Powered monitor problems - Yamaha MSR400

[bkahuna]

Has the output of your +15v reg dropped again, or is it solid even when the unit is failing?

[kkatarn327]

More likely that there is another more common and much simpler explanation.
If the signal gets week and distorted until a certain high level is reached in gain, and it suddenly works correctly is best explained by oxidation, probably of inserts if it has them (not familiar with the particular model) or a connector needing to be de-oxidized. That is the expected behavior of an oxidizes connection or jack.

Would signal clipping as a result of oxidization be reflected by the unit's peak indicator? Because that's what's happening... most of the time it doesn't really sound like it's clipping (probably because it's so quiet) but the peak indicator is going nuts. Either way, as per your input I deep cleaned the unit with electrical deoxidizer. Thanks for the tip!

Has the output of your +15v reg dropped again, or is it solid even when the unit is failing?

Apparently not. There is +14.9VDC at pin 3 of the +15V regulator and -14.8VDC at pin 3 of the -15V regulator. However, this time around there seems to be no voltage present at pin 1 of the -15V regulator, whereas pin one of the +15V regulator shows 24.1VDC. I checked the caps again (C244 and C245) and they both had 24VDC +/- 0.2VDC across them. I also checked the transformer secondaries again and they both check out fine, as they did before.

I did notice however that after leaving it unplugged overnight it worked great the next day. Left it unplugged for another night and it began to work fine that next day as well, but started to go out (not as bad as the initial problem though).

[Enzo]

Pin 1 is not the input of a -15v regulator, the input will be on the center pin. The fact that you have both +15 and -15 means both regulators have full voltage inputs.

You have both 15v supplies at test, but when the amp starts to fade into distortion is the voltage dropping off still? Oh, kahuna just asked the same thing, sorry.

This is where freeze spray is handy. Play some music through the system, and when it starts to crap out, spray some freezer on that-15v regulator and see if the sound comes back. Of course there could be something loading the regulator down, but if the system has to warm up to the failure mode, then freezing the board in general may reveal a sensitive part.

[kkatarn327]

Ok so there is some interesting stuff going on. First of all, the unit doesn't have to warm up to failure anymore. Now it clips as soon as you start the input and goes into "thump thump" mode soon thereafter (unlike before, when it would work great at first then start to clip). Basically I'm back to square one. The freeze spray didn't help either but thanks for the tip Enzo.

Secondly, I went to test the 15V supplies when it distorts as per your suggestions. Now the way this unit was made, it's nearly impossible to gain access to the PCBs while the speaker lead connector is plugged in to the board. So I figured I would just plug in my iPod, turn the level up high, watch for the peak indicator and test the regulators when it's going off. As I turned the level up with the iPod plugged in (and the speakers disconnected), the peak indicator didn't go off until the last notch on the knob, as it would when it works normally. I then plugged the speakers back in and tried again but it went right back into failure. It would seem that without the speakers plugged in, the unit works fine, but is this expected behavior? This whole time I have been testing the unit with the speakers disconnected because like I said, it's impossible to gain workable access to the PCBs when the leads are connected. Has this been skewing my results?

I tested the 15V regulators again as well. Both the outputs test out well but there is still 30VAC on pin 3 of IC201 (the +15V reg) and 52VAC on pin 1.

EDIT: I just checked the DC resistance of the speakers, I thought maybe one of them shorted and that's causing a load problem with the amp circuit. The tweeter shows 13Ω and the woofer shows 3.5Ω. The service manual says the tweeter is rated at 16Ω and the woofer is rated at 4Ω so this seems to be within acceptable range.

[Enzo]

Do you have any clip leads? I put amp modules like that on my bench every day, I use clips to connect to the little output connectors. Or alternatively, pull the speaker wires from the cabinet, connet them to the amp, then connect extensions from there back to the cab.

I don't know that the amp works fine without a load, all we know is without the load current, the thing can;t motorboat.

