chris1954 12/9/2017 10:29 AM
BA115 DC on speaker output
Hi - I've found multiple issues with this amp....replaced the zeners (D8 and D9) - one was shorted out - now the LED comes on versus no LED prior. I've also replaced Q2 and Q5. But I'm seeing -34v at the speaker and when I run a signal into the amp, I see it die around Q6 area. Any ideas would be helpful! Thanks!
 
Enzo 12/9/2017 11:14 AM
Work with no load until the amp is stable and not making DC. Don't even think about signal while there is 34vDC on the output. D8,9 shouldn't affect the power light, but do you have good power rails? No system works if the power supply isn't right. You have +40 and -40, more or less? I don't care if they are 42v or 37v, as long as both are about the same. And do you have both +16 and -16? Easiest way is to find the two large resistors, R66,68, 270 ohm 5 watt. on each, 40v at one end and 16v on the other. If a 16v is missing, that will shove U3 in the power amp over to one side. So what is on pins 1 and 7 of that IC? Whatever is on pin 7 is likely to be what is on the speaker too. You had to replace the outputs? OK< any time you do that in any amp, the drivers are instantly suspect, as they likely were stressed by the bad power transistors. Also always check the associated resistors, in this case: 47 ohm and 220 ohm. I usually replace the drivers even if they SEEM to be OK. Once the amp settles down to zero volts DC on the output, THEN worry about signal.
 
chris1954 12/9/2017 11:18 AM
Thanks! The only reason I was worrying about signal was that I thought the amp was fixed. I had no LED until I replaced the Zeners. So I hooked up a crappy speaker and wham - it blew. Checked for DC and there is was. I'll look into your suggestions....super thanks for your reply!! I'll report back...
 
chris1954 12/9/2017 12:45 PM
Hi Enzo - Here is what I found.... D8,9 shouldn't affect the power light, but do you have good power rails? No system works if the power supply isn't right. You have +40 and -40, more or less? I don't care if they are 42v or 37v, as long as both are about the same. >>>>>>>>>Chris: Yes. + and - 35v or so. And do you have both +16 and -16? Easiest way is to find the two large resistors, R66,68, 270 ohm 5 watt. on each, 40v at one end and 16v on the other. >>>>>>>>>Chris: No - Here is where things are wacky. R68 is 10.7v / 35v (and running hot), R66 is -16v / -35v (warm too). The 10.7 is suspect...but I'll listen to your thoughts! If a 16v is missing, that will shove U3 in the power amp over to one side. So what is on pins 1 and 7 of that IC? Whatever is on pin 7 is likely to be what is on the speaker too. >>>>>>>>>Chris: U3 pin 1 is 11v, pin 7 is 4v, pin 8 is 11v, and pin 2 is 11v. Speaker is 35v You had to replace the outputs? OK< any time you do that in any amp, the drivers are instantly suspect, as they likely were stressed by the bad power transistors. Also always check the associated resistors, in this case: 47 ohm and 220 ohm. I usually replace the drivers even if they SEEM to be OK. >>>>>>>>>Chris: I measured R34, 41, 35, 31 - all are 220 Ohms. R33 and 49 measure 47 ohms.
 
Enzo 12/9/2017 2:31 PM
The two resistors DO run hot normally, but if the one with 10v on it is extra hot, it is because more current is drawn through it pulling down the 16v. They dissipate more than 2 watts, which is plenty to get hot. My first suspect on the 10v instead of 16 is the zener is dying. otherwise something in the circuit could be loading it down. Such a loading would most likely be a bad op amp, any of them getting hot?
 
chris1954 12/9/2017 2:39 PM
Are you referring to D6, D7, D8 and D9 on the zener's? I didn't feel any of the op amps getting hot...but I'll check again. How can I test if an op amp is bad? I can't remember how to do this....from my school days....a long long time ago! And yes - R68 with the 10V is for sure running hotter. Again - I really really appreciate your help. Big time.
 
Enzo 12/9/2017 2:54 PM
No, the zeners in the power supplies that set the voltage, in this case 16v zeners. In this case D11, D12. If D11 collapses, then instead of -16v, you get -10 or -8 or -5 or whatever. If it outright shorts, you get zero volts there and the resistor gets REAL hot. I test op amps by function, they work or not, and they don't have voltage on their output that doesn't belong there. I can unsolder it and pull it out and see if the voltage in the circuit rebounds to 16v. I still suspect the zener first.
 
chris1954 12/9/2017 4:09 PM
Checked D11 and D12 - both are fine. Even replaced them and same issue. No op amps are warm to the touch. There is a consistent 10v on each op amp....this makes sense....and everywhere where there should be 16v - there is 10v. Makes sense too... It would seem to me that where the 16v is created is where the issue might be....but again, I look to you, the expert. Thinking out loud, +40v is dropped to +16v. I'm now wondering if R68 is the issue. It measures ok, but it seems like after R68, we only have the 10v. But again, I'm not an expert. What creates the voltage drop from +40 to +16v? Also could the driver transistors be an issue? Q4 and Q1? hmmm....just thought of that.
 
