Yamaha MT-50 4 track recorder

[dkevin]

Hello,
I bought a "fixer" Yamaha 4 track recorder model MT-50. It has a peculiar issue and I am hoping someone can give me an idea of where to start troubleshooting it. The L.E.D. meter for track two pegs from the moment power is supplied to the unit. It doesn't fluctuate with the sound being input or played back through the system. I tried adjusting the playback volume for track 2 but it made no difference. As an additional note, the seller and I had a spirited discussion about the A/C adapter he sent with it. The MT-50 manual specifies 12V @ 600 mA . The adapter he sent was a cell phone charger that supplied 6V @ 300mA. If he used this adapter with the recorder what sort of damage would it have caused? The two issues may well be unrelated. As of now, I am thinking that a capacitor or two has failed in the channel 2 circuitry and is causing the meter to rise to the top of its scale. I can hear a bit of white noise in my headphones and I wonder if it is related to a capacitor failure as well. I do not have a schematic for this device. In fact, I have not ever seen a schematic for it. If you have any clues, tips, wisdom to share about these issues, please join in. Thank for your time!

[The Dude]

NOTE: Without a schematic, this is a bit of generic info and speculation.

My first suspect would be a shorted op amp on the channel with the pegging meter. On many of these machines, the inputs share op amps. In other words CH1 and CH2 will share a chip. CH3 and CH4 will share a chip, etc. Look for an op amp with supply voltage or DC voltage on its output pins. You can compare the suspect op amp voltages with the working channel(s) op amp(s). If you can't find a schematic, find a datasheet for your IC's. Off the top of my head; I'm not sure, but my recollection is that these Yamaha's use SIP op amps (in case your trying to find them looking at the bottom of the board).

On the power supply: It would be more likely that the supply would be damaged than the 4-track if it wasn't rated for proper current. At any rate, if the device calls for a 600mA supply, you should be using at least a 600mA supply. I doubt that is what caused your problem, but it's still not a supply you should be using.

[Jazz P Bass]

I had no luck finding the MT 50 service manual.
Here is a page from the MT 400 LED circuit.
MT400 LED.pdf
The MT 400 service manual can be found here: https://www.manualslib.com/manual/12...aha-Mt400.html

[dkevin]

Thanks for your responses! I am attaching a picture of the insides of the unit. The channels are laid out horizontally from left to right. I enlarged the picture so it is a bit blurry. I will take some close up pictures of the actual unit and try to concentrate on the possible areas of concern.

[dkevin]

I am attaching a link to a photo site that has pictures of the victim. I tried to label them but they seem to be rotated inconsistently. Take a good look and advise as possible. Thanks again!https://imgur.com/a/2rXUwEn

[g1]

You may not get very far on this one without the service manual. Unless you have already identified the Meter drive IC and are working from a pin-out. Then you can follow back from the IC if it is getting faulty input.
Otherwise, for anyone to help you here, there is not much that can be done without the service manual.
They do sell them: https://usa.yamaha.com/support/parts_manuals/index.html

[dkevin]

I have entered the information in their suggested format (S M MT50) but they do not show any such manual. I tried in several different categories but no luck. I do not seem to have luck looking for a service manual on Ebay or Google either. I may have to wing it....

[g1]

Try searching the IC part numbers til you find the LED meter driver.
If it's BA6137, the MT400 manual might help. If it's IR2E27A, the MT120 manual might help.
From the driver you can check back to what The Dude mentioned in post #2.

[Enzo]

Often such chip data sheets have sample circuits, and many designers pretty much just lift the sample.

Forget the internet, CALL Yamaha and ask if they can provide the file.

[dkevin]

I will call Yamaha US support tomorrow morning. I do not expect great help. I took your advice when looking for a schematic for a Fostex X-18H schematic and called to ask for help. They referred me to a Fostex repair facility who no longer worked on that device. On another note, I did locate and download a copy of the MT-100 service manual. It contained a parts list and the larger processors on the circuit board are also on the MT100 board. One is a AN 6292 NK which is a DBX II chip and is made by Panasonic. The second is a TC4066 BP which is referred to as a CMOS quad-bilateral switch. I don't know how they factor into the channel 2 circuit but at least I have the data sheets and can check some voltages. Who knows, maybe Yamaha will come through and provide the documentation I need.

[olddawg]

I was an Authorized Yamaha Tech for many years but am not familiar with this model off the top of my head. First.. make sure you have a proper power supply at the right voltage and well filtered. Extra current on tap won’t matter much but an inadequate supply will. I doubt if under voltage damaged it. It would just not work well if at all. The first thing I would do is see if there are any mechanical record/playback/assignment switches associated with that channel. If so clean the with Caig Deoxit and toggle them several times. You may have high frequency oscillation pegging that meter out of the audible range or dc jumping a dirty switch contact. After that every channel should be a duplicate of the other. If you are patient you can start at the input and follow the signal. Varying the signal on a good channel should identify what is driving the VU meter. A scope would be helpful. Calling Yamaha will cost you nothing but don’t hold your breath..

[Enzo]

Calling sometimes works and sometimes not, but it is a step many neglect.

4066 is a quad switch. Used to be the 4016 was what we used. 4066 is a common CMOS chip. it has four independent sections. Each section has an in and an out (either way, it is bidirectional) and a control pin. The control pin decides if the in and out are connected together or not. SO it acts as a switch. Used to route analog signals among other things.


I checked my dealer/support portal, and there is no service manual/documents there. So that is a strike against you, but they still could help, never know.

[dkevin]

I called Yamaha support and they were helpful. I got transferred to the service dept. and they are going to send me a copy of their one (and only) hard copy of the service manual for the MT-50. I was not expecting much so this is a very pleasant surprise. Of course, they are all working remotely due to the _virus and it will have to wait until they are able to go to the office and lay hands on the manual. I will continue, until then, to try and trace the circuit and see if I can isolate the component that has failed. There are several test points along the circuit's path and I can use a healthy functioning track's test voltages to set a benchmark by which to measure the faulty track's voltages. Any more words of wisdom before I dive in??? (what could go wrong?)

[Simonp]

Is there any chance you could point me to that MT-100 service manual? I've been unable to find a copy. I've bought a used machine thats having transport problems. It will play, but the stop button sporadically fails to retract the head. I already replaced the belts and have been stymied by the lack of info on this machine.

[dkevin]

https://reverb.com/item/35681428-yam...SABEgKsmvD_BwE