I recently picked up an adcom 555-II in excellent physical condition, but only one channel works.
When I checked the fuses, the working channel rail fuses are 10 amps instead of 7, the non-working side has no fuses.
After all the reading I have done about these amps, I don't want to just put fuses in and see what happens. I do not see any obviously damaged components.
Right now everything inside looks original, all ADC 185 labeled ouputs. The only strange looking thing is at the bottom of each output board there are three pins sticking straight out with leads tacked on at a 90 degree angle going to some flat package device mounted to the same heatsink as the outputs.
I sent an e-mail to Adcom asking how to get a service manual, although in this case if I new there was a good repair/refurb shop within driving distance of West Palm Beach, FL, I would consider handing it over to a pro.
Any troubleshooting ideas before pulling out all the outputs on the bad side? I have seen mention of checking the emitter resistors, the resistors from the input grounds, etc. The offset on the good channel is right around 5mv.
I have seen mention of different ideal replacement outputs, I also understand that the drivers may need to be replaced. What about the differential pair? And all the sets need to be matched?
My end goal is to be able to drive old low impedance speakers loudly.
I am leary of there not being any protection in place. If I were to get this beast working I would like to add (possibly externally) at least DC offset protection to save my speakers from certain death in the event of a failure.
Where do I start?
When I checked the fuses, the working channel rail fuses are 10 amps instead of 7, the non-working side has no fuses.
After all the reading I have done about these amps, I don't want to just put fuses in and see what happens. I do not see any obviously damaged components.
Right now everything inside looks original, all ADC 185 labeled ouputs. The only strange looking thing is at the bottom of each output board there are three pins sticking straight out with leads tacked on at a 90 degree angle going to some flat package device mounted to the same heatsink as the outputs.
I sent an e-mail to Adcom asking how to get a service manual, although in this case if I new there was a good repair/refurb shop within driving distance of West Palm Beach, FL, I would consider handing it over to a pro.
Any troubleshooting ideas before pulling out all the outputs on the bad side? I have seen mention of checking the emitter resistors, the resistors from the input grounds, etc. The offset on the good channel is right around 5mv.
I have seen mention of different ideal replacement outputs, I also understand that the drivers may need to be replaced. What about the differential pair? And all the sets need to be matched?
My end goal is to be able to drive old low impedance speakers loudly.
I am leary of there not being any protection in place. If I were to get this beast working I would like to add (possibly externally) at least DC offset protection to save my speakers from certain death in the event of a failure.
Where do I start?
Start by checking for a shorted output on the side with no fuses... I would not put fuses and power on just like that.
Use the bulb in series with the AC mains trick to tell you of a faulty side.
The flat pack (3 pins) is the driver, usually it does not go on the Mk.II.... (on original 555's they'd almost always short out)
If you cannot get ADC parts, you can sub MJ15025/24 because I suspect thats what Adcom used and relabelled as ADC-18x parts once they ran out of suppliers for the D424/B554 parts.
Adcom will send u the SM, they are also available on ebay...
Hopefully this will get you started.... on the Mk.II's 70% it is 2-3 outputs shorted , the rest of the components are good...
Use the bulb in series with the AC mains trick to tell you of a faulty side.
The flat pack (3 pins) is the driver, usually it does not go on the Mk.II.... (on original 555's they'd almost always short out)
If you cannot get ADC parts, you can sub MJ15025/24 because I suspect thats what Adcom used and relabelled as ADC-18x parts once they ran out of suppliers for the D424/B554 parts.
Adcom will send u the SM, they are also available on ebay...
Hopefully this will get you started.... on the Mk.II's 70% it is 2-3 outputs shorted , the rest of the components are good...
I found a ADC-D42 that is shorted.
I just happen to have 4 new MJ21193G and 4 21194G's sitting here from another project I have not finished.
These are not matched but were all shipped together from Mouser at the same time.
If they are not matched what would be the outcome?
Other things I should check?
Also - knowing that my goal is long life and driving low impedance loads, are the 93/94 or 95/96 a better match?
I just happen to have 4 new MJ21193G and 4 21194G's sitting here from another project I have not finished.
These are not matched but were all shipped together from Mouser at the same time.
If they are not matched what would be the outcome?
Other things I should check?
Also - knowing that my goal is long life and driving low impedance loads, are the 93/94 or 95/96 a better match?
Sometimes matching can be overrated... 
Go for it !!
In this amp they are perhaps equally good. 95/96 better maybe when it come to SOAR at peak loads delivered by the 555ii PSU... 93/94 better with higher voltages (ironically) but the 93/94 are much more robust than the originals OR the ADC parts...
Go for it !!
Go for it !!
