Yamaha Stagepas 600i - Rchan keeps blowing

[Journey11]

Ah crap ... I kinda thot so ... I was looking at that thing and for the life of me cannot figure out how the dang input signal gets through to the mains ! And ya too much feedback gets cut off when those 0.022's get removed. I'm gonna flip those babies over so we have a lot of separation between the lands. This will guarantee that there will be NO shorting at all, no matter how poorly aligned. Last time I installed them I was very careful and yet I looked at one and it could have been just enough out that it may not have shorted but could arc. Problem was NO one has the correct size to replace them ... the new ones are 2 watts ... originals were 1 watt. They sold me 2010 wides and I need 2550's (metrics) and even those may not work. You know what ... I hate this crap ! This stuff should be mind blowingly EASY ! and they screw it up so guys like us can't fix anything. I thought about using a big fat axial one but not enough clearance for it since it's on the bottom side and not enough room. Digikey may just have one that will work but I know if I flip the ones I have over, they will WORK ! no problem ... after that I will re-assemble and do the light bulb AND the Variac trick.
My only issue is finding test points on the top side of the pcb for the critical signal points. The main trannies are not a problem but if I have to go back further then it could be difficult finding spots to test.

 

[Journey11]

So what was yer thots on the low start up resistance with the light bulb thingy ? I found it rather surprising that a 100 watt bulb had such low resistance COLD ... 13 ohms ... what's up with that ? If I wanted to limit current ... I would use min 500 ohm but do not have a 100 watt 500 ohm resistor. .. .best I got is 500 ohm at 25 watt. Minimum for low risk I would think is around 200 or 250 ohm. The rating on the amp at 1/8 power is 100 watts. And how can that be when the amp is rated for 340 watts per channel ?

 

[PETERDECO]

I wouldn't use a resistor - just a light bulb in series with the AC line to keep those FET's from blowing. If they blow within 5 seconds when you turn it on without the light bulb, you have a short. Although there are better ways, I would use an ohmmeter to check all the transistor junctions. Emitter to base, base to collector should behave like diodes - conduct only in one direction. Then emitter to collector. Compare the readings with the same transistor in the good channel. A super low resistance in the emitter to collector junction shows something is shorted. Remove the transistor and test it again AND the same points it was soldered to. There might be something on the board that's shorted and not the transistors. Check the diodes too.

 

[Journey11]

YA I agree ... after looking at the circuit forever it seems. Well I have it all re-assembled and rigged for running with a 100 watt bulb and plugged into variac. Still haven't done it yet but I'm thinking tomorrow I'll get up the courage to attack it.
I was looking at the datasheet for those output trannies and continuous drain current is rated for 23 AMPS !!!
92 for pulsed ! I'm starting to think that they didn't blow from over current but from over voltage maybe ?
If that circuit path actually pulled 23 or more amps ... I'm sure it should have would have blown the main AC fuse ?

 

[Journey11]

Hi Peter,
I missed yer latest reply here before I posted just now .... DUDE ! I have checked every semi-conductor ... most of the caps across power/signal to GND and a lot of resistors as well 3 times over and there are NO shorts to be found. This why I am so puzzled by this repair job which should have been super straight forward.

 

[Martaine2005]

I’m trying to keep up with this thread but am getting confused.
Are you actually taking Peter’s advice?.
Have you powered it up with the series current limiting bulb yet?. Incandescent bulb of course. If a short appears, take the output transistors out of circuit and see if the short goes away.
This could be an intermittent fault.
Leave it connected through the series bulb and with a wooden or plastic prod, gently tap and put pressure on components and solder joints. If youre lucky, the bulb will indicate by either dimming or brightening on the offending area/part.
Use freezer spray too.
Clearly something is shorting. You need to find out methodically whether it’s a thermal issue or other failure.

Martin

 

[WHONOES]

I would be inclined to remove all the transistors on the errant channel. Then go round with a DMM and check that all the resistor are as they should be. Then assuming all is well put a brand new set of transistors in and check very very carefully, using a magnifying glass if necessary, for any shorts or broken tracks before you even consider applying power.
The best way to test power amplifiers is by using a current limiting power supply. That is what I use and it has saved my bacon many times.
As an after thought, the amp could be oscillating when switched on. If so, that will wreck the output stage in no time at all.

