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Did I wreck the amp?
Did I wreck the amp?
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Old 11th September 2019, 12:31 AM   #1
Bud Barnez is offline Bud Barnez  United Kingdom
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Default Did I wreck the amp?

Was just attempting to check a suspected bias immbalance on a working Speakercraft BB2125 2-chanel amplifier I have. I foolishly managed to short out one of the emiter resistors in the process. There was a bit of a pop and the fuse in the back panel blew (fuse in mains plug ok).

Meter probe must have connected one leg of an emitter resistor to a wire link on the PCB. The wire link disentigrated and now the shorted resistor reads 0.9ohms, not 0.4 like the rest. Red line in picture shows what I shorted and green line shows where the wire link ran.

Schematics for these amps are hard to come by and even with one I'd likely struggle.

I tried testing the transistors (still on the board) using multi-meter in diode test mode. I'm very much a novice but recorded the results which look to me like all the left chanel transistors are damaged (all tests showed less than 0.02V); right side I think is ok although may not have recorded the results properly.

So, I guess I'm asking the wise folk here if there's any hope for this amplifier now? If I replaced the left chanel output devices might it be ok, or is the damage likely to be more extrensive.

Grateful for any help or recommendations on further diagnosis.
Attached Images
File Type: jpg Speakercraft damge.jpg (344.7 KB, 238 views)
File Type: jpg Overview.jpg (382.0 KB, 239 views)
File Type: jpg Transistor test.JPG (73.9 KB, 254 views)
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Old 11th September 2019, 01:42 AM   #2
jwilhelm is offline jwilhelm  Canada
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Anything can be fixed, it depends how much time and money you want to put into it. Normally you replace one stage before the damaged parts. If your outputs are blown and the drivers are fine, replace the outputs and drivers. If the drivers are damaged replace the pre-drivers or VAS transistors, depending on the circuit.
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Old 11th September 2019, 05:57 AM   #3
ubergeeknz is online now ubergeeknz  New Zealand
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Quote:
Originally Posted by jwilhelm View Post
Normally you replace one stage before the damaged parts. If your outputs are blown and the drivers are fine, replace the outputs and drivers. If the drivers are damaged replace the pre-drivers or VAS transistors, depending on the circuit.
Why, if they're good? I'm not sure I follow the logic here.
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Old 11th September 2019, 06:52 AM   #4
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by ubergeeknz View Post
Why, if they're good? I'm not sure I follow the logic here.

He's going by the usual proper toubleshooting techniques, as us techs have done from experience.
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Old 11th September 2019, 06:58 AM   #5
ubergeeknz is online now ubergeeknz  New Zealand
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Originally Posted by wiseoldtech View Post
He's going by the usual proper toubleshooting techniques, as us techs have done from experience.
Is it because they are likely to have been stressed and may fail soon anyway?
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Old 11th September 2019, 08:43 AM   #6
indianajo is offline indianajo  United States
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Roger that. Plus taking the case off & on again is $s more than some 80 cent drivers or op amps. Bad drivers can damage your $4 apiece output transistors again.
To be explicit bud barnet, yes you wrecked the amp. Those left channel output transistors are garbage.
Buy output transistors in sets, from the same vendor on the same day, and for best match buy in quantities of 25 (a whole tube). If you can't afford that many, buy 2 or 3 more than you need, measure Vbe and don't use the ones that are not the same value as most of them.
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Last edited by indianajo; 11th September 2019 at 08:49 AM.
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Old 11th September 2019, 08:50 AM   #7
Mooly is offline Mooly  United Kingdom
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Did I wreck the amp?
You need to remove and test the transistors. Remember that they are effectively in parallel and so if just one failed it would affect the readings seen across the others.

Such paralleled devices usually mean that they should all be replaced as a set of matched devices, or at least be from the same batch.

The advice to replace the drivers is absolutely sound and should be followed.

The bias should be turned to minimum before powering the amp up and a current limited supply (bulb tester) used at all times whilst working on this.
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Old 11th September 2019, 09:20 AM   #8
googlyone is offline googlyone  Australia
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Did I wreck the amp?
The initial measurements indicate shorted output devices. This generally takes out the drivers also.

The advice to replace everything after the VAS is not bad advice, though i have to admit that if I were trying to save a buck I'd just do the output devices and drivers.
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Old 11th September 2019, 10:33 AM   #9
jwilhelm is offline jwilhelm  Canada
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Originally Posted by ubergeeknz View Post
Why, if they're good? I'm not sure I follow the logic here.
When parts fail there is usually high current and voltage produced. In the case of arcing like this extremely high voltages are produced. Whatever is driving the damaged parts would have taken a hard hit and are likely damaged even if it's not evident.
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Old 11th September 2019, 10:36 AM   #10
jwilhelm is offline jwilhelm  Canada
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Originally Posted by googlyone View Post
The initial measurements indicate shorted output devices. This generally takes out the drivers also.

The advice to replace everything after the VAS is not bad advice, though i have to admit that if I were trying to save a buck I'd just do the output devices and drivers.
In most cases you might be saving literally a buck. Is it worth tearing down a second time to save $1? I value my time a lot more than that!
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