I have here a music man 65 reverb amp head (2275-65) with a very quiet and fizzy, distorted output. Brand new EL34s & Capacitors have been replaced but problem still persists.
Here’s where things get strange however, the amp seems to work well when i remove the op amp chip (1458). Volume jumps up and i get a very clean crisp signal. But if i put the op amp back in, it’s back to low fizziness. I figured the op amp was fried so i bought a new one, and installed it, but the new one is behaving the same as the old one!
Does anybody know what’s causing this? or how I can fix this? Please and thank you!
First thought is...
...something leading into or out of the 1458 opamp (IC-8) is bad...but you probably figured that part out already!
If you are comfortable working safely inside these amps, power off, unplug from wall, discharge caps, etc.
Pull the opamp and clean its socket with DeoxIT or CRC contact cleaner or similar, then reinstall and test.
Then I would check voltages at the opamp, 16v+ on one side, 16v- on the other, and that the grounds are good. See the schematic for details.
If that is ok,next use an audio probe to trace out just where the signal goes bad. That will help pinpoint any component that needs to be replaced. I would start with the components around IC-8.
Just for grins, clean and exercise your Deep switch and Master Volume pot. Sometimes you get lucky and it's something simple.
Thanks for your response and
Thanks for your response and your help! After taking some voltages, it seems my op amp chip is getting 30v+ and 30v- ... im taking a look at the schematic and i cant figure out why it’s not getting the 16v. Any ideas of where to look? or what to check?
(Here is some side information to note: the two heatsinks on the DB-2 board are not heating up, and the tremolo also doesn’t work, not sure if that information will help us diagnose the main issue, but just thought i’d share)
This makes no sense
Hi there,
you should not get any signal if you remove the phase splitter chip.
In regards to your voltages: What sort of voltages are you getting around the 16V Zener diodes?
Sounds like you may need to take this amp to a tech.
Sincerely,
Lars Verholt
SOLVED
Hey all! Big thanks to both of you guys for your help. After countless hours of research, schematic examination, and trial & error, the culprit was clear. This amp had been repaired with incorrect components before i purchased it; the replacement zener diodes they used capped off at around 30v instead of the 16v it should be. Hence frying my LM1458 chip. Put in the correct 16v zeners and a new LM1458 and voila she works flawlessly.