If you have access to a scope, you need to monitor the output at switch on and see if this is so.
If oscillation is taking place then you need to check any compensation components.

Also, it would be worth checking the out put offset voltage when you power up. It should be less than 100mV and if you can at the same time, check the quiescent current which should be less than 50mA I should imagine.

 

[Journey11]

I’m trying to keep up with this thread but am getting confused.
Are you actually taking Peter’s advice?.
Have you powered it up with the series current limiting bulb yet?. Incandescent bulb of course. If a short appears, take the output transistors out of circuit and see if the short goes away.
This could be an intermittent fault.
Leave it connected through the series bulb and with a wooden or plastic prod, gently tap and put pressure on components and solder joints. If youre lucky, the bulb will indicate by either dimming or brightening on the offending area/part.
Use freezer spray too.
Clearly something is shorting. You need to find out methodically whether it’s a thermal issue or other failure.

Martin

Hi Martin,
Thanks for chiming in here ... Peter has been helpful but sometimes he loses track of where I am at. I am ready to power up using series 100W bulb and have a variac to use as well. As far as intermittent fault goes which Peter did mention could be the issue ... I replied by saying if it was intermittent, I would have noticed this after replacing the output trannies twice now. Each time it blew in about 4 seconds both times, doesn't seem intermittent to me. Way back in the thread I mentioned that the new replacement 0.022 current sensing resistors had a very narrow spacing of the pads (pics above) and it may have been possible that one or both had been installed and misaligned just enough to cause a short or maybe an arc ... to remedy this I re-installed those resistors upside down so there is absolutely NO chance of shorting due to misalignment. Very confident now that this will garantee no chance of a short across those units. As I have explained to Peter a few times now ... I have gone over the pcb and checked ALL semi-conductors 3 times over or more now, carefully checking off on a diagram so as not to miss a single one. Also checked many caps that tie power or signal to ground ... no bad caps. Also checked a few resistors here and there ... I usually ignore resistors at my own peril since they hardly ever fail, especially high ohms values ... so focused more on the lower value ones.
Again, I thank you for chiming in since Peter has been pretty the only one so far save for WhoNoes a while back now. I see he has msged again and will respond to him next. I am also considering what WhoNoes said ... replace all semi's ssince the cost will be pretty cheap after putting a shopping cart together ... still less than 30 buck so far. Some caps I'd like to replace ... but I can't get the exact replacements as far as temp co-efficient ratings go.
I don't think I have ever seen a cantancorous amp as this one at least in a long long while.
Actually ... now I think of it ... I just repaired a Behringer BX4410a that was a real bitch ... turned out it was a high value resistor 150K that went open ... or very high Mohm range. Just something that almost NEVER happens especially with high value ohms value. Seen lots and lots of 10 ohmers fried to hell. Low ohms, high current ... lots of heat for cooking eggs.

At any rate ... I'll keep you updated if and when I solve this conundrum ...

 

[Journey11]

I would be inclined to remove all the transistors on the errant channel. Then go round with a DMM and check that all the resistor are as they should be. Then assuming all is well put a brand new set of transistors in and check very very carefully, using a magnifying glass if necessary, for any shorts or broken tracks before you even consider applying power.
The best way to test power amplifiers is by using a current limiting power supply. That is what I use and it has saved my bacon many times.
As an after thought, the amp could be oscillating when switched on. If so, that will wreck the output stage in no time at all.

If you have access to a scope, you need to monitor the output at switch on and see if this is so.
If oscillation is taking place then you need to check any compensation components.

Also, it would be worth checking the out put offset voltage when you power up. It should be less than 100mV and if you can at the same time, check the quiescent current which should be less than 50mA I should imagine.

