Another Yamaha EM 150 question...

[Drummer53]

Hello All! Newbie here!

I just got a Yamaha EM 150 (for really really cheap!) and for the most part it all works, and great at that I might add, but the one exception is the R main speaker out side is not working but when I plug a cable into it, it gives out a "power hum" and not a "ground hum" if that makes any sense. I'm no tech guy but I did swap the "circuit boards" that are in the "heatsink" type enclosures and the non-working side worked, so I'm thinking that I've isolated the problem to something on the one circuit board, but my problem is, what do I do next? I've tried finding a Yamaha EM 150 Service Manual but have not found anything other than Owner's Manual's. Any tips, advice, step by step instructions, will be and forever greatly appreciated!!!

Also, would this one circuit board cause the whole side (R or L) to malfunction as in the Aux, Rec, Monitor, Echo, etc.? When I plugged (stereo) into the Aux for testing the board, only one side of it worked also, so I was just wondering if all the "R" side stuff is within the individual circuit board or not.

Thanks!
Steve

[Enzo]

First, do not connect a speaker to the loud hum channel until we KNOW it is not producing DC voltage. Measure for DC across the speaker jack, or plug a cord into the jack and measure at the free end for voltage between tip and sleeve. Not worried about a fraction of a volt, we do not want to see a substantial voltage. It will destroy speakers.

DC on an output means a blown amp channel. Could be shorted transistors, coul dbe open resistors, could be a number of things.

If you swapped the amplifier boards and the problem moved with them, then that should narrow it down to the board itself. On your photo I can see the three legs of the output transistors poking through the board left and right sides. Check them for shorts. Check the two white rectangular resistors for opens. In my experience, the majority of these Yamaha power amp modules are fixed by replacing bad outputs, seldom anything more.

[Drummer53]

Thanks Enzo, will do and hope to reply with positive news!!

[Drummer53]

well, gettin a little closer but not there yet, is there some kind of cross reference for TCG TCG181 8011 or SK3297 RCA H 8047. These are the transistors I'm going to replace. Or does anyone know the "ratings" / schematics for these? Thanks, Steve

[Enzo]

Those are generic replacement series types for whatever used to be there. Let us look for real parts instead.


here is your service manual attached below

Good luck finding 2SC1079, but if it were mine, I;d probably just stick a couple MJ15003 in their place and move on with my life.

[Drummer53]

Dang! I just ordered a pack of 4 of the 2N3055 after cross referencing the TCG 181. So will the 2N3055 be compatible to the MJ15003's? And 10,000 thanks for the service manual!!! I wish I had an "Enzo App" on my (android) phone!

[Enzo]

2N3055 was THE workhorse transistor of the 1960s or 1970s. Originally a 60v part, they later had higher voltage versions. Your power amp is single sides, so whatever that main power supply puts out is what your transistors need to be rated for. 75v? 80v? You tell me.

Look up the ratings on the 2N3055s you bought compared to the MJ15003. The 15003 are 140v 20 amp parts. They will handle whatever this amp will throw at them.


A discussion:
You ran into the problem of generic replacements. Your ECG/NTE/TCG/XYZ181 is a replacement for many parts. It has ratings of whatever you found, but that doesn't mean the original parts had that rating. All it means is that is what NTE suggest you use in place of the originals. So the NTE181 is a 90 volt 30 amp 200 watt transistor. The original 2SC1079 was a 150v 12A 100W part (that actually crosses to an NTE280) Problem is things don't operate in reverse. What we cannot assume is that everything the NTE181 replaces has the same specs. For example, the 90v 30A part might be used in place of an 80v 20A part, so we cannot use THAT part in all places the 181 might go.

[Drummer53]

Hopefully I cancelled the order in time and will be ordering the MJ15003's. By the way, and this may sound stupid, but in my attempt at isolating the problem even further, after replacing the two 5 w 0.47 ohmK white rectangular resistors on the "bad" circuit board with two new ones, I did a little experimenting and tried swapping one of the presumably "good" SK3297 RCA H 8047 transistors. So basically, I had one "good" and one "bad" on both circuit boards. It obvioulsy didn't work, so I swapped both transistors to opposite boards and still nothing. Well, like I said, it was stupid, cuz' now, both sides are making the hum noise, but now it seems like more of a "ground noise hum". Hence, the ordering of 4 new transistors! If I got paid to be stupid, I'd be rich!

