s150 blown neg rail fuse

joegio · 7th June 2010, 03:38 PM

Hi

I've been enjoying audio for some years now, and have even built my own pro-ac r2 clones and recently the ALPHA LS. I've never had any amp issues (until now) so I'm quite new to the whole circuit/smoke thing. My Threshold S-150 just quite working on me yesterday. I was pushing it a bit hard but not enough to get it hot, just over warm. One channel now blows the neg rail fuse on power up. I'd like to try to repair myself as I can not afford to send out for repair at this time. A visual inspection shows no signs of burnt areas. My hope is someone who might know how/where to start trouble shooting with the particular model could get me started.

Thanks
Joe

Nelson Pass · 7th June 2010, 07:41 PM

Occasionally a fuse just gets old and starts to have greater
resistance internally or in the mechanical connection, and
then it heats up and dies.

I would replace it with a fast blow fuse of a lower value and
turn on the amp with nothing connected to it, and see if the
fuse blows again.

joegio · 7th June 2010, 08:08 PM

I powered up for 30 sec then down with a new 3 amp fuse with no input or outputs connected, then checked all fuses, the left neg fuse and it is now blown.

Joe

Nelson Pass · 7th June 2010, 10:48 PM

That means the amp is broken. Probably failed output devices.

If you check Collector - Emitter with an ohmmeter, the failed
device will be a probably be shorted. Compare that with other
devices, and the good ones will measure about an ohm higher
because of the emitter resistors on the parts.

Unplug the amp and pull the output fuses before you do this.

nigelwright7557 · 7th June 2010, 11:45 PM

You will probably need to check the driver transistors too as sometimes an output transistor will take a driver transistor with it.

I would check them all before considering powering it up again.

Dont forget to rebias the amp if new transistors are fitted.

joegio · 8th June 2010, 01:11 AM

OK just to be clear ...

Can I check the devices on the board or do I have unbolt and de-solder the devices ?

I have seen other posts that state that the correct bias is achieved when the heat sinks are at a certain temperature range, what would be a "safe" bias current to get started at ? Where/how should this value be measured ?

What would the correct replacement device(s) part number be ?

I'm using the s300-SII schematic as a reference assuming that it is similar minus 4 output devices (Q22, Q23, Q29, Q30).

At this point all I can offer is my thanks for your patience.

Thanks
Joe

7th June 2010, 03:38 PM	#1
joegio diyAudio Member Join Date: Oct 2008	s150 blown neg rail fuse Hi I've been enjoying audio for some years now, and have even built my own pro-ac r2 clones and recently the ALPHA LS. I've never had any amp issues (until now) so I'm quite new to the whole circuit/smoke thing. My Threshold S-150 just quite working on me yesterday. I was pushing it a bit hard but not enough to get it hot, just over warm. One channel now blows the neg rail fuse on power up. I'd like to try to repair myself as I can not afford to send out for repair at this time. A visual inspection shows no signs of burnt areas. My hope is someone who might know how/where to start trouble shooting with the particular model could get me started. Thanks Joe

7th June 2010, 11:45 PM	#5
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	You will probably need to check the driver transistors too as sometimes an output transistor will take a driver transistor with it. I would check them all before considering powering it up again. Dont forget to rebias the amp if new transistors are fitted. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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7th June 2010, 07:41 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Occasionally a fuse just gets old and starts to have greater resistance internally or in the mechanical connection, and then it heats up and dies. I would replace it with a fast blow fuse of a lower value and turn on the amp with nothing connected to it, and see if the fuse blows again.

7th June 2010, 08:08 PM	#3
joegio diyAudio Member Join Date: Oct 2008	I powered up for 30 sec then down with a new 3 amp fuse with no input or outputs connected, then checked all fuses, the left neg fuse and it is now blown. Joe

7th June 2010, 10:48 PM	#4
Nelson Pass The one and only Join Date: Mar 2001	That means the amp is broken. Probably failed output devices. If you check Collector - Emitter with an ohmmeter, the failed device will be a probably be shorted. Compare that with other devices, and the good ones will measure about an ohm higher because of the emitter resistors on the parts. Unplug the amp and pull the output fuses before you do this.

8th June 2010, 01:11 AM	#6
joegio diyAudio Member Join Date: Oct 2008	OK just to be clear ... Can I check the devices on the board or do I have unbolt and de-solder the devices ? I have seen other posts that state that the correct bias is achieved when the heat sinks are at a certain temperature range, what would be a "safe" bias current to get started at ? Where/how should this value be measured ? What would the correct replacement device(s) part number be ? I'm using the s300-SII schematic as a reference assuming that it is similar minus 4 output devices (Q22, Q23, Q29, Q30). At this point all I can offer is my thanks for your patience. Thanks Joe

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