In this amp they are perhaps equally good. 95/96 better maybe when it come to SOAR at peak loads delivered by the 555ii PSU... 93/94 better with higher voltages (ironically) but the 93/94 are much more robust than the originals OR the ADC parts...
Go for it !!
It's working. I ran it on a dimbulb tester (250 watt) for a while. I then plugged it into the mains to do the initial idle current setting - it scared me, when I first turned it on it tripped the 1500 VA UPS on the other side of the room which goes KUNNNNNNNNG when it goes online.
The original side idle current after 5 min was about 5mv? I don't know why - I cranked it up a little bit, but not a lot in case there is an open transistor on that side (I didn't check yet). My focus right now is the newly repaired side. As long as it keeps working I will order the parts and make the other side match.
Now I am letting it build some heat with the ~66 watt output so I can do the next step.
I will give another update in an hour or so. So far so good!
Thanks!!!
The original side idle current after 5 min was about 5mv? I don't know why - I cranked it up a little bit, but not a lot in case there is an open transistor on that side (I didn't check yet). My focus right now is the newly repaired side. As long as it keeps working I will order the parts and make the other side match.
Now I am letting it build some heat with the ~66 watt output so I can do the next step.
I will give another update in an hour or so. So far so good!
Thanks!!!
Still working. a continuous 66 watts of music is fairly loud. The only 8 ohm speakers I have are a pair of Seas 8 inch woofers - fairly efficient and well ventilated. I put them in the closet and played between 50 and 100 watts on my RatShack meter for 10 minutes - set the bias at ~12mv and put the screws back in.
The heatsinks didn't get very hot during the 10 minutes of heat up time.
I went ahead and ordered the pieces to do the other side so they match as well as the little electrolytics. I will use it on my little 6 ohm (really 4ohm) infinities for a few days before I subject it to the 2 ohm load of the bigger ones. If it takes out a speaker I would rather lose a little one.
It sounds great - I can't tell a difference between the two sides but I am more comfortable with new parts, and both sides are working which is nice.
Once I put the new parts in the other side I will set the bias perfectly.
It has been playing music now for a few hours and the heatsinks are just a little bit above room temp.
Thanks again!
The heatsinks didn't get very hot during the 10 minutes of heat up time.
I went ahead and ordered the pieces to do the other side so they match as well as the little electrolytics. I will use it on my little 6 ohm (really 4ohm) infinities for a few days before I subject it to the 2 ohm load of the bigger ones. If it takes out a speaker I would rather lose a little one.
It sounds great - I can't tell a difference between the two sides but I am more comfortable with new parts, and both sides are working which is nice.
Once I put the new parts in the other side I will set the bias perfectly.
It has been playing music now for a few hours and the heatsinks are just a little bit above room temp.
Thanks again!
I tend to find the mids on the overbiased side sound less harsh... not sure what speakers you are using.
Just a word of caution... the 555ii is not designed for base 2 ohm loads (which can dip down further). But maybe in a bandpass scenario it might survive....
If you do dare... then keep a vigilant eye on heatsink temps... at no point should u exceed 65C if it does.... you are killing the amp.
Just a word of caution... the 555ii is not designed for base 2 ohm loads (which can dip down further). But maybe in a bandpass scenario it might survive....
If you do dare... then keep a vigilant eye on heatsink temps... at no point should u exceed 65C if it does.... you are killing the amp.
Setting Bias?
Everything is still working great.
I recieved and replaced the outputs on the second channel with the same transistors, so now both sides are identical. I also replaced all the small electrolytics.
I set the bias according to the FSM procedure.
Then I redid it and set it to 12mv instead of 10 - Better?
I put the amp in it's home along with wires running to 2 DMM's connected to the test points so I could monitor it temporarily. I have found that the thermal tracking is fairly slow - I guess because of the mass of the heatsinks, or maybe the bais diodes or thermistor location is not ideal on the heatsink.
All of the following conditions occur after running the amp for many hours in it's fairly open cabinet.
Playing music that has a couple seconds of loud, low frequency bass (amp drawing about 1100 watts from AC outlet) the bias will be very low for about 30 seconds. in the 4-6mv range, then it will slowly climb back up to 12mv. An example of this would be during a couple of James Horner tracks that have a few moments of loud bass in the 20-30hz region, but are otherwise light and orchestral.
If I play normal loud music (rock, dance) - drawing about 250-300 watts from the wall for an hour or so, if I pause the bias will be at 12, then slowly start crawling up to about 15mv.
So it seems that when the transistors are hotter than the sink the bias is low, when the heatsink is hotter than the transistors (or keeping them hot) the bias is high.
The reason mention all this is that (and this may be totally in my head) any time the amp sounds particularly good and clear in the upper frequencies, if I stop the input the bias settles around 14-15mv but then sinks back down to 12 as everything stabilizes.
