Strange Nakamichi tuner issue

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Hello all!
First time poster here, so please be gentle... I've owned a Nakamichi TA-4A receiver for a few years and absolutely love it. Alot of equipment has come and gone over the years, but this one always ends up cranking out the tunes. A few months ago, the tuner quit working with similar symptoms as in this thread. I got tired of not having the tuner, so I purchased a new Yamaha R-S700 to replace it. It seemed like a great recever, but it honestly was no match for the Nak. Side by side listening revealed the Nak was more powerful and had better bass, even though they're rated at the same power. The Yamaha was boxed up and returned. I decided to repair the tuner and enjoy it a few more years. With the help of the service manual, I tracked down the problem to the voltage regulator supplying the tuner board. I got that fixed and replaced the backup battery. The tuner came back to life and works perfect, except for one thing. The digital display shows .1 mhz higher than actually tuned. If I want to listen to 93.1, tuning to 93.1 will result in no lock and no stereo. Tuning to 93.2 will result in a lock and stereo. When scanning in auto, all of the stations grabbed are even numbers, always displayed .1 too high (93.2, 98.0, 103.6, etc). It does this throughout the entire band. I'm really stumped on this. There is no mention of any adjustments in the service manual. Any suggestions would be appreciated...
 
There are a number things that can cause this.
Possibly a shift of frequency do to the repaired voltage regulator or the reference oscillator crystal has shift out of tolerence which is highly unlikely but it does happen.

Set the tuner to your desired frequency say 93.1mhz and try tweaking the local oscillator coil very very gently until it locks on the signal.
It is possible the the coil got misaligned slightly during the repair job causing the phase lock loop circuit to make up the difference and causing an improper display.

I could verify this procedure for you on my Technics tuner if you wish.
Once I determine which coil is the local oscillator coil as I don't have the schematic for it and will have to do some searching to find it. jer
 
Here is some more info.
It suggest that the discreminator coil could be misaligned.
This is correct and not the local oscillator coil.
Sorry for the misinformation it has been quite a long time since I have worked on any receiver circuits as they rarley go bad.

If you try adust the LO coil the PLL will just compensate for the change and the results will remain the same.


http://www.diyaudio.com/forums/anal...n-tuner-problems-please-help.html#post1424817
 
I would not suggested messing with it if it works as I just shattered the slug by turning it to far.

Don't worry I will fix it as it was just a spare tuner and a freebe any way.
It is not the first one I have broken from not having the proper tool. jer
 
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Hi JHanko,
These Nakamichi receivers had a circuit modification that dealt with that fault. I no longer have my service manuals, but an ex-Nakamichi service center would be able to perform the changes you need.

Do not fiddle with any adjustments! That is not your problem, you have a circuit failure. BTW, I am using a TA-2 in the bedroom and agree with your findings about the sound quality with the Nak. Because it is a Stassis type output stage, you absolutely do need about 6" clear air above this receiver.

Hi geraldfryjr,
Please tell me you are joking! Everything you just posted here is not simply pure silliness, it's just plain wrong! Never mess with any adjustments without having the proper tools and service information.

