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Old 3rd December 2004, 07:11 AM   #1
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Default Adcom 555 II channel failure

I was just given an Adcom 555 MkII to see if I can repair it for a friend. The left channel is dead. I have searched thru all the 555 threads (and saw the 555 schematic) to see if anyone else has posted about a similar problem. I dont want to modify it (yet) just get it going. The B+ power supply is fine. No fuses are blown.
What are the common failures on these amps? There are also 2 IC's that I dont see on the posted schematic marked ADCOM 3A as well as 9039. I'm assuming the 555 II is similar to the 555 based on posts I have read here. Any comments or ideas appreciated.
Thanks
BL
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Old 3rd December 2004, 08:18 PM   #2
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Further comments:
I have to admit I didnt search very well on this board before I posted my question. I used 555 without the II and came up with lost more information. I have now learned about the op-amps, what they do and what their replacement is. Also have the bias procedure for when I do get it fixed. The 555 does not have these IC's. I'm still poking around inside and the problem is
in/around/near the drivers. I still need a schematic for the 555II
or at least expected voltages at key points

Thanks
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Old 3rd December 2004, 08:36 PM   #3
jam is offline jam  United States
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Look at the driver transistors.
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Old 17th December 2004, 08:50 PM   #4
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Default Need source or equivalents

I have found the bad device in the 555 II I am reparing - 2SA1210 PNP 200v 140ma

What I need is a source that will sell these in small numbers OR I need an equivalent. Having searched these forms and elsewhere they all advise to avoid NTE/ECG so the possible subs I am looking at are MJE350 2SA1540 2SA1209 2SA939S and 2SA1698.

Honestly I don't know enough to say which one is the better choice or if any of these are suitable. The failed device is Q105 on the schematic.

Adcom did e-mail me a schematic but it is almost unreadable.
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Old 18th December 2004, 12:56 PM   #5
Giaime is offline Giaime  Italy
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If you replace ALL of the drivers with the same type of matched pairs (pnp+npn, don't replace only the pnps), you should choose the couple that has these characteristics: (in order of importance)

- max Ic
- max Vce
- DC hfe
- if you can, even the frequency response
(obvius, don't take RF or switching units)

And if you can, buy the same case style of the originals. That will help with the pinout and the appearance.

I have a Marantz 1060 that has the same problems, and the only matched pair I could get was a T0126, not the beautiful T05 metal can like the originals. Obviusly the pinout was different, and I had to stretch the pins in the right places. Obviusly a pin broked just out of the plastic box, and I tried to resolder it. It looked well, but when I plugged the unit and turned the power on it burned a bunch of components.
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Old 18th December 2004, 12:57 PM   #6
Giaime is offline Giaime  Italy
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I'm still dealing with that Marantz... It makes me insane...
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Old 7th January 2005, 07:50 AM   #7
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Getting back to look at this thing again after the holidays. Since this is not my amp and am repairing it for a friend, He contacted ADCOM and they mailed him 2SA1380/2SC3502 as subs for the
2SA1210/2SC2912. Sanyo datasheet shows these as "Ultrahigh Definition CTR Display,Video Output Applications". Max Ic is less than the originals..are these suitable? Thanks BL
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Old 10th January 2005, 12:44 PM   #8
Giaime is offline Giaime  Italy
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Bah, if Adcom says they're right....
What are the Icmax ratings of the originals and the new ones?
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Old 11th January 2005, 08:21 AM   #9
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I cant find a datasheet for the 2SA1210. The 1209 is rated at 140ma and the 1208 at 70ma so I'm thinking the 1210 is higher than 140ma. The 2SA1380 is rated at 100ma.
BL
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Old 11th January 2005, 01:04 PM   #10
K-amps is offline K-amps  United States
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Go ahead and try them, jus have a 100 watt bulb in series with the AC fuse so that there are no "KABOOMS"

Also see if any of the outputs are shorted out. I have never repaired an Adcom whose outputs were fine....

Also check all diodes especially the zeners, those have a tendancy to fail.


I have original Adcom 555II drivers and outputs if you need them. Email me. I might even have the A1210. email me... too lazy to do to the basement and check.
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