Another Yamaha EM 150 question...

[Drummer53]

welp, here's the results, let me know if I did it right or wrong, but if I did it right, there's definitely some serious DC coming out of the jacks!

[Drummer53]

oh yeah, the "-30.xx" reading is with the power off, if that matters. And I'm a Harbor Freight junkie too, Insiders Club rocks!

[Jazz P Bass]

Get some voltage readings directly off of the transitor cases.
All of them.Then I will relate what they should be.
(I am on the input repair at the moment)
I replaced all four (R & L Channels) transistors & I have zero volts on the outputs.
The left channel had a shorted transistor & one that read .120mv. That channel read 80 volts on the output.
The right channel (which kinda worked) had 40 volts on it.
Upon removal of Ch 1 's transistors, one 'diode checked' good (a proper.500 volt forward drop) while the other one read .115mv.
That ain't good.
That is why I replaced them all.

[Drummer53]

welp, I did a DC check on the 4 transistors (the ones I've ordered to replace with the MJ15003's) and it gave me various very "low" number readings. I then did an AC check and they were all pretty much at 0 (zero). Don't know if it effects the readings or not but I also had the master volume all the way up too. Oh yeah, I still need to check the TR 12, 13 & 11 too, will post later when I do.

[Jazz P Bass]

I am not too sure what you are checking.
DC check.
AC check.
Do you have a meter that has the 'diode check' function?

[Enzo]

Let's all get on the same page.

DC voltage check. COmpare to your good channel as reference. On the case of TR14 should see about +88vDC, and on the case of TR15 about +44vDC. (I just spotted the voltage points on the drawing) VOlume settings should have no bearing on your DC measurements.

AC voltage checks would detect hum/noise or signal. If you are not amplifying signal, we expect low or zero AC volts in the circuit. On the main +88v rail we might see a small amount of ripple.

DIode test. This is what we use to test individual transistors. A crude test but very useful.

Don't try to fix it all at once, cure the cancer, THEn go back and fix the hangnail. You need to get the amp stable and NOT making DC before worrying about noise or distortion.

[Drummer53]

Welp, I think a little whoo hoo! is in order! I just got my MJ15003's today and put them in and both speakers work and no hum when I power it on!!! But....(you know there's always a but!)...now I've got cracklin' comin' out of the speakers which wasn't there before and my VU meter's don't seem to be registering anything whereas they were before. I experimented with my smartphone goin in the Aux (and both sides work there to now too!) and if I keep it at a really low volume, no cracklin', but when I increase it, it starts cracklin', on both speakers. Then I plugged a mic in with the smartphone still going and every time I test checked with vocal, the music from the Aux would "lose power" in a sense and kinda fade out then fade back in after the vocal mic check. The speakers aren't blown or anything so at this point I'm thinkin' dirty pots? bad cables? or am I losing power or signal somewhere else? Or is there more checking on diodes, transistors, capacitors, etc, that I need to be doing? I'm just excited that both sides are working now!!!!

[Enzo]

OK< so it works, but over a certain level a cracking sound accompanies the music signal. it doesn;t make noise just sitting there, it is more like distorted peaks?

Plug a signal into an input channel, pan to center, speakers on both amps, does the crackle come out both speakers together? Or is it independent on each channel, crackling at different times? DO all the channel inputs result in the same crackling? Does killing the reverb make a difference? Does turning the graphic EQ off make a difference? You have left and right monitor out jacks, run a cord from them over to some other amp for a listen, crackles? We need to isolate just where the problems are and are not.

You just replaced output transistors, did you set the bias on that channel? Did you check the bias on the other channel?

Your meters are wired right across the output (with a few small parts) So if they stopped working, I might suspect a missing or loose comnnection to them.

Crackling could be clipping, so did you check the power supplies? You need to have roughly 90v on the main supply, and the output bus of each channel ought to sit at half that, roughly 45v. DO they? And your low voltage circuits need to have good clean +22vDC, got that?


Your power amps plug in with 6-pin connectors, look closely at the solder on the male pins and also check the solder on the female receptacles.

AT this age it is perfectly reasonable for the electrolytics to be dried out. And in this single supply design, that includes the speaker caps at the output.

[Jazz P Bass]

I would like to add that by going in reverb return jack you will bypass the whole preamp.
The reverb return pot will then work with the master volume.

[Drummer53]

you said "got that?" .......HA! I got raw milk! and love it!......I'm seriously trying to keep up with you but it is mind boggling at times and I have to remind myself that the intracacies of circuitry is a bit more than building desktop computers, I don't do that for a living, but have built my own desktop computers for the past 20 years. I do love learning about all this and can't thank you and Jazz enough. It's kinda like your a car mechanic and I understand the "shade tree mechanic" basics, and can even fool the best of them with some "old school mechanic lingo" but when you start talkin about the internals of a motor and all the parts needed to make another part work, I tend to get, well, humbled and put in my place!

