Hello all,
I was following a thread that was posted about this amp and decided to register because I don't know whats going on with the silly thing. Let me summarize as best I can, hopefully I don't leave anything important out.
The back story is "I can't believe what big companies will put their names on!" I bought a 100W Crate (don't remember the model number) and it started acting up, drove the 50! mil round trip to take it to their certified repair guy who worked above his garage (how is he THE guy again?) He failed to fix it twice so they replaced it with the BA115 which played for about 3 months then blew the outputs. High quality stuff here...
Anyway, I'm an electronics tech but I really don't have the equipment here at home to properly test everything. I replaced the output FETs and tested everything I could reasonably test with a soldering iron and a DMM. The Driver transistors read okay, I even popped them out to see if I could test them in my DMM's transistor slot, but the leads were cut so short that not only could i not check it that way, but I checked, double, and triple checked them when I soldered them back onto the board. Surprised they even made contact. Checked all the zeners, checked caps for opens and shorts, did the usual visual tests and after I replaced the outputs, I hooked it up through a dim bulb tester (a real makeshift piece of work if i do say so myself) and that was just fine (bulb flared and settled as I would expect it to).
So I plugged it in, and nothing blew so thats a good sign. I proceeded to set the bias resistor to 5mV across R61 as the schematic says. Alright good. Hooked it back up to the speaker cab and besides some crackling while it warmed up, not problems. However, when I plugged in, it was VERY quiet. Played with the volume a bit but still way too quiet for 100W.
Shut it down, took the amp back off the speaker cab and checked the Bias again, it had dropped to 3mV! Explains why it was so quiet... so this time I took a little longer and set the Bias to exactly 5.0mV. However, I left the probes there a little longer and watched the Bias start to climb (mind you this is just plugged in, no load (speaker) and no signal applied. Went from 5 to 7 mV in about 2 minutes. I guess this explains why the outputs might have blown in the first place.
Shut it down, checked the "cold" resistance of the bias pot and decided to see if it would change if it sat there for a while and it didn't. Fired it back up (still no load, no signal) and checked the BIAS again, rose from about 3mV to 6mV in a few minutes. So I checked the resistance (while it was on this time) of the pot, and it was moving a bit (also about tripled its cold value) but nothing I would be worried about, just fluctuating up and down. However, the bias is still changing so something is "leaking."
I was wondering if any of the more experienced people here might know whats going on here or offer a bit of advice. I already took the "loss" on the amp and bought another one, this is merely a learning exercise.
Interesting side note, the Fender I bought to replace it has been replaced once already, I point back to the original background quote
Thanks!
Adam
