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Old 9th January 2009, 09:30 PM   #1
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Unhappy 300DC repair

Okay guys, my 300DC has been on the fritz for some time now and I was so embarrassed by what I did to break it, that i couldn't bear to seek help, but here goes.

I decided to test some power transistors (2SB645s and 2SD665s) that I suspected to be bad by swapping them into the left channel of my 300DC. My thinking was, if the transistors were bad, the protection relay would simply not click-on and I'd now if the transistors were bad.

The relay didn't click-on and nothing smoked, so I threw out the transistors and swapped the good ones in. The protection relay still stays turned off though. I left the amplifier on for a little bit to see if anything would smoke. Q711 awarded me with a nice POP and some smoke.

Click the image to open in full size. <HOTLINKED, SORRY>

I don't have the service manual for the 300dc, so I don't know enough to identify the part to replace it, nor why it would blow.

I just need some help with how to proceed with getting the channel fixed. I'm quite a novice when it comes to fixing my own equipment beyond identifying blown components and poor solder joints. Thanks guys.
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Old 10th January 2009, 05:52 PM   #2
Bigred is offline Bigred  Canada
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Default Re: 300DC repair

Quote:
Originally posted by mechanicalman
I decided to test some power transistors (2SB645s and 2SD665s) that I suspected to be bad by swapping them into the left channel of my 300DC.


Your first mistake. This is VERY poor troubleshooting practise.

Quote:
I left the amplifier on for a little bit to see if anything would smoke. Q711 awarded me with a nice POP and some smoke.
Knowing there is a problem and waiting for the magic smoke to release?

Quote:
I don't have the service manual for the 300dc, so I don't know enough to identify the part to replace it, nor why it would blow.

I just need some help with how to proceed with getting the channel fixed. I'm quite a novice when it comes to fixing my own equipment beyond identifying blown components and poor solder joints. Thanks guys.
The best advice and help on how to proceed I could give after reading all the above is to find yourself a good AUDIO TECH. IMO this amp is VERY MUCH worth having repaired by a pro and definitely not something I'd recommend you be learning on.
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Old 10th January 2009, 08:04 PM   #3
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yeah, the guy i used to go to in San Raphael, closed his shop without warning. i drove up with my amp in tow only to find "Annie's Hair and Beauty" where "European Electronics" used to stand (where i bought my very first marantz receiver).
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Old 10th January 2009, 08:14 PM   #4
Bigred is offline Bigred  Canada
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Well I know San Diego is quit a distance away from you but I can recommend a good shop there for you. You could always ship it. Maybe some others will chime in with some places closer. Nice 2130 BTW, you gotta love them scope tuners.
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Old 11th January 2009, 01:42 AM   #5
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i'm very nervous about shipping it.

i know it's done, but the thing weighs close to 70lbs and the is cosmetically a 9/10.

i'm pretty sure any music store i take it to around here wouldn't take the time to match semi-conductors and would simply affect repairs i could do myself. i am very proficient with a soldering iron

btw, i've since rigged up my Hfe capable multimeter to a custom made socket, so i can test the transistors without ruining more 300DC amps.

i'm still confident that this problem isn't beyond my abilities if someone will help me identify what could be wrong with the left channel. i had a similar protection problem in my 1300dc that i traced to a faulty cap in the right channel (c723).

and i LOVE that 2130 so much. i never listen to the radio for anything anymore, but i promise you, it's the LAST component i'll ever part with.
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Old 12th January 2009, 06:22 PM   #6
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could someone maybe send me the electrical parts list for the p700 board, so i can begin looking up datasheets to find good replacements for the semiconductors?
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Old 16th January 2009, 02:14 AM   #7
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the q711 transistor is labled a977-r. i would assume that's an 2sa977-r.

after poking around here and looking at some datasheets, i was going to replace it with either a 2sa1209 and replace the complimentary PNP with a 2sa1211 (q712).

i was also considering shoe-horning a 2sa1535a (220 package) and a 2sc3944a pair. i'm having trouble locating somebody with the 2sc3944a though.

i'm pretty sure at this point, that the cause of the damage was a shorted driver (q003 or q004).
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Old 16th January 2009, 05:26 AM   #8
SQLGuy is offline SQLGuy  United States
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The 2SA1209 looks like a good sub.

I assume, since you have an intact right channel, that you are comparing resistance measurements between to two channels to find other likely damaged components. There is a decent amount of circuitry between the bad outputs you were testing and Q711, and all of it's suspect at this point.

Cheers,
Paul
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Old 16th January 2009, 06:24 AM   #9
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no, i hadn't actually considered investigating further. the rest of the board looked pristine and the back of the board didn't have any cooked pathways (it looks downright pretty).

i'll make sure to start comparing things before reinstall the board.

i'd kind of rather use the 2sa1535a/2sc3944a pair, just because the heatsink for q711/q714 tends to get really hot. i feel like a 220 package would do better than a 126 style transistor with all that heat. do you think there'd be anything wrong with using this pair as opposed to the drop-in replacement (2sa1209/2sa1211)?
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Old 16th January 2009, 07:44 AM   #10
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Default against what big red will say .....

i have a couple of suggestions ....but of course this depends on your skills ......

There is an alternative way to do measuring when you have one channel working and one not .....usually when you measure things you use a multi meter for example to rail voltage and ground ...... to find out what your rail voltage is .....


well not in this case..... at this time you go to the collector of transistor bla of left working chanel to the same exact spot of the collector of transistor bla bla on the right not working chanel

the indication you get should be zero or with in a few millivolts .... big diference of 5-10-20 volts will point to you that the particular area has some kind of trouble .....

focuson protection circuits , seperate the protection from the amplifier and start trouble shooting the amp as is ( forget that your amp uses protection for a minute cause the fault might be on the protection circuits

a good start will be also to verify all the voltages mentioned in your schematic if something is wrong there this is also a fault indication

finally you may wana take a look here

vintage amplifier repair/upgrade manual

happy regards sakis
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