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Old 7th June 2010, 03:38 PM   #1
joegio is offline joegio  United States
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Default 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
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Old 7th June 2010, 07:41 PM   #2
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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.

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Old 7th June 2010, 08:08 PM   #3
joegio is offline joegio  United States
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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
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Old 7th June 2010, 10:48 PM   #4
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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.

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Old 7th June 2010, 11:45 PM   #5
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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.
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Old 8th June 2010, 01:11 AM   #6
joegio is offline joegio  United States
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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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Old 6th February 2013, 12:18 AM   #7
joegio is offline joegio  United States
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Smile its alive

So its been a while but I finally got the never to dig in.

First I poked around with a multimeter to see if I could find some shorted transistors.
Nothing really jumped out except that I was getting a low resistance (4 ohms or so) across the diode in the output transistor bank (or so it seemed).

Next I used a variac to get the channel up without blowing a fuse, this was about 4 volts DC. I noticed that the rail that was blowing the fuse a larger voltage drop than the other 3 good rails.I checked the voltage drop across each emitter resistor, I found one that had about twice as much drop than any of the others.

I removed the suspect transistor and powered it back up for to 4 volts, now all resistors had about the same drop and now the rail that the same voltage drop as the other 3 rails. I replaced the NPN transistor with an ON 15022G for $5.00 and now it works and sounds great. The suspect transistor had about 4 ohms of resistance every which way I checked it.

I set the bias so that I have about 1.5 mV across each emitter resistor, and the heat sinks get warm but not hot after an hour.

I also noticed that before the transistor went I had a hum in the PS transformer that has now seemed to have gone away.

Thanks for the help.

Joe
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Old 6th February 2013, 12:28 AM   #8
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Well done. Its very satisfying to fix an amp.

I have seen me resort to taking every transistor and diode out to check them.
On one fault it was the last transistor I took out that was faulty.
While all the semiconductors are out I check all the passive components.

Another trick is to feedback the output of the VAS back into the LTP and this allows powering up the amp without blowing fuses.

One problem with power amps is because the amp is a loop a fault anywhere can feed right around the loop putting all voltages into unusual states.

Your problem would have showed up on a multimeter ohms check of the output transistors.
I usually find transistors have shorted D-S or C-E but I have known G-S shorts that took out a class d chip with it.

Its all good clean fun so long as you don't blow yourself up !
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Old 6th February 2013, 12:33 AM   #9
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That's great. It's a challenge to diagnose and fix an amp.

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