Acoustic 470

[gbono]

Acoustic 470 which seems to have several issues. First off, the vibrato is bleeding though even with the intensity turned down and footswitch used to turn the feature off. You can just barely hear it audibly and it shows up on a scope modulating the input signal. I believe the feature is always “on” and the footswitch turns it off?

Next problem and it’s possibly related to the above issue, is that there is a residual distortion that can be heard especially after playing single notes. Sounds like very mild crossover distortion. The distortion pot is switched all the way off – switch engaged. Sinewave input doesn’t show crossover distortion at the output.

The preamp boards are a real pain to probe in the chassis since there are a few dozen wires soldered on the edge terminations and they like to break when moving the boards. Looking from the input onto the 170043 board, it looks like I shouldn’t see a change in the signal comparing the outputs of Q102 and Q104 – that’s my first measurement. Any other ideas?

[soundguruman]

Well the acoustic amps* are the most reliable amps ever made. Perhaps the makers of Crate amps could learn something.
But even at some point, stuff starts to fail. I mean, that thing is really old, but not as old as me.
But they had edge card connectors, that somebody removed, and soldered the wires directly to the board. This is because after 30 years the connectors probably turned green and failed.
So what you really need to do is disable the vibrato completely. And then, get the bias working again.
Seems like the bias is going too high and causing the crossover distortion.
Maybe the foot switch has failed?
Seems like there is an adjustable bias control already on the power amp board (label "view A") and an output drive balance control.
And it seems like those controls need to be cleaned really well, (maybe replaced) then adjusted using a scope. The controls are probably corroded, therefore you have lost your bias adjustment.
* please do not confuse the REAL Acoustic amps with the NEW acoustic amps. The NEW acoustic amps are not really acoustic amps, but a cheap imitation.
You can build a crate amp and put an "acoustic" name plate on it, but it's still just a crate amp.

Another distinct possibility is that the capacitors are too old and you are losing your voltages, the voltages are too low. That would explain it for sure. You may need to replace all the old electrolytic capacitors, so that the voltages will come back up again.

[gbono]

There is a bias adjustment in the tremolo osc section - I'll check this out. The PA board was completely rebuilt by me and the trimmers for sym and bias are new. I thought I set this up already but I'll check again with scope and KillaWatt. I'll also recheck the "sag" on the PS - it wasn't an issue before but I haven't worked on this thing in months.

Yes the "new" Acoustic company (something to do with Guitar Center as I remember) seems to have only reproduced the "brand" in name and not the spirit of the 1970's organization. BTW have you seen the other "new" Acoustic company that is remaking the 360/361? I wouldn't think that a $5K quasi horn loaded cabinet would be that popular these days.

[gbono]

Fixed the distortion issue by readjusting the bias pot with the help of a KillaWatt.
Now I'm trying to dampen the tremolo bleed thru - R236 on the 170046 appears to have a big effect on the overall preamp gain. Assum that's because the tremolo is the last stage before the line amp and output to PA board. The preamp (170043) shows 15 mV at the input and it looks like .26V at the tremolo section output Q206 (170046). Anyone have an idea how to set this up?

I'm also seeing some weird osc on the output of this amp. It is isolated and generated by the preamp and not a problem on the power amp. Looks like it's above 16kHZ. Seems to decrease when the reverb is turned up? I'm guessing its also compounded by the tremolo bias/gain issue - too high of gian in this section?

Multiple problems as usual.

[soundguruman]

Fixed the distortion issue by readjusting the bias pot with the help of a KillaWatt.
Now I'm trying to dampen the tremolo bleed thru - R236 on the 170046 appears to have a big effect on the overall preamp gain. Assum that's because the tremolo is the last stage before the line amp and output to PA board. The preamp (170043) shows 15 mV at the input and it looks like .26V at the tremolo section output Q206 (170046). Anyone have an idea how to set this up?

I'm also seeing some weird osc on the output of this amp. It is isolated and generated by the preamp and not a problem on the power amp. Looks like it's above 16kHZ. Seems to decrease when the reverb is turned up? I'm guessing its also compounded by the tremolo bias/gain issue - too high of gian in this section?

Multiple problems as usual.

The footswitch is supposed to ground the cathode of CR 201, which is supposed to be shutting off the collector current of Q 205....apparently the footswitch is not shutting it off?
Or...the transistor is staying on by itself, or partially on enough to make the pulsing noise?
Try to verify the parts in that circuit starting with the footswitch. Is one side of R 231 grounded when you operate the footswitch?

[gbono]

The footswitch is supposed to ground the cathode of CR 201, which is supposed to be shutting off the collector current of Q 205....apparently the footswitch is not shutting it off?
Or...the transistor is staying on by itself, or partially on enough to make the pulsing noise?
Try to verify the parts in that circuit starting with the footswitch. Is one side of R 231 grounded when you operate the footswitch?

The amp seems to have been designed so that the features are normally on and the footswitch is used to turn them off. In other words the features (tremolo, reverb & distortion) are all enabled when the footswitch isn't present. BTW the original footswitch for this model is almost worth more than the amp itself.

I'm charging on with the tremolo bias adjustment experiment. THX

[soundguruman]

Any footswitch can be substituted, with some creativity, or the footswitch jack wire on the unit must be grounded.
Ground that wire, you see it on the schematic?
Have you CONFIRMED that the cathode of CR201 is grounded? That should shut it OFF.
Do this FIRST. Does grounding that wire shut it OFF or NOT?
Yes you can adjust the bias of the tremolo, BUT the pulse for the tremolo should shut OFF and the pulse should STOP.