BA115 DC on speaker output

[chris1954]

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]

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

[chris1954]

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]

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...

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]

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.

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]

How can I test that? Interesting thought. And huge thanks!!!

[The Dude]

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]

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.

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]

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.

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]

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]

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]

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.

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]

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]

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.

http://music-electronics-forum.com/t46197/

[chris1954]

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.