| Author | Message | ||
| Doug Elick |
The tremelo is dead on my HD-130 (early 12AX7 model). Adjusting the knobs does nothing but deaden the volume of the amp and make it sound scratchy. Any common culprits? Just to be sure, if no pedal is plugged in, the trem and vib. controls are always in an "on" state, correct? Thanks, Doug | ||
| michael kaus |
The foot switch is NOT needed on MM amps to make it work. There is a po t on the inside of the chassis that does provide some adjustment for the trem. Be carefull not to get the bias adjust pot- it's right there too> Mike. | ||
| Doug Elick |
Something definately sounds blown. As the intensity setting approaches 5, the volume decreases and gets very distorted---like a destroyed speaker. The rate knob does nothing. My research on this site and elsewhere has turned up the LM307 or the 2n4091 & 2n3391 transistors as common failure points. Anything else I should look for? Just trying to get a plan of action together before I open this thing up. Doug | ||
| Steve Kennedy |
The 2N3391 is your tremelo oscillator. If that is producing the cyclic changes associated with Tremelo, then that circuits is working. No audio passes through that part of the circuit. The oscillator output signal is applied to the gate of the FET 2N4091 (TR-1) which "modulates" the output of the reverb circuit (making the audio signal louder and softer by bleeding signal to ground, the automatic equivalent of cranking a volume pot up and down. This should also not cause distortion (as audio is not passed through the FET) UNLESS it is leaky and is passing DC voltage to the op-amp input (through a leaky cap C25) which is where it is most likely your distortion is occurring (even though the op-amp is probably working OK). The FET and/or C25 would be my first guess and the LM307 op-amp that follows would be my second guess. A third guess would be to check the LM307 op-amp BEFORE the tremelo circuit (reverb circuit output). Steve | ||
| Doug Elick |
Actually, there is no cyclical effect at all; just an increasingly weak, distorted sound as the intensity control is moved up to 5, then improving again as it is moved towards 10. Perhaps 2N4091 is a smoking wreck and is bleeding DC to the OP amp while effecting no changes to the audio signal because it's in effect, "ignoring" 2n3391. Or maybe the whole &^!$ circuit is toast. Only one way to tell; I suppose it's time to get some more desoldering braid. As a note, reverb works beautifully (after I fixed a broken wire in the tank). Thanks, Doug | ||
| Doug Elick |
P.S. Where would you test for the oscillating voltage being generated by the 2N3391? Thanks, Doug | ||
| Steve Kennedy |
I'd look with a scope (if you have one) at the gate of the FET, where C28, R42 & R43 all come together. The only problem is that you may not see it at all if the FET (2N4091) is shorted. Steve |
Tue, 03/25/2014 - 09:59
#1