Technics SU-A900Mk2

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Info. needed on the above, please - I was not sure whether to post this under 'Solid State' or 'Chip Amps' - anyhow a circuit diagram, plus info.(pin-outs & block diagram) for the RSN6000B Mosfet Module(IC502) would be helpful. I am in process of repairing an SUA900Mk2 in which the mains fuse T1 (3.15AT) had blown, the only other known fault being that D702(original type P300D) was short circuit. D 701, 03, & 04 'cold' check OK, but I have replaced all 4 with 1n5408 diodes. I don't know why one diode of 4 in the main Bridge rectifier would fail s/c, and have yet to power up and test the amp.
 
Problem solved! After making some further checks, I found that the only fault was the short-circuit rectifier diode and blown mains fuse. I replaced all 4 original diodes with 1N5408s, and the blown fuse. The amplifier is now working perfectly on test as I type this, but I still do not know why ONE diode of four in a conventional Bridge Rectifier should go s/c
 
If the failure occurred, for instance, due to an output short, then it depends on the phase of the input AC supply at the moment the short occurred. The two input rectifiers that were conducting at the time of the short are likely to fail first - in this case, one of them failed first, followed by the fuse blowing and protecting the others. That's why the others did not fail.
 
I know this is an old thread but I need some help
My SU-A900MK2 is gone to hell.
After some investigation I found 1 diode shorted (replaced all 4 ) and a blown fuse.
It now power on (Stand by) but at soon as I press it the relay that connects the power supply rails goes on and off that is with the light bulb test thingy on the live line without it it just blows the fuse.
Any suggestion ?
 
The only thing I can suggest, RS232, is to replace also the ceramic capacitors connected in parallel with the rectifier diodes, as suggested by Sakis. I don't have a circuit diagram to hand, but it's possible that the RSN6000B module(IC502)is faulty. This, IIRC, is the Power amplifier IC, so a fault here could blow the fuse
 
Just for the statistics ...

In the age of 47 i make repair work from the age of 16 ( !!) well today i repair an average of 300-400 amplifiers per year .... I am not just cocking ....this info might be useful

so in all my repair career i have repaired just a few of them lets say 4-5 ( keep in mind that this series was a top seller in Greece ) but the latest 6 months i had to repair around 10 with most of them belonging to 40-50 years old costumer so no party animals there ..just casual listeners .

It seems to me that the notorious and now days costly if not unobtainable IC have reached the end of life . The numbers around those amps just dont fit .

If you ever find somewhere the IC please let me know i have 4 of them here waiting before i mode them to real amplifiers .


Kind regards
sakis
 
Got amp up and running.
Found out the problem with the on and off, if power up with the light bulb protection there is not enough juice for the relays to latch and the empty capacitors would cause a big inrush of current and make the light bulb go on if left like that after 6 or 7 latches it would stay on.
Now something else...
If I go close to the 11 o'clock position on the dial the amp just disconnects the speakers and I'm starting to give up on it.
Speakers ? they are some vintage Leak ones rated at 50W if I'm not mistaken.
Will try later with other ones and see what happens.
Any suggestion ?

Ric
 
Problem solved.
The amp was connected to the line without the bulb limiter and to a CD player.
The problem was the connector from the mainboard to the front panel.
Connected the amp to a set of speakers that can handle the power of the amp gave it a good blast (past the 12 o'clock mark) and no more problems
If someone is repairing this 900MK2 please make sure the 4 connectors are properly connected to the front panel boards.

Ric
 
Problem solved.
The amp was connected to the line without the bulb limiter and to a CD player.
The problem was the connector from the mainboard to the front panel.
Connected the amp to a set of speakers that can handle the power of the amp gave it a good blast (past the 12 o'clock mark) and no more problems
If someone is repairing this 900MK2 please make sure the 4 connectors are properly connected to the front panel boards.

Ric

Presumably there were dry solder joints between the connectors and the PCB, Ric?
 
Presumably there were dry solder joints between the connectors and the PCB, Ric?

Actually there are no dry solder.
The plastic part of the connector(s) are "L" shaped and they have some up/down movement and is what causes the problem.
After putting the front panel back in place just press the 4 connector into the front panel board and that's it (well in my case it solved the problem).

Ric
 
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