oh boy - everytime i have to pick this thing up im afraid its going to be the last time i pick anything up!
schematic can be downloaded here: https://elektrotanya.com/yamaha_pc4002_pc4002m_sch.pdf/download.html
anyway, here's a brief (i guess not that brief - buckle up) rundown:
1) belonged to a studio - sat around for a long time - inherited by another studio.
2) was going into thermal protection - i traced the issue to the left channel
3) took a look at the board - seemed from sitting around some solder connections looked cracked and not so good - did a general clean/resolder
4) went out of thermal - worked fine - i tested it running at 100 watts for thirty minutes or so - no problems
5) went back to the studio - worked for about 25 minutes then went back into thermal mode
6) got it back on the bench - once again issue in the left channel
7) this time, however, i noticed the fuse powering the left channel was blown
8) transistors measured okay - i plugged it in with a light bulb limiter and it worked fine - no thermal mode, but (because of the light bulb) the voltages were low so it wasnt quite in full operating mode
9) ran some signal through it - no problems
10) took the limiter off and plugged it in - worked fine for about 5 minutes at 10 watts then blew another fuse
11) check and got some bad measurements on output transistors - one PNP and two NPNs had blown
12) i took those out ran it without them and it worked fine at lowish wattage (10-25) for enough time to convince me it wasnt going to blow
13) OKAY i figured it was some bad outputs. ordered some new ones (these are hard to find dammit!), put them in, it worked fine for about 5 minutes then BLAM - blew a fuse and more outputs. these time they were different ones and in different locations.
any thoughts what might be going on here? i measured all the bias and load resistors they are fine - im particularly puzzled as to why its running perfectly fine for a few minutes and THEN blowing up - i dont wanna keep ordering these expensive resistors without a better game plan -
thoughts?
Quotei measured all the bias and load resistors they are fine
What exactly does this mean? What are bias resistors?
MY first reaction is the thing is biased too hot. Monitor mains current. You have a mains fuse for each channel. Pull the fuse on the bad channel and run only the good channel. No signal, just idle. How much mains current is drawn? Now remove the good channel fuse and install a fuse in the bad channel. How much mains current is drawn? If substantially higher, try knocking the bias control down to see if it relents.
hm good thought -
im waiting for replacement output transistors for the 'bad' channel, but in the meantime i plugged it in with only the 'good' channel hooked up and measured output mains current - at first it seemed okay but as i let it sit (no load, no input) it slowly started to increase - after a little while it was pulling 1.4 amps or so - and it was hot to the touch! seems like there might be a bias issue on the 'good' channel, as well, so very likely there's one on the 'bad' -
there are two bias trimmers in there, what's the best way to adjust the bias on this thing?
I am sure there is a spec in the manual, but here is the quick and dirty way:
Monitor mains current, set the bias all the way cold. I am sure there is crossover distortion that way. Now advance the bias control until the mains current JUST starts to climb, then back off just a hair. Close enough for now. If you were to scope the output with a sine wave flowing, adjusting the bias until the notch just disappears should more or less coincide with the method I just described. At the point the notch goes away, the current starts to climb with the bias control.
I like the Yamaha amplifiers, but I did often find they needed bias adjustment when they came through the shop.
thanks, enzo -
biased the 'good' channel properly and its staying stable!
replaced the transistors in the 'bad' channel intending to try to bias it and BOOM - blew them - this time in a split second.....
hmmmm. i saw that this time they took R239 with them and also R241 - R246. i noticed while replacing R241, etc, that these are 4.7 ohm resistors in this yamaha. on the schematic, the PNPs have 4.7 ohms in this spot and the NPNs have 47 ohms in this spot.....why does my amp have 4.7 ohms in both??
Probably because the difference is a typo. The schematic is very clear, but if I were to wager, I'd say the 4.7 was the choice anyway, and since that is what is already in there, I feel justified. But mainly, if you look lower left corner of the drawing, note 7 is the flame proof resistors, and it lists all of those as 4.7 ohm.
I hope we are still NOT using a load on the amp. NO LOAD until we know the amp is stable.
As always, check all transistors there, plus ALL the resistors associated. Did you swap out the drivers? Even if they test good, blown outputs stress drivers. That is Q219,220. In fact I'd probably change the predrivers as well.
how crucial is having matched output transistors in this amp?
They need not be matched but should all be the same type, no mixing.
edit:
lemme run a thorough go through then ill get back to you!
hm.
thought I was being thorough!
replaced the outputs. tested everything very carefully. with no load made sure voltages on the bad channel matched the good channel as close I could. plugged it into an ammeter (no load still) and watched it sit - just the bad channel - quite stable at about 0.64 amps (just like the good channel).
plugged in a load - carefully and slowly turned it up, thinking to just run it at a couple watts and watch the current - and before I knew it BAM. again....fuse and outputs blown....
