto the roof, it was evident that the masthead amp had caught fire, and completeley burned out, burning through the cables for Sky and terrestrial.

I originally thought it may have been due to a rope which was holding the chimney cowl in place, but i doubt that even if the rope caught fire, it would have generated enough heat to melt the amp boxes and burn out the circuit boards.

This is the first time I have ever seen this happening. I assumed the 12v line power supply would not have generated enough heat to melt and burn the boxes. Thankfully the amps were located outside and there was only minimal scortching on the roof. I dread to think what would have happened were they located within the building.

The amps were (to the best of my knowledge and memory) an aniference masthead amp, and an antiference OX3 distribution amp, both in the region of about 20 years old.

To the best of my knowledge, there were no shorts in any of the cables.

I hope someone can shed a little light on the subject for me.

Thanks in advance

Alisdair.

Nigel Goodwin · 07-03-2010, 22:12

Mast head amplifier power supplies are low current only, so it's doubful that there's enough power there to damage anything. A FAR more likely scenario would be lightning damage?.

wideboyal · 07-03-2010, 23:13

No lighting at all in the area. Customer was in the house when the signals went off (5.30pm) and would have heard a lighnting bolt.

Certainly one of the most confusing, and scary problems I have had to deal with.

prking · 07-03-2010, 23:33

To melt through the insulation of the aerial and satellite coax would need a temperature of about 180 C. If the metal was melted as well then you are talking >500 C
I would agree with Nigel that lightning is more likely than the amp catching fire. What remained of the mast head amp? (its difficult to burn a circuit board as they are effectively self-extinguishing, and generally show just scorching)

Soruk · 07-03-2010, 23:58

Have you checked the power supply for the masthead amp is putting out the right voltage/current?

wideboyal · 08-03-2010, 00:09

The circuit board was still there, but had basicly turned into a charred mess. I will post a picture tomorrow of the remains.

I know lignting seems the most logical thing, but there wan't any at all when the problem occurred.

The customer is a long time family friend and i know for a fact that he would have told me if there was anything such as a lightning strike.

prking · 08-03-2010, 07:23

It would certainly be interesting to find out the source of ignition.

radioredcat · 08-03-2010, 11:38

I agree with all the coments so far but i would open up the psu and examine it for damage and the fuse in the plug (should be 3 amp) the transformer in it will have a thermal fuse.

If the psu is ok then lightning is only possible reason unless someone has put an accelerant and set it alight.

Some pics of the psu would be good.

andy

rastus · 08-03-2010, 12:09

Quote:

Originally Posted by radioredcat

...open up the psu and examine it for damage and the fuse in the plug (should be 3 amp) the transformer in it will have a thermal fuse....

I would certainly be anxious to examine that PSU quite closely. Without knowing exactly how the thing is arranged, it's foolish to be pedantic: but just suppose that the mains transformer had suffered an internal short between the secondary winding and the 'hot' end of the primary - you could end up with mains voltage on the co-ax feeder up to the head amp. A thermal fuse wouldn't necessarily stop this sort of thing happening, and even if the thing is fused at 3 amps, that means that you're entitled to dissipate around 750 watts of heat before the fuse even thinks about blowing.

If the co-ax outer is earthed at any point, of course, this isn't likely to happen and the fuse will blow. But it may not have been earthed...

I once had an elderly Sanyo rx from which I could draw sparks from the aerial plug whilst inserting: it had a switched-mode PSU, and nothing inside the set was tied back to earth potential. It relied entirely on the isolating capacitors inside the aerial socket to stop quite high voltages from passing outwards up the aerial feeder. My feeder was earthed - hence the sparks. But if those isolating caps had failed, I don't like to think about what it would have done to my aerial pre-amp (or, for that matter, to the set-top box which later appeared between the two...)

Nigel Goodwin · 08-03-2010, 14:48

Quote:

Originally Posted by rastus

I once had an elderly Sanyo rx from which I could draw sparks from the aerial plug whilst inserting: it had a switched-mode PSU, and nothing inside the set was tied back to earth potential. It relied entirely on the isolating capacitors inside the aerial socket to stop quite high voltages from passing outwards up the aerial feeder. My feeder was earthed - hence the sparks. But if those isolating caps had failed, I don't like to think about what it would have done to my aerial pre-amp (or, for that matter, to the set-top box which later appeared between the two...)

It does nothing to the aerial preamp, it doesn't work like that.

It's the poor aerial installer who gets affected by it

But isolated aerial sockets disappeared MANY years ago, such sets have been almost non-existent since the mid 80's.

albertd · 08-03-2010, 15:10

Quote:

Originally Posted by radioredcat

If the psu is ok then lightning is only possible reason unless someone has put an accelerant and set it alight.

