Help with a Yamaha P-250

[JZRepair]

Hey all,

I've got a P-250 on my bench with a bad transformer.  I can't seem to find any information on this and I have verified the one winding on the secondary is bad.  One leg is producing ~8Vac, the other 0.8Vac.  I've scoured the internet for a replacement, with no luck.  Service manual doesn't give voltages, but I can probably have one made up by a local transformer company.  I just need to know what all the voltages on the windings are.

So, does anyone have one of these or has had one of these that can verify what the voltages on the windings should be?  Y-Y is my issue, but if you can verify the R-B-R winding, that would be great.  I get 20V-0V-20V on that winding and that winding seems to be functioning properly.  Any help would be appreciated.

EDIT- I tried uploading the service manual, but it's too large.  It can be found searching for it.

(edit- accidentally hit Modify -- PRR)

[PRR]

> ....service manual, but it's too large.  It can be found searching for it.

Post your link.

I find sites with broken security, or dead, or promoting the Owners Manual (and two others), and a LOT of other Yamaha "250"s from motorcycles to outboard motorboat engines.

[JZRepair]

Honestly, I don't remember where I got it from.  It was probably a sketchy site.  Here's a screenshot though.

[sluckey]

You must read the voltage ***BETWEEN*** the two yellow leads, not referenced to chassis.

[JZRepair]

You know, that threw me off with the primary, but I have ground connections on the secondary, so those shouldn't be isolated like they are on the primary.  Unless I'm missing something, I should be able to read the secondary winding's AC voltages in reference to ground, since they tie to GND through the filter caps.

[sluckey]

Unless I'm missing something, I should be able to read the secondary winding's AC voltages in reference to ground, since they tie to GND through the filter caps.

Not so with the yellow wires. The ground for the DC supply is the bridge negative terminal. If you want a correct voltage reading, you must connect the meter leads ***BETWEEN*** the two yellow leads. NOT CHASSIS GROUND!

[JZRepair]

Not so with the yellow wires. The ground for the DC supply is the bridge negative terminal. If you want a correct voltage reading, you must connect the meter leads ***BETWEEN*** the two yellow leads. NOT CHASSIS GROUND!

Wait, even if that's connected to chassis ground?  That should tie it to ground as all the grounds are connected through the chassis.  The only thing that's isolated from the chassis is the mains coming into the unit.  If that's not the case, then it still doesn't explain why the DC voltage would be so off.

[sluckey]

Not so with the yellow wires. The ground for the DC supply is the bridge negative terminal. If you want a correct voltage reading, you must connect the meter leads ***BETWEEN*** the two yellow leads. NOT CHASSIS GROUND!

Wait, even if that's connected to chassis ground?  That should tie it to ground as all the grounds are connected through the chassis.  The only thing that's isolated from the chassis is the mains coming into the unit.  If that's not the case, then it still doesn't explain why the DC voltage would be so off.

That's right. If you connect a scope to either yellow wire you will see that the voltage is no longer AC. It's pulsating DC voltage. Your meter doesn't know what to think about that!

We need more info. At this point we don't know what the AC voltage to the bridge is. And we are missing a big part of the power supply so we don't know what the DC voltage should be. Post another snapshot but this time include the rest of the power supply and we may be able to help a little better.

[PRR]

> we are missing a big part of the power supply so we don't know what the DC voltage should be.

Yeah. If we know the DC voltages the AC can be figured. But no clue in that image.