(I had ran the good channel for a while - totally stable totally linear at up to 100 watts - didn't push it past that yet)
I did not replaced the drivers - but they tested fine in and out of circuit!
what do you think, Enzo, should I just replace all the drivers and pre-drivers (and anything else, transistor-wise) even if they are testing good? I'm pretty baffled at this point....
You need a lamp limiter or better, otherwise you will just go in circles.
Phil.
https://www.ssguitar.com/index.php?topic=2093.0
Whenever I have to replace outputs, I change the drivers, yes even if they test OK. They are getting stressed by the power transistor failures. And your meter puts a tiny current at a low voltage through the part. The circuit puts substantial voltage on the parts and draws substantial current through them. Your meter can't test for that.
And I agree, you should be using a bulb limiter or similar.
k ill change Q219 and Q220 - what about Q215-Q218? those measure okay, too, but wondering if it'd be safer to swap them, as well.....
and, just as a note: i have been using a bulb limiter (see the rest of my post above), but i can't put the amp into 'real' operational mode with the limiter inline because all the voltages are about halved by the bulb.
As I said, I swap out the drivers, the predrivers are small parts are they not, like 50 cents or something? I'd just add them to the list, don't be cheap for a dollar.
Quote from: Enzo on July 11, 2017, 08:56:44 PMWhenever I have to replace outputs, I change the drivers, yes even if they test OK. They are getting stressed by the power transistor failures. And your meter puts a tiny current at a low voltage through the part. The circuit puts substantial voltage on the parts and draws substantial current through them. Your meter can't test for that.
Amen to that one brother!! The idea is that you are already in there so you might as well. With amp repair we constantly hear don't just replace parts such as throw in a new set of caps before isolate and fix the REAL problem. But when you have transistors near a point of failure it sometimes pays off big time replacing those questionable parts. I owe a lot to the pro repair guys online as I just know to replace those pre-drivers after a major failure. Their experience saved me from finding out the hard way many times. :trouble
sounds good!
im having a hard time sourcing the 2sa1294 and 2sc3263
are these okay replacements?
http://www.mouser.com/ProductDetail/Toshiba/2SA1943NS1ES/?qs=%2fha2pyFaduidnB2ikiWN6Mj%2fNiMyHui3NfG44QxZh5k%3d
http://www.mouser.com/ProductDetail/Toshiba/2SC5200-OQ/?qs=sGAEpiMZZMtQ1ytNl72Bk61bpVKF%252bR%252bC
Try this out. It seems Digikey is now my go to for Sanken type transistors.
https://www.digikey.com/products/en?keywords=2sa1294
https://www.digikey.com/products/en?keywords=2sc3263
any reason those ones i posted links to wouldn't be suitable replacements?
They would probably be fine, just do not mix the types.
alright guys we are almost there!
re-transistored the whole amp (the bad channel, that is) and we seem to be powering up and no-load idling just fine at the same idle draw as the other channel.
only thing is, before i send anything through it, i'd like to replace the green thermal tape behind the output transistors. its gone through some trials and tribulations and there was at least one bare spot that shorting B+ to B- through the case of one of the output transistors. i dont have any of this stuff around - RIP radioshack and the electronics store in town doesn't carry it -
any suggestions where to look for it (non-internet) or what kind of tape works for this purpose?
no one have any thoughts on where to get green heatsink tape?
all the stuff I'm finding is double-sided and made for ICs - comes in squares
the tape on the Yamaha output transistor heatsink is greenish and one-sided and clearly comes in a roll -
Quote from: ilyaa on August 07, 2017, 11:51:08 AM
no one have any thoughts on where to get green heatsink tape?
all the stuff I'm finding is double-sided and made for ICs - comes in squares
the tape on the Yamaha output transistor heatsink is greenish and one-sided and clearly comes in a roll -
Maybe some Kapton tape? probably not available local to you...
How about some Mica insulators?
Well don't call it tape, that will surely confuse the search.
Bergquist SilPads is probably the best know brand name.
Transistor mounting supplies
Transistor insulators
I have never seen it in rolls. I usually see them precut for various transistor sizes, complete with hole for mounting screw.
It also comes in sheets so you can cut it to size.
