Ion Obelisk 3

[Spev]

Hi All

Just taken delivery of a faulty obelisk 3. One of the power amp stage resistors is badly fried and the output cap on that channel is bulging. Photo and schematic are attached. R17 is the burnt out resistor. Thought I'd throw this out to the masses for any input/advice on problem solving before I dive in with my probes...

http://www.growthconsulting.frost.co..._power_amp.gif

http://www.growthconsulting.frost.co.../$File/ion.jpg

cheers in advance

James

[homertooties]

hi there wondering did you win this amp on ebay?

[Spev]

er, yeah, that'd be the one - wasn't from you was it?

couldnt resist a new project at that price...

[homertooties]

What a lovely designed amp simple and pure sound in mind similar to cyrus ones and twos,your gonna have to get your meter in there looks like the amp has had a major dc problem running round the board where it shouldnt,check ouput stage,drivers,diodes across output if any,resistors,psu caps,all elects in board ,the bias might be knacked to ,might aswell replace bias pots as these cause a lot of greif when the alter value,or do it the cyrus way,if you fix the amp and can stabilse the bias measure the pots and replace with a fixed value,always go fo lower than higher,something to do for the winter.good luck

[homertooties]

Hi no i did not sell it but i was watching it small world eh

[homertooties]

sorry forgot to say driver transistors and predrivers with all connecting parts,time consuming but worth it if you power up stick a 6o watt light bulb in series with mains to limit current,if bulb stays on switch off and check amp,if goes out straight away your on your way to having a piece of british hifi resserection

[Spev]

cheers, checking dc around the board was first on my list...

I've worked out for £30 I can completely rebuild the power amp section from scratch, so that always an option...

some of the other resistors in the bad channel measure the wrong value - I can only assume they've changed due to excessive temp over time.

there's what looks like condensation on most of the board - I'm assuming that's probably electrolyte from the fried output cap?

need to get a replacement cap and resistor in there and then I can start measuring...

[Tube_Dude]

Originally posted by Spev
Hi All

Just taken delivery of a faulty obelisk 3. One of the power amp stage resistors is badly fried and the output cap on that channel is bulging. Photo and schematic are attached. R17 is the burnt out resistor. Thought I'd throw this out to the masses for any input/advice on problem solving before I dive in with my probes...

James

Hi james

It look like your output darlingtons TR8 and TR9 are burned and the output is at 60 volts , the positive voltage rail.

As the output capacitor C18 is only 35 volts rated (bad design practice ) , it exploded..

So , inspect output Darlingtons and associated resistors in the output stage , and put 63 volts rated capacitors at the output...

Cheers

[Spev]

the output transistors actually look fine from an initial visual inspection - not really sure what signs they'd show though - would they appear blackened if fried?

having said that - it would make sense for that to be the reason if the output cap and R17 are the destroyed items...

I need to check the voltage coming in to the power amp as well as the psu caps are only 63V rated... not sure if they're the original caps or if someone has been tinkering (looks like it from the glue around the power cable socket...)

[Spev]

Right, I've rebuilt the power amp section and got everything up and running. The only thing that's concerning me is the bias setting. On the above schematic is suggests 10mV drop between the two points shown.

Now, if I set it to that I get distortion at any level above very quiet. When I received the amp the variable resistors were set such that both channels were roughly 200mV. So I've set the bias to roughly 90mV, which gets rid of distortion but makes the amp run hot - the heatsink gets really hot... I can only imagine how hot it would have got with the bias at 200mV (which is probably why one channel fried...). Without a scope I obviously can't tell what the inaudible distortion is like.

Does this sound about right? I'm wary of it running hot, but the mV value given in the schematic just makes this thing distort...

Cheers

James