Enzo 12/9/2017 9:48 PM
The zener and the op amps are all connected together. The zener decides the voltage. You can test the zener as a diode to see if it is shorted, but the only way to see if it functions as a zener is to apply working voltage to it, something your meter does not do. If the diode is bad and only makes 10v instead of 16v, then everywhere 16v would be there would be 10v instead. Of course something on the 16v line could be dragging it down, like a stuck parking brake. What creates the voltage drop? On a plain resistor, the current through it, that is Ohm's Law. But here, the end voltage is not set by current, but by the zener diode voltage. If the resistor measures 270 ohms or so, then it is fine. You had 16v 5 watt zeners in stock?? If you replaced the zener and still get 10v, and the resistor is 270 ohms or even remotely close, then we look at the load. I see five op amp ICs. I see a couple connections to the power amp, but they are through 1k resistors, which tells me a dead short to ground through 1k would only draw 16ma, not enough to load it down. Couple diodes at the input jack, I doubt they are shorted, but easily checked. C39 could be bad. If after sitting there a few minutes, my op amps were not getting warm, then I would probably start lifting them from the circuit to see if the voltage recovers.
 
chris1954 12/10/2017 10:30 AM
Progress.....I pulled out the drivers. After I pulled out Q1, +16v appeared. Gonna order new Q1 and Q4 transistors! Also when I put Q4 back in and left out Q1 - no DC at the speaker. I did ohm both Q1 and Q4, and they both look ok...but I'm just gonna replace both of them. Turns out that lightning took out this amp.... Does this all make sense?
 
Enzo 12/10/2017 12:58 PM
Not quite, by "ohming" I assume you used resistance measures? To see if the part was shorted? What about open? You should use the diode test function of your meter to test the base to collector and base to emitter junctions Then also emitter to collector both directions to make sure no short or leakage exists. Q1 was killing your 16v, so obviously it has a fault that your meter is not detecting. I wouldn't have thought Q1 could kill the 16v, but I guess I was wrong. At least I was right to suspect the drivers.
 
g1 12/10/2017 1:04 PM
[QUOTE=Enzo;473280] I wouldn't have thought Q1 could kill the 16v, but I guess I was wrong. [/QUOTE] I didn't think so either. An extra 16mA? It must be running very close to capacity under normal conditions. Or perhaps bad solder at the R68 connections?
 
chris1954 12/10/2017 1:16 PM
Sorry - I did both...looked for open and the diode function. Agree on the fault the meter is not detecting....unless there is something else.... Someone also asked about solder at R68 - I resoldered both R68 and 66. Also checked C39 and everything along the +16v line..... Hoping new drivers fix the problem....
 
chris1954 12/12/2017 2:52 PM
Update - Not good. Replaced the drivers - 340 and 350. Now I'm blowing fuses....argh!
 
chris1954 2/1/2018 1:30 PM
Hi Enzo and all - I gave up for a few months...but back at it. Looks like one of the mosfets IRFP-140R (Q5) is shorted. Must have been bad - I got it from eBay...after reading, seems like that is NOT the place to buy parts...so I have 2 new MOSFETS on order - Q2 and Q5.
 
chris1954 2/12/2018 10:35 AM
HI Enzo - I've found one of the MOSFETS being bad as I said. I pulled out both MOSFETS, and have received new - decided to get both - Q2 and Q5. So I have both Q2 and Q5 out of the circuit, along with Q1 and Q4 - (tested them and they are both good) and I pulled out Q3 - which tests good too. When I power up the board with these 5 Q's out, R68 still gets hot. Seems like this should not happen. Before I put the Q's back in - I'm thinking I need to figure out why R68 is hot. My voltages are fine and no DC on the speaker output....again, with the 5 Q's out. Would really appreciate your thoughts on trying to troubleshoot if there is an issue as to why R68 gets so hot. My gut says there is still an issue. Tired of blowing out MOSFETS - specifically Q5.
 