Search "thermal interface products" at Mouser.
k think I'm gonna go with the insulators -
man this amp fix is never ending!
we did it!
alright rebuilt most of this damn thing -
the heatsink may have been a big issue - i noticed some spots where it had either melted or blown up a little bit and i think this may have been causing some arcing/shorting!
but ran both channels at 75 watts for quite a while and we're all good.
NOW for the final question:
i was fixing this up for a studio in town. they are using it as a - and i'm not kidding - headphone amp. im wondering, should i talk them out of this usage altogether? or is there a way to make it work? they had their tech put a 4 ohm 25W resistor in series with the + speaker lead and otherwise just hook it directly into headphones. the headphones they are using are 38 ohms that can handle 1.5W. now if they run it at low wattage it should be okay, BUT
1) the VUs on the front are for watts/8ohms - with this setupt the amp is seeing 42 ohms - is there a way to calculate the adjusted wattage
2) would it make more sense to also wire a high wattage 8 ohm resistor across the output jacks so that the impedance matches more closely?
3) is this a silly path to take? i think the engineer just really likes this amp but im not sure i see the point of using a high wattage power amp on headphones....
One pair of phones, or a whole string of them?
if I needed a phones amp, I'd grab me one of those little small mixers, the ones with maybe one or two XLR inputs and a couple line inputs, they are cheap, and have headphones amps in them.
You could also buy a real headphone amp too, but those only do that and nothing else. My little mixer can be used for many other things that arise.
I think 1-3 phones -
but the impression I am getting is I should talk him out of using the Yamaha as a headphone amp?
I am not totally sure why he is hell bent on that, but seems to me like not a great idea, if not a terrible one.
Many amps drop main out to phones with a resistor, ther is precedent. Seems like overkill to me.
Quotethey had their tech put a 4 ohm 25W resistor in series with the + speaker lead and otherwise just hook it directly into headphones. the headphones they are using are 38 ohms that can handle 1.5W.
Madness.
They need to add a couple 220 or 470 ohm resistors, one in series with each earphone, at *each* headphone out jack.
As is, they will blow earphones, as in catching fire, and kill or gravely damage users.
You must feed *milli* watts to earphones, not full watts (not even 1 W).
J M,
a 220 or 470 in series with the + lead of each earphone jack or with both the + and - lead of each jack?
what wattage would you recommend for those 220s or 470s? 10? 20? and are you saying instead of the 4 ohm or in addition to?
No you don't need 10 or 20 watt resistors,, maybe 2 watt resistors will be fine. :tu:
Here is my Laney KB schematic clearly showing both a HP output as well as a line out (DI) from the power amp section. There would be many SS amps with similar setups if you search. 8|
Phil.
and that rig will be sufficient even for a much higher power output amp like this Yamaha?
Well try a 220R then if the signal is too hot swap to 470R.
Like most of these things you have to Select on test.
Phil.
so this thing was working fine and then blew again WTF...
doing some research I came across this:
"The one limiting factor is the design of the amplifier output. If it is a single-ended design, meaning that the negative side of both the left and right output channels have a common reference (ground) and as a result the positive side of each channel is the only audio energy source, all you have to do is connect the sleeve to the common reference, the ring to the right channel positive speaker output, and the tip to the left channel positive speaker output. But if the amp is a dual ended design and both the positive and negative speaker outputs are presenting audio energy, you cannot sum the negative outputs from each channel (which is what happens when you connect to the sleeve of the headphones) or you will fry the amp."
(found on this thread: https://www.gearslutz.com/board/geekslutz-forum/54486-powering-headphones-power-amps-how.html)
now this Yamaha is a dual-ended design and it was being used in the studio for the stereo headphone mix - meaning that the sleeve of the headphone jack on the output was indeed connecting both negative poles of the Yamaha's output. is this why the amp is blowing up???
I think what they mean is most amps we see have a split supply, as in V+ and V-, all referenced to ground. The output is ground and hot, the hot swings pos and neg.
But some amps are bridged in nature. ON a conventional amp in bridge, we use the hots of the two channels out of phase. neither speaker post is ground. Some amps are built this way for each channel. Some popular Crown amps come to mind. Each channel is already two power amp stages facing each other, and neither speaker post is ground. In a stereo amp, then the negative speaker posts are NOT ground and you cannot connect the left and right together.
Your amp looks to me to be a conventional plain old vanilla V+ and V- design. The output is ground and hot. SO I do not think you are facing the phones grounding something.
Generally when an amp works for years, then blows up, you fix it, and it keeps blowing up, it means you are not completely repairing it each time it blows up. I have no idea what you are missing. I always check and resolder the drivers, if I didn;t already replace them. That is Q219,220. How are you setting bias? What is idle current draw from the mains?