HI Whonoes ! ... Thanks again for coming back with comments.
Peter actually recommended just changing out all semi's which I'm still considering since my shopping cart for all that is still under $30 CAD. Just a lot of work to do it all but I have done crazier things for a small amount of pay.
Unfortunately I do not own a nice current limiting PS ... would just LOVE to have one !
As far as I know ... the possible cause of the initial failure was due to a speaker wire extension being needed and he maybe shorted out the speaker line and viola POP ! I replaced the output stage once ... happened immediately again. Then got the whole rig into the shop and replaced again ... 4 seconds after power up POP ! If you refer to the previous post to Martin ... I mention a possible screw up on my part installing the current sensing resistors. This has been addressed and totally eliminated as a cause for failure again.
again referring to above response to Martin ... I have spent many hrs checking and triple checking ALL semi-conductors in the amp ... none found to be bad. However, all checks were made in circuit ... your idea of removing all semi's is a great idea but a lot of work.
With all the hrs of work I have put in to date ... I think my hourly wage is about 50 cents per hr ! at this point ...
But it's become PERSONAL now ! and I rarely concede defeat to an electronics device !
I do have a scope to see if the amp is oscillating but it blows so fast I'm not sure I could make that test fast enough and if I could ... you say look for compensating components ? I'm not sure I would know which components to even start looking at ... any ideas ?
I think I will start with the light bulb power up to start with using the variac as well. Only issue I might see with using variac is the switching power supply won't start working properly until it gets to the right voltage ... then it may pop again possibly ?
I also mentioned a post or two ago that the datasheet for the mains trannies says continuious drain current rated 23 amps ! If these went over that rating I would have to think the main AC fuse should have been taken out for sure. What would your thoughts be about that ?
At any rate ... I will attempt the bulb trick and keep y'all updated ...

 

[PETERDECO]

My internet just came back on yesterday after the storm and am catching up on things. If you follow the posts we made, I suggested over 2 weeks ago to use the light bulb to check for excessive current draw and also remove the outputs so they won't blow, check voltages and compare with the good channel. Being a stereo amp with a working channel, you have an advantage to be able to track down the problem. Do the light bulb thing and tell us what you find.

 

[WHONOES]

Current limiting power supplies are not that expensive these days particularly the Chinese ones, scuse me while I wash my mouth out. Once you get one, or two for split supplies, you will wonder how you ever managed without.
Perhaps there is a project in there somewhere. Will see what I can rustle up.

 

[Journey11]

Hi there again ... ah ya I'd prefer to make my own unit ... that way I can fix it if it ever goes down and i know exactly what is what. If you get innovative and have some motivation ... drawing up a schematic would be helpful ... I'm sure there are already lots out there but I'm more of a fix it guy rather than design/engineering. I'm absolutely sure if I hit the books I have stashed in the basement .... I could design one ... probably might even make a damned good engineer with all I know about Tesla tech.

 

[WHONOES]

Working on a design for current limited PSU. Watch this space

 

[WHONOES]

See attached a schematic for a 50V 2A variable power supply.
2 will be needed if you want a ± supply.
Carefully read the notes on the schematic.

Edit. Seem to have got 2 copies of the schematic. Perhaps a moderator could remove one.
Moderators note : removed double on request.

 

[Journey11]

Hello Lads ...
I finally got the balls to power up the Stagepas 600i using the light bulb trick and no action on the light BUT and this is a big BUT ... I only switched it on for lets say approx. 2 seconds then off ... because in the past as you all know .... it took about 5 seconds to blow out the new components.
So I had an idea after the 1st power up with light bulb ... I soldered 4 wires across each of the two 0.22 ohm resistors (on the topside where +HV comes in) to see what voltage drop was across them when powered up. One set on the good side and one set on the problem side.
They both read pretty much ZERO after power up for a few seconds ... my best guess is that there should be no voltage across those resistors if all is good. No current flow until a signal is applied and the amp starts working...correct ? If I'm not mistaken ... if there is a crap load of current thru these resistors the voltage would be at whatever Vcc was attached to them ?
That seems correct to me according to ohms law.
If I am right ... I think I fixed it ... and the reason for the previous failures has to have been ... the fact that either one of those 2 resistors had a direct short across them due to footprint misalignment as I said earlier in the post.

I think my next test will be this ... run an audio signal into the bad channel and see IF ... on power up I get some sound ... if only for the few seconds I turn it on for.

What Ya'll think ?

Journey