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Old 4th November 2014, 12:28 AM   #1
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Default Nakamichi 730 issues-help!?!?!

Hello everyone,

I have a Nakamichi 730 I just acquired. The receiver was working excellent, sounding great, and it even still looks great! The first day of using the receiver, I played it for around 5 hours with issues.
The second day of use, The receiver started making a crackling and rustling noise (snap-crackle- and small pops) and then the receiver just shut off. This was after 15-20 minutes of use. I can turn the receiver back on immediately and it just sounds great for the next 15 minutes.
Now on the third day, the receiver shuts off after 10 minutes of use.

If I have the receiver volume low, the receiver will still do the snap crackle and pop, the receiver will stay on, but the audio cuts out. The audio will then come back on for a minute or so(depending on how much cracking and popping) and then it will totally shut down.
Each time I can turn the receiver back on and she starts playing music right away.
Any ideas??
Michael
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Old 4th November 2014, 06:32 AM   #2
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dry joints ?
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Old 4th November 2014, 08:04 AM   #3
Mooly is offline Mooly  United Kingdom
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As above.

Also could be a transistor breaking down (surprisingly common on older stuff). Typical failure mode is B-E junction becoming open or high intermittently.
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Old 4th November 2014, 11:32 AM   #4
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Well, it looks like I will be working on this so any help would be appreciated. Mooly- can you tell me what the B-E junction is and where to look for that? I have the schematic, but haven't really even looked at it. I Old 'n' Cranky- when you say "dry joints", do you mean cracked solder joints? I need to be doing this since there are no technicians who will look at this. It's such a lovely sounding receiver I want to be able to resurrect it.
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Old 4th November 2014, 11:34 AM   #5
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The first thing I will do is look at the transistors, and test them. Correct?
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Old 4th November 2014, 11:45 AM   #6
irribeo is offline irribeo  Netherlands
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Any pcb trimmer/pots of the open/unsealed type ? Cabinet is very full, does it get warm?
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Old 4th November 2014, 12:44 PM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by michael88 View Post
Well, it looks like I will be working on this so any help would be appreciated. Mooly- can you tell me what the B-E junction is and where to look for that? I have the schematic, but haven't really even looked at it. I Old 'n' Cranky- when you say "dry joints", do you mean cracked solder joints? I need to be doing this since there are no technicians who will look at this. It's such a lovely sounding receiver I want to be able to resurrect it.
B-E is the base emitter junction of any of the transistors. Typically any that are giving trouble will be "driver transistors", the VAS transistor (voltage amplifier stage) or any used as regulators in the power supply. All these are locations that can run hot.

Dries can be hard to spot. Look at the last pictures in post #1 here,
Sony CDP790 and KSS240 Restoration Project

Quote:
Originally Posted by michael88 View Post
The first thing I will do is look at the transistors, and test them. Correct?
Almost certainly the transistors will test OK on a cold static test. Its when they heat that they go intermittent. The clue to finding these is to measure the base to emitter differential which will always be in the 0.6 to 0.7 volt region. When they go intermittent that voltage difference shoots up.

Be very careful, one slip of the meter leads when it is on will cause much damage.
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Old 5th November 2014, 01:51 PM   #8
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Thanks gentlemen! This is some great information and now I know where to start looking for the problem!!
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Old 5th November 2014, 02:37 PM   #9
Mooly is offline Mooly  United Kingdom
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A tin of freezer spray can be helpful with problems like this. The secret is to literally apply it drop by drop to a part rather than blast a whole area.

Its quite expensive nowadays (it consists of R134A refridgerant) but you might be able to use a cheapo air duster can used upside down (butane) which chills.
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Old 5th November 2014, 11:09 PM   #10
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One way to help find bad solder joints is to use something like a toothbrush (insulated plastic) and gently push on different areas of the circuit board while the unit is operating.

Not always successful, but it's a quick cheap method that can help show up joint or track failures.

bad joints is where experience counts.
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