Looking for (info) Adcom 555 Mk2 Factory output trannys - diyAudio
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Old 30th August 2004, 03:29 AM   #1
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Default Looking for (info) Adcom 555 Mk2 Factory output trannys

Hi guys (OH, and ladies!)
I'm new to the furum, it's a great thing going here!

I picked up a 555 mk2 on ebay recently. It had been dropped and hed 1 side out. Hmm .. I didn't think dropping would damage the outputs, so wasn't I surprised that was the problem! Anyway, every other Adcom I have had used Toshiba outputs (you know the ones). This thing has TO-3's with what I am assuming is an internal Adcom part #.. ADC-B55 (9342 185W) and ADC-D42 (9340 185W). This kinda P'd me off!!

I am trying to avoid changing all of the outputs, I am not expecting this amp will be working very hard (maybe to rears). Not to mention I am sometimes really cheap! Does anyone know what these are? Obviously someone's gotta make 'em. Also. I am SURE someone out there has done a complete replacment of these in the past , and may have some they want to sell.

Thanks in advance,
marino@charter.net
Marc
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Old 30th August 2004, 03:36 AM   #2
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Oh, I have 2 pair down but I wouldn't mind finding a whole side's worth..Thanx
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Old 30th August 2004, 06:45 AM   #3
djk is offline djk
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ADC-B55 (9342 185W) and ADC-D42 (9340 185W).

2SB554
2SD424

$4 each from

http://www.audiolabga.com
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Old 30th August 2004, 02:31 PM   #4
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Thanks for the info. I never compared the part numbers, they are close! I must ask, are you 100% sure they are the exact same component? I mean, really, really, really sure!

Thanks again!
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Old 31st August 2004, 08:15 AM   #5
djk is offline djk
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That doesn't deserve a reply.
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Old 31st August 2004, 05:24 PM   #6
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djk,
If you found my post insulting, I did not mean it that way. I was simply wondering if they were the EXACT same transistor, or is the adcom piece a knock off with the same specs. In other words, are the adcom pieces built by toshiba. Obviously you are sure...
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Old 31st August 2004, 06:06 PM   #7
markp is offline markp  United States
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They are the same part relabeled for Adcom to overcharge for.
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If it sounds good... it is good!
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Old 4th September 2004, 11:46 AM   #8
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Thank djk and Markp. I appreciate it very much.
Marc
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Old 11th October 2004, 03:51 PM   #9
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Default Results..Thank you

This furum is superb, as are the people on it!
My question is at the end.

I thought it would be nice to share some observations on mods to this amp (Adcom 555mk2) and what I heard.

I decided to replace all of the outputs on my 555 mk2 with ON MJ15024/25 rather than overpay for possibly counterfeit toshibas.

I put a 100 watt light bulb in series w/ the ac line on initial power up. A practice I learned here and will use it every time as I KNOW it saved the new transistors I just installed.

Thanks to the forum I knew what to look for on power up after replacing the toshibas on the bad side. I got a scope, ran a 1k sine, and took a look. I found severe oscillations on the side that had blown outputs while running the new transisters(gee, I wonder what killed the old ones..sarcasm).. I checked further and found 2 open resistors and 1 tired resistor on the front end board. I replaced the electrolytics at the output board w/120uF 400v panasonics I had laying around because the 160v factory jobs must be ready to go by now. I bypassed those with .01 polys. I fired it up and it looked great on the scope... with no dc offset. The bias seemed high then, judging by the heat. I backed the bias down until it felt right.

I've got to say..I then A/B'd the new channel with the old good channel and hear a HUGE inprovement. I know this has been said a thousand times but...It's like someone took a blanket off of my speakers and replaced the tweeters. It's that much better! It sounds like a different amp..smoother..more detailed etc.etc.. The bass seems just as good as before.

I then replaced the other side..

Now the question:
How does one properly bias this amp? I looked for crossover distortion on a 1k sine and saw none at any bias level. The only thing that changes is idle heat. Voltage across the emitter resistors only is at about .001v to .002v at any bias adj. Can I get away with my 'heat method'? I do have an accurate IR thermometer.

P.S. I'm hoping my 555 series 1 breaks now..so I can 'fix' that one too!
Thanks in advance,
Marc
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Old 11th October 2004, 07:06 PM   #10
anatech is offline anatech  Canada
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Hi mpmarino,
Connect a voltmeter on the mV range across TP 201 and 301 on the output PCB's (251 & 351 on the other channel). Let the amp idle with no signal and no load on the outputs for approx 5 min. Adjust bias control R119 (R169 other channel) for 10 mV. Allow the amplifier to stabilize and readjust for 10 mV.
Now run the amp. around 66W into 8 ohms for 10 min with the cover on, both channels driven. Disconnect the signal and speaker loads and allow the amplifier the restabilze, readjust the bias trimmers to obtain 10 mV across the test points. This should set the bias to around 23mA per transistor assuming they are matched.
This is the factory specified procedure.

BTW, Adcom always sends the output transistors as a matched NPN & PNP set. I do not think you appreciate the value of this. They do not overcharge for their parts, in my experience, compared to a few other manufacturers. I really think markp's statements are ill informed and should be retracted.
-Chris
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