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Post subject: Fender Acoustasonic SFX 2 'heat' problem...
Posted: Wed Sep 20, 2017 6:48 pm
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Hi,
Hope you will take some time to read my problem here..
It's a long one but clearly explained..

I'm trying to fix a 'heat' problem inside a Fender Acoustasonic SFX 2.
The One with 2 channels like this one:
http://media.musiciansfriend.com/is/ima ... 00x500.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:
http://www.umlib.com/brand/Fender/categ ... ent/313992

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..

Thank you.
J-Pierre


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Post subject: Re: Fender Acoustasonic SFX 2 'heat' problem...
Posted: Sat Sep 23, 2017 8:33 am
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Joined: Thu Aug 07, 2014 3:25 pm
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The test points 72-75 at 1.15 vdc agrees with the schematic. Since both channels are similar, it is probably a design issue, as you suspect.
That's not to say it's an error. A hot bias improves sonic quality, so it may be intentional.
That is not to say you can’t cool down the amp. There's a recent post on this forum about installing a fan to prevent thermal shutdown.
The other approach is to lower the bias by adjusting the resistor divider network around the bias control transistors, Q15&16. These transistors need to be turned on a little more to lower the bias.
R253 is a 33 ohm resistor feeding the base. Its a good candidate to get a feel for how much you will have to change values.
Lowering this value will turn down the bias. As a test, you can momentarily short r253 to measure the effect on bias with a clip lead. You can also parallel another resistor across R251 to lower the bias. Start with 100k. You can also raise the value of r255, but that probably requires pulling the board.
While the test points will show bias changes, the commonest method of measuring bias is at the emitter resistors of the output transistors. Clip a dc voltmeter across the emitter resistor to measure the voltage drop. The current is inferred by the voltage drop and will be in the low millivolt range. This model has relatively low values at .15 ohms. Most use .5 or .22 ohms.
An amp with .5 ohm resistors runs nominally at .003 to .01 volts drop across the emitter resistor.
With .15 ohms, a nominal value would be .001 to .004 volts drop. You are looking for enough bias to prevent crossover distortion and a cooler heat sink at idle.
All bias tests should be from a cold start. There are bias diodes mounted in the heat sink. These should be coated with silicone grease to track thermal changes rapidly. The bias will rise as the heat sink warms up.
Good luck.


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Post subject: Re: Fender Acoustasonic SFX 2 'heat' problem...
Posted: Sun Sep 24, 2017 9:22 am
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Joined: Fri Dec 05, 2014 11:40 am
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TimsAudio wrote:
The test points 72-75 at 1.15 vdc agrees with the schematic. Since both channels are similar, it is probably a design issue, as you suspect.
That's not to say it's an error. A hot bias improves sonic quality, so it may be intentional.
That is not to say you can’t cool down the amp. There's a recent post on this forum about installing a fan to prevent thermal shutdown.
The other approach is to lower the bias by adjusting the resistor divider network around the bias control transistors, Q15&16. These transistors need to be turned on a little more to lower the bias.
R253 is a 33 ohm resistor feeding the base. Its a good candidate to get a feel for how much you will have to change values.
Lowering this value will turn down the bias. As a test, you can momentarily short r253 to measure the effect on bias with a clip lead. You can also parallel another resistor across R251 to lower the bias. Start with 100k. You can also raise the value of r255, but that probably requires pulling the board.
While the test points will show bias changes, the commonest method of measuring bias is at the emitter resistors of the output transistors. Clip a dc voltmeter across the emitter resistor to measure the voltage drop. The current is inferred by the voltage drop and will be in the low millivolt range. This model has relatively low values at .15 ohms. Most use .5 or .22 ohms.
An amp with .5 ohm resistors runs nominally at .003 to .01 volts drop across the emitter resistor.
With .15 ohms, a nominal value would be .001 to .004 volts drop. You are looking for enough bias to prevent crossover distortion and a cooler heat sink at idle.
All bias tests should be from a cold start. There are bias diodes mounted in the heat sink. These should be coated with silicone grease to track thermal changes rapidly. The bias will rise as the heat sink warms up.
Good luck.


I'll try to fiddle with both 2.43k 1% and 33ohms resistor feeding a small sine signal
to see what it causes as crossover distortion vs lowest idle current.
I'll get back to you..


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Post subject: Re: Fender Acoustasonic SFX 2 'heat' problem...
Posted: Wed Sep 27, 2017 3:48 pm
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Joined: Fri Dec 05, 2014 11:40 am
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I finaly solved that design problem..
I started to place some resistor values in parallel
with each R250 and R251 (2.43k 1%) and I finaly
found the sweet spot resistor which is 10k 1%.
So placing one 10k 1% resistor in parallel with each
R250 and R251 drops the power transistors idle current to normal
and sets metal bar's temperature cold at idle (no signal applied).
I guess the schematic value of 2.43k 1% worked for some
SFX 2 amps but is aside with some other and those will
get VERY HOT at idle !! Be aware.


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Post subject: Re: Fender Acoustasonic SFX 2 'heat' problem...
Posted: Thu Sep 28, 2017 8:07 pm
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Joined: Thu Aug 07, 2014 3:25 pm
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So have you noticed a difference in sound quality?
Fender solid state amps seldom have adjustable bias. It prevent more problems than it causes, usually.
Most run at a low bias. Yours may have had a change in gain to the bias transistor. If it lost hfe, the bias would go up.
The bias transistor is critical to the stability of the amp as it warms up. The output transistor gain goes up as it heats up. The bias transistor also heats up and raises its gain as well. As the bias transistor turns on a little more with heating, it lowers the DC on the base of both driver transistors to compensate for their higher gain.
The bias transistor is selected for its gain curve to match the gain curve of the outputs. Its a dynamic relationship between them. The transistor specs can't be allowed to drift and you can't use substitute numbers. Even the same number does not mean its the same gain as the installed version's original gain was. Different manufacturing batches have different hfe ratings.
If the specs have drifted, the transistor will not track the bias properly. While it may be ok cold, rising temps will send the bias way too high when warmed up and rockin.
So set the bias cold. Run a signal through to warm it up. Turn off the signal and measure the bias again. Then turn it up and get the heatsink hot and measure the bias to see how well it tracks.
A drift from .003vdc cold across the emitter resistors to .01 hot is not a problem. If it goes over .05, it could be.


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