Nakamichi BX-100 repair

[col]

I recently found a Nakamichi BX-100 that someone had chucked out on the street.

Cosmetically, it is in very good condition. Looking inside the cassette tray the heads seem to be in good condition too and the rubber wheels don't look that worn. When I plug it in and turn it on though not much happens the 2 lowest leds of the level lights come on but none of the play/stop/rew buttons work.

I checked the power supply at the transformer with my DMM and it has around 18v on each rail (AC).

I would dearly like to fix this as I have a HUGE box of tapes that have been sitting on a top shelf for about 7 years and I'm quite keen to be reunited with some of their contents

Also, I recently brought my first oscilloscope so this will be good fodder for it. Iv'e never used one before though and apart from watching a few tutorials on youtube don't really know what I'm doing with it yet.

I guess a good place to start would be to try and get a service manual?

If there is any tape deck experts out there that could give me some advice on how to proceed I would much appreciate it.

[kevinkr]

First step is to find the service manual and schematics online.

Depending on your level of patience and ability to do diagnostics you may or may not be able to fix it. Lack of experience may be a big handicap here, provided you can solder, think logically and actually understand the schematics you may have a chance. Hopefully exotic spare parts are not required.

Given that it is generally dead from description the first area I would check would be all of the power supplies.

[col]

Hi Kevinkr,

Thanks for your reply.

My level of expertise with soldering and making up kits is good and I know the basics on how to check power supplies, passive components etc. Thats about it though.

Today I stripped the panels away from the BX100 PCBs and have started probing it with my scope The power supply is +/-8v and the caps and rectifier bridge seem to be working fine.

I pulled out the main switch panel and depressing the switches with the probe attached seems to do something as the volts change. I'm wondering if the problem could be in the microprocessor that seems to control everything. as the fault doesn't seem to be confined to one circuit but to everything. I can't get any of the switches to have any affect at all. None of the motors have started to spin yet.

Yes, I should go and do some more internet searching see if i can find a schematic. My preliminary searching has only come up with a user manual.


col.

[Mooly]

Hi,
You need a circuit for sure. As to the processor being duff, this is the last thing to go for. 99% of faults on consumer gear are usually something simple like a fuse, circuit protector or safety resistor open circuit.
Even when you are convinced the processor is the problem a check on ALL the pins will usually reveal the fault as some input as not being correct.
Edit, Try and trace all the rails, look for large caps that might be rail decoupling and see if they have sensible voltages across them. Look for low value "safety" resistors in the PSU that may be open circuit.
5 volts for the logic seems resonable etc.

[kevinkr]

Originally posted by Mooly
Hi,
You need a circuit for sure. As to the processor being duff, this is the last thing to go for. 99% of faults on consumer gear are usually something simple like a fuse, circuit protector or safety resistor open circuit.
Even when you are convinced the processor is the problem a check on ALL the pins will usually reveal the fault as some input as not being correct.
Edit, Try and trace all the rails, look for large caps that might be rail decoupling and see if they have sensible voltages across them. Look for low value "safety" resistors in the PSU that may be open circuit.
5 volts for the logic seems resonable etc.

Generally agree here, only exception would be in the instance of a nearby lightening strike, because of the small die geometry in modern digital ICs their tolerance to transient events makes them vulnerable to relatively low voltages. In a previous job I did some lightening strike surge testing on a consumer audio product I helped to design, and despite independent voltage regulators and careful ground plane design the micro-controller (and only the micro-controller) died repeatedly when subjected to a distant simulated lightening strike to ac mains. (~6KV spike for some hundreds of nS) The fix required some minor design changes mostly to the ac input filtering. (Once it's in, its in.. )

I'd say the first thing to check for is presence of proper supply voltage at the uP (older products probably 5V) and next the check the clock. As Mooly says take a look at the issues he raises.

[Cobra2]

I think the capstan motor should start as soon as player is turned on. There may be a door-switch or "casette in tray" detector that "kills" all commands.

Arne K

[col]

Still have not been able to find a service manual. Digging around on the net just seems to come up with commercial sites trying to sell manuals for mega-bux. Hard to believe someone hasn't loaded all the service manuals into a free website making money off the adverts, gawd knows there would be enough audio nerds visiting to make a fortune. Anyway, if anyone out there has a service manual I can stick the pdf on my website for free

Iv'e been randomly probing resistors and diodes to see if I can find a dead one. Some of them seem to have been purposely soldered to the main board with the legs showing, maybe to dissipate heat? There is one big resistor next to the crystal which strikes me as odd, it seems to be 0 ohms. With my DMM/scope if something is wrong with a component it usually shows O.L (for overload) but that one comes up with 0 ohms. I checked it several times always the same.

I looked at the door switch and that seems to be fine. The one thing that is telling is that as soon as i apply the power the 2 lowest leds on the level meters light up and stay alight.

I also probed all the pins on the microprocessor with power applied and there is definitely stuff flowing through it, so at least it isn't completely dead. Wether the stuff is the right stuff I have no idea.

col.

[col]

Iv'e put a few photos up, click on "Full Size" for a good view. I will upload some better close up shots later, these are just to give a general idea, it's only 7:30am here at the moment and I haven't had any coffee yet.

http://pix.minirig.org.au/main.php?g2_itemId=408

col.

[stevecole]

Hi There

I have a BX125, so if we assume that model and the '100 behave in a similar way, then here are some observations that might help you. On the 125, in the 'just powered up' state, you do indeed get the bottom 2 LEDs of each level meter illuminated. Also, the 'up' LED of the electronic 'master fader' (which I believe the '100 has too) should be 'on'. The capstan motor only runs during 'playback' and 'record'. The transport has a further 2 motors, one for tape spooling, and another smaller one with a worm-gear on it that drives a cam-system which I think moves the headblock. I have a vague memory of that bit of my unit getting stuck occasionally - you could try giving it a helping hand.

I need to sort the tape-counter out on mine sometime soon so the lid will be off - if I can do some measurements for comparative purposes let me know, however I don't have a 'scope so that might limit things.

BTW, these machines are in a different league to units I've fiddled with by the likes of Technics or Denon, but at the end of the day they are common on a certain on-line auction site, so if you are defeated you should be able to get another for little outlay. I've been in contact with the UK service agent recently, and parts are now difficult to get and are pricey too, so there might be someone near to you who could use the unit for spares, so don't skip it!

Regards

Steve

[col]

Hi Steve,

Thanks for that. Yes, if you take the lid off your BX125 maybe you could measure the voltage across the two biggest caps. directly after the rectifier. One is 4700uF 25v (I am getting a reading of 23v there) and the other one is 6800uF 16v (that one is only reading around 1v). You can check them with a standard multimeter. I'm mostly interested in the voltage across the second one.

cheers,
col.