Nakamichi 480 low playback line output right channel

[gnexus]

I'm gonna start a new thread on this strange issue. It should prove helpful to others who might encounter it, in the unlikely event it ever gets resolved.

This is a quite strange issue that has proven to be incredibly difficult to diagnose without an oscilloscope. The reason is that it it ONLY occurs with tape playback and only on line out. Headphone output is even and has the high frequencies. But the right channel line out is very low and is missing the highs.

In addition, the problem only occurs on playback. It doesn't occur in record mode. Since this is a 2-head deck the record monitoring is from line in and not from the tape itself.

The issue also isn't from the transport or tape heads. Headphone out is fine.

A lot of diagnostic work has already been done with help from dankik, who has been very helpful. But the issue persists..

The following post will outline the diagnostics done so far

 

[gnexus]

The 480 also has very low output on the RIGHT channel. Moving that discussion here from the 580 thread:

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

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.

The capacitor C245 also goes to VR205, is then amplified, and goes to the VU meter.
Does the VU meter for the R channel show low volume also, or is it OK and only the Line Out is low?
If not OK, then check C248 and VR205.

View attachment 3821274

The VU level is ok.

Further testing will need to wait. IC401 accidentally got shorted when the DMM ground clip jumped off. Now the 480 is dead and needs a new voltage regulator IC.

That is what happens when not using a proper workbench... Won't do that again! But badly need better tools and setup for this work.

The low line out right channel on the 480 is BACK!

Same tape! No changes to RCA connections or anything else!

No changes whatsoever EXCEPT yesterday tried REC again to see if still no REC on right channel (still nothing). So maybe it has something to do with the play/record switch?
Edit: no! Sprayed more deoxit and wiggled the switch many times when off with no change afterwards to right channel output.

So, anyway, the new voltage regulator obviously didn't fix the low right channel. Instead it's something that seems to be (occasionally) intermittent...

 

[gnexus]

The following has also been done:

clean & check play/rec switch
clean & check tape selection switches
replace both Dolby ICs
swap all Dolby circuit PP caps
swap C220/120
swap C213/113
swap C245/145
checked Electrolytics in line amp
checked Electrolytics in play amp
checked line amp voltages w/ manual spec

Now waiting to get an oscilloscope for further tests.

 

[gnexus]

Looking at the schematic again it seems the issue MUST originate in the playback eq. circuit and not the line amp.

In record mode the playback circuit is bypassed and the output signal comes from line-in. Record mode line output is fine. So the origin must be in the playback circuit.

I don't then understand how headphone out in play sounds okay. But the issue seems to have to be the playback circuit if record mode is ok.

Can someone please verify that in case I made a mistake reading the schematic? Dankik, what's your opinion? Thanks in advance.

 

[gnexus]

Just completed more (hack) diagnostics:

Checked all electrolytics in playback circuit. Both channels same and within spec.

Swapped out Q101/102 with Q201/202. Q202 swap seems to have made very slight improvement.

Balanced VR101/201 levels on VU using test tape made on the 580 at 0dB. The tape outputs only -5dB on the 480. But at least now the VUs are equal on most tapes made on the 480.

Now the L&R line outputs seem fairly equal. But right channel is very "muddied" and lacking any high frequencies. A frequency response test tape fails to output anything above 1000Hz. Headphone output is perfectly equal to the limits of my hearing, which unfortunately isn't that much anymore...

So somehow, somewhere, there's an invisible low pass filter in the line out right channel. But only on tape playback and not with record line monitor.

 

[gnexus]

Success!!!
After extensive checks of all the caps, transistors & resistors in both the playback and line amp circuits it turned out the cause was simply mechanical and electronic adjustments that were just WAY the hell off!

It already sounded fantastic with headphones - basically CD quality. So I didn't want to touch any adjustments. I was waiting for that until everything was done and I had the proper alignment tapes, an M300, and test equipment...

No adjustments have been done previously. The settings were from the factory, as this deck was purchased new. So that just shows you that when you take apart the transport to fix seized capstans, and also replace a few PCB components, that it just screws up everything and you'll be very lucky if it even works after that...