Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: magicD16 on June 13, 2019, 11:15:50 PM

Title: Fender Acoustasonic Jr. Squeal
Post by: magicD16 on June 13, 2019, 11:15:50 PM
Hello everybody,

I'm having an issue with a Fender Acoustasonic Jr (pre-DSP). For some reason, this thing squeals terribly if I have the master reverb over 4 AND press the "Master phase" switch.
The reverb sounds fine at max, as long as I don't press the "Master phase" switch. Any ideas what could cause this?
Title: Re: Fender Acoustasonic Jr. Squeal
Post by: phatt on June 14, 2019, 03:17:40 AM
Welcome D16,, :tu:
I assume you have an electric Acoustic plugged in when this problem happens and you are likely close to the amplifier?

If so then the *Phase switch is working as it should*,, that is why it's there to help stop Feedback. When using E/Acoustic guitar in close proximity to a speaker Feedback is always bound to happen. At higher volumes the phase switch may not be enough and it will squeal like a pig no matter what.  :'(

If it squeals with no guitar plugged in then Yes the Amp may have a problem, if not then the issue is likely the guitar not the amp.
All E/Acoustics exhibit this curse because they have a hollow cavity which will oscillate at it's resonant frequency. Larger bowels are more likely to squeal than smaller ones.
It may not be obvious to the casual observer but Ovation guitars have far less Feedback problems due to the rounded bowel.
The classic wood guitars may well give a better tones but with 2 opposing parallel sides the chances of feedback are very high once you amplify it.

I personally use a bowel back guitar for these reasons. As I'm mostly playing through amps Sure the unplugged sound is not as good as other guitars but once I'm plugged it's a big sound. For me THAT is when I want it to sound Great.

In My Observation if you want to play Amped Acoustics then Test for Fback problems before you buy.
Title: Re: Fender Acoustasonic Jr. Squeal
Post by: magicD16 on June 14, 2019, 11:21:57 PM
Thank you for your reply, Phil!

I was testing this amp with an electric guitar. It belongs to a friend and I am working on it for him. It came to me for non functioning reverb, which ended up a bad connection on the connector inside of the reverb tank that connects to the output coil.

I thought that it was a feedback problem created by my guitar, but it still squeals without anything plugged in to the input jack. That is, if you depress the "master phase" switch and turn the master reverb past 4. I might drive myself crazy with this issue.  :loco

I've attached the service manual/schematic. Any suggestions would be greatly appreciated!
Title: Re: Fender Acoustasonic Jr. Squeal
Post by: phatt on June 15, 2019, 09:07:04 AM
OK Obviously a Electrical problem and not acoustic FBack.

Ok the reverb was faulty and you fixed it but if you are not familiar with reverb circuits there are a few thing that can catch you out.
1/ check the tank leads for correct in/out. The owner may have tried to fix it and messed up the plug connections. So don't assume it was correct when you got it.
(some rev circuit will squeal if plugs are reversed)
2/ check your work and make sure you did not inadvertently ground the wiring on the drive end of the tank.

Note that the driver circuit (U9A) output is not grounded,, while (U9B) the Pickup circuit IS Grounded.  If the leads have been reversed it will likely squeal like a pig.

The Ground lifted output of U9A has to be reflected in the tank connections as well.
If you follow the ground connections in the tank you will (should) find that the tank Case is only grounded on the Pickup end.

If unsure how to establish correct in/out then rule of thumb is the drive transducer will be the lower resistance. Ball park readings would be ~30-Ohms for drive Tx and ~200-Ohms for PU Tx.
Only the Pu Tx is grounded to reverb tank if both Tx's are grounded to case then you may have  wired it incorrectly when you fixed it.
Title: Re: Fender Acoustasonic Jr. Squeal
Post by: g1 on June 15, 2019, 01:20:58 PM
Try again how you did it with the electric guitar plugged in, but set all the guitar controls (and reverb) to zero.  Still squeal?