To properly adjust a tuner, you're going to need far more test equipment than an accurate meter. The odds are great that you'll really degrade the performance of the tuner, and the charges to put it right can easily be high.
Possibly a shift of frequency do to the repaired voltage regulator
Huh?
or the reference oscillator crystal has shift out of tolerence which is highly unlikely but it does happen.
This is an extremely rare event, and probably caused by something else entirely.
Set the tuner to your desired frequency say 93.1mhz and try tweaking the local oscillator coil very very gently until it locks on the signal.
Ahhh, no.
It suggest that the discreminator coil could be misaligned
Good Lord! No!!!! Never mess with adjustments when a device stops working properly. They don't drift that far with time for one, and never change suddenly for two.
This is correct and not the local oscillator coil.
STOP. Do not adjust your tuner either.
Sorry for the misinformation it has been quite a long time since I have worked on any receiver circuits as they rarley go bad.
They are often out of alignment, some worse than others. This does not cause the tuner to not work. Try using some logic please, especially when advising other people!
If you try adust the LO coil the PLL will just compensate for the change and the results will remain the same.
No it will not. What will happen is the performance will be strongly degraded - if it still functions! I'm now in a state of shock due to your suggestions.
I found the discreminator coil on my receiver but don't have an alignment tool to fit in the tiny square hole in the slug.
As soon as I find or make one, I will verify that this procedure works.
Thank goodness for small miracles. You were meant not to have the proper tool for a reason.
I would not suggested messing with it if it works as I just shattered the slug by turning it to far.
Completely predictable.
Ferrite is brittle and the cores are tight. The slug shattered because the tool may not have been a perfect fit, or you did indeed turn it too far. If the alignment is out, it will not be out by very much at all. Keep that in mind.
Don't worry I will fix it as it was just a spare tuner and a freebe any way.
It is not the first one I have broken from not having the proper tool.
And that's a good reason or justification for trashing another tuner? You may well break the next core as well, and they are not always the same.

When aligning the detector of an FM receiver, one of the things you absolutely need in addition to a good meter is a good harmonic distortion analyzer. You also will need a very good FM generator that can be modulated with a constant tone (1 KHz is a good choice). You also will require a 19 KHz pilot (at 9%) and an MPX modulator to produce a stereo signal with tones.

JHanko has an electrical failure that caused this fault. In no way was messing around with the alignment indicated by anything.

-Chris
 
I do sincerely apologize for my misinformation as it has been along time ( 25 years) since I have done any work on receiver circuits. I do know the procedures and consequence's of not having the proper tools.

I normaly don't post any information unless I am absolutely sure it is correct.

I got a little ahead of myself and after doing some research it all started to come back and I had realized that I was wrong,by then it was to late to retract my erronous posts.

I'm very sorry.

I do hope there there is a lesson to be learned to those who don't have the proper tools or knowledge too leave these types of repairs to a qualified tech.

As far my cheapy tuner,I am quite capable of repairing it ,but you are right it would not have been broke had I have had the patients and searched for the proper tool buried deep in my garage.

I am very sorry and I wil do my best not allow this type of mistake happen again as I take great pride in helping others by giving the proper information ,unlike some of the other jokesters in some of these theads.

I'm truley sorry and I hope there wasn't damages done due to my misinformation. jer
 
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I was going to say I remember a marathon thread of mine on a Nak tuner but see there is a link to it hidden in the first posting. I couldn't remember the exact outcome tbh but see we got there in the end. A lot happens on DIY in three months :)

As this voltage regulator is perhaps a common problem (as anatech mentions there being a stock fault on these... if that is the fault Chris means) I wondered in a hazy moment if the original "calibration" could be returned by tweaking this supply... either a variable reg or using a 7805 in "variable" mode by adding a pot lift to the ground connection.

Do the two diodes in the original reg arrangement allow for temp compensation to maintain calibration as the receiver warms... does the alignment depend on supply voltage in a critical way.
 
geraldfryjr: Thank you for taking the time to help. I'm sure you had the best intentions. No harm done.

anatech: Thanks for the reply. What you say makes sense. It's quite possible that the tuner fault is what caused the regulator to open, but I would think the fuse would have prevented this. The tuner still sounds great, so I guess I'll leave it alone. I can live with that extra digit. It's not like I'm trying to sell it. This receiver lives in my shop, where it runs 8-10 hours everyday. I use the tuner alot. I would love to get it aligned, but I wouldn't even know where to begin looking for an ex-Nakamichi service center.

Mooly: I thought anatech meant that the tuner/display being out of sync is the fault. I could be wrong though. I can't answer if the PS has temp compensation or if the alignment is voltage dependant. All I know for sure is that after the repair, the voltage was well within 5% of the service manual spec. The only adjustment I made was L305 (center voltage), as this is all I have equipment for. This was only off by a very small amount (~30 mv.). I got it set to 0v ±5mv. This made no noticable difference in reception or sound.

