Well, crap! Yamaha P-751 stuck in 45

Well folks, I can't say I did anything today regarding the TT diagnose/repair. As some of you already know from the solar thread in the Pub, the electricity went off today shortly after 10, and since my solar backup is a minimal system at the moment, I wasn't willing to sacrifice soldering-gun wattage from the battery no knowing how long I'd be without. It did finally come back on around 5 or so.

My Mitsu uses a sensor to read the strobe markings on the platter ...

mitsu-strobe-001.jpg


Maybe something similar? Try dragging a finger on the platter to slow it down and see if maybe you can trigger a response. That also has LEDs that can fail and fubar the controls.

I have that table but it's boxed up for awhile.

Check that the switch is working by use of a dmm. Stuck on 45 could mean a permanent ground or logic 1 high voltage signal is present.

As I've received a couple good suggestions quoted right there as far as where to start looking, so this will make a nice little project to work on during the week after work. And now that I've got some suggestions as to where yo start looking... Thanks guys. :thumbsup: I'll have a look at that and let you know. I don't recall ever seeing any strobe-tracks on the inside of the platter so they might be under that big square cover where the drive motor is. I'll have to see.

Maybe the problem is the music that you're playing.

View attachment 749627

The table rejected Barry

I can say with a significant degree of certainty that it probably wasn't Barry that's responsible for this since the first record I tried to play yesterday when this happened was Ella and Basie!, and there's no reason ever for any TT to reject a voice a sweet as hers or a band as awesome as his. Sooo....

Here's to everyone having a great night! :beerchug:
 
Currently finally working on the P-751.

Got that board/motor out and flipped over to see if there are any cracked solder joints. Nothing stands out as obvious so far, so I'm just following the 33/45 circuit and reflowing solder at the moment. We'll see.

E666993D-404E-4469-80C4-77E0DFD271B6_zpsdgwmc0rs.jpg
 
Last edited:
i would first try removing power and testing the speed selector switch to be sure its working .if so power up in 33rpm mode and see what happens . am thinking it might be latching in 45 mode but might release when power off and go into 33rpm when powered back on .. just a theory ..bearing in mind i haven't seen the schematic yet .
 
i would first try removing power and testing the speed selector switch to be sure its working .if so power up in 33rpm mode and see what happens . am thinking it might be latching in 45 mode but might release when power off and go into 33rpm when powered back on .. just a theory ..bearing in mind i haven't seen the schematic yet .

5C96F597-EB4C-4724-A8B8-C62A7CE49D49_zps9pz3jfhh.png

E2925F75-0534-414F-932E-67B4BD70EF30_zpsuyfmpcoo.png

Thank you for your suggestion. Looks like the switch closes the circuit for 33 and leaves it open for 45.

I've put a jumper over connecting the pins on the board, effectively bypassing the switch altogether, but it makes no difference. It remains in 45.
 
Last edited:
that is strange as i thought the switch might work like that .
next job is check every voltage on the chip the switch connects to when in 45 and 33 mode. i suspect the chip has latched into 45 mode . and may need replacing. or simply a bad solder connection .
 
Well, I've been all around that board reflowing all the solder joints.

No luck.

I hate to give up on this TT because it's been great the last few years I've owned it. I guess I'll send a message to the folks at organ donor to see if they've got that whole motor assembly or maybe can track it down.

Thanks for your input, guys. As always, I appreciate it. :thmbsup:
 
Well, I found a replacement motor over on the auction site. Coincidently, connected to a fully operable, and in really nice condition Yamaha P-751 - exactly what I already have minus the fully operable part. LOL Since it was about the same price as what OrganDonor is selling a different model but similar motor for, I decided to just go ahead and get this replacement TT and be done with it.

I realise some of you would have considered just getting an entirely different TT altogether, but I already have a few headshells, the alignment tool, and already know that light tonearm works very well with my Sonus Gold (which the YP-701 doesn't because it's too heavy). Besides, like I mentioned earlier in this thread, I really like this range of TTs anyway.

So here it is - just arrived this afternoon.

