Fender Champion 40 schematic needed.

[caverat]

Need schematic for Fender Champion 40. NOTE: This is a new amp, not the original series from Fender. This amp uses a switch mode power supply and IC Output. The Fender website does not list a schematic, and I was told they do not repair, only replace under warranty. (The amp I have was one of those discarded under warranty) The amp sounds fine at first, but runs hot. After about 15 minutes it overheats and shuts down. Need schematic to determine proper voltages from power supply as it may be running too high.

[nosaj]

Need schematic for Fender Champion 40. NOTE: This is a new amp, not the original series from Fender. This amp uses a switch mode power supply and IC Output. The Fender website does not list a schematic, and I was told they do not repair, only replace under warranty. (The amp I have was one of those discarded under warranty) The amp sounds fine at first, but runs hot. After about 15 minutes it overheats and shuts down. Need schematic to determine proper voltages from power supply as it may be running too high.

What do you see when you look at it? Anything discolored? Heatsinks not properly mounted with grease? etc.

In lieu of a schematic things can still be done. Like looking up the IC datasheet which would tell you approximate voltages it should see. Sometimes its a lot of research checking datasheets, but persistence will sometimes win out.

Check thise solder joints maybe one is opening when it gets hot.

nosaj

[J M Fahey]

Agree.
WHAT IC does it use?
Not impossible but rather than a rogue supply, I would expect, say, an unstable oscillating power amp or other conventional problems.

What happens if you disconnect speaker?

[caverat]

Nothing obvious appears burnt / loose / out of the ordinary. Amp is a combination of discrete components and surface mount so that is also a factor. I did pull up the data sheet on the output IC. Type is a TDA 7294. supply voltages are max + /- 50 VDC. The power supply is putting 45V plus and 47 V neg on the chip; there is some instability, however if the current draw is excessive that may account for that. Speaker was changed out just to verify no shorted turns in VC. No help there. One thing of interest is the amp will run OK as long as no signal is applied, but even with minimal output it heats up very fast. Also confirmed no parasitic oscillations on a scope, output is very clean with no clipping or other issues. ( I am aware that the TDA 7294 is prone to oscillations if caps are bad, was hoping to get schematic to help identify which caps are which in the circuit. May just have to do some reverse engineering if one is not available.)

I would like to isolate what is wrong if for no other reason than to identify it. (Don't really need an amplifier, but since this is a strange problem I would like to know what the fix is in case I run into a similar problem. Besides, the amp didn't cost me anything; it was new, and discarded per Fender warranty instructions)

[Enzo]

Did you contact fender and ASK if they could provide the schematic? Do not repair lists only mean they won;t pay for warranty repair work, not that the amps are not repairable or that they will not support them. I have schematics for most of the do not repair stuff I ever encountered. And warranties expire. There will be amps needing repair.

Please do not tell Fender you have a "discarded under warranty" unit. The rules are that dealers or service centers are to DESTROY the dead units after harvesting any useful parts. You can tell them you need to repair this for someone whop has no warranty or for a relative.

An overheating amp is probably not due to "wrong" voltages. If your amp produced 55v instead of 45v on the main rails, what would you do about it anyway?

You should be able to determine what is overheating, most likely the power amp IC. It may work, sound OK, and otherwise seem good, but it still can be defective. I'd be inclined to try a new IC there.

[Jazz P Bass]

I have seen the TDA series ic's 'pull down' the +Vdc rail when things are going south internally. (ie: not fully blown)

Don't know why that is though.

[g1]

Is the output IC correctly mounted to the heatsink? Is there heatsink grease or a silpad?
If there was a goof up at the factory it sounds like it probably would have worked long enough to pass QC.
Agree with above though, if voltage is being pulled down, the IC is probably bad.

[caverat]

I will order in an output IC, only $6.60 from Digi-Key on my next part order. Worth taking a chance, worst that can happen is I wind up with a spare output I.C. The thing that gets me is why it doesn't overheat till a signal is applied. Usually overheating is caused by a loss of the internal bias network which causes excessive stand by current flow through the output transistors. But I suppose leakage is possible in either. Whatever the reason, it is all inside the chip; no fixing that!

As a tech it just interests me as what is wrong, not that I have any use for a "destroyed" Fender amp. Call it a learning experience; I probably will still wind up harvesting usable parts in the end anyway..... Fender does not supply a schematic or replacement parts. Since this is a current model with a 5 year warranty they just do as Enzo said, replace to the customer and destroy the old one. Thus no repair is anticipated.

