Powered monitor problems - Yamaha MSR400

[kkatarn327]

We have several Yamaha MSR400 powered monitors we use for our setup and after a gig one night, one of them started acting up. The signal would clip at really low volumes (both for line level and amplified inputs) with the peak indicator lighting up and everything.

After a while it developed a new problem as well. It will start making a steady "thump thump thump" sound at about 75 bmp, unaffected by the volume or EQ controls, or by unplugging the input; the only way to make it stop is to turn the unit off. I noticed however that as long as the volume is all the way down, or no sound is coming through, it's fine. But about 10 seconds or so after I'd start talking into the mic (plugged directly into the unit) it'll start again. Also, if there's nothing plugged in and the volume is turned all the way up, the noise will start. All the while the unit makes this noise, the peak light is constantly lit.

Any ideas would be greatly appreciated. Thanks!

[mozwell]

Service manual is here Audio Yamaha : Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

This site has a huge amount of service manuals available.....

First thing is to remove the amplifier & check the power supplies, the power amp + & - supplies & the low level +/- 15V supplies.
It is ok to run this amplifier without speakers connected.
Is there a DC offset on the output.
There seem to be two small fuses inside, check these are both ok. If one of these has "opened" you will lose the +15V or -15V supply.
This will cause lots of problems with the line level input signal circuitry and may cause the problems you have seen. The "thump" may be a protection circuit operating without the +15V or -15V supply

Also, a caution, if you are not familiar with electronics, then i woudl get a service tech to look at this. There are high DC voltages around the power amps & mains voltage, & i dont want you to get a shock.

[Enzo]

And my first reaction was also to check the +/-15VDC supplies

[kkatarn327]

Thanks for the replies! All the fuses are intact, and the -15V supply checks out but the +15V comes up short, around +10.5V, and I'm not sure if this is within the threshold of normal variance. Haven't had a chance to check the PT secondaries or DC offset at output yet but I will do it first thing when I get back.

[Enzo]

The same winding of the transformer makes both 15v supplies, it won;t be the transformer. I don't know why folks dwell on them.


No, 5v low is not within normal variances for a 15v regulator. Either the regulator is shot, or the filter cap before it is.

Set your meter to AC volts and remeasure. If you get several volts of AC, then teh filter cap is dead., but if your 10vDC has no AC on it to speak of, then the regulator is suspect.

[kkatarn327]

Ok, had a busy weekend but I finally got around to doing some more checking.

There is about 3 mV offset at the tweeter output and 5 mV offset at the woofer output. I'm assuming this is within acceptable range.

Checking the +15V regulator, there is about 50VAC on pin 1 and 30VAC on pin 3. On the -15V regulator, there are absolutely no VAC to be found. I suppose this means it's the filter cap. Does this also however mean that the regulator should be replaced?

[Enzo]

Replace the cap and find out.

[kkatarn327]

So according to the following diagram it would be C244?

[Enzo]

Looks like to me.

[mozwell]

The low DC offset on the speaker terminals woudl seem to suggest the power amps are working ok, so we only need to look at the low level signals.
Measure the AC into the bridge rectifier D227 to D230. This is the orange & white wires on header CN202. Both orange wires shoudl measure the same AC voltage to the white wire.
Measure the DC voltage across C244 & C245, or on the input to each voltage regulator
(i agree if you have big AC voltages on C244 that is where the problem is likely to be).
Measure the DC voltages on the outputs of the two voltage regulators IC201 & IC202. Are either of these getting hot ?
It may be prudent to replace C244 & IC201 if you have problems here with the AC ripple & a low +15V rail

If you really have 30VAC on the output of the +15V regulator, there is a chance all the ic's are blown that connect to the 15V rail. Hopefully this is not the case.

Also, double check the point you are using for 0V when you are measuring voltages. CN202 (white) anode of D224.
Check too that the tab of IC201 is at 0V

One thing to consider, which may not be obvious.... The voltage regulator TO-220 devices on this board seem to have no heatsink on them. It is possible (and i have seen this actually happen) is that when there is vibration, one of the legs of the TO-220 devices can shear where it connects to the plastic body. It "looks" ok, but there is no connection. Replacing both voltage regulators should confirm if this is the case.
If you replace these, some of the "electrical silicon" around the leads ( and all the way up to the body of the device) will act as a support for the body of the TO-220 devices & stop them failing in this way.

With no signal in, measure the outputs of each op amp, are they all close to 0V (AC & DC)

[kkatarn327]

mozwell,

Thanks for your continued help. Both orange wires measure 17VAC against the common (white).
The DCV across C244 and C245 is both a steady 24.3V. However I did notice when discharging these caps, C245 retained a significantly higher voltage than did C244. Is this evidence of anything?

At this point I am planning on replacing both C244 and IC201 and I will order the parts tonight. Hopefully this will fix the problem

[kkatarn327]

Ok, update! The parts showed up today and I replaced C244 and IC201, both with the same type. At first it worked like a dream, I could get the volume up to instant feedback levels with little to no clipping. Then (within 5 minutes) it started acting weird, where it would be really quiet and clip at first but after you pushed it past a certain level all the volume would come back, without the clipping problem (like it was clearing its throat, that's the best way I can describe it lol). After a few minutes of that, it went right back to the original problem, where it would clip at really low levels then do the "thump thump" thing after going past a certain level. Also, after having heard the full volume of the unit again, I realize that when this problem is happening, the overall volume is VERY quiet.

The only thing I can think of now is that there is another faulty component that's causing C244 or IC201 or both to fail. Any ideas?

Also, mozwell, thanks for the tip on the TO-220 packages. I checked the old one after I took it out and all the legs seemed to be intact.

[bkahuna]

There may be something down the line (opamp or decoupling cap) that is shorted and trying to draw too much current from the regulator. After the unit cools down for awhile (powered down) does it work for awhile again?

[kkatarn327]

Nope. I unplugged it from the AC and left it alone for hours, and when I tried again it went right back and did the same thing.

I did notice that the solder leads of one of the big caps, C252, had visible scuff marks (not quite burns) around it. This is the only visual 'damage' on the entire board. Is this a possibility? Any other ideas?

[km6xz]

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.