Troubleshooting BA115

Clintstone

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Not sure if this is the correct forum, please tell me if it is not. So my bass player buddy brought me his Ampeg BA115 because it popped the fuse, at least he was honest with me a bolt what happened though. His buddy's kid was using it and told him it just quit working, further investigation revealed the kid had rolled all the knows to 10 and basically murdered it lol. I agreed to have a quick look, at it for him thinking it might be something simple.
So I started with the basic stuff, unplugged the PT output and checked for voltage, all good. I checked the rectifier and its not shorted so thats ok. After a little research I discovered these models are famous for blowing output Mosfets, so I desoldered them to test, sure enough one was shorted. Now in times of Covid these were not easy to find, I ended up ordering them from China and waiting 3 months but they finally did arrive. One of the forums I found online had a decent description of this failure, so following their advice I desoldered and checked the other 2 driver transistors and the bias transistor, they were ok by my multimeter check. I soldered in the new outputs, replaced the fuse and it popped as soon as I hit the power switch.
So my guess is something is obviously shorted to ground which is popping the fuse, I got busy with other projects and put it on the shelf for a couple of months.
Last night I decided to give it another look and see what I could figure out. I put a new fuse in and carefully watched as I hit the power switch, to my amazement the fuse did not blow, for about ten seconds then it popped. Now my guess is that while the amp sat for a couple months the caps discharged, and once they charged up poof the fuse went.
I'm kinda stumped on which way to go next as far as diagnosis, this amp is 15 years old so would it be wise to re-cap before going any farther (would only cost like 20 bucks and would probably be good anyways)
Any tips on where to look next, I do have the schematic for it
 

tubedude

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Power MOSFETS usually have to be replaced in matched pairs. Tighter matching than tubes.😁
I assume the site you referenced covered all the essentials for the replacement process.
Since it took 10 seconds, I'd say the replacement went well. If one of the MOSFETS were shorted it would have likely failed immediately.
If your power supply checks good,
can you isolate the power rail from the sections of the circuitry to locate the problematic area? Adding sections at a time and waiting for failure.
Posting the schematic will help.
 

Jon Snell

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If, and it is quite likely, the bias pot has got/had a dirty spot on the track, the bias will go to maximum, enough to draw lots of current and pop the fuse.
The Fets used here are quite robust and probably are OK.
Check the pre set and set the bias to zero, (zero resistance across the pot). If the amp is OK, that is probably all it was.
Replace the pot and set up acording to the specification; 50mA quiescent current at idle.
Otherwise remove the output Fets and check the drivers etc.

I wouldn't trust Chinesium Fets, you have no idea what they actually are.


Available through Mouser, RS, Farnel, Element 17 etc etc etc.
 

Clintstone

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Bias pot measured 470 ohms across the pot, so I adjusted it to 0 resistance, diodes D6 to D9 test ok. I plugged into my light bulb current limiter and it glows bright, so dead short somewhere. Going to pull out the FET's and test them again.
 

Jon Snell

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I would remove both FETs, link as shown and see if the centre voltage is zero and the current draw is as it should be. If it is, then you have some duff FETs.
 

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Clintstone

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Pulled the mosfets out and tested them again, sure enough Q5 was shorted. I replaced it again and turned the bias pot to 0 ohms like Jon Snell suggested. Turned on the amp with the current limiter in place and I was rewarded with a bright bulb for half a second and then it dimmed down to virtually nothing. This made me happy, so I checked my rail voltages and ended up with +- 32V. Taking a leap of faith i plugged the amp in and turned it on without the limiter, success no blown fuse. Now the instructions on the schematic say to set the bias as 5mV across R61 which is a 0.1 ohm 3watt resistor, I did this and so far so good, no smoke no fire, and no blown fuses. Dare I hook the speaker and test it?
 

Clintstone

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If, and it is quite likely, the bias pot has got/had a dirty spot on the track, the bias will go to maximum, enough to draw lots of current and pop the fuse.
The Fets used here are quite robust and probably are OK.
Check the pre set and set the bias to zero, (zero resistance across the pot). If the amp is OK, that is probably all it was.
Replace the pot and set up acording to the specification; 50mA quiescent current at idle.
Otherwise remove the output Fets and check the drivers etc.

I wouldn't trust Chinesium Fets, you have no idea what they actually are.


Available through Mouser, RS, Farnel, Element 17 etc etc etc.
Where did you find the spec of 50mA
 

Clintstone

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D'oh, slapps forehead, that makes perfect sense now. I hooked it up and tried it out, it works, sound is clean but has very low output, like 10 sounds like about 2. This one has me ready to pull the last of my hair out.
 

Clintstone

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Ya I checked the headphone socket, even desoldered it so I could get a better look, it looked and tested fine. Hooked up my phone to the cd input and it definitely seems louder
 




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