Fender Acoustasonic II SFX problem

[RichT777]

I have a Fender Acoustasonic II SFX, brand new from GC. I tried to use it for its first performance on Christmas Eve at a church service. After it had been on for 30 minutes, with my iPod playing though the little RCA in section, the amp would go silent for 3 or 4 seconds, and then come right back for another minute, then cut out for another 3 or 4 seconds, etc. I tried it the next day and had the same deal after it had warmed up. Any suggestions on what might be wrong. And, anyone else have this problem with this model? Thanks for the enlightenment.

[Enzo]

Ball up your fist and whack the top of it. ANy effect?

This is a brand new amp with 5 year warranty. it doesn;t work right. Take it back for an exchange or repair under warranty. The dealer should be able to exchange from stock, especiallu in the first 30 days, but ANY authorized Fender repair shop can handle this.

[Dennisjo]

I have an Acoustasonic SFX, and it's audo also cuts out every few minutes, and then comes right back on again. It seems to cut out quicker if I'm playing it loud. TP14 does change state when this occurs. The main outputs and heat sink are almost too hot to touch. This particular problem didn't occur until I had it all apart fixing a pot problem. Could lathering on heat sink compound help this or I'm open to any suggestions? Thank you.

[52 Bill]

Could lathering on heat sink compound help this or I'm open to any suggestions? Thank you.

I doubt that more heatsink grease will help your problem. When you reassembled the amp did you correctly install the heatsink bar so that it and the output panel lies flat to the chassis?

[jpdesroc]

I have the same heating problem with a Customer Acoustasonic SFX 2..
It's a long one but clearly explained..

I'm trying to fix a 'heat' problem inside it.
The One with 2 channels like this one:
http://media.musiciansfriend.com/is/...00-500x500.jpg

The customer send it for repair because he was experiencing
intermittent pops and crackle in both channels..
Interestingly the front brown plastic layout were all the pots are
was badly disformed at some places like it had been heated
by some sort of reason..

So I downloaded the schematic for it here:
PDF manual for Fender Amp Acoustasonic SFX II

So I had a look inside to find that some of the electrolytic capacitors
had their outside plastic covering bent, melted and shrinked due to extensive heat
inside the amplifier.. (mostly the two 6800uf/63v power supply caps).
So I replaced these capacitors and put the amp on test
with 2 x 8 ohms dummy loads with some signal applied at around 60% of max power
one speaker on green/blue leads and the other speaker on black/white leads.
Well this amplifier got very hot rapidely..(after 1 minute)
The sine waveform applied was very nice at the output (no distortion).
But the inside thermal metal bar that all the power transistor are screwed on
and transfer the heat to the chassis was VERY HOT !
So I shut the amp off and waited for it to get colder..
Then I tried it ON with no signal applied.
Same thing.. very hot rapidely..(after 1 minute)
So I took some DC measurement at test points for each power amp (no signal applied)
and I found that the power transistors were drawing too much current at rest..
Each power amp have the same components arrangements.
4 power transistors on each amps.

On the first amp the 4 power transistors drawned each around 166mA on (+/-41vdc rails)
That's 41v x .166a = 6.8watts each at rest !!! 27 watts total of dissipation without any signal applied (??)
Around the same values were measured on the second power amp..
So, overall the metal bar that is used to dissipate the heat
has around 2 x 27 watts to dissipate at rest (no signal applied)!
--> This is why the amp inside was getting so HOT and the front plastic
--> layout shrunk and melted...
Here are the DC measurements I found:
Power amp #1 Power amp #2
Schematic value reading value Schematic value reading value
+Supply rail +46vdc +42vdc +46vdc same as amp#1
-Supply rail -46vdc -42vdc -46vdc same as amp#1
TP68 +43.0vdc +41.0vdc TP69:+43.0vdc +41.0vdc
TP70 -43.0vdc -41.0vdc TP71:-43.0vdc -41.0vdc
TP72 +1.15vdc +1.25vdc TP73:+1.15vdc +1.25vdc
TP74 -1.15vdc -1.23vdc TP75:-1.15vdc -1.23vdc

TP14 -34vdc -34vdc

Current in both R264 & R266: 166mA (6.8watts dissipated by each transistors Q30 & Q32)
Current in both R268 & R270: 213mA (8.9watts dissipated by each transistors Q34 & Q36)
Current in both R265 & R267: 166mA (6.8watts dissipated by each transistors Q29 & Q31)
Current in both R269 & R271: 166mA (6.8watts dissipated by each transistors Q33 & Q35)

All transistors drivers have been tested OK.
Again when a signal is applied both amplifiers gives a nice output waveform
but the amp at rest is almost smoking..

Can you help me on something I did not see ?
It looks like a design problem since both amps behave exactly the same way..

Any help ????
Jean-Pierre Desrochers

[Enzo]

It is idling too hot, look into adjusting the bias. Q15, Q16 are the bias circuits for each power amp. Note the bias is essentially the voltage across emitter to collector of each. The drawing specifies 2.3v. You have about 2.5v.You want to reduce that. If it is higher, bring it down to spec. if already there, try a little lower.