[Jazz P Bass]

You may or may not have damaged the circuit.
If you swapped a shorted transistor to the good side, it may have taken out one of the driver transistors.
Use your DVM & look at TR 12, 13 & 11.
Unfortunately, now you do not have a 'good' side to compare to.
Slow down, quick swapping parts.
This output design is tricky & everything has to be correct to get a hum free balanced output.
There are only seven active parts in the circuit.
But they all have a specific function.

[Drummer53]

Thanks for the help Jazz! and I will take your advice and run with it too. But does anybody know of a reputable website that sales transistors? I came across Futurlec but haven't ordered anything yet. Don't know if I trust the eBay sellers on these. Also I stumbled across this older website that discuss's counterfeit transistors, specifically the MJ15003's.

Counterfeit Transistors

[Enzo]

Big old reliable parts houses like Mouser do a good job.


As to the hum, disconnect your speakers, and measure to see if the amps are putting DC voltage on the output.

[Jazz P Bass]

Just wanted to say thanks for the Yamaha EM-150 schematic, Enzo.
Guess what showed up at my door today?
3 inputs are dead & the left channel is out.

[Drummer53]

you guys are awesome! But I do have another question of which I feel is going to make me look even more "tech illiterate", and trust me, I'm trying to remove the cob webs in my brain as I think I used to know how to do this (back in the 70's) but when you said Enzo "Measure for DC across the speaker jack, or plug a cord into the jack and measure at the free end for voltage between tip and sleeve", how does one exactly do that? Another reason I'm a bit skeptical is because I did fry/blow a multimeter not to long ago and the new one I have is, well, fairly new and I really like it! and wouldn't want to risk frying it! A bit "gun shy" I'll admit! So I guess what I'm asking is, what setting do I need to put the multimeter on and does it matter where the red and black probes go? Thanks again for your patience and teaching!

[Enzo]

Oh geez, Jazz, I was going to guess you opened the door to see that girl you met at the Molly Hatchet concert with a child that looks like you....


You want to know if any amp is putting unwanted DC voltage on an output. If a speaker gets DC, it will damage it, but in the meantime, the load that puts on your power supply will cause high ripple, so the speaker makes hum. Your system has some voltage, I have no idea what, you tell me. What voltage does the power amp run on? It is single ended, so 75-80vDC? Find out. It is on the main filter caps and the channel fuses. THAT is the most that can come out the amp. Oh, well, I suppose there could be some hideous short inside and the main 120vAC from the wall could be there, but mot likely. SO set your meter up for 100vDC. Or start with AC if you want, then switch to DC.

All I wondered was whether there was DC on the output. That means the jack where the speaker cord plugs in. You could measure right at the jack from the inside, but it is not easy to do from the outside. SO I suggested just plug a cord into the jack, now the other end of the cord provides a convenient place to take a reading. That free end has a plug with a tip, and the shaft (sleeve) of the plug. Black probe on the sleeve, and red probe on the tip. DC or not.

Want a litle more technical? Once it passes that test - no DC - then we check the amp itself. This is a single sided amp. Assuming it runs on 80v, that means the output of the amp sits at 40v. There is a big cap at the jack to block the DC and just allows the audio throuigh to the speaker. SO if that output point is anything other than 40v (or whatever half the main power rail is) we need to know why. Thyat is pin 6 of the 6-pin connector on the power amp module.


I have used the same Fluke meter now the last 20 years, but I also go to Harbor Freight a lot, and they have little meters that normally sell for $10, but are often the free item on their coupon sheet, and are also on sale a lot for only $2. SO grab one or two of those and have them around. If you are worried about your good meter, use one of the cheap HArbor Freight things first. They work well enough, especially just for gross checks. I keep them scattered about the warehouse, so a meter is always handy. Great for basic voltage readings or checking speakers for continuity or whatever.

[Jazz P Bass]

The EM-150 that showed up has the left channel out.
It has the full 80 volt supply on the output jack.
The right channel sites at 40 volts.
I removed the output transistors & one was bad. (NTE280).
Both channels had a hodgepodge of output transistors from previous repairs. I replaced all four with what I had on hand. (MJ21194)
The driver transistors all check good on 'diode check'.
All resistors check good.
Resoldered both output boards.
Powered it up & all is good.
Now to repair the three bad inputs.