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Author Topic: Yamaha PC4002M blowing fuses/output transistors  (Read 6884 times)

ilyaa

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Yamaha PC4002M blowing fuses/output transistors
« on: June 21, 2017, 01:32:48 PM »
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?

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #1 on: June 21, 2017, 06:03:52 PM »
Quote
i 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.

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #2 on: July 02, 2017, 12:03:37 PM »
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?

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #3 on: July 02, 2017, 01:25:04 PM »
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.

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #4 on: July 09, 2017, 04:48:02 AM »
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??

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #5 on: July 09, 2017, 05:28:12 AM »
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.
« Last Edit: July 09, 2017, 05:31:32 AM by Enzo »

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #6 on: July 10, 2017, 02:53:04 PM »
how crucial is having matched output transistors in this amp?

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #7 on: July 10, 2017, 03:00:03 PM »
They need not be matched but should all be the same type, no mixing.

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #8 on: July 10, 2017, 03:58:04 PM »
edit:

lemme run a thorough go through then ill get back to you!
« Last Edit: July 10, 2017, 05:36:22 PM by ilyaa »

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #9 on: July 11, 2017, 08:02:36 PM »
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....

phatt

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #10 on: July 11, 2017, 08:54:47 PM »
You need a lamp limiter or better, otherwise you will just go in circles.
Phil.

https://www.ssguitar.com/index.php?topic=2093.0

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #11 on: July 11, 2017, 08:56:44 PM »
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.

ilyaa

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #12 on: July 12, 2017, 02:07:22 PM »
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.

Enzo

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #13 on: July 12, 2017, 07:07:42 PM »
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.

DrGonz78

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Re: Yamaha PC4002M blowing fuses/output transistors
« Reply #14 on: July 12, 2017, 10:17:55 PM »
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.

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
“A person who never made a mistake never tried anything new.” -Albert Einstein