Author Topic: Amplifier Repair - Need Output Transistors  (Read 3923 times)

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Offline sheek88

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Amplifier Repair - Need Output Transistors
« on: March 31, 2016, 10:46:49 pm »
Hey everyone,

I have a Denon SA-3900 stereo amplifier that I'm trying to repair. It blew up a few years ago, and i'm just getting around to fixing it now. I've measured the output transistors in circuit, and at least one of them is blown (short circuit). I'm having trouble finding replacement transistors for it. The model numbers are NEC B617 and D587 (they're a matched pair). There's two pairs, and if I can find replacements I'll replace all 4 just to make sure the circuit is balanced.

The closest replacement I can find are these: http://www.ebay.com.au/itm/1pair-Transistor-NEC-MT-100-2SB617-2SD587-B617-D587-/272042053403
However the bottom numbers are different to mine. Does this matter, or will they work? I was hoping to find a newer transistor with the same specs to replace with, but I'm having trouble finding any data on these transistors.

The only data i've been able to find is here: http://www.hitech-egy.com/product-details-en.php?id=274
But again, the bottom numbers are different.

Any help would be appreciated :)

« Last Edit: March 31, 2016, 10:49:12 pm by sheek88 »
 

Offline klunkerbus

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Re: Amplifier Repair - Need Output Transistors
« Reply #1 on: March 31, 2016, 11:08:41 pm »
Bottom numbers are likely a date code, lot code, batch number, or something like that.
 

Offline sheek88

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Re: Amplifier Repair - Need Output Transistors
« Reply #2 on: March 31, 2016, 11:17:58 pm »
Thanks, I thought they might be something like that, but wasn't too sure. So they eBay ones I linked should be fine? Would I be safe to replace just one pair if the other seems OK, or should I be safe and replace both pairs?
 

Offline klunkerbus

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Re: Amplifier Repair - Need Output Transistors
« Reply #3 on: April 01, 2016, 12:20:09 am »
Of course, I can't truly speak to the suitability of the parts.  The ones you found look OK, but I'm not familiar with them.  There's also a lot of fake/counterfeit stuff on eBay that always raises some concerns. 

If it were me, I'd replace both pairs, just to cover the case that whatever damaged the one channel also did some latent damage on the one that still works. 
 

Offline sheek88

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Re: Amplifier Repair - Need Output Transistors
« Reply #4 on: April 01, 2016, 12:25:22 am »
Of course, I can't truly speak to the suitability of the parts.  The ones you found look OK, but I'm not familiar with them.  There's also a lot of fake/counterfeit stuff on eBay that always raises some concerns. 

If it were me, I'd replace both pairs, just to cover the case that whatever damaged the one channel also did some latent damage on the one that still works.

Thanks, I've decided to give the eBay ones a go, and will replace all 4. Hopefully they didn't take out too much else when they blew...
 

Offline Anks

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Re: Amplifier Repair - Need Output Transistors
« Reply #5 on: April 01, 2016, 07:24:14 am »
Also check the bias string and transistor along with the drivers ,emitter resistors and surrounding components. Nothing worse than putting new outputs in just to find them shorted after first turn on.

As a thought I have never seen fake transistors in that type of package not to say they don't exist.
 

Online mzacharias

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Re: Amplifier Repair - Need Output Transistors
« Reply #6 on: April 02, 2016, 02:40:20 pm »
Be wary of bias diode(s) if any. Monitor DC voltage across it and if it appears unstable or open circuit, shut down immediately. Also monitor millivolts across the emitter resistors. Any indication of excessive idle current - shut down immediately. A variac is most helpful in these situations.
 

Offline sheek88

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Re: Amplifier Repair - Need Output Transistors
« Reply #7 on: April 03, 2016, 03:51:28 am »
Be wary of bias diode(s) if any. Monitor DC voltage across it and if it appears unstable or open circuit, shut down immediately. Also monitor millivolts across the emitter resistors. Any indication of excessive idle current - shut down immediately. A variac is most helpful in these situations.

Thanks for the input. Every time I turn it on, there's two resistors that start smoking within a few seconds, so I'm leaving it off for now until the new transistors arrive. I can't find any other components that seem to be damaged yet. I de-soldered the two smoking resistors and they're surprisingly still within spec (although I'll replace them when I replace the transistors anyway). I think one of the blown output transistors is sending too much current through the circuit. I'll post an update when the new transistors arrive :)
 

Offline DrGeoff

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Re: Amplifier Repair - Need Output Transistors
« Reply #8 on: April 03, 2016, 06:07:41 am »
To help with this, try putting 10 ohm 5W resistors in series with the power leads whilst checking the DC state of the power amplifier section.
If there's a problem they will get very hot, but will drop the voltage quickly so as to prevent damage to your devices. Otherwise you can estimate the bias current by monitoring the voltage across the resistors and also detect for broken upstream driver devices.
Was it really supposed to do that?
 

Offline bdivi

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Re: Amplifier Repair - Need Output Transistors
« Reply #9 on: April 03, 2016, 06:36:21 am »
The transistors are actually the Japanese notation 2SB617 and 2SD587. This should help with your search.
 

Offline sheek88

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Re: Amplifier Repair - Need Output Transistors
« Reply #10 on: May 14, 2016, 11:57:02 am »
Thanks for everyone's help, new transistors arrived and amp is fixed. I almost gave up fixing it after I put in the new transistors, as it wasn't working. Everytime I turned it on it blew the bridge rectifier....turned out I shorted the negative rail to ground by mounting the new transistors without the mica insulator!

Going to use it for a week and make sure it's still good, then will probably replace all the caps with Panasonic FC or similar. Current caps are around 40 years old, but out of the 15 or so I measured, both ESR and capacitance values were in spec.

« Last Edit: May 14, 2016, 11:59:41 am by sheek88 »
 


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