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#241803 - 2015-10-22 00:16 More 556 Power-Amp help
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
556 has been unused for two years. Powered it up, operated for ~ 30 seconds, then blew a fuse. Disconnected all inputs and outputs, changed fuse. Fuse blew in a few seconds.

How do I disassemble so I can view and measure inside? Screws near top between heatsink fins? Screws on bottom? Remove screws on back, then remove back?

There are no longer any dealers that I know of in USA.
I am a electronics engineer with much experience in repairing similar amps.

Please, just a simple clue. Thanks


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#241810 - 2015-10-22 06:34 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2004-08-07
Posts: 7,833
Ian Offline
President of the Imperial Galactic Government
Ian Offline
President of the Imperial Galactic Government

Registered: 2004-08-07
Posts: 7,833
Loc: Surrey, England, UK
Try Marc Koval on this forum.


Meridian owner since 1992
Prime & PSU, Focal Elear headphones, roon (ROCK 8Gi5 Nuc), Explorer 1 & 2, F80. 200/203, MC200 and various Sonos.
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#241834 - 2015-10-22 14:32 Re: More 556 Power-Amp help [Re: Ian]
Registered: 2010-02-03
Posts: 846
Mtns Offline
Pan-dimensional being
Mtns Offline
Pan-dimensional being

Registered: 2010-02-03
Posts: 846
Loc: North Carolina, USA
He is at CEDIA in Dallas thru the weekend and I believe has curtailed his email activity.


James
NC1: 861v4+ID40, 218, 251(to outdoors), DSP8000SE (upgraded). 5500VC, 5500s, DirecTV, Oppo BD103, QNAP251, Roon
NC2-see FL 2 with Triad Silver Speakers and Acurus Amps

FL1: G61R, 218, DSP8000SEs, 5500HC, DSP5000s, DirecTv, Oppo BD103, QNAP469, Roon
FL2: Marantz 7005, M60s, M60C, M33s, Oppo BDP-93, MS200, Roon
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#241844 - 2015-10-22 18:02 Re: More 556 Power-Amp help [Re: Mtns]
Registered: 2002-08-28
Posts: 4,532
Mr Meridian Online content
President of the Imperial Galactic Government
Mr Meridian Online content
President of the Imperial Galactic Government

Registered: 2002-08-28
Posts: 4,532
Loc: Perthshire, Scotland
Hi James

IIRC, it's the three screws along the top of the rear face which need removed. I think the top plate, including the bonded glass panel, lifts from the rear and slides backwards from a full width horizontal slot inside the front panel.

I don't remember any side screws holding it, just the front slot and three rear screws. There's a certain amount of compression fit from the heatsinks at each side, so it is held quite firmly and may need some jiggling to release it.

Cheers

George

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#241880 - 2015-10-23 18:35 Re: More 556 Power-Amp help [Re: Mr Meridian]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
Hi George,

Yes, three screws at top of back panel need to be removed, but also two screws on top left and two screws on top right through heatsinks.

Top is now removed. Measured 0 ohms across one of the four 6800uF, 63V filter capacitors. Next step is to remove capacitor to determine if short is internal to capacitor or output transistor short.

Thanks for your assistance in top cover removal.


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#241990 - 2015-10-26 21:14 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2000-06-08
Posts: 314
Marc Koval Offline
Hitchhiker
Marc Koval Offline
Hitchhiker

Registered: 2000-06-08
Posts: 314
Loc: Dglv, Georgia, USA
It will be one of the output transistors. not a lot of fun getting everything lined up.

Let me know if you need any help

Marc


Marc Koval
The Repair Shop
404-542-4902
marc<at>the-repair-shop<dot>com

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#242155 - 2015-10-30 23:18 Re: More 556 Power-Amp help [Re: Marc Koval]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
Hi Marc,

I've started to test amp. Unplugged power transformer wires from amp boards. Have powered one of amp boards by supplying (current limited) dc voltages to pins that secondary wires were connected.

Have slowly increased voltages from +- 5vdc to +-40vdc over a period of 2 hours. Measuring 75mA power supply current and +18mV dc offset from output terminals.

How much higher should I take supply voltages? 50vdc? 55vdc? 60vdc?

How much supply current?

Limit of acceptable dc offset?

Thanks


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#242182 - 2015-10-31 18:25 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
Connected first channel back to power transformer secondary windings. +v = 55.4 vdc, -v = 54.8 vdc. Output offset voltage = 14mV.

Connected second channel to current limited variable power supplies. At +v = +1.2v and -V = -1.2v, supply current is ~ 100mA. Output is at -1.2vdc.

As I increase supply voltages (I stopped at 2.5vdc), output continues to track -v and supply current increases linearly.

My conclusion, one of the 5 pnp output transistors is shorted.

Anyone willing to concur with my conclusion? Anyone able to describe output circuit?


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#242191 - 2015-10-31 22:40 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
One last help please.

Output transistors - PNP and NPN
2Nxxxx and 2Nyyyy

What are the correct part numbers?


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#242206 - 2015-11-01 18:35 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2010-01-21
Posts: 63
JamesBrennan4 Offline
Mostly harmless
JamesBrennan4 Offline
Mostly harmless

Registered: 2010-01-21
Posts: 63
Loc: South Carolina, USA
To answer my own question.

PNP BD744C
NPN BD743C

TO-220 package
BVcbo = 110v
BVceo = 100v

Ic max = 15A

90 watts


200, 501, 505, 506, 508.20, 541, 551, 555, 556, 557, 561, Allison IC-20
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#288722 - 2019-11-04 09:51 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2012-09-23
Posts: 10
Zam Offline
Harmless
Zam Offline
Harmless

Registered: 2012-09-23
Posts: 10
Loc: Sydney, Australia
Hi James, did you ever get your 556 power amp going?

I'm experiencing a very similar problem with mine blowing fuses very quickly.
I'm looking at my TO-220 transistors, but they are LM317 and 337. I can't see any of the transistors you mention?

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#288723 - 2019-11-04 09:52 Re: More 556 Power-Amp help [Re: JamesBrennan4]
Registered: 2012-09-23
Posts: 10
Zam Offline
Harmless
Zam Offline
Harmless

Registered: 2012-09-23
Posts: 10
Loc: Sydney, Australia
I can't see BD744 transistors anywhere?? Is it bcos my unit is a 240V version?

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