[J M Fahey]

Just for mental curiosity:
IF amp sits idle with no problem, but when fed audio it starts overheating, then some change must exist.
Heating means energy is being delivered to the amp and being dissipated as heat, energy which was not getting into it otherwise.

* One possibility might be that it´s unstable and starts oscillating, but you already ruled that out. Mentioned only to avoid losing count.

* or output rail might be shorted somehow.
Just for kicks disconnect speaker and remeasure its VC resistance, we expect around 6 or 7 ohms if 8 ohms or around 3 if 4 ohms.

or output connector/wire might be shorted.
Turn amp OFF, wait for few minutes for all caps to discharge and measure resistance from IC speaker out pin to ground.

Then turn amp ON, still without speker, and check for DC at the output, we expect less than 100mV DC

That said, *may be* the amp is bad but muted or in standby, and when receiving audio it enables itself (or is enabled by some auxiliary circuit) and then the fat falls in the fire.

Anotherb test: plug the amp in a lamp bulb limiter.
Per your description I guess bulb will blink and barely glow afterwards.

Now send signal, still without speaker: does bulb stay about the same or it jumps brightness up?
We are using it as a crude mains current meter.

[Enzo]

My similar thought was to monitor the mains current draw for real. The bulb is quick and simple but might mask an instability. But a current meter in the mains line should be transparent enough to see if the presence of signal spikes current.

[caverat]

Just to fill you in on what I have done today:

I broke the supply leads to the TDA chip on both rails and inserted a 1 ohm resistor in each to allow for voltage drop/current measurements. I also added a 1000 uF cap on the chip side of each for decoupling to prevent possible oscillations. The amp was powered and appeared to work as previously; heating up quickly when a signal was applied.

Next step was to monitor the current flow on each rail under those conditions. Current rapidly increased as signal level increased, however the speaker bootstrap remained near ground potential. The signal was measured at .25VAC across the speaker and audio was clean. The draw from the supply was well above what should have been present for that audio level, almost 750 ma. thus it appears the signal was affecting the bias on the output transistors inside the chip. Reducing the signal level allowed this current to also drop, however it did not fall to original levels when first powered up. I again raised the signal level while monitoring the current. It continued to increase as the chip heated up. Eventually a point was reached where cutting the signal had no affect; and the chip went into a thermal runaway condition. At this point I discontinued the test, will replace the chip and see if this fixes the problem.

Another question was also resolved, I wondered why this did not cause a catastrophic failure as one might expect in a standard amplifier configuration. I found that as the load increased the supply voltage dropped considerably. Normally with a transformer power supply this would be where the resistors smoke or the fuse blows. But in this supply the voltage dropped which seems to have provided a form of current limiting. Thus no smoke. I will see what happens once I obtain and replace the IC and try again.....

[caverat]

Just as an update to help anyone who may encounter a similar problem. The amp is now working after the output IC was replaced. This was a case of oscillation, that was not detected on a scope earlier. I did check the output under normal operation and everything appeared normal. In fact what was happening was the scope itself as soon as it was connected provided sufficient load to block the oscillation. As soon as it was removed the HF returned.

I did a similar measurement, only placing a 1 meg resistor in series with the scope lead and caught it. What was seen was a ringing on top of the audio signal. This ringing was at about 50 kHz, well outside audible range. I used a Tek 2215 scope to catch it; that scpe has a 1 meg input impedence, but apparently the capacitance of the scope lead was enough to kill the HF, making it appear the amp output was clean.

As of now the amp is working good, no HF ringing or other anomalies, output IC gets warm under load, normal though, not overheating with runaway. Something to consider if you encounter something strange like this; decouple your probe at the point of contact ahead of the lead. While this will skew the accuracy of amplitude / voltage readings, the presence of an unwanted signal may be found. Such a condition is in error if ANY signal like that is present; voltage level not with standing.

[g1]

Were you using a 10x probe? They will greatly reduce the probe capacitance.

[cgoepel]

Hi folks,
I also have a Champion 40 that shuts down after a few minutes. Following the thread above, I replaced the TDA7294, an analog output IC. Replacing the output IC didn't fix the problem. The amp shut down after a few minutes. Even the pilot light goes out. The power supply is a switcher. All rails go to zero when the unit shuts down. Apparently the power supply goes into protect mode. I don't have a schematic.
Any ideas???

[Enzo]

Did you CALL Fender and ask for a schematic?