Marshall bass amp 3520 (1988)

[rudutch]

picked this up used / in-op - it blew the fuse instantly when the switch turned on.
jumpered the fuse holder and got a nice 60hz hum and some visible arcing in the power transformer.

any idea where to find one?

it's a large round (torroidal) with no part numbers

thanks in advance

[rudutch]

sorry, Drake 07 197/6

[g1]

You likely fried the power transformer by jumpering the fuse holder. There will also be other problems that caused the fuse to blow, like the power amp could be fried.
Figure out whether it is worth spending a lot of money on.

[rudutch]

You likely fried the power transformer by jumpering the fuse holder. There will also be other problems that caused the fuse to blow, like the power amp could be fried.
Figure out whether it is worth spending a lot of money on.

less than 2 seconds -

[g1]

Yup, the circuit breaker for your AC outlet will allow 15 amps. Stuff can blow up real fast.

[olddawg]

picked this up used / in-op - it blew the fuse instantly when the switch turned on.
jumpered the fuse holder and got a nice 60hz hum and some visible arcing in the power transformer.

any idea where to find one?

it's a large round (torroidal) with no part numbers

thanks in advance

Argh! Why do people do this? I've seen 1/4 aluminum dowel cut to fuse lengths before. It almost always turns a simple repair for a qualified tech into a not economically repairable situation.

[Enzo]

Two seconds is a long time to electricity. Electricity moves at the speed of light, which means it can go all the way around this Earth of ours about seven and a half times in one second. Takes a lot less time to go from one end of a transformer to the other.

[J M Fahey]

jumpered the fuse holder

WHY would you do this?
I mean, it can be done somewhat safely on a perfectly working amplifier , but ... on an amplifier which has just killed one .....
Like walking into mine field and somebody telling you: "look!!, there's a mine visible there !! don't step on it !!!" and you: "I'm not so sure it's a mine, I'll step hard on it to check !!"
Well, check you will, for sure !!

[rudutch]

WHY would you do this?
I mean, it can be done somewhat safely on a perfectly working amplifier , but ... on an amplifier which has just killed one .....
Like walking into mine field and somebody telling you: "look!!, there's a mine visible there !! don't step on it !!!" and you: "I'm not so sure it's a mine, I'll step hard on it to check !!"
Well, check you will, for sure !!

okay, does anyone know the output voltage (s) found several but don't care to guess

[MarkusBass]

And you know that you need to fix the amp, not only to replace the transformer, right?
You can look here; http://music-electronics-forum.com/t7764/ or contact the manufacurer: DP Energy Services - World Class Transformers
I don't know the exact voltages but the power supply of the power amp was abot +/-55V. This is DC voltage - you have to calculate AC on your own.

Mark

[J M Fahey]

1) *Maybe* (with a Big "MAY" ahead) the transformer still isn't dead.
Check it on its own.
Build a lamp bulb limiter (search for it), plug the amp there (with proper fuse of course) , *disconnect* all transformer secondaries , tape their ends for safety, and turn the amp on.
*If* the 60/100W bulb blinks and turns off or remains dark red or orange, there is still some hope.
If it shows nothing, the transformer primary blew open.
If it shines brightly, itīs shorted.
Any of two latter cases, it's junk.
Pray it's like the first one.
Post your results.
The power amp and PSU schematic is:

Drake will ignore you, because since 1986 they must have made 20 Million other transformers for thousands of customers, no kidding.
*Maybe* Marshall has one left over in a dusty shelf, wouldn't count too much on that.
Keep the old one just to take some physical measurements to compare to possible generic replacements.
Good luck.
PS: those MosFets are hard to find and expensive, add them to your cost estimates
PS2: post a couple guts pictures showing the space available and parts size.

[rudutch]

isolated the xfmr output at the rectifier, reads 97vac with a 35 ohm load

seems a bit high

[g1]

97VAC or approx. 50VAC on either side of the center tap? What voltage rating are the main filter caps (c37&38) ?

[rudutch]

97VAC or approx. 50VAC on either side of the center tap? What voltage rating are the main filter caps (c37&38) ?

2 leads going to the rectifier, 93vac

Marshall 3520 pictures by rudutch - Photobucket

[J M Fahey]

isolated the xfmr output at the rectifier, reads 97vac with a 35 ohm load

What does this mean?
35 ohms where?