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
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
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
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
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
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
adcom transistors
Hi,
Just thought I'd throw in my 2c here. I just finished refurbishing a trio of adcom 565 amps, the output transistors being 10 sets of the now (in)famous 424/554 pairs. While the 424s and 554s were all matched with each other pretty well, the 2 polarity sets didn't come close. The NPN parts were all about half the beta of the PNP parts, at least using low current tests...and it was true for all 3 amps...I've owned 2 of the 3 from new, so I know they are the original transistors, so perhaps matching NPN to PNP is not that important in the context of the 5x5 series amps.
Stuart
Hi,
Just thought I'd throw in my 2c here. I just finished refurbishing a trio of adcom 565 amps, the output transistors being 10 sets of the now (in)famous 424/554 pairs. While the 424s and 554s were all matched with each other pretty well, the 2 polarity sets didn't come close. The NPN parts were all about half the beta of the PNP parts, at least using low current tests...and it was true for all 3 amps...I've owned 2 of the 3 from new, so I know they are the original transistors, so perhaps matching NPN to PNP is not that important in the context of the 5x5 series amps.
Stuart
Hi Stuart,
The NPN's and PNP's need not be matched, and seldom are in production. I do when servicing some amplifiers (eg. Marantz 500). It is important (within reason) to match NPN's to each other. The same is true for the PNP sets.
At higher currents, the emitter resistors do a reasonable job of enforcing current sharing. When driven, the error amp stage will adjust the amount of drive to compensate for differing beta's in positive and negative transitions.
I've found that when all sections are matched (PNP to NPN also), outputs, drivers .. on & on. You will get a more stable amplifier that sounds better.
My main issue is with parts jammed in with no regard to matching. I have witnessed one amp using four different case styles of outputs! (as an extreme example, it came in after an entire year after that service job!!) I was really just trying to point out that Adcoms parts prices were value added. Admittedly, they don't want to be parts suppliers either.
-Chris
The NPN's and PNP's need not be matched, and seldom are in production. I do when servicing some amplifiers (eg. Marantz 500). It is important (within reason) to match NPN's to each other. The same is true for the PNP sets.
At higher currents, the emitter resistors do a reasonable job of enforcing current sharing. When driven, the error amp stage will adjust the amount of drive to compensate for differing beta's in positive and negative transitions.
I've found that when all sections are matched (PNP to NPN also), outputs, drivers .. on & on. You will get a more stable amplifier that sounds better.
My main issue is with parts jammed in with no regard to matching. I have witnessed one amp using four different case styles of outputs! (as an extreme example, it came in after an entire year after that service job!!) I was really just trying to point out that Adcoms parts prices were value added. Admittedly, they don't want to be parts suppliers either.
-Chris
Hi mpmarino,
No sweat, glad it helped.
Hi Zero Cool,
D427A/B557A, used in smaller Marantz receivers 120V, 8A, 80W, hFE=40-140.
D424/B554, used in bigger amplifiers incl. Marantz 180V, 15A,150W, hFE=40-140. Real nice transistors, fT=6MHz.
From '86 Japanese Transistor Manual. Watch out for remarks on these! Complete with date codes. Originally by Toshiba.
-Chris
No sweat, glad it helped.
Hi Zero Cool,
D427A/B557A, used in smaller Marantz receivers 120V, 8A, 80W, hFE=40-140.
D424/B554, used in bigger amplifiers incl. Marantz 180V, 15A,150W, hFE=40-140. Real nice transistors, fT=6MHz.
From '86 Japanese Transistor Manual. Watch out for remarks on these! Complete with date codes. Originally by Toshiba.
-Chris
Hi Markp,
Since when do you buy matched transistors from jobbers??? The one's I've received from Adcom in the past were matched (graded), much like the old Marantz parts. Evidently this is of no value to you.
When I ran a fair sized warranty depot, I bought $5K or more a year in transistors. Matching the common ones was not a problem. I gladly purchased matched parts for numbers I didn't normally carry. Now that I've sold the place, I'm in the same boat as all DIYer's. I have to buy many parts to get a matched set.
So, how much is getting a matched set worth? The same as individuals with random parameters? Excluding your time, and the cost of the equipment, how about the extra parts that don't match? Or do you execute an improperly done repair. I hope you do not charge for your services. If it's your own piece, it's your decision.
Part of my rant here is due to all the attention paid to wire, capacitors and resistors. If the circuit isn't set up properly, the results outweigh all the fancy parts you can list. Assuming the circuit has been properly designed. So do your bit on the bench first.
-Chris
Since when do you buy matched transistors from jobbers??? The one's I've received from Adcom in the past were matched (graded), much like the old Marantz parts. Evidently this is of no value to you.
When I ran a fair sized warranty depot, I bought $5K or more a year in transistors. Matching the common ones was not a problem. I gladly purchased matched parts for numbers I didn't normally carry. Now that I've sold the place, I'm in the same boat as all DIYer's. I have to buy many parts to get a matched set.
So, how much is getting a matched set worth? The same as individuals with random parameters? Excluding your time, and the cost of the equipment, how about the extra parts that don't match? Or do you execute an improperly done repair. I hope you do not charge for your services. If it's your own piece, it's your decision.
Part of my rant here is due to all the attention paid to wire, capacitors and resistors. If the circuit isn't set up properly, the results outweigh all the fancy parts you can list. Assuming the circuit has been properly designed. So do your bit on the bench first.
-Chris
mpmarino said:
Marc,
I have been away from the forum for a few months and this replay is belated but for whatever it is worth... I have worked with 555's as long as anyone. I have many parts lying around including:
Original Transformers
TOS 2sd424/2sb554 pairs (several pairs) Original TOS.
ADC-42 and ADC55
By the way the ADC parts are NOT from Toshiba, they are actually Motorola MJ15022/23's, trust me on this, they are NOT toshiba parts relabelled.
cheers!
PS: Don't worry about djk, he is a great guy and very knowledgable, just that he gets irked very quickly. ;-)
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