Is the mysterious AC voltage still on the +15 IC when the load is removed? I can't imagine where that is coming from.

[kkatarn327]

I do have clip leads but all four leads are on the same connector so that might be a little tricky. I'll try to rig something up though.

And yes, the AC voltage is still there without the load, even after having replaced C244. Out of curiosity, I checked for AC on some other major components and got some odd results.

***all the following voltages were taken with no output load connected***
***all AC voltages POSITIVE***

First of all, the positive end of C252 (one of two big caps, 6800uF @80V) showed 155VAC. The positive end of C253, the other big cap, showed no AC voltage.

Second, the positive end of C244 (1000uF @35V), the cap that was in question earlier and which I replaced, shows 50VAC. The positive end of C245, the same cap for the negative side, shows no AC voltage.

Moving on to the big transistors, on the edge of the main PCB and mounted to the heatsink:

D236, pin 2: 11VAC

Q241N, pin 1: 12VAC
Q241N, pin 2: 155VAC
Q241N, pin 3: 12VAC

Q238N, pin 1: 11VAC
Q238N, pin 2: 155VAC
Q238N, pin 3: 11VAC

Q229N, pin 2: 11VAC

Q215, pin 2: 3VAC

Q227N, pin 1: 1VAC
Q227N, pin 2: 11VAC

On the positive side, only one transistor showed AC voltage:
Q217, pin 2: 3VAC

Here are the AC readings on the diode bridge, not sure if they are normal or not:
D238, pin 1: 155VAC
D238, pin 2: 50VAC
D238, pin 3: 50VAC
(pins 2 and 3 are connected directly to the power transformer secondaries so I guess that's normal, but they're both showing up as +50, is that a problem?)

So clearly there is a lot of AC voltage on the negative side that shouldn't be there, which is odd because it was the +15V reg that showed AC voltage. Does this mean all the transistors on the negative side are fried? Is it even worth replacing them all at this point (especially since I still don't know what's causing the AC voltage)?

Here is the diagram of the component side of the main PCB included in the service manual for reference. I labeled all the relevant components thus far to be spotted easier.



EDIT: Doing some backtracking on the board, all the ~155VAC spots can be traced directly to pin 1 of the diode bridge. I'm assuming the diode bridge is at fault. But even if I were to replace it, wouldn't all the components that were affected by the AC voltage need to be replaced as well? And even if I replaced all the affected components but missed only one, that could fail as well and I'll be back to square one yet again. I'm beginning to think it's not going to be worth fully repairing this board. Anyways, I'll keep working to try and trace back the ~12VAC spots and the ~3VAC spots (these are a little more difficult because there are a lot of other components in the path).

EDIT (again): The ~50VAC on the positive end of C244 is coming from the cathode of D227. There's 17VAC on the anode which makes sense because that's coming straight from the low voltage transformer secondary (which also shows 17VAC) but somehow there's 50VAC on the other end. Is this normal operation?

[kkatarn327]

Disregard my comments in the previous post about "positive" AC voltage, wasn't thinking straight, sorry.

[Enzo]

I am having a very hard time with any of this AC voltage. There is nothing in this amp with 155 volts on it AC or DC, so I don't know how you get some in there. I have to wonder where the black probe of your meter is sitting.


Do you have a bench speaker or a guitar or bass cab handy? Might be easier to connect one of them instead of the speaker in the 400. And all four? Would it be sufficient to just connect the woofer and leave the tweeter out of it for now?

And for that matter, we don;t need a speaker load at all until we get these voltages sorted out.