0788BD9F-38C3-4AC6-87EF-3311FD3D841D_zps24vtwpic.jpg

Still needs to be gone over and shined up a bit, but it's working nicely - so no complaints. It even came with a Grado GF3, however good that is, which I luckily have a replacement stylus for. The Grado is not in the pic, but I've already checked its alignment and it's currently playing as I type - sounds good, too!

So now I've got 3 headshells and cartridges for this TT: the Sonus Gold, the AT3450 with an E stylus, and now this Grado. Also a parts TT in the event I need it, or maybe I'll end up figuring out what's wrong with that other one and have it running again.

Did I mention it came with functional hinges and a better condition dustcover? Awesome!

Cheers folks! I'll be busy for a little while getting this one wiped off and shined up.

Total score in my opinion.
 
Last edited:
good move on the new acquisition.

I cannot read the schematic very well but whatever IC is connected to the 33/45 RPM selector is likely to be suspect.

If the switch is good, and if C14, R24, and C17 all measure good, and if there is continuity to the right places, that will lead you to that IC. Most likely a custom part, you would need the whole schematic and follow the logic through the chip and to the motor at some point.

One of the problems with the Japanese Direct Drives is finding parts.

Now that you have two 751's you have a donor.

I had that table really singing with a Denon DL 110 on it once upon a time.
 
Hi there, our Rey de la Playa (beach king, sounds like me :beerchug:) had directed me to this thread. I have the same darn problem! And I am sitting in Brazil looking over the beach without a chance to get a quick fix from eBay or a part from anywhere.

But I am curious about these photos of the schematics. Someone has a maintenance and repair manual with schematics? Or is this from the user manual? You really think that IC is a custom chip? Have you been able to identify it?
 
Hi there, our Rey de la Playa (beach king, sounds like me :beerchug:) had directed me to this thread. I have the same darn problem! And I am sitting in Brazil looking over the beach without a chance to get a quick fix from eBay or a part from anywhere.

But I am curious about these photos of the schematics. Someone has a maintenance and repair manual with schematics? Or is this from the user manual? You really think that IC is a custom chip? Have you been able to identify it?


The P-750 and P-751 are the same turntable. Over on Vinylengine, they have the owner's manual, parts list, and service manual (for the 751, but like I said, they're the same).

https://www.vinylengine.com/library/yamaha/p-751.shtml

You'll need to set up a free account like you did here to gain access to the files.

Check the two capacitors and one resistor that bobins08 mentioned in the quote below. I didn't do that on mine because I ended up finding that other P-751, but it could be one of those little parts is bad.

As for the IC chip, for some reason I have an IC chip in my eBay watch list. I found it so long ago I don't remember why I put it there - it could be for this TT, but it could be for an amp I was messing around with.

So where are you from in Brazil? I've got friends in Belo Horizonte who keep telling me to come visit. I really should go see them soon.

I cannot read the schematic very well but whatever IC is connected to the 33/45 RPM selector is likely to be suspect.

If the switch is good, and if C14, R24, and C17 all measure good, and if there is continuity to the right places, that will lead you to that IC. Most likely a custom part, you would need the whole schematic and follow the logic through the chip and to the motor at some point.
 
Ah great! Already had an account there. They even have the service manual! Wonderful!

I am not "from" Brazil. I sort of live there now. On the hill overlooking the ocean. Which beach is your kingdom?
 
I cannot read the schematic very well but whatever IC is connected to the 33/45 RPM selector is likely to be suspect.

If the switch is good, and if C14, R24, and C17 all measure good, and if there is continuity to the right places, that will lead you to that IC.
I can't find R24 at all, but the schematics are hard to read indeed.

I see C14, R14, and C17 involved. Now what could go wrong with these little capacitors? The worst that could happen is that they become conductors. If C14 became a conductor it would be similar to the switch closing, so it should be stuck in 33 or fry completely. So my next easiest guess is that R14 has interrupted continuity. Now how can I figure this out with while not finding my voltmeter ... :(
 
OK, found the meter. And did a few measurements. I see that one of the wires to the switch go on the pin labeled (-). Between the (+) pin and the (-) pin there is 25V. Between the other wire going to the switch and the (-) pin (to which the switch is connected) I measure only 30 mV. 30 mV seems like a ridiculously low level for something used in a switching / regulator circuit. That is suspect. I would have expected something like 5 V or so for TTL levels. But 30 mV is ridiculous. So we are looking at the R14/C14 complex. Either R14 being disconnected or C14 having become a conductor could explain what we are seeing. As C14 goes to (-) like all the other little 22 nF capacitors.

