Fender Acoustasonic - Low Volume On One Speaker

[earache]

Hi

I have a Fender Acoustasonic SFX that has a couple issues. It came to me with the complaint that the amp was "buzzy". A quick look and listen determined that the 8" upper speaker was blown. I verified this by swapping in a known good speaker.

While doing the speaker swaps it became evident that the 10" bottom speaker has very low output. Swapped in place of the 8" speaker it seems to have the appropriate volume, so it seems that the issue with the 10" speaker is the circuit supplying it, not the speaker itself.

I am curious as to what could cause the speaker to not have the expected output, any ideas?

I have the schematic and started to look at test points. Unfortunately I don't have a signal generator and so couldn't follow the print's instruction on how to test past Test Point 7. The results I did get for some of the test points seem weird though..

TP1 and TP2 are at the rectifier diodes. The schematic says 20.2VAC, I measured 39.5VAC

TP3 and TP4 should be +28.4VDC and -28.4VDC respectively. I measured +52 and -52VDC.

TP5 expects +15.6VDC and I measured -15.8VDC. I wonder if there is a typo on the schematic?

TP6 measured good at -10.3VDC TP7 measured good at -43.7

Hope someone can suggest some areas to look at to fix these issues, which (to review) are low output from one speaker and possibly strange voltage readings.

Thanks in advance for the help!

Acoustasonic_SFX_schematic.pdf

[The Dude]

I can't see the schematic link you posted, but I think you have the wrong one. Try this.

http://support.fender.com/schematics..._schematic.pdf

Edit: After review, that one isn't jiving either. I still contend you have the wrong schematic for the amp you're working on. What's the exact model?

[Jazz P Bass]

Here is the Acoustasonic SFX.

[earache]

Perhaps you are correct Dude, tell me if this jives with the schematic I have...the front of the amp says "Acoustasonic SFX" and the serial number tag says "PR 370". This is where I got the one I posted:

http://support.fender.com/schematics..._schematic.pdf

[g1]

Your schematic from post #4 should be the correct one. Look at page 4 for your TP1-4 readings. As for TP5, check pins 4 and 8 on one of the 4560 Ic's. If pin 8 is +16V, and pin4 is -16V, then the +/-16V supplies are fine and you don't need to worry about tp5 (but let us know if it's a typo).

Edit: I see there is a problem as they are using same TP numbers for different spots depending on whether you are looking at page 1 or page 4 of pdf.
I think the TP5 you measured is from pg4, as are your TP6 & 7 readings.

[earache]

g-one - you have pointed me to the correct page and now the measurements I made for TP1 thru TP5 all match the test points measurements as listed on page 4.

I measured Pin4 and Pin8 on U2 of the top pre-amp board and got -15.8 and +15.7 respectively, so the +/- 15 rails look ok.

So to ask the main question again, what would cause low output of the bottom 10" speaker - what to troubleshoot?

[The Dude]

I know you said you swapped speakers and the 10" speaker seemed fine in place of the 8" speaker. But, both the 8" and the horn are likely crossed over at some frequency. I am wondering, especially since the 8" was dead, if maybe the 10" has a shifted gap/frozen cone. It would make some sound at higher frequencies, but because of limited cone movement, not work at full range. Before we go on to troubleshooting the amp, can you make sure that the cone is not frozen on the 10" speaker? I am wondering if the amp was dropped.

[Jazz P Bass]

Also, it would help if you could place a quantity on 'low output'.

I know you said that you do not have a signal generator.
But a guitar will work fine if all you want to measure is the output voltage.

Place your meter leads on the 10" apeaker output wires, set your meter to read Volts/AC, turn up the amp half way & bang away on an E chord.
Then do the same for the front 8" speaker amp.

Let us know what you can come up with.

[earache]

After playing with this amp for a while and getting more familiar with it, I am beginning to think that the amp is working as it was meant to, and that the 10" speaker is meant to have less output.

When it came to me and I found the 8" speaker blown and the 10" speaker seemingly quieter than the 8", I assumed that the 10" had a problem which overloaded the 8" and caused it to blow. I mean, it's a 10" speaker, it's gotta be as loud or louder than the 8", right?

I have been using it with a good speaker clipped in place of the bad 8". I have discovered that the more you turn up the DSP return control, the 10" speaker gets louder.
I believe the 10" speaker serves the DSP function so as to add some stereo quality and more "air", and is not mean to be as loud as the forward facing "main" speaker.

There is nothing in the amp's manual that covers this feature...so I assumed there was something wrong with the 10" speaker. Now I don't think there is.

Anyone else reading this post have any experience with this amp?

I believe that the amp will be back to 100% once the replacement speaker I ordered arrives and is installed. Thanks for all your help and effort everyone!

[The Dude]

I don't recall working on one of these, but that means little. I see a lot of gear and have a horrible memory.

If you look at the schematic, the 8" and the horn run off of one amp and the 10" runs off of another. They are independent of each other in that respect. So, whatever happened to the 8" likely had little effect on the 10". The horn output is then further knocked down by R135 and likely there's a crossover cap on the horn driver or it's a piezo. (speculating) The 10" driver would likely be driven by a low pass filtered signal and used as a woofer. It would not seem as loud unless you fed the amp with a signal that contained a lot of low frequency content. You could verify this with a sweep generator or even music that has a lot of low end kick drum or something of the sort. Then, listen to the 10" driver and see if it starts moving some air. It probably doesn't do much with a guitar.

[Jazz P Bass]

[QUOTE=earache;364213]I believe the 10" speaker serves the DSP function so as to add some stereo quality and more "air", and is not mean to be as loud as the forward facing "main" speaker.

There is nothing in the amp's manual that covers this feature...so I assumed there was something wrong with the 10" speaker. Now I don't think there is.))

Actually the DSP function IS covered in the manual.
http://support.fender.com/manuals/gu...asonic_SFX.pdf
Page 6.
The DSP Instrument SEND and the DSP Return controls must be turned up past 1.
If you have no Send signal ,then the Return will do nothing.
It will be a Dry signal.
Likewise, if the Send is up then the Return mixes the Dry (no effect) & the Wet signal.

According to the Test points, both amplifiers should have the same output power.

[earache]

It's just counterintuitive. The main forward-facing speaker is smaller than the side mounted secondary speaker. What happens is that as you turn up the DSP return the side speaker comes into play more. For instance, the repeats of the slap back echo come thru the side speaker. It's actually a cool effect.

[Jazz P Bass]

I guess the amp would sound the best in a corner?