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Old 13th March 2011, 10:23 PM   #1
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Default SWR Basic Black fried PT

First off - I am a newbie to solid state amps.
My friends Basic Black bass amp has a cooked power transformer.
We were playing a gig and we could smell something burning and it blew the fuse.
Little did I know, he had it plugged into a 2 ohm cabinet!
I took it home and put in a new fuse and it promtly blew the internal fuse.
I check the resistance in the primary and it is about 1.4 ohms.
The red wires on the output (91 v) look like they overheated.
The areas of the board where the transformer wires are attached and around the output transistors are attached look like they got hot.
The transistors themselves LOOK ok.

My question is; can the PT get fried from overload and the rest of the components survive?

I can't seem to find a schematic that includes the power supply section.
The PT is a Mercury Magnetics 560005.
I called them and they are $150 and they would need to wind one since they don't have them in stock.
The spec sheet they sent me says the PT has one primary but the actual transformer has 3. a blue set of wires is not used, and the primary wires used are actually two sets of black and white (one set has a stripe).

Does anyone know where to get a generic transformer?
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Old 13th March 2011, 10:47 PM   #2
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you must first check the diodes and especialy the output devices first.
Otherwise you will be replacing another transformer.
First disconnect the transformer from the rest of the circuit to verify if indeed the transformer is actualy blown.
I have seen some survive in such a case. jer

p.s. even if they look okay they very well may be shorted.

Last edited by geraldfryjr; 13th March 2011 at 10:49 PM.
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Old 13th March 2011, 10:53 PM   #3
jaycee is offline jaycee  United Kingdom
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Paste up the spec sheet and we can help better.

BTW, I would be very surprised if the output transistors survived this. You need to check them.
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Old 13th March 2011, 10:57 PM   #4
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also if you can find out what the voltage and power rating of the transformer .
I'm sure one could find a suitable replacement for alot less.

I did the same thing to my ampeg V4-B ( sad) and cooked the output transformer but there is no way around that one.
$200 + for a new one and just as much (if not more) to have it rewound. jer

Last edited by geraldfryjr; 13th March 2011 at 11:02 PM.
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Old 14th March 2011, 05:34 AM   #5
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In regards to the PT - I will post a pic of the spec sheet.
I don't know why it shows a single primary coil in the description but in real life there is actually 3 primary windings.
Click the image to open in full size.
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Old 14th March 2011, 05:37 AM   #6
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In regard to the output transistors - how do I check them?
Here's what it looks like at the leads going to the board...
Click the image to open in full size.
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Old 14th March 2011, 05:40 AM   #7
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The transformer looks like this...
Click the image to open in full size.

And since the resistance at the primary (two sets are used) shows as almost 1 ohm I figured no matter what I do to the secondary it will draw tons of current.
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Old 14th March 2011, 05:46 AM   #8
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I expected to see burned components but it looked rather good.
Here are some pics...
Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

These are trying to show the areas that look like they got hot.
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Old 14th March 2011, 05:48 AM   #9
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This 5 watt resistor looks suspect.
Paert of the power supply that I can't find a schematic for.
Click the image to open in full size.
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Old 14th March 2011, 06:00 AM   #10
Pano is offline Pano  United States
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Stevie B. Nice photos, well done, but please see this thread.
Pictures -- Why Not attach Them ??
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