Music Man HD 130 reverb head.
Tremolo effect becomes increasingly more subtle as the Volume is advanced. The turning point seems to be at about 3 on the Volume, after that the Tremolo effect is increasingly more subtle. This is like having a wet/dry blend control set mostly dry.
At and before 3 on the Volume produces a strong tremolo effect.
Not sure if this is normal function?
Referencing schematics 2475-130 & 2275-130, the Tremolo intensity output is fed to the non-inverting input of an op-amp while the audio signal is fed to the inverting input of the same op-amp. Also there is a 4k7 resistor from the Tremolo intensity output to the wiper of the volume pot. (forming a voltage divider?)
Thinking this issue or normal function is due to interaction with the volume pot via the 4k7 or function of the the op-amp's differential inputs.
I'm lost here trying to analyze this circuit. Any help even if it's just 'yes, that is normal' would be appreciated, though I would like to understand technically how this circuit works.
Thanks!
Did you try substituting a
Did you try substituting a known "good" IC-7?
Thanks for the reply RD112.
Thanks for the reply RD112. Good idea, I have not tried subbing the IC. I will try this tonight, and report back with results. I'm assuming you feel my amp is not functioning properly?
Substituting a "know good" IC
Substituting a "know good" IC into IC-7 proved no change in function. I borrowed the IC from the normal channel, which is working properly, to sub into the IC-7 position.
FET?
It may be that what you're describing is normal behavior. But if it isn't "normal", it's been my experience with other brands that the FET is the weak link. BTW: I was hoping someone that has your amp would chime in. What happens when you increase the tremolo intensity and then play with the volume control? Another guessing game question: has the volume pot been replaced? If yes, was it with the correct spec and taper pot? Good Luck and let us know if you find anything definitive. -mgriffin
You may be right on. I did a
You may be right on. I did a continuity test on the 2n4091 jfet, out of circuit. I'm getting a 4k ohm reading gate to drain when it should be OL. I ordered replacement 2n4091's to sub in. Also I scoped an audio signal at both inputs of IC-7. The balance of amplitude between both inputs changes as the volume is increased, the dry signal becomes multiple times greater in amplitude than the tremolo signal. Removing the 2n4091 restores amplitude balance to both inputs, obviously the tremolo effect is disconnected. I'm thinking the 2n4091 is allowing more signal to ground then intended and not allowing the amplitude to increase with the dry signal amplitude. I could be way off here, like I said in my O.P. I do not understand this circuit.
I will post back with the results when I receive the new 2n4091's. I was also hoping someone with this amp would chime in on normal behavior too. Thanks for all the help guys!
Update, subbed in NOS 2n4091
No change in function with the 'new' 2n4091. Proves the original part is good, also to back this up I did another continuity test on the original and both 'new' 2n4091's, all results are equal. I must have made a mistake with my original measurements.
I am back to square one on this, not knowing where to go next or if I'm chasing my tail on something that is normal function.
Thanks again to anyone involved in this thread and trying to help.
C25?
Another guy had a similar problem. He said C25 after the FET went bad. http://www.pacair.com/mmamps3/node/1866 Go to the end. Good luck. -mgriffin
Thanks for the reply and good
Thanks for the reply and good find. I tried subbing C25 with no success.
Dual Volume Pot?
Any luck figuring out the strange tremolo behavior? Question: has the volume control double gang pot been replaced or rewired or possibly miswired? One side is a 10k Linear taper and the other side is a 10k Analog taper. If the wires got swapped or mixed up or the dual gang pot was replaced with one with incorrect taper(s), it would work but would operate strangely. -mgriffin