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#288726 - 2019-11-04 11:14 556 Blowing Fuses
Registered: 2012-09-23
Posts: 10
Zam Offline
Harmless
Zam Offline
Harmless

Registered: 2012-09-23
Posts: 10
Loc: Sydney, Australia
My 556 power amp (240V version) has just developed a problem with blowing the 3.15A fuse. I've checked the power supply and it still outputs the correct AC voltage. I have isolated the problem to just one of the boards (right channel). As soon as the troublesome board is connected the fuse will blow. There are absolutely no visible signs of burning out or deterioration on the PCB or the components. I am expecting to replace the electrolytic capacitors anyhow, as the unit is over 20 years of age now.

I have read from another user that it could be a bad output transistor. However, the transistors he was identifying didn't match any of the transistors I can see on the board, or on the circuit diagram. Is anyone able to shed any light on this? Apparently, there are a total of 5 output transistors in TO-220 form. I just can't see those there. Any help provided will be appreciated.

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#288896 - 2019-11-12 22:26 Re: 556 Blowing Fuses [Re: Zam]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
LM317 and LM337 are 15v voltage regulators.

The output transistors are BD743C (NPN) and BD744C (PNP). Number is listed on Meridian schematic and on my actual 556 amplifier. There are actually 6 NPN transistors and 6 PNP transistors per output. 5 of 6 are for output and 1 is used for driver for those 5 transistors. All 12 transistors are secured to heatsink plate on side of unit.

I found a collector to emitter short. The capacitors were good.


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#288933 - 2019-11-14 12:20 Re: 556 Blowing Fuses [Re: JamesBrennan4]
Registered: 2012-09-23
Posts: 10
Zam Offline
Harmless
Zam Offline
Harmless

Registered: 2012-09-23
Posts: 10
Loc: Sydney, Australia
Doh, now I know why nobody posted any ideas on this one. I must seem like the dumbest user on this site. I can see where those transistors are now. I’ll post my findings once I get them out of circuit and tested.

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