BTW, the IC is a VC4046 for which I could find a source even in Brazil. It is apparently used in a number of other turntables from other manufacturers also.
 
So, my next step should be to measure on IC2 the pin #6. If it has more than 30 mV then I have the culprit. If it also has such a measly level, then the chip must be bad or the C13 shorted, which also might have killed IC2 anyway. Unfortunately I had killed the Ohm-meter part of my meter, so I won't be able to measure out the R14's 390 kOhm.
 
I can't find R24 at all, but the schematics are hard to read indeed.

I see C14, R14, and C17 involved. Now what could go wrong with these little capacitors? The worst that could happen is that they become conductors. If C14 became a conductor it would be similar to the switch closing, so it should be stuck in 33 or fry completely. So my next easiest guess is that R14 has interrupted continuity. Now how can I figure this out with while not finding my voltmeter ... :(

R24 is at the bottom left corner by TP1 - start at C17 and move right - it's the 6th/last component in that group.

5C96F597-EB4C-4724-A8B8-C62A7CE49D49_zps9pz3jfhh.png

If you have figured this problem out, you definitely have won the internet for the day! Good luck!

I wish I understood electronics like you guys do!
 
Last edited:
I can't find R24 at all, but the schematics are hard to read indeed.

I see C14, R14, and C17 involved. Now what could go wrong with these little capacitors? The worst that could happen is that they become conductors. If C14 became a conductor it would be similar to the switch closing, so it should be stuck in 33 or fry completely. So my next easiest guess is that R14 has interrupted continuity. Now how can I figure this out with while not finding my voltmeter ... :(

Test the switch with an ohmmeter to verify opens and closes. The check continuity of the circuit from the switch thermals to the IC pins to verify no open traces.

To test the cap you lift one lead off of the board and then verify it not shorted using an ohmmeter. It should read infinite. If you have a capacitance meter on your DMM you can verify the value of the cap. Most likely the caps are good.

For the resistor lift one lead and verity its value as opposed to an open (infinity), a short (zero) or an out of spec value.

With a scope you can verify the voltage on various IC pins to see if the switch is having any effect. I would also check the oscillator crystal ... if either of these caps near it are shorted it could be a problem although with this problem most likely 33 and 45 would not work.
 
Oops, I wrote C13 above, I meant C14. (Is there no way to edit my posts in this forum?).

But I might cry "Eureka" now, because the IC2's pin #6 carries some 3.9 V, which is a lot more reasonable. So, if I could just find a resistor of roughly 390 kOhm to connect this pin to (-) I should have broken the spell! Where will I find one? Do I have some old device I can gut? Hmmm....
 
Now this is strange ...

There is a strange labeling mismatch. R14 and R15 seem to be swapped. Check this out. The schematics say that IC2 pin 6 connects to C14 and R14, which goes through to the 33/45 rpm switch. It also says that IC2 pin 7 connects to C13, R13, and R15. R15 connects to IC2 pin 1 and R13 to the connector at the edge of the board. Now check out the photo of the PCB. Here R14 and R15 are swapped! R14 bridges between pin 7 and pin 1, while R15 connects to the 33/45 pin on the edge of the board! It gets even more confusing when swapping the board over. As now R14 is indeed the 390 kOhm, orange (3), white (9), yellow (10^4), while R15 is 100 Ohm, brown (1), black (0), brown (10^1). 100 Ohm makes a lot more sense also in a switch circuit, just enough to avoid a huge current to flow through (-) when the switch is closed (and also, this might be the reason it burned out, because the turntable is constantly on 33 rpm.) So anyway, the schematics has R14 and R15 swapped completely, not just the label, but also the spec. The PCB is right.

I don';t know how I can share Google photos directly here. But I show the whole thing in this publicly shared album:

https://goo.gl/photos/fRLHesqrJ6Qr7MNC9
 
Back
Top Bottom