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Old 28th June 2007, 03:27 PM   #1
hewlew1 is offline hewlew1  United States
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Default Transistor Substitution

I have aquired a Carver M500 amp that is in need of output transistors 2SD745/2SB705 made by NEC. I have found a substitute which is 2SC2921/2SA1215 made by Sanken. Has anybody ever used these substitutes in this situation? Did they hold up and was there any sacrifice to performance of the amp? Any help is appreciated . Thanks, Harry
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Old 28th June 2007, 03:40 PM   #2
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Hi Harry,

One of our moderators (Anatech) used to be a certified Carver repair person. I have alerted him to this post and when he is next online he will be able to assist you.
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Old 28th June 2007, 04:30 PM   #3
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That's what I call "SERVICE".

Keep up the good work.
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Old 28th June 2007, 06:28 PM   #4
hewlew1 is offline hewlew1  United States
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Appreciate your help Cal and I look forward to his input. Thanks again, Harry
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Old 28th June 2007, 06:36 PM   #5
anatech is offline anatech  Canada
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Hi Harry,
Those should work fine as long as they fit. I once used a sub (could have been those) that I had to drill new mounting holes into the heatsink. They were close, but not close enough. If you have to do this, be sure to clean and sand the surface flat. My final sand is with #600 wet/dry. It's not as bad as it sounds.

As long as it doesn't oscillate and you get the bias right, the amp will meet all its specs. If the outputs were blown, be sure to replace the drivers as well. Also make sure you didn't lose any commutators or blocking diodes. You do know this is not a normal amp, right? I also think this has a mag coil, so the triac isn't a switch, it's a voltage regulator. DO NOT SHORT IT and apply over 1/2 full supply voltage! If you do, you will run everything at far too high a voltage (not to mention the high current draw and angry hum just before the fuse blows).

-Chris
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Old 29th June 2007, 01:11 AM   #6
hewlew1 is offline hewlew1  United States
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Hi Chris. I may be biting off more than I can chew with this amp and I may possess just enough knowledge to be dangerous. First off I bought this amp broken from a self professed tech he stated it had many new parts that had been replaced. The filter caps in the power supply had wires soldered to them and were flopping around inside the case when I first opened it up. After anchoring the caps I used my variac to fire it up with an amp meter in place of the fuse current draw was what I thought to be accessive so I shut it down. I disconnected the output voltages from the power supply to the amp board and fired it again this time supply appears to be OK brought her up to ac line voltage and current draw was minimal. Have started testing components on amp board and so far have 1 pair of blown outputs and blown drivers 4 transistors total tested bad. I have replaced the baising resistors for the outputs with 1/2 watt units and changed every electrolytic on the amp board I intend to replace every diode as well. If I have done anything improper up to this point I would appreciate some guidance from you. There are other issues with this amp as well it has some sort of solid state timer wire tied inside that appears to energize the relay for the speakers. Oh well one thing at a time right now I need to order the transistors that I need. Remember my statement about limited knowledge I know what an electrical commutator so I am assuming it is another way to reference the blocking diode. Thanks, Harry
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Old 29th June 2007, 06:35 PM   #7
anatech is offline anatech  Canada
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Hi Harry,
Quote:
I bought this amp broken from a self professed tech he stated it had many new parts that had been replaced. The filter caps in the power supply had wires soldered to them and were flopping around inside the case when I first opened it up.
Oh Gawd!!!!!!!!!!!!!!
So you are behind the 8-ball already. I hope it was next to free!
Quote:
it is another way to reference the blocking diode.
Yes.

All right now. Are there any ECG or NTE replacement parts in there anywhere?

Do you have the schematic?

-Chris
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Old 30th June 2007, 02:43 AM   #8
hewlew1 is offline hewlew1  United States
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Hi Chris, Yes I have a schematic and I dont see any ECG or NTE replacement devices. What I do see is an SK 9446 replacing 2SD745 in the channel that I am assuming functions properly, all transistors in that channel tested OK. What I am proposing is to buy 4 Sanken replacements and use them for the blown channel replace the SK9446 with the 2sd745 and having all NEC devices in the other channel or do you feel it would be better to replace them all with the Sankens? Thanks, Harry
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Old 2nd July 2007, 01:57 AM   #9
hewlew1 is offline hewlew1  United States
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Chris, This amp has 4 Zener Diodes on the amp board 2 of them are rated 11.7 volt the other 2 are rated 14.7 volt. Can these be replaced with 12 and 15 volt units? I am not having any luck finding exact replacements.Thanks, Harry
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Old 2nd July 2007, 02:07 AM   #10
anatech is offline anatech  Canada
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Hi Harry,
The SK's have to go, and the rest in that channel. Sorry, older bad repair job.

You can use different devices between channels. You can not reuse any part that was installed in a damaged channel. Replace drivers as well. Match the outputs in each channel.

The 12 and 15 V zeners will be fine. Many have a 5 % or 10 % tolerance. The differences are then well within the possible error band for each.

-Chris
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