I did the cracklin' checks and the cracklin' does come out of both speakers, panning and not panning, it does however seem to crackle sporadically in that sometimes it happens on both speakers at the same time and also at different times, nothing consistent though one way or the other.

All 6 channels are doing the same thing.

Killing the reverb doesn't seem to have an effect on the cracklin', turning the EQ off does seem to affect it, but the cracklin' is still there whether the EQ is on or off, it's just not as predominant when it's turned off.

The VU meters appear to be working, kinda strange though, that when I plug one speaker cable in (via the Aux input), that side of the VU meter works, but when I plug both speakers in, neither one of them are registering anything.

Did another DC check on the speaker jacks (while music was playing) and got hardly any DC voltage at all so I'm hoping that's a good thing.

I am curious as to how to set the bias, been looking on the internet for "how to's" cuz' I didn't want to bug you about it but couldn't find nothing in simplified plain english, so I guess I have to bug you on how to do that too.

The 6-pin connectors, male and female, all look good, didn't see anything that stuck out as being bad.

Still have to do the monitor to alternate amp, power supplies, and low voltage circuits checks

And Jazz, I don't know if you meant "Echo" instead of "Reverb" return, but I did run the music thru the Echo return and the main volume worked like you said, but the cracklin' was still there.

But what I'm intrigued about is the bias settings, how does one do that on a mixer like this?

[Jazz P Bass]

The bias setting is clearly stated in the manual. (attached page)
There are two test pins sticking out of each output section module.
Attach your meter leads there. The adjust potentiometer is right next to it.
The VU meters are set using a signal generator & measuring the output voltage to a dummy load.
The meters are a visual indicator of the output, so it does not make any sense that one would kill the other.
Unless you are losing the signal going to that amp side.
The one I just repaired had extremely crackly control pots.
I cleaned them all with Deoxit. (Radio Shack has it.)
Did you use the 'reverb Return' jack as an input?
That will eliminate the preamp controls.
Which simply leaves the master volume control & the reverb return control.
My feelings are that you are not set up for this kind of repair.
So you have to take it one step at a time.

[Drummer53]

Thanks Jazz, and I kinda figured it out after looking thru the manual Enzo provided, that and the help of my (step) son coming in and pointing out the places I need to be checking, between the both of us we are kinda stumbling thru it. But I did discover something, when I accidentally tapped the left channel circuit board itself, the speakers gave off a "pop" (see video). So that tells me there may be a short somewhere. I took the board out and checked everything but couldn't find anything that stuck out as causing the problem. The right channel is still cracklin' too, but it doesn't "pop" when I touch the circuit board.

And you're probably right about it being over my head in repairs, but $20 for the mixer, and a few bucks in transistors, and for what I'm planning on using it for, I figure it's at least worth trying to fix myself, plus, when I bought it, it did work great, with the exception of the one side, and now I'm just trying to get it back to where it was now that both sides are working. Plus, after the past few weeks of stress I've had, I'm finding this very therapeutic! reminding me of my roadie/sound man days of the 70's as a teen! I need to be doing this to remain sane!

I feel like I'm so close to having it repaired but it's just those "little things" that are important that I don't know that are holding me back! And trust me, I do appreciate yours and Enzo's help, teaching, and patience!!!

poppin and cracklin' - YouTube

[Drummer53]

ok, now I'm puzzled, got some Deoxit (dang stuff ain't cheap) and been checkin' and cleanin' things, and now, when ever I tap any of the control knobs, the "tap sound" is comes thru the speakers!? Don' think I've ever seen that one before. The one circuit board is still given off a "pop" to the speakers too when ever I tap the edge of it. In my limited knowledge, I keep thinkin it's a ground issue, but everything appears to be grounded unless I'm missing something. My son did "tap" around on the circuit board and we found one small resistor poppin' every time he tapped, so I removed the whole circuit board, check the soldering, etc., check the ohms, etc. and everything appears to be working on it, so I hooked everything back up and still got the pops and cracklin'.

Got an off the wall question to, how bad does weather and/or humidity affect these mixers? Would near freezin temps have any efffect on what's goin on with this board?

Hey Jazz, you ever hear of Butler, PA? My wife is from that area.

[Jazz P Bass]

If I have a board out, in my hand, I resolder it.
You had both module boards out.
Did you resolder them?
That Deoxit is great stuff.
The pins where the modules sit on. Wipe a little on them.
I must stress an 'Enzoism.'
Narrow down the problem.
If you have many issues, pick one to eliminate.
Then move on to the next one.

[52 Bill]

Jazz is right, sometimes you can't see a bad solder joint. If you have an area that is responding to physical stimulation, go ahead and resolder that area and you may find that even though it looked fine there was something loose there.