Audio Research D125 blown up

[Sal Paradise]

Dear Hive mind,
I have an AR D125 and there has been a component that has burnt out.
I have traced it to being R174 400 Ohm, 7W OR F3 1/8A fuse.
Are these the same component? It is a blue glass device that does not look like a resistor.

I have attached pictures of the schematic and the unit with the compenent in question.
I need to replace this device so any help would be greatly appreciated.
Thanks








 
 

 

[George 47]

You may get more responses if people did not have to join DiY Audio.

 

[Jazid]

I can't tell what it was from the information and pics you've provided. I reckon DIY Audio is the place most likely to generate a useful response since there may well be people who've worked on this amp around there. My only opinion is that these components shouldn't have burnt out like that, so they are the sacrificial link but the rest of the circuit needs to be checked over because something is clearly wrong. Replacing like for like can be expected to lead to the same result again, also if the component is designed as sacrificial it may have an unusual temperature coefficient that needs to be followed. I'd certainly suggest checking the operation of the MOSFets and surrounding ciruit for starters, as well as any power valves attached.

 

[uzzy]

Owners manual and circuit diagrams here  https://www.arcdb.ws/Database/D125/ARC_D125_manual.pdf

 

[Birdbrain]

A 30-year old valve amplifier which gets hot.

Has it ever been properly serviced?

I would find the best person to give it a thorough check-up.

Just replacing a fuse probably isn't a great idea.

 

[uzzy]

A 30-year old valve amplifier which gets hot.

Has it ever been properly serviced?

I would find the best person to give it a thorough check-up.

Just replacing a fuse probably isn't a great idea.

What he said .. it is a valuable bit of kit that needs loving care - 

 

[mac72]

It looks like fusible 7W resistors AudioResearch, McIntosh and other american brands used  quite often in a screen voltage circuit . It blew as one or more of the screens started conducting too much current or one of the fet's or diodes failed , 

 

[rabski]

It looks like fusible 7W resistors AudioResearch, McIntosh and other american brands used  quite often in a screen voltage circuit . It blew as one or more of the screens started conducting too much current or one of the fet's or diodes failed , 

At a guess, I think you're spot on Slav.

Looking at the circuit board, it's been seriously hot, and not just for a few seconds either. Whatever caused it needs to be sorted. Just replacing it is a bad idea, as it'll either be a waste of time, or may allow the fault to worsen.

 

[mac72]

At a guess, I think you're spot on Slav.

Looking at the circuit board, it's been seriously hot, and not just for a few seconds either. Whatever caused it needs to be sorted. Just replacing it is a bad idea, as it'll either be a waste of time, or may allow the fault to worsen.

That's right , it needs someone competent to find source of the fault , overheated/burnt mark is a proof that resistor must have been working for a long time at the border of it's working range , replacing it won't cure the problem .

 

[mac72]

Dear Hive mind,
I have an AR D125 and there has been a component that has burnt out.
I have traced it to being R174 400 Ohm, 7W OR F3 1/8A fuse.
Are these the same component? It is a blue glass device that does not look like a resistor.

I have attached pictures of the schematic and the unit with the compenent in question.
I need to replace this device so any help would be greatly appreciated.
Thanks








 
 

Replied to your query on DIYAudio , don't use standard resistor in this position and see what happens as suggested by someone over there , if you decide to do so use "flame proof fusible resistor 400R/7W" as it would "fuse" at around 0.125A as on the diagram , standard resistor would just get extremely hot and keep working for way longer - you might do more damage that  way . Check all transistor, diodes and power tubes before you attempt to power it up