Again, thanks to all for the help. If anyone has any more suggestions on how to get this fixed, please reply. If there's no other options, I'll just leave it alone and hope it doesn't get worse...
 
Thanks ,JHanko, for your kind response.
I am very happy there hasn't been any damage done to my poor judgement.

However it has certainly gotten my curiousity going as I have wittnessed this problem my self in several different pieces of equipment.

Most people write this off to the station changing their frequency and not changing there advertising.

I don't believe this to be true as the radio in my mothers 2003 buick rendevous is now 2 points off and I don't believe it was when it was new when she drove it off of the lot.

I would have noticed it and persuaded her to get it fixed while it was still under waranty.

I do remember back in the day when digital tuners started coming out that some had an AFC (automatic frequency control) button and would cause this same situation.

For that reason alone a had refused to get a digital tuner as I lived in the fringe areas and I liked to do some DXing on the fm band as well.

So,an analog tuner was the only thing I could use sucessfully.

I have the schematic of your tuner now and will keep studying it.

So I will keep searching for the answer and when/if I come up with something I will deffintely post it here on DIY audio.

Cheers and Good Luck. jer
 
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Hi jer,
Well, my response was a bit strong. For that, I'll apologize. The fact you are trying to help is commendable, and that is what community is all about. I make mistakes as well.

My biggest concern is that people work both intelligently, and carefully so as to avoid extra problems. For a sudden fault, adjusting things will only make it worse.
I do remember back in the day when digital tuners started coming out that some had an AFC (automatic frequency control) button and would cause this same situation.
Now that is clearly a fault with the detector (quadrature or whatever method is used). The DC output is used to center the AFC dead on the station. If it's working properly - fantastic! If not, forget listening to FM. Also, the quality of the charge pump (to derive tuning voltage from a variable duty cycle pulse train from the controller) will greatly affect the tuning. I can see here a leaky capacitor could cause some of what you had observed. I have found those in some products. If you look at the output from the charge pump, it should be a smooth DC. If you inject music on that, it will come out the speakers. Cool tricks.

Hi Mooley,
There are a number of faults I was considering. The known fault is a B+ switch that can confuse the processor, among other things. It's a PNP transistor north of the tuner controller on my TA-2. They replace it with a more heavily spec'd part. In fact, there may be more than one involved on the TA-4. I would recommend that all transistors in that circuit used to switch power be replaced with parts a little higher in current rating, with around the same beta. It's cheap insurance given that I can't remember the details exactly. Sorry I could help more.

There is also a resistor in each amplifier channel in the power amp that fails. It goes high resistance and will shut that channel down. It may even trip the protection when it's partially open. Replace that with either a higher power metal oxide resistor (3 watt is good). It will be more of a problem with the TA-3 and TA-4 as most of the open area above is blocked with the tuner PCB as it gets larger. For those of you who want to improve performance a little, use a current source in those locations.

-Chris
 
No problem,anatech, all is good.

I have been looking over the charge pump circuit and its associated 28v regulator.
Which is why I had questioned the repaired regulator.

However, it was never stated as to which regulator was repaired.

A leaky cap could very well be causing a drop in the tuning voltage making the PLL to over correct as well as the posibilty of the 28v regulator not being exactly 28v.
The PLL is ruled out (IMO) becuase they either work or they don't. jer
 
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Hi jer,
The PLL is ruled out (IMO) becuase they either work or they don't.
Nothing is ruled out until the problem has been found. :)

Keeping an open mind while troubleshooting is the hard part. PLL circuits can do really odd things depending on what is wrong. Generally though, I agree. It's locked or it's not. But what is the pull in range?

-Chris
 
The regulator that failed with no output was U951, a 78m12 device. After replacing, the output was 13.3v, measured with a recently calibrated Fluke 87.
 

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That voltage is correct.
It is 1.3v volts because the two diodes raise the ground reference of the LM7812 regulator by the total voltage drop across the diodes ( 2 X .65v).
For an example if you added another diode the output would be 13.95v and if you took one diode out (or just shorted one) the voltage output would be 12.65v. jer
 
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