Nakamichi 480 power supply issue (control cam)

[dankik]

Check the voltage at VR101 and see if it matches the table below and is stable.
If it is not stable coming out of IC402 it will move the motor.
The collector Q406/Q406 outputs need to be stable through R423 to the motor.
If you say that the motor keeps moving back-forth, then Q405/Q406 may be bad or IC402 is bad.

 

[gnexus]

Sounds like a cold solder joint?

Thanks for comment!
But that was one of the first things I checked...
This PCB looks new. Deck is unmolested and obviously never touched after factory. This board is later than 580. So it's machine assembled with immaculate solder work except the hand soldered wires. Q407/408 also have flux residue. But that's factory work.

 

[dankik]

All I am saying is to put your DMM on the R423 and monitor the voltage at different function positions. Once it gets to a position, the voltage should be stable and not vary. Maybe you have some AC on the DC signal that is causing the slight movement. Put your DMM on ACV and look at that the motor voltage or VR. There should not be AC once the motor goes to the new position.

Being a factory solder does not mean it cannot develop a crack in the joint over time (40 years) and introduce extra resistance or an intermittent connection. Also, at times leads can become oxidized and cause problems with continuity.

I would touch up the solder joints at IC402, Q405, Q406, R423, and VR601.
Did you say you cleaned VR601 contacts?

 

[gnexus]

Check the voltage at VR101 and see if it matches the table below and is stable.
If it is not stable coming out of IC402 it will move the motor.
The collector Q406/Q406 outputs need to be stable through R423 to the motor.
If you say that the motor keeps moving back-forth, then Q405/Q406 may be bad or IC402 is bad.

VR101 was another item first on the checklist. But so far checked only after the oscillations stopped as it takes a minute or less to stabilize. So too little time.

I assumed, if it is a power supply issue, which it must be because the transport is verified in the other 480, then better to first check at the source.

The voltages at VR101 are correct EXCEPT for pause. Pause is WAY off! So I will obtain the correct new value for R454 tomorrow using a trimpot. Then can buy and install the correct resistor.

I just replaced IC402 since I have plenty and they seem to often go bad. Probably a good idea to just routinely replace those. So far no oscillations...

But we'll see tomorrow if it actually helps...

 

[gnexus]

All I am saying is to put your DMM on the R423 and monitor the voltage at different function positions. Once it gets to a position, the voltage should be stable and not vary. Maybe you have some AC on the DC signal that is causing the slight movement. Put your DMM on ACV and look at that the motor voltage or VR. There should not be AC once the motor goes to the new position.

I'll definitely do that tomorrow if the issue is still present!

Please note this issue is not "slight movement." It's a rapid oscillation at stop position. I don't know a way to measure the oscillation frequency. But it's less than 60Hz. It's maybe 30Hz or so and lasts between 15-30 seconds or so now. It used to be much longer before the new power caps.

 

[gnexus]

Being a factory solder does not mean it cannot develop a crack in the joint over time (40 years) and introduce extra resistance or an intermittent connection. Also, at times leads can become oxidized and cause problems with continuity.

I would touch up the solder joints at IC402, Q405, Q406, R423, and VR601.
Did you say you cleaned VR601 contacts?

Very true!
But if it's a solder issue then it goes away very rapidly when the joint heats up. Then later, after the deck is powered off and fully cooled down, it's still gone for at least 6 or more hours. Then the issue returns the next day...

Well IC402 is now definitely touched up as it's new!

It can't be VR601 because the same transport works fine in the other deck.

Edit:
Forgot to mention that the voltage on IC402(7) was fluctuating rapidly after power on when checked today. That's main reason it got replaced. IC402(1) voltage was perfectly stable. No time to check anything else before issue went away.

 

[gnexus]

Absolutely no control cam movement today!
Had DMM on R423 at power on. The voltage slowly rose from 11.8V to 15V.

Maybe replacing IC402 fixed the control cam issue?
(But I'm dubious at this point after so many false starts...)

We'll see for certain after a few days. But looks very hopeful so far.

 

[gnexus]

Otherwise, except for control cam pause calibration, his 480 is DONE!
All other calibrations, including EX, SX and ZX recording levels and bias, are now done. Only remaining things this deck needs are cosmetic issues like on the 580.

Both decks also need new black zip ties inside...

 

[gnexus]

Just received the 1M ohm trimpots. So replaced R454 with one. The pause position was much better at max travel with stop & play correctly set, but STILL WAY OFF!

So had to add R454 in series with the 1M trimpot. Then the trimpot had to be set at approx. the centered position! So the correct value of R454 for this 480 is actually about 500k higher than the specified value!

Now all control cam positions are perfectly set!
That will permanently resolve the incompletely engaged record switch position. That in turn resolves the supposed "blown recording input" problem of very low and distorted input, which was originally incorrectly diagnosed.

So far no more control cam oscillations. It now definitely appears that IC402 was the actual cause. It was a power supply issue. But not a electrolytic cap problem as originally suspected.

But at least now it's got all new power caps!

 

[gnexus]

Thanks very much to both @dankik and @tadawson
their help in fixing this deck.

 

[gnexus]

Getting very tired of the way this deck plays games...

Today it had no control cam oscillations. But the stop and pause calibrations were quite far off. It's REALLY bad when the stop position is off because then the cassette door won't open. That's the big problem with the oscillations. The cam often then doesn't end up in the correct position until you power the deck on and off several times...

It's very nice to now have the trimpot to also adjust the pause position! That should have been there from the factory.

I squirted more deoxit in VR601 just in case it has unseen issues. But that transport works fine in the other 480.

 

[gnexus]

The control cam oscillation is still there!

It now only happens if the deck is powered off for 8 or more hours. The oscillation is quite short now. It only lasts a few seconds. But then the cam often doesn't end up in the correct stop position. Then the power must be cycled several times to allow the cassette door to open. The cam calibrations are EXACTLY correct in both position and voltage.

The voltage at IC402 pin 7 varies rapidly upon power on. It settles at 15V as the oscillations stop.

Next step is to locate which input causing is causing the op amp output voltage to fluctuate. Must be either IC402(5) or (6) because IC402 is now known good.

 

[dankik]

Check the 24V voltage going to IC402 pin 8. Also, check for 12V at R420. They should be rock steady just after turning on power. If not, then you have a power supply problem.
Check the output voltage of IC401 regulator or the filter capacitor C405, 3300uF/25V (note: it should probably be increased from a 25V to 35V or 50V value because it is regulating down to 24V, and C404 2200uF/50V.

 

[gnexus]

Check the 24V voltage going to IC402 pin 8. Also, check for 12V at R420. They should be rock steady just after turning on power. If not, then you have a power supply problem.
Check the output voltage of IC401 regulator or the filter capacitor C405, 3300uF/25V (note: it should probably be increased from a 25V to 35V or 50V value because it is regulating down to 24V, and C404 2200uF/50V.

Thanks!

But I've already done ALL those steps before swapping out IC402...

There are no power supply issues anymore AFAIK. Both 24V and 12V are absolutely rock steady just after turning on the power. So it's something else.

This morning no oscillations...

Power at IC402(8) is 23.82V instantly after power on.
It's 14.4V at pin 5 and 14.6V at pin 6 instantly after power on. It's ~10V at pin 7. ~12V at R423 and 14.85V at VR601.

The voltage at IC402(7) fluctuates when there are cam oscillations. But that day is not this day...