Adcom 555 II advice - repair? test? - diyAudio
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Old 14th November 2008, 12:50 PM   #1
CBRworm is offline CBRworm  United States
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Default Adcom 555 II advice - repair? test?

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?
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Old 14th November 2008, 01:13 PM   #2
K-amps is offline K-amps  United States
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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...

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Old 14th November 2008, 01:40 PM   #3
CBRworm is offline CBRworm  United States
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sounds like a good start.

To begin with I guess I will pull the fuses out of the working side.

I am thinking that if there are some shorted outputs I should probably replace all of them on that channel (assuming they were all abused equally)?

What about matching these outputs?
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Old 14th November 2008, 02:12 PM   #4
K-amps is offline K-amps  United States
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Match them if you can...

by the way I always preferred the sound of the original Toshiba's than the ADC-parts... so if you can get those, better.

If not, rebuild it with MJ21193/4/5/6's. I have done it many times, they fit in well and sound more like the TOS's than the ADC's
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Old 14th November 2008, 02:20 PM   #5
CBRworm is offline CBRworm  United States
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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?
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Old 14th November 2008, 02:33 PM   #6
K-amps is offline K-amps  United States
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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 !!
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Old 14th November 2008, 02:40 PM   #7
CBRworm is offline CBRworm  United States
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Alright - I'll give it a shot and keep you posted.
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Old 14th November 2008, 02:51 PM   #8
K-amps is offline K-amps  United States
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Bias the new side a few mV's higher than stock. +3 to 5mV higher across the TP's compared to the working / stock side...

when it all works I suspect you will find that overbiased new side sounds better than the original... :-)
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Old 14th November 2008, 04:44 PM   #9
CBRworm is offline CBRworm  United States
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Is it ok to reuse the original yellow insulater pads? I have new mica pads if I need them. No grease in here now - should I use some?
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Old 14th November 2008, 05:28 PM   #10
K-amps is offline K-amps  United States
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Yellows are better.

Just do a continuity test after installation to ensure they are still insulating.
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