Ion Obelisk 2 keeps blowing fuses

[cchean]

My Obelisk 2 was playing fine until it blew the internal fuse. This fuse is located between the mains and power transformer. it blows upon turning the amp on, regardless if there are speakers connected or not.

What should I check first? I have the schematics, downloaded from vinyl engine.

 

[a.palfreyman]

I have an Obelisk 3 that I haven't used for many years now but have done some work on it in the past.
First off, I assume you mean you have replaced the fuse and it blows straight away? Did you use the correct value / type?
I believe the obelisk 2 and 3 are very similar. Ob3 schematic says 2.5AT for 110V usage.
When it first blew, did you have a chance to 'feel' the amp casing? Was it cool of hot?

 

[cchean]

I have an Obelisk 3 that I haven't used for many years now but have done some work on it in the past.
First off, I assume you mean you have replaced the fuse and it blows straight away? Did you use the correct value / type?
I believe the obelisk 2 and 3 are very similar. Ob3 schematic says 2.5AT for 110V usage.
When it first blew, did you have a chance to 'feel' the amp casing? Was it cool of hot?

Yes, it blows right away. Mine uses 1.6A 250V, I replaced for the same value.

I did not feel the amp casing, I don't think it was too hot. The only thing worth mentioning is that the amp was in storage for a few years.

 

[MUTTY1]

Sleeping for a few years then the full mains! I know nothing but am empathetic I believe there are ways of waking up stuff slowly that can be done without much cost….

 

[a.palfreyman]

Aren't you US based? 110/120V mains?
From your original post, it was working, then stopped having blown a fuse, which it now continues to do. That indicates it was OK but not now. If however you have too small a fuse, it could be that in itself. According to the Ob3 schematic, 110/120v operation requires a 2.5AT, being 2.5A and the T means timed or slow blow. If you have a quick blow, especially if this is under rated, the in-rush current will just blow the fuse.

 

[Arkless Electronics]

/\ This.USA 110V mains has to give over double the current of our 240V mains for the same power, hence the fuse needs to be about twice the rating. Obvs it could have another issue but I'd check this first!

 

[cchean]

Sleeping for a few years then the full mains! I know nothing but am empathetic I believe there are ways of waking up stuff slowly that can be done without much cost….

Yeah, my fault, I know. Shoud've used a variac...

 

[cchean]

Aren't you US based? 110/120V mains?
From your original post, it was working, then stopped having blown a fuse, which it now continues to do. That indicates it was OK but not now. If however you have too small a fuse, it could be that in itself. According to the Ob3 schematic, 110/120v operation requires a 2.5AT, being 2.5A and the T means timed or slow blow. If you have a quick blow, especially if this is under rated, the in-rush current will just blow the fuse.

I am using slo-blo fuse, rated @ 1.6A. I could try using a 2.5A but I remember the amp working fine before I put it in storage.

 

[Arkless Electronics]

I am using slo-blo fuse, rated @ 1.6A. I could try using a 2.5A but I remember the amp working fine before I put it in storage.

Unless you have 240V mains in North Carolina... you need to be using a 2.5AT fuse. If it still blows then it could be something more.

 

[S-Man]

Sleeping for a few years then the full mains! I know nothing but am empathetic I believe there are ways of waking up stuff slowly that can be done without much cost….

Yes!

The main smoothing caps probably need reforming after a hibernation.
Wake it up with a variac, ideally monitoring current so that the caps can be reformed safely. If the variac will do it, take the voltage higher than your highest mains (for a while).

 

[a.palfreyman]

Yes, precisely guys. Was trying to find out if it was blowing a properly rated fuse before we go any further. IF IT DOES BLOW A PROPERLY RATED FUSE, then we can look if it has a shorted rectifier diode, or see if the caps are just leaky, but need to establish the absolute basic possible fuse problem first.

 

[john.luckins]

I have mended a couple of Obelisk one's that exhibited the same symptoms. In both cases it was caused by thermal runaway on one of the output channels. The heatsinks are IMHO too small and the thermal coupling to the current limiting and bias setting transistors is insufficient. There is also a clever delay circuit to the application of full DC voltage to the output stage drive which relies on a 22uF capacitor. If this goes open circuit with age then the slow switch on fails and this sees off the outputs stage transistors.

I understand the Obelisk 2 added a larger heatsink and coupled the limiting transistors to it better but I don't know if the delay circuit was changed. It was a clever little addition so I suspect not.

It would be handy if you could unplug and open the amp up to take some photos and post them here. There might be signs of damage around the output stage or elsewhere. If you have a meter you might be able to measure for a shorted rectifier diode or two using the resistance range. If you have solder skills and kit it may also be possible to isolate the fault further as the amp boards in the later models were powered by soldered wire links.

Finally, if this particular amp is as original and has had no recap done before, then this is highly recommended at this age, but not before the amp itself checks out OK. These are nice amps and well worth the effort to keep them going. My sisters sounds quite marvellous through Rogers LS7's.

John

 

[cchean]

I have mended a couple of Obelisk one's that exhibited the same symptoms. In both cases it was caused by thermal runaway on one of the output channels. The heatsinks are IMHO too small and the thermal coupling to the current limiting and bias setting transistors is insufficient. There is also a clever delay circuit to the application of full DC voltage to the output stage drive which relies on a 22uF capacitor. If this goes open circuit with age then the slow switch on fails and this sees off the outputs stage transistors.

I understand the Obelisk 2 added a larger heatsink and coupled the limiting transistors to it better but I don't know if the delay circuit was changed. It was a clever little addition so I suspect not.

It would be handy if you could unplug and open the amp up to take some photos and post them here. There might be signs of damage around the output stage or elsewhere. If you have a meter you might be able to measure for a shorted rectifier diode or two using the resistance range. If you have solder skills and kit it may also be possible to isolate the fault further as the amp boards in the later models were powered by soldered wire links.

Finally, if this particular amp is as original and has had no recap done before, then this is highly recommended at this age, but not before the amp itself checks out OK. These are nice amps and well worth the effort to keep them going. My sisters sounds quite marvellous through Rogers LS7's.

John

Hello John and thanks for your feedback. I opened up the amp and took a few pictures, but I could not see any physical damage. I've already ordered some 2.5A fuses as suggested by other members, but if that does not work, I have solder skills and can take measurements if you are kind enough to help me and guide me through.