The OP has said that there was no lightning about, and assuming that we are talking about Sunday, that is borne out by this:

Lightning Plot for 1730 GMT 07/03/10

which shows no sign of lightning anywhere in the UK.

MartinPickering · 08-03-2010, 15:22

I may be able to explain this:
http://www.glodark.com/design/cappics.htm

Note that my investigations were into new (or near new) equipment just over 20 years ago.
We could reproduce the effect in the laboratory but it was extremely rare in service.

Nonetheless, it goes to show that you shouldn't simply toss an amplifier amongst your cardboard boxes in the loft!

albertd · 10-03-2010, 11:08

Quote:

Originally Posted by albertd

Lightning Plot for 1730 GMT 07/03/10

That link has now expired.

07-03-2010, 21:44	#1
wideboyal Forum Member Join Date: Jan 2006 Posts: 27	Masthead Amp Fire. SERIOUS PROBLEM! Had a call out last week to a customer who had lost all terrestrial and Satellite signals. When I got onto the roof, it was evident that the masthead amp had caught fire, and completeley burned out, burning through the cables for Sky and terrestrial. I originally thought it may have been due to a rope which was holding the chimney cowl in place, but i doubt that even if the rope caught fire, it would have generated enough heat to melt the amp boxes and burn out the circuit boards. This is the first time I have ever seen this happening. I assumed the 12v line power supply would not have generated enough heat to melt and burn the boxes. Thankfully the amps were located outside and there was only minimal scortching on the roof. I dread to think what would have happened were they located within the building. The amps were (to the best of my knowledge and memory) an aniference masthead amp, and an antiference OX3 distribution amp, both in the region of about 20 years old. To the best of my knowledge, there were no shorts in any of the cables. I hope someone can shed a little light on the subject for me. Thanks in advance Alisdair.

07-03-2010, 22:12	#2
Nigel Goodwin Forum Member Join Date: Apr 2006 Location: North Derbyshire Posts: 23,051	Mast head amplifier power supplies are low current only, so it's doubful that there's enough power there to damage anything. A FAR more likely scenario would be lightning damage?.

07-03-2010, 23:13	#3
wideboyal Forum Member Join Date: Jan 2006 Posts: 27	No lighting at all in the area. Customer was in the house when the signals went off (5.30pm) and would have heard a lighnting bolt. Certainly one of the most confusing, and scary problems I have had to deal with.

07-03-2010, 23:33	#4
prking Forum Member Join Date: Sep 2002 Gender: Male Location: Weston-super-Mare Services: Freeview (T845PVR) Posts: 7,999	To melt through the insulation of the aerial and satellite coax would need a temperature of about 180 C. If the metal was melted as well then you are talking >500 C I would agree with Nigel that lightning is more likely than the amp catching fire. What remained of the mast head amp? (its difficult to burn a circuit board as they are effectively self-extinguishing, and generally show just scorching)

07-03-2010, 23:58	#5
Soruk Forum Member Join Date: Feb 2004 Location: Basingstoke (Hannington TX) Services: Freeview (Nokia 221T, Woolies WorthIt), Demon ADSL, Orange ED50 Posts: 226	Have you checked the power supply for the masthead amp is putting out the right voltage/current?

08-03-2010, 00:09	#6
wideboyal Forum Member Join Date: Jan 2006 Posts: 27	The circuit board was still there, but had basicly turned into a charred mess. I will post a picture tomorrow of the remains. I know lignting seems the most logical thing, but there wan't any at all when the problem occurred. The customer is a long time family friend and i know for a fact that he would have told me if there was anything such as a lightning strike.

08-03-2010, 07:23	#7
prking Forum Member Join Date: Sep 2002 Gender: Male Location: Weston-super-Mare Services: Freeview (T845PVR) Posts: 7,999	It would certainly be interesting to find out the source of ignition.

08-03-2010, 11:38	#8
radioredcat Forum Member Join Date: Aug 2008 Location: st helens Services: Winter hill,The Wrekin,Free from sky,VM XL,10M Broadband Posts: 834	I agree with all the coments so far but i would open up the psu and examine it for damage and the fuse in the plug (should be 3 amp) the transformer in it will have a thermal fuse. If the psu is ok then lightning is only possible reason unless someone has put an accelerant and set it alight. Some pics of the psu would be good. andy

08-03-2010, 15:22	#12
MartinPickering Forum Member Join Date: Sep 2003 Location: A small Greek island Services: Slingbox/Freesat Posts: 523	I may be able to explain this: http://www.glodark.com/design/cappics.htm Note that my investigations were into new (or near new) equipment just over 20 years ago. We could reproduce the effect in the laboratory but it was extremely rare in service. Nonetheless, it goes to show that you shouldn't simply toss an amplifier amongst your cardboard boxes in the loft!