The factory service manual calls for 10+/-1
It was mentioned that I could set it 3-5mv higher than stock.
Is setting it higher required because of the slightly different output transistors, or it just personal experience that this amp might sound better with a little more current.
The working stock side originally was set at 17mv with the stock outputs. I didn't change it initially since I knew I was going to replace the outputs within a week. Because it was so high I didn't use the add 3 or 5 to the working sides value suggestion.
The heatsinks seem to stabilize at just warm after the amp sits for a while.
In my brain it seems to sound best when the bias is between 13 and 15mv - for all I know I may be hearing distortion that I like (I am using these with Emit tweeters which may add their own HD that I like), or it may just be the 'more is better' mentality.
Most of the time this amp will run at output levels of less than 10watts, many hours of the day. Occasionally it will be cranked to near deafening levels, but just to the point that the distortion LED's barely flicker.
What is a safe range across the test points, and what would be the ramifications if I had it set too high long term? I assume that at some point thermal runaway could be a risk, but it seems to cool pretty quickly right now anytime the input is turned down. The hottest it has gotten so far was uncomfortable to hold the heatsink, but not 'burning hot'. And that was during the bias setting procedure running a 120hz sinewave at 66 watts for about double the recommended time (I got interrupted just as I was letting the amp stabilize the first time and had to start over).
Ultimately I would like to set this up (with a complex series of timers and relays) so that for normal listening the speakers would be powered by my reciever and only turn on the Adcom for loud music - but at this point that is just a pipe dream. As a part of that same pipe dream I would like to add DC output protection to save the speakers in case of an amp failure. While I am dreaming a soft start circuit would be nice too.
Everything is still working great.
I recieved and replaced the outputs on the second channel with the same transistors, so now both sides are identical. I also replaced all the small electrolytics.
I set the bias according to the FSM procedure.
Then I redid it and set it to 12mv instead of 10 - Better?
I put the amp in it's home along with wires running to 2 DMM's connected to the test points so I could monitor it temporarily. I have found that the thermal tracking is fairly slow - I guess because of the mass of the heatsinks, or maybe the bais diodes or thermistor location is not ideal on the heatsink.
All of the following conditions occur after running the amp for many hours in it's fairly open cabinet.
Playing music that has a couple seconds of loud, low frequency bass (amp drawing about 1100 watts from AC outlet) the bias will be very low for about 30 seconds. in the 4-6mv range, then it will slowly climb back up to 12mv. An example of this would be during a couple of James Horner tracks that have a few moments of loud bass in the 20-30hz region, but are otherwise light and orchestral.
If I play normal loud music (rock, dance) - drawing about 250-300 watts from the wall for an hour or so, if I pause the bias will be at 12, then slowly start crawling up to about 15mv.
So it seems that when the transistors are hotter than the sink the bias is low, when the heatsink is hotter than the transistors (or keeping them hot) the bias is high.
The reason mention all this is that (and this may be totally in my head) any time the amp sounds particularly good and clear in the upper frequencies, if I stop the input the bias settles around 14-15mv but then sinks back down to 12 as everything stabilizes.
The factory service manual calls for 10+/-1
It was mentioned that I could set it 3-5mv higher than stock.
Is setting it higher required because of the slightly different output transistors, or it just personal experience that this amp might sound better with a little more current.
The working stock side originally was set at 17mv with the stock outputs. I didn't change it initially since I knew I was going to replace the outputs within a week. Because it was so high I didn't use the add 3 or 5 to the working sides value suggestion.
The heatsinks seem to stabilize at just warm after the amp sits for a while.
In my brain it seems to sound best when the bias is between 13 and 15mv - for all I know I may be hearing distortion that I like (I am using these with Emit tweeters which may add their own HD that I like), or it may just be the 'more is better' mentality.
Most of the time this amp will run at output levels of less than 10watts, many hours of the day. Occasionally it will be cranked to near deafening levels, but just to the point that the distortion LED's barely flicker.
What is a safe range across the test points, and what would be the ramifications if I had it set too high long term? I assume that at some point thermal runaway could be a risk, but it seems to cool pretty quickly right now anytime the input is turned down. The hottest it has gotten so far was uncomfortable to hold the heatsink, but not 'burning hot'. And that was during the bias setting procedure running a 120hz sinewave at 66 watts for about double the recommended time (I got interrupted just as I was letting the amp stabilize the first time and had to start over).
Ultimately I would like to set this up (with a complex series of timers and relays) so that for normal listening the speakers would be powered by my reciever and only turn on the Adcom for loud music - but at this point that is just a pipe dream. As a part of that same pipe dream I would like to add DC output protection to save the speakers in case of an amp failure. While I am dreaming a soft start circuit would be nice too.
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