Nakamichi TA-2A No FM lock until warmed up

nickhead

Active Member
Hi all. I am moving this thread from the solid state forum to the tuner forum as it might be more appropriate here. I have experience working on older audio equipment, but generally don't work on these "newer" IC controlled machines. I got this one to give it a shot. The problem, as described to me when I got it, was that it would not lock on FM stations, even strong local ones. I found online that quite a few others have this same issue with this tuner, but no one seems to know the solution. While troubleshooting I found that the problem will go away in about 15 minutes and lock stations in stereo if it was left on. Nothing obvious stuck out as I looked it over.

Voltages all looked good with the exception of pin 9 of U104 it's a PLL FM MPX demodulator LA3400N that I can't find a datasheet on (a main reason why I don't like working on these machines with the mysterious little black boxes all over inside them). The schematic says it should be 0V, I see 3.25V. The only thing I can find on that pin is in the service manual and all it says is "Vcc on muting". I have 3.25V there all the time in every mode. Not sure if that's an issue or if the schematic is just wrong.

I also saw voltage missing at the collector of Q114. I reflowed him. No change. Removed him and he tested good. Re-installed and now have the correct voltage there. Didn't fix the problem though.

I've found that I can "fix" the problem for a few seconds by pressing the "memory" button. When it lights up and is waiting for you to press the button you want to store the station in everything works perfectly and sounds great. After it exits memory mode back to static and no lock. Weird...must be them mysterious little black boxes...

This tuner/amp is otherwise in great shape. Seems like it hasn't been abused. The normal heat related issues, like the fried underrated resistors, bad solder connections, etc. do not exist on this amp. Does anyone have any ideas on what could cause this issue and where to look to try and fix it?

Nick
 
Muting issues are fun. Make sure the alignment is correct first.

Since works at warm up some cold spray may help determine the problem. If something is bit out of on the alignment.
 
Do you know how to align one of these? I have the service manual, but don't see alignment in there. I'm by no means a tuner expert. I mainly work on amps. I've already used half a can of compressed air trying to freeze it back to fail, but once it starts working I can't get it to fail again without turning it off for a while.
 
Alignment instructions are in Section 4.4.2 of the SM. Locations shown in Fig. 3. You'll need a scope, signal generator, etc.

But it is also worth checking one other known problem: Q525 on the Logic board (shares the top board with the tuner). If you got the SM from HiFiEngine, the very last page has a service addendum. The description of the problem is rather cryptic (if your customer reports transistor damage...), but a look at the schematic shows Q525 is involved in the +/-18V supply. If yours is misbehaving, it is possible it or some derivative voltage from it creates some wonkiness in some of the logic circuits, and perhaps ultimately affecting things in the tuning circuit (the latter just a guess).

I replaced Q525 on both of the TA units I have as a matter of course when I got them a few years ago. The original is a garden variety small transistor (2SA733), but apparently was somewhat undersized. The suggested replacement is now obsolete, but based on info given in another thread, I used a PN2907A-AP, but had to twist the pins to get a matching ECB order.

If you make any changes, I also suggest upgrading those 22KΩ resistors (R309L and R309R) to at least 1W versions (I went with 2W metal-oxide resistors). These are used as constant current sources.
 
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I did look at the muting level adjustment, adjusted up and down with no change to the problem. So I set it back to where it was. I've been concentrating my efforts on the tuner circuit, but everything is pointing to this not being a problem there. I've been over just about every component in there and I feel like I know them all intimately. So looking at the logic board would be the next logical move :) The service bulletin is not very informative - "if you receive a complaint about transistor damage"??? LOL how many people bring in a unit for repair and say it has transistor damage?? That circuit doesn't appear to be related to this issue, but I'll take a look at it after work today. I'm sure I have something that I can put in Q525's place to see what happens.
 
On the logic board C506, a ceramic capacitor that sits between the output of U501 and the input on U502 was bad. I don't have a clue what the purpose of this cap is, but I swapped him out and all is good. Does anyone know what the purpose is of this cap?
 