DrGonz78 2/12/2018 12:48 PM
R68 and R66 should get hot as they are dropping resistors. On one side of R68 you should have 40v and 16v on the other side, measure that to confirm. Also, check voltages on each side of R66 for -40v and -16v on each side of that resistor. BTW have you built a Light Bulb Limiter??? You probably don't own a variac with ammeter to monitor current draw from mains when you first power up the repaired amp. A light bulb limiter will help you limit excessive current from damaging your repair if there is still a short in the amp. So the idea is to replace all known blown parts in the amp and then you first power up the repaired amp by way of the light bulb limiter. If the bulb stays bright when turned on then there is likely still a shorted part or connection to be found. If the bulb goes dim at turn on of the repaired amp then it is likely not drawing excessive current any more. Another added benefit to using the light bulb limiter is it can be possible to perform simple voltage checks on the amp instead of just blowing out transistors and fuses. It's not a perfect tool but it does come in handy. [url]https://www.ssguitar.com/index.php?topic=2093.0[/url]
 
chris1954 2/12/2018 2:02 PM
Thanks - cool - or rather warm...the R's are ok then. I checked voltages and all is good. I do have a LB limiter...but it's not working as it should...hahaha....just a bad electronics day all around! I'll look at your link and see why my new limiter is not working..... Aside - I put in the new parts and no more blown fuses....so looks like no shorts....but when I measure ground to the plus side of the speaker output, I see 30+ volts....strange....back to the original issue. Both Q2 and Q5 are still ok...too. When I power up, I don't have a load on the speaker out - nothing is hooked up. With non tube amps, I seem to remember that this is ok...or am I wrong? I'm about to give up and get the amp back to the owner....my bud. or is this voltage ok on the speaker output? I doubt it....I think it should be like mv - small small small...
 
DrGonz78 2/12/2018 7:41 PM
[QUOTE=chris1954;479943]When I power up, I don't have a load on the speaker out - nothing is hooked up. With non tube amps, I seem to remember that this is ok...or am I wrong? I'm about to give up and get the amp back to the owner....my bud. or is this voltage ok on the speaker output? I doubt it....I think it should be like mv - small small small...[/QUOTE] Yeah no speaker connected until you have close to 0v DC on the output the amp, small millivolts. Solid state amps are typically fine to run without a load unlike tube amps. Please test Q1 and Q4, also make sure Q3 is good too.
 
DrGonz78 2/12/2018 8:55 PM
[QUOTE=chris1954;479925]I pulled out both MOSFETS, and have received new - decided to get both - Q2 and Q5. So I have both Q2 and Q5 out of the circuit, along with Q1 and Q4 - (tested them and they are both good) and I pulled out Q3 - which tests good too.[/QUOTE] Re-reading your post I must have missed that you DID already test those transistors. Sorry I should have noticed that... Anyway I would still make sure the insulators are good on Q2 and Q5.
 
chris1954 2/12/2018 9:19 PM
How can I test that? Interesting thought. And huge thanks!!!
 
The Dude 2/12/2018 9:47 PM
To test that, measure the resistance from the Drain (center leg) of the MOSFET to the heat sink and make sure there is not a short. The center leg is internally attached to the metal case on the back of the device. If your insulator is not doing its job the MOSFET will be shorted to the heat sink, which is normally ground.
 
chris1954 2/13/2018 7:07 AM
[QUOTE=The Dude;479989]To test that, measure the resistance from the Drain (center leg) of the MOSFET to the heat sink and make sure there is not a short. The center leg is internally attached to the metal case on the back of the device. If your insulator is not doing its job the MOSFET will be shorted to the heat sink, which is normally ground.[/QUOTE] Looks like the insulators are fine. No short. Argh! This thing is driving me nuts! lol I also noticed that on both Q2 and Q5, while I was checking for Drain shorted to ground, the Source on both were shorted to ground for a moment....after a moment or so, they were not shorted. I waited a bit and tried again and it was the same thing. So I must have been charging up the MOSFET or something....normal? I looked at the schematic and Q2 Source should be +40v - which it is when turned on, and Q5 Source is on the -40 side but has R61 to -40v. Seems odd that these two Source pins would be shorted with no power....I was getting the short with the unit turned off. and Yes, I'm grasping at straws!!!
 
chris1954 2/13/2018 7:15 AM
[QUOTE=DrGonz78;479978]Yeah no speaker connected until you have close to 0v DC on the output the amp, small millivolts. Solid state amps are typically fine to run without a load unlike tube amps. Please test Q1 and Q4, also make sure Q3 is good too.[/QUOTE] Yep Q1 3 and 4 are ok - I checked them before putting the new Q2 and Q5 in. I actually replaced Q1 and Q4 with new Q's....
 
chris1954 2/17/2018 8:34 AM
So I'm still stuck. Still seeing rail voltage (40v) at the speaker output. Q1 Q3 Q4 are all ok - been replaced. Insulators are fine - no shorts. I noticed that on both Q2 and Q5, while I was checking for Drain shorted to ground, the Source on both were shorted to ground for a moment....after a moment or so, they were not shorted. I waited a bit and tried again and it was the same thing. So I must have been charging up the MOSFET or something....normal? I looked at the schematic and Q2 Source should be +40v - which it is when turned on, and Q5 Source is on the -40 side but has R61 to -40v. Seems odd that these two Source pins would be shorted with no power....I was getting the short with the unit turned off. Really could use some help on what I should attack next...please?
 
