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Old 23rd Jun 2019, 6:13 pm   #1
Focus Diode
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Default Thorn 1400 fuse blowing again.

Some may recall me mentioning a Ferguson 3645 16" portable I obtained which was found to have a blown fuse despite no apparent shorts. A replacement blew instantly. The HT rectifier and mains filter capacitor were replaced in case they were breaking down under load. Fitting a new dropper and powering up via a lamp limiter the fuse held with no further problems when full mains was applied.

The set hadn't been switched on for a few months, but on powering up this evening I had a dead set. The 1.5A mains fuse was found blown again!

The system switch has been ruled out as it was beyond repair, so systems are now changed manually.

With the usual suspects replaced I can perhaps suspect the main smoothing can or on off switch perhaps?

Clearly, the original fuse blowing problem has yet to be rectified! Any thoughts?
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Old 23rd Jun 2019, 6:23 pm   #2
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Default Re: Thorn 1400 fuse blowing again.

As it had been standing for a while I would suspect carbonization somewhere on the PC board.

It might save a lot of head scratching if you simply put a large fuse in [5amp?] and wait for it to pop something. Not a lot of damage can be caused in the 1400. Very simple circuit mains input wise. I have never had intermittent fuse blowing on a 1400 and I must have repaired literally 100's!
The on/off switch is a possibility, it may have broken down due to a faulty mains filter and have an internal conductive coating due to the flash. John.
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Old 23rd Jun 2019, 8:13 pm   #3
toshiba tony
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Default Re: Thorn 1400 fuse blowing again.

Isn't the fuse after the switch? From memory it is.
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Old 23rd Jun 2019, 8:48 pm   #4
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Default Re: Thorn 1400 fuse blowing again.

Quote:
Originally Posted by toshiba tony View Post
Isn't the fuse after the switch? From memory it is.
Yes it is in the circuits I have seen.
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Old 23rd Jun 2019, 8:57 pm   #5
ms660
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Default Re: Thorn 1400 fuse blowing again.

Schematic and other info for the BRC 1400 chassis:

https://archive.org/details/RadioTel...dels/page/n521

Lawrence.
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Old 6th Jul 2019, 6:24 pm   #6
Focus Diode
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Default Re: Thorn 1400 fuse blowing again.

Hi John,
Many thanks. Nothing wrong with the set as things turned out. Perhaps I should try an anti surge type replacement fuse when I get hold of some.

Still some issues to address with the set such as lack of width and the classic UHF aerial socket dry joint.

Cracking picture though. Photos taken on 625.
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Old 6th Jul 2019, 7:13 pm   #7
Philips210
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Default Re: Thorn 1400 fuse blowing again.

Hi.

I'm probably wrong here but I had a case of frequent fuse blowing in an alarm system but couldn't find anything wrong. When I tried a fuse of the same ratings but brand new rather than new old stock, the problem disappeared. So I checked my batch of NOS fuses and found that on close examination, there were signs of corrosion on the fuse wire. No doubt these fuses must have been in damp conditions and I guess effectively derated hence the fuse blowing.

Thinking of frequent fuse blowing, the first generation Thorn TX9 with its thyristor power supply could be a nightmare to solve. Thankfully the problem was put to bed when Thorn went over to a chopper power supply.

Regards,
Symon
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