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Nakamichi 480 not recording on right channel

G

gnexus

Active Member
The Line inputs are okay. It has perfectly even line-in audio levels (adjusted with level controls) and equal audio output and VU levels on both channels. This is from a 400 Hz test tone on line-in.

The Left channel playback works fine. But right channel has no audible output after record (it's only a 2 head deck so no actual tape monitor output during recording...). The tape was also tested on different deck (580) and only the left channel was recorded.

This is with a standard 400Hz test tone calibrated to the red lines next to -10dB on the VU meters. Left channel output corresponds to set record level. Right channel no output and meter wiggling at -40 dB.

Update:
After making the recording I just now noticed that the right VU level and playback audio output is now very low on a tape with known even channels. This wasn't previously an issue. So the issue is very likely to be the record/playback switch. I'll spray some contact cleaner in it and update this thread accordingly once more info is known.

Update 2:
Spraying contact cleaner in the REC/PLAY switch returned the PLAY output levels to even. Confirmed with 400Hz test tape from other deck that has even VU levels.

BUT, the recorded 400 Hz output is incredibly low. Left VU is at -8dB (or whatever the red mark is) while right channels is -40dB. Dolby in/out has no effect. Raising the record input level to 0 raises the left previously recorded output, but it has no effect on the right channel.
 
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The 480 also has very low output on the RIGHT channel. Moving that discussion here from the 580 thread:
dankik said:
The 480 is similar to the 580. The headphone output takes the same Line Out signal and goes through one additional amplification stage before going to the headphone jack. Since you say that the headphone output is good, then the output from the first amplification stage must be because it is common to both the headphone output and line output.

The ONLY difference is that the line out goes through one more capacitor (C245) before going to the Line Out RCA jack.
Check C245 (C145) to make sure it is good. You can also check the board traces and solder.

View attachment 3820864
Click to expand...
 
gnexus said:
Then something strange is going on. I pulled both C245 and 145. Both had identical capacitance of 13uF. I swapped the two when reinstalled. Still has same issue with very low right channel.

This board is very clean. Just like new. So no issues with the traces, etc. I extensively cleaned the RCA jacks by scrubbing them with deoxit. The jacks are enclosed in plastic. But has 0 ohms between RCA and PCB.
Click to expand...
 
Turns out the voltage regulator IC was somehow responsible for the low right channel line output. Replacement of the voltage regulator IC also dramatically improved FF and REW speeds! :)
 
Assume now that the issue with no recording on the right channel is due to a defective Dolby IC. Most other possible causes have already been eliminated. Will try swapping Dolby ICs to confirm if defective.
 
Turns out the voltage regulator IC was NOT responsible for the low right channel line output. The following day the low right output was back! :(

Replaced the 2 Dolby uA7300 ICs with LA2730s. That accomplished NOTHING, unfortunately. (Hopefully at least slightly improves the Dolby NR).
Still no right channel recording..

So then swapped ALL polypropylene caps in Dolby circuit with left channel. All caps had similar and okay values. So no effect on right channel recording. :(

Also noted that, unlike tape playback, the right channel line output is fine for recording monitoring.

Now running out of possibilities to resolve these problems without a scope...

Any ideas anyone?
 
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gnexus said:
Turns out the voltage regulator IC was NOT responsible for the low right channel line output. The following day the low right output was back! :(

Replaced the 2 Dolby uA7300 ICs with LA2730s. That accomplished NOTHING, unfortunately. (Hopefully at least slightly improves the Dolby NR).
Still no right channel recording..

So then swapped ALL polypropylene caps in Dolby circuit with left channel. All caps had similar and okay values. So no effect on right channel recording. :(

Also noted that, unlike tape playback, the right channel line output is fine for recording monitoring.

Now running out of possibilities to resolve these problems without a scope...

Any ideas anyone?
Click to expand...
Get a scope . . . you have already wasted far more time than it would cost you.
 
The problem with no recording in right channel was the transistor in the record amp right channel - Q203 was dead.

Problem resolved :)

So end of this thread.
 
The problem with no recording in right channel is now BACK!

Unbelievable!!! :(
It worked fine for a day. Then when powered off and then powered on the next day it was back! Just like the low volume issue on same channel...

I tried swapping Q203 with another 945 transistor to no effect. Also checked the adjacent capacitors. No luck. I already ordered new KSC1845 low noise transistors to replace the 945s. Maybe that'll fix it again? Maybe not... :/

Think this deck is cursed! A 480 is not worth all this effort... :(
 
This poor old very long neglected 480 is doing much better now. :)

Playback on line out is now even and CD quality. Recording on BOTH channels is also working now.
It seems it somehow got WAY out of both mechanical and electronic adjustments just from sitting so long in non-climate storage.. I don't yet have test tapes. So can't do final adjustments, but at least it's much better now. :)

Unfortunately the right channel recording is still very low. It's 0 dB on left using 400Hz test tone. But right is at -20dB with the recording level pot maxed out :(

Not sure of the cause yet. Playback is very even now. So wouldn't think it's an alignment issue with a 2-head deck...
 
The low recording level on the right channel seems to be a mechanical alignment issue as the recording level voltages are even at the tape head. It seems that alignment for recording is much more critical than for playback.

Unfortunately no alignment tapes or a M300 jig to properly set the alignment.
 
It's not any mechanical alignments. Most all electronic components have also been tested. So now thinking the playback /recording head could be bad. That would explain a lot - like the low right channel output on old tapes previously recorded on this deck in addition to the low right channel recording level. Will swap transports to test...
 
It's not the head. Your problem is intermittent. Since playback is always ok, the head is not intermittent. Check the schematic to see if this uses a relay to switch the head from playback to record.
 
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