[JZRepair]

Yes, I know that's not the complete picture.  But that bridge's negative is tied to GND.  All the GNDs are tied to the chassis, PSU is connected to a foil tape on the wood, and when the unit is closed, the screw mounts complete the GND connection through the whole unit (minus mains).  I just uploaded the service manual to elektrotanya, but don't know if it'll be available anytime soon.  I did find it elsewhere, so I'll share that link.

https://www.owner-manuals.com/P250-service-manual-YAMAHA.html

Hopefully that will help clear up some things.  From my understanding, the primary is completely isolated from the circuit, due to not having a tie to the chassis in the form of a grounding wire, resistor to chassis, or death cap.  So the secondary, since all grounds are tied together makes it its' own complete system and I should be able to reference the AC voltages to ground and get accurate readings.  Thus, common probe to ground, and the other probe to AC should give me each leg of the AC voltages accurately.  Again, correct me if I'm wrong.  I deal with systems that are almost always tied to GND, so this one is throwing me for a loop from the beginning.  Everything I can see points to the one secondary winding of the transformer being at fault, because I should have equal AC voltages on those legs.

[sluckey]

https://www.owner-manuals.com/P250-service-manual-YAMAHA.html

Bad link for me.

Thus, common probe to ground, and the other probe to AC should give me each leg of the AC voltages accurately.

That works fine if the winding has a center tap connected to chassis. This winding does not.

Again, correct me if I'm wrong.  I deal with systems that are almost always tied to GND, so this one is throwing me for a loop from the beginning.  Everything I can see points to the one secondary winding of the transformer being at fault, because I should have equal AC voltages on those legs.

For the third time, you are wrong. Three strikes and you are out. Are you just too stubborn to simply put one meter probe on one yellow wire and the other meter probe on the other yellow wire??? 

[JZRepair]

Again, correct me if I'm wrong.  I deal with systems that are almost always tied to GND, so this one is throwing me for a loop from the beginning.  Everything I can see points to the one secondary winding of the transformer being at fault, because I should have equal AC voltages on those legs.

For the third time, you are wrong. Three strikes and you are out. Are you just too stubborn to simply put one meter probe on one yellow wire and the other meter probe on the other yellow wire??? 
[/quote]

Dude, I'm trying to understand and you're not explaining very well.  So, why only when it has a center tap?  And, wouldn't that throw off the rest of the DC readings then, because they share the same ground?  On top of that, if I'm reading DC voltages, does it skew the DC readings once it's been rectified and smoothed?  At in inlet of the 3v3 and the 5v regulators, I'm not getting enough DC voltage to even come close to turning these on.  So are you saying even reading DC, I need to have one probe on the wire opposite of the one I'm trying to read filtered clean DC from?  Again, I've never seen this before, so I'm trying to learn and your patience is appreciated.

[sluckey]

Sigh... Just buy a new transformer.

[PRR]

https://www.owner-manuals.com/P250-service-manual-YAMAHA.html

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[DummyLoad]

Found the entire manual here.

No part of the 5V/9V PT secondary (yellow leads) touches ground - Period! It's a FWB with a floating full wave tap for the 9V supply - AC rating for that secondary would probably be in the neighborhood of 7VRMS at rated load. 


--Pete

[PRR]

> Found the entire manual here.

Thanks.

The digital supply is half drawn "upside down"; at least most drawers put plus-up.

+5V and +3.3V DC, no surprise. At least 8VDC before the regulator so 7V to 10V AC with rectifier losses. (Guess low and you will have mystery faults from ripple glitch.) A discrete transistor on the +3.3V so this is probably significant current.

+ and - about 24V DC for the audio power amp. Figure 17-0-17 (34VCT) AC at about 3 Amps for constant MAX volume, though the PT and the speakers are small so 1.5A AC may play any reasonable performance.

The PT secondaries are fused at 5A which is much more than it could ever need; these are just to limit the size of the fire if something shorts.

You are NOT likely to find a part with these voltages. Two PTs may be easy. A re-wind may be possible. (In fact careful disection and gauging and counting would tell all, without electrical brain-pain.)

The "+B" is an under-filtered feed probably for graceful shut-down.

[PRR]

One leg is producing ~8Vac, the other 0.8Vac.

OK. One leg of the 8V DC feed does go to system common (ground), but the relationship to transformer winding is not as simple as we might like. One leg never goes below -1V relative to GND line, and then not all the time. This may well look-like "0.8VAC". Truth comes from measuring Yell to Yell. But if that makes you uncomfortable, don't bother.