Well, obviously, the purpose of that cap is to prevent problems with FM lock-up. ;)

Glad you got it fixed, and hopefully this will be helpful to others who experience similar issues. Seems odd that a ceramic cap would go south like that, but perhaps it is more common than my uninformed perception is. It does seem there is a lot of "handshaking" or interconnectedness, for lack of a better term, that goes on among the ICs in the tuner, display modules, and protection modules in these more modern designs, and one small fault brings the whole thing down.

It reminds me of a superb electronics tech (non-audio) I knew in a former life, who held some patents on digital machine control going back to the late '60s, who once remarked to me that the lab instruments we were working on (mass spectrometers that he was involved in the design of) were always on "the verge of operation", and any small thing like a compromised cap or resistor could bring the very expensive instrument (about a megabuck in today's dollars) to its knees.
 
I never would have guessed a ceramic cap was the problem. I know they do go bad, but I've never seen one act like this before.
 
On the logic board C506, a ceramic capacitor that sits between the output of U501 and the input on U502 was bad. I don't have a clue what the purpose of this cap is, but I swapped him out and all is good. Does anyone know what the purpose is of this cap?
Good find. That signal is the FM LO VCO that is divided down by the pre-scaler, U501, since U502 can not handle 100MHz, it needs something divided down by 100 or so.
C506, is used as a DC block and AC signal coupler into the PLL synth chip, U502. Yes it is unusual that a ceramic disk cap would fail but so be it.
 
Thanks for that info rcs16. Some additional information and a question: This cap when "cold" was well within specs. It's a 103Z. Capacitance measured at 12nF cold. Heating it with an iron dropped the capacitance down to less than 1nF, which I believe would be out of spec. But it worked when heated, failed when cold. Do you think this circuit needs a different capacitor? Maybe a 1nF? (Which is what I replaced it with), or is there another problem in the circuit that just happens to be covered up with a lower capacitance component?
 
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I think it is the temp coefficient of the cap, a Z5U class dielectric? A "103" is 10n, best is to use NPO(COG) dielectric type for temp sensitive applications.
It depends on where in the ckt the part is used, temp co might not be important, it will never be operating at soldering iron temp.
Kinda strange that you measure cold and it is within spec but does not op in ckt, but if you heat it up, the C goes down and the ckt now works?
 
Yeah, that's why I was wondering if there is another problem. But I know they used multiple questionable components in this unit, is this just another? I'd like to here from others that had this problem to see if this cap is their issue too.
 
Hmm, I guess I am wondering about rcs16's point as well. Any possibility that the issue was extrinsic to the cap, such as a cold joint on its leads?

Nick, your diagnostic skills seem excellent, so if you don't mind, I am also intrigued how you identified C506 as the issue, given that it actually tests ok at ambient temp. I am trying to learn here.
 
Not cold solder. I always re-flow components before I remove them to test. I found it by warming up components in that area. I was actually looking at U501 as the problem, but when my iron skimmed over that cap all my static went away. That cap got nowhere near soldering iron temperature, just get the iron close to it and the problem went away. That's a .01uF cap and my meter showed capacitance of .012uF. When I put the iron near it, it dropped to .001uF. I don't believe this circuit gets hot, so my guess would be the heat was coming from below, and maybe it just needed a little bit to drop it to .010uF and start working. Either way I replaced it with a .001uF and all works well.
 
For those that want some in depth knowledge of how these pll synthesizers work, xraytonyb just posted a great video that explains it very well. It's called Realistic 2200 part 2 - PLL Synthesized Tuner Theory. I think I know now why replacing that cap fixed the issue.
 
I wish I had the talent to tackle this issue and others with my TA-2A...I have to find someone I can trust to change this (although I will continue to see if I can educate myself to do it) but I tried something on a throw away Home Theater receiver to practice and I killed it...

Thanks everyone...I continue to learn!

Bill
 
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