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Power Issues Nakamichi SR-4A
Power Issues Nakamichi SR-4A
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Old 14th December 2013, 06:54 AM   #11
Mooly is offline Mooly  United Kingdom
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Power Issues Nakamichi SR-4A
Its definitely open circuit with 4 volts across B-E. The 2SC1345 should be OK.

I'm wondering whether its worth replacing the other one too. Those type (generally) have a habit of failing intermittently... I'm wondering if that has happened and that is what has killed the small driver. Any suitable NPN power device would be OK.

Cracked joints are common on any older equipment, particularly around parts that run hot. It can be worth just going over a whole area if there is any suspicion of anything intermittent. The foam is possibly to stop the board being pressed down and shorting... hard to say... but foam, glue, cable ties, all are seen in commercial gear.

As to the original problem... as it is the driver that is zapped, it makes sense in this case to replace both.
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Old 14th December 2013, 10:10 AM   #12
DreadPirate is offline DreadPirate  United States
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I'll replace both. Might also just go ahead and recap the board. The foam is strangely dried out and deteriorated and I'm also seeing possible signs of cap leakage and corrosion. The heat sinks appear to have been glued down and I'm seeing corrosion on the associated transistor leads. I bet the vents were blocked on this thing.
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Old 14th December 2013, 10:22 AM   #13
Mooly is offline Mooly  United Kingdom
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Power Issues Nakamichi SR-4A
I would initially just try and fix all the faults before doing a general recap etc. That way you know the unit is OK and then if something goes awry with the recap you know where to look. Do too much work, and then hit a problem and its more difficult finding what went wrong.
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Old 14th December 2013, 02:17 PM   #14
Mannegizen is offline Mannegizen  Sweden
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Couldn't it just be a case of bad soldering of the transistor?

I've encountered a few cases where the soldering on a visual inspection looked just fine, but must have had a layer of oxidation between the circuit trace and solder.

(dunno if I makes sense, English is not my native language.)
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Old 14th December 2013, 03:24 PM   #15
Mooly is offline Mooly  United Kingdom
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Power Issues Nakamichi SR-4A
Quote:
Originally Posted by Mannegizen View Post
Couldn't it just be a case of bad soldering of the transistor?

I've encountered a few cases where the soldering on a visual inspection looked just fine, but must have had a layer of oxidation between the circuit trace and solder.

(dunno if I makes sense, English is not my native language.)
Unlikely given the high output voltage of the reg. Only a dry on the zener could give the raised output voltage. One damaging scenario is where a dry on the collector of the series pass device would see the driver trying to supply all the current via the B-E junction of the pass device.

Oxidation of component leads is a real problem. When you reflow the joints the solder doesn't "take".

Your English is perfect
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Old 15th December 2013, 02:54 PM   #16
DreadPirate is offline DreadPirate  United States
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I'll be going with KSD2012 and KSC945 for the replacements.
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Old 15th December 2013, 04:07 PM   #17
Mooly is offline Mooly  United Kingdom
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Power Issues Nakamichi SR-4A
They should be fine.
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Old 16th December 2013, 05:24 PM   #18
Mannegizen is offline Mannegizen  Sweden
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KSC945(C) or you will have to do some origami folding with the legs

(C) as in center collector.
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Old 16th December 2013, 06:24 PM   #19
DreadPirate is offline DreadPirate  United States
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Quote:
Originally Posted by Mannegizen View Post
KSC945(C) or you will have to do some origami folding with the legs

(C) as in center collector.
Good point!
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Old 15th February 2014, 11:09 PM   #20
DreadPirate is offline DreadPirate  United States
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Got the new parts in. Q402 was confirmed defective. Q402, Q403, and ZD403 replaced. I am now getting 5.6V on CN12 which is correct (I was getting 10V before), but I'm reading 13.9V instead of 10V on this connector. This fault was also present before repair, but I thought it would also be corrected with replacement of Q402. I believe it is causing the issues I'm experiencing on the tuner. No AM at all, and on FM, signal meter is stuck on 3 lights and only one station comes in no matter the tuning position. TP1 is reading .5V (should be 20mV) and TP2 is reading 6V (should be 22V). I am at a loss as to why I'm getting 13.9V instead of 10V at CN12.
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