[kkatarn327]

For the AC voltages I took the readings with the black probe on the ground of the IEC mains socket as well as the center tap of the respective PT secondary and both locations result in identical readings (for the capacitors I just test across the terminals and get the same readings as well). I don't know if I mentioned this in one of my edits but all the AC voltages trace back to the two rectifiers on the board. One of them is a 4-pin bridge rectifier IC (model D10xB20, circuit designation D238). This one shows 0VAC on pin 4, 50VAC on pins 2 and 3 (all normal) but somehow 150VAC on pin 1. The other rectifier is four 1N4004L diodes (circuit designations D227, D228, D229, and D230). There is 52VAC on the cathodes of D227 and D228. As far as I can tell this means both rectifiers have failed (possibly due to power surge?) and plan on replacing the bridge rectifier IC as well as D227 and D228 but that might take a while because these parts are not locally available for me. What do you think?

Thanks again for all your continued help Enzo, really appreciate it.

[Enzo]

With power removed and main unplugged, look at resistances between that IEC mains ground, the center tap of the power transformer secondary - I closed the schematic, is that the white wire? - and ground within the circuit. Seems to me the +15v regulator is a 7815 or similar, yes? And th center pin would be ground, meaning the tab on top is also ground. Is there continuity between that point and the earth ground and the center tap?

Do you see my dilema? The amp is running on 120VAC, or are you in 240VAC land, and that is the only place in the unit where AC voltages remotely as high as 155 are. If you have shorted rectifiers, you will be blowing fuses. If you have open rectifiers, you will have no voltages. But whatever the rectifiers are doing, how can they make 155VAC out of what they have?
]
You mentioned one end of the main bridge had zero volts? 50 on the AC legs and 155 on the other end? That is very suspicious.

[kkatarn327]

Yes, there is continuity between the IEC ground, both center taps, the ground legs on both the +15V and the -15V reg (and yes, it's a 7815 and a 7915), and common ground points on the board with minimal resistance.

The same thing was confusing me too, especially on the 1N4004L bridge, where one end got 50VAC from 17VAC. And yes, the D10xB20 4-pin bridge had 155VAC on pin 1 (positive DC supply), 50VAC on pins 2 and 3 (AC in, which is normal since they're wired straight from the taps which are rated for 50VAC) and no AC voltage on pin 4 (negative DC supply). I thought as well that if it was diode failure then the unit's malfunction would be far more catastrophic but since all the AC voltages on the board can be traced back to the rectifier I don't know what else to think. For all I know these rampant AC voltages could be totally unrelated to the main issue with the unit.

[Enzo]

What is your meter? ANy chance it is being confused? Do you have a scope?

My problem is getting past the existence of 155VAC at all. I can't imagine how it could be generated in any way. If there really was 155v there, things would be blowing up and fuses screaming into the night.

DO this for me - humor an old man - reverse your meter leads, ground the red one and redo these measurements. Does the same end of the bridge show zero and the other end still 155VAC, or do the ends now swap places?


And I just looked back, you have 50VDC on the AC leads of the big bridge? As well as 50VAC? I am not sure I can even imagine that, not on a scope anyway.

[kkatarn327]

Unfortunately I don't have a scope. I used to have access to scopes and signal generators at the university but those labs are now closed.

I have a digital RadioShack meter, not the top of the line but it works. I tested all the functions on known values (resistance, DCV and ACV on all ranges) and it seems to work fine.

No, I never checked the DC on the bridge nor on most of the points I checked for AC voltage. Now checking the DCV on the bridge rectifier, I get +71.5VDC on pin 1 (the positive DC supply), nothing on pins 2 and 3, and -71.5VDC on pin 4 (the negative DC supply). This seems to be in order.

After swapping the leads (connecting the red probe to the IEC ground) I get 0VAC on pin 1, 50VAC on pins 2 and 3, and 150VAC on pin 4. It would seem that with the probes reversed, so do the ACV readings on pins 1 and 4. Is this normal?

[Enzo]

No, but it does tell us the 155VAC is in the mind of your meter, and not in your amp. If AC voltage was really present there, your meter would measure it no matter which way the leads were attached, AC has no polarity. You have your +70 and -70.

ANy chance your meter has a weak battery?

[kkatarn327]

Just put in a fresh 9V battery (even tested it beforehand to be sure, reads 9.5V) and the same AC readings are all still in the same place.