Enzo 2/17/2018 9:06 AM
MOSFETs don't charge, but the filter caps in the power supply do. Your meter is trying to do that. With no speaker, you can turn the amp on and find +40 on the output? And no excess mains current flow? (Meaning it doesn't blow fuses, at least with no load?) If so great. Now is there +40 and -40 on the sources of the MOSFETs? If yes, then move on, all that shorted power supply stuff is just confusion with your meter. R61 is 1/10 of one ohm, it disappears as far as the circuit cares. So now look at this as a circuit, not a bunch of parts. Just because we have 40v on the output, doesn't mean the outputs are at fault. OH they could be, but you replaced them. SO what is on pin 7 of the op amp? That after all is what controls the voltage on the speaker. And for that matter go back to pin 1. have you subbed another op amp into that position? The MOSFETs may be allowing 40v out because something earlier is telling them to. Qs 1 and 4 have 1k base resistors to +/-16v. DO they have similar voltages on the bases? Other than polarity? Speaking of parts though, have you verified the 220 ohm resistors Q1,4 use? The 47 ohm gate resistors for Q2,5? How about the four 15v zeners at the gates of Q2,5? Just test them as diodes, they won't be zening other than on peaks.
 
chris1954 2/17/2018 2:04 PM
[QUOTE=Enzo;480431]MOSFETs don't charge, but the filter caps in the power supply do. Your meter is trying to do that. With no speaker, you can turn the amp on and find +40 on the output? And no excess mains current flow? (Meaning it doesn't blow fuses, at least with no load?) If so great. Now is there +40 and -40 on the sources of the MOSFETs? If yes, then move on, all that shorted power supply stuff is just confusion with your meter. R61 is 1/10 of one ohm, it disappears as far as the circuit cares. So now look at this as a circuit, not a bunch of parts. Just because we have 40v on the output, doesn't mean the outputs are at fault. OH they could be, but you replaced them. SO what is on pin 7 of the op amp? That after all is what controls the voltage on the speaker. And for that matter go back to pin 1. have you subbed another op amp into that position? The MOSFETs may be allowing 40v out because something earlier is telling them to. Qs 1 and 4 have 1k base resistors to +/-16v. DO they have similar voltages on the bases? Other than polarity? Speaking of parts though, have you verified the 220 ohm resistors Q1,4 use? The 47 ohm gate resistors for Q2,5? How about the four 15v zeners at the gates of Q2,5? Just test them as diodes, they won't be zening other than on peaks.[/QUOTE] 1. +34v at the speaker...I know this voltage can be something close to 40v..the 34v is what I'm getting at the source...FYI. 2. Sources of MOSFETS Q2/Q5 - Yes there is 34v on both. 3. U3 OP Amp - Pin 1=+12.6v and Pin 7 = +10.9v. (I've not subbed another op amp) 4. Verified the 220 ohm Resistors for Q1 and Q4. And verified 47 ohms for R33 and R49 5. Q4 base voltage = 10.4 6. Q1 base voltage = 1.5. Hmmmm might this be an issue? not equal to Q4 base voltage. 7. I changed out the zoners and retested - they are fine. Changed out all 4 of them. So it looks like the Q1 and Q4 base voltage delta is an issue? Q1 runs hotter in temperature than Q4 - FYI.
 
Zouto 5/2/2018 10:12 AM
Hi Chris, could you fix it in the meantime, did you find the culprits? I just got a BA115 HP with similar problems and a blown tuner module. I could not check parts and voltages etc. up to now, but heard that terrible DC-on- Speaker -sound, directly after turning it on. Thanks for informations, hints and help Zouto
 
Sowhat 5/2/2018 10:29 PM
This sounds eerily like a post I made on pretty much the same thing a few weeks ago... it got resolved and in a way not mentioned so far in this thread. [URL]http://music-electronics-forum.com/t46197/[/URL]
 
chris1954 5/5/2018 6:28 AM
Hi - actually I gave up! dumped the amp. couldn't figure it out and decided to move on. glad I did. was driving me nuts. Sorry I can't help. Good luck! I gave up since I'm a gigging musician playing 2-3 times a week with no extra time. Otherwise I would have continued to work on the amp.