TDA7293 supply voltage bypass caps

[J M Fahey]

Not so good, eh!!
Yours got a little larger !! :lmao:
I would google for some 7294 board and just use it, if space inside the chassis permits it.
NOTE: you got *very* lucky, Protoboards are *not* meant for Power electronics, since the hole contacts are relatively weak.
Now that you know what the real problem is, stop using that Proto in that way and go for the real world PCB.
Congratulations. :dbtu:
PS: post some amp chassis guts pictures, to see the heatsinks and have a general idea of the available space inside.
Put a (readable) ruler inside so we can have an idea of actual distances.

[JBro]

Yeh J M, that was just a test circuit that i built to troubleshoot the amp. I never intended to use it as anything else, and only very briefly at that! Pretty nasty looking, heh?  But it did the trick. Quick and dirty.  :cheesy:

Anyway, there's lots of room in there for the new power amp board. I was thinking of using a pre-punched padded circuit board which i have a piece of leftover from something.  But i'll check into that board you suggested. I thought about etching a pcb, but...I don't think I need to go that far.

Parts are ordered.

[JBro]

Big Bummer... The amp is NOT fixed. Its better, but not ALL better. The replacement circuit is built and installed. The sound is perfect at low volumes, but as the volume is increased and/or the harder I play, the more the light bulb glows on the limiter, though the sound stays good with limiter in line; It can get pretty bright.

To my inexperienced mind, the power amp circuit is fine, but another circuit elsewhere in the amp feeding the output stage is broken...I'm hoping I damaged only a discrete component when I sparked the heat sink on the chassis a while back.

I suppose I will have to reverse engineer some more...not looking forward to that. In fact, not sure its worth it to go much further and am contemplating putting this project on hold as I am super busy with other things and just wanna PLAY in my free time.

I will go as far as breaking down the PS Board (see post 1 for a description of boards)...maybe there is a bad diode on there? That would be so easy...

Any suggestions on how to trouble shoot from here would be appreciated. I am hoping for simple, small steps right now...guess I'm trying to stay off the DSP Board.

[J M Fahey]

The sound is perfect at low volumes, but as the volume is increased and/or the harder I play, the more the light bulb glows on the limiter, though the sound stays good with limiter in line; It can get pretty bright.

YOUR AMP IS FINE.

The limiter is doing its job, limiting current fed to it.
It sounds good but you probably can´t pull more than, say, 15 W out of it.
Plug it straight into the wall outlet and post results.
Good luck.

[JBro]

Hey J M,

Thanks for responding. I thought when I saw the bulb burning brightly, that it meant something was wrong with the amp so I stopped playing immediately and shut off the amp.

After reading your response, I decided to be cautious and play louder and longer thru the limiter before plugging straight into the wall. Here's what happened:

I slowly turned up the channel volume, master and drive and played harder over time. The bulb responded by burning brighter as things got louder. I had all three controls at almost 12 o'clock and was striking the strings hard. Eventually the amp's output went from a clean sound with a bright white bulb to distorted and soft with a dimly lit orange bulb. As the output degraded more and more, the bulb burned dimmer and dimmer.

Apparently, something somewhere is failing as things warm up.

[J M Fahey]

A:

The bulb responded by burning brighter as things got louder.

B:

Eventually the amp's output went from a clean sound with a bright white bulb to distorted and soft with a dimly lit orange bulb.

These 2 statements are contradictory.
A is right.
B is wrong, it´s the exact opposite.
It should go from low power, clean sound,dim bulb, to higher power, mushy sound, brighter bulb, in rhythm with the music.
Please confirm.

PS: is the Power chip firmly bolted to heatsink, with proper grease and mica?
Does it get warm or unbearably hot?
Please post a picture.
Good luck (I think your amp is fine, just you are being over cautious)

[phatt]

I realize that these things are hard to describe and I maybe misreading but
From what you describe it Sounds Normal?

BTW if you keep the light bulb in place you have power supply sag just like early Valve Amps.

Phil.

[JBro]

Thanks guys.

I am being cautious. Its my last chipamp.

To clarify, drive, channel, master...all at 12 o'clock. Playing bar chords hard.
Things start out normal: clean sound, bright bulb. With a little time: distorted sound, low volume, dim orange bulb.... all without changing the controls. (In time with strumming).
Guess I do not wanna plug into the wall yet without confirming. Seems like something is failing with higher power in response to heat.

The power chip is in place, clamped firmly to the heat sink, with insulator in place, just like the original was. Don't know temperature as I have the chasis covered....will check.

[phatt]

Hi Jbro,
         I'll assume you have the *Whole Amp* connected when you do this light bulb test?

My guess is the preamp section is *Regulated* and the symptoms you describe tell me the preamp section is running out of regulation and the signal will be very distorted and very little energy will get to the power amp anyway but that does not mean the poweramp is dying.

If the power Amp was drawing too much current the bulb would glow very bright.
Which is why Mr Fahey asked for clarification.

Here's my thoughts,


Disconnect the preamps and all power supplies *except for PowerAmp*.
Now with the limiter bulb still connected Send audio into power amp chip (via an ipod or something)

(The idea here is to Remove the preamp circuitry from the test as you are trying to test the power chip,,, NOT the whole circuit.

The bulb should glow brighter on strong signals and just a dull glow with no signal.

You won't be able to get full power with a limiter in place and it will probably start to distort if the input audio is too strong.

Now with a medium strength signal If it still does the big loss of power and very dim bulb then something deeper maybe wrong.

Example;
if you slowly increase input signal and the Lamp suddenly glows really bright and severe loss of output power (often accompanied with humming power transformers due to big current drain on the power rails) then you have a big problem.

But if you slowly increase input signal and the bulb gets brighter and then the signal starts to distort a bit but the bulb just stays at a steady glow then the Amp is acting normally and the limiter is doing it's job.

IF that's the case then remove limiter,, play audio again and you should get full clean power.

At that point re-connect the rest of the preamp section back up and cross your fingers.

Of course I reserve the right to have missed something. wink.
best of luck,,,Phil

[JBro]

Thanx Phil,

Don't think I know enough to bypass the preamp at this time without schematics of the amp.
I don't know how to deal with the inputs on the header (4 into 2) cause I don't know whats on the other side and do not know enough to come up with an input solution at this time.
There is no effects loop either. The preamp is on the DSP pcb.

J M,
The chip does get hot...I am guessing too hot for the amount of signal its been getting from the guitar.
This makes me think there is too much feedback gain.

I took a closer look at the schematic i came up with and compared it to the one teemuk posted.
I don't like the large values I came up with for C6 and C7. They are HUGE in comparison.
I am changing them out for smaller ones as a first resort to see if this cuts back on the heat.

[J M Fahey]

Post your C6 and C7 values, I don´t know what you are talking about, anyway they do not influence heat but frequency response.

[JBro]

OK, J M... just thought the larger cap values might have an affect on the amount of current and voltage (thus power) feeding back into the amp.

New values:
C6 = 140pF
c7 = 57pF

After replacing the caps, I plugged the amp into the wall and slowly turned up the drive, channel and master controls letting the chip heat up slowly. Heat dissipated nicely into the sink. 15 minutes later, at 1 o'clock for all knobs the sound degraded into low volume and distortion.

Next, i let the amp cool down completely over night, leaving the settings at 1 o'clock. I then turned the amp on and played. The output degraded after a minute or so. The heat sink barely started to warm up where the amp connects to it. In fact, almost all of it was still cool.

So, being plugged into the wall, does this avoid the preamp from "running out of regulation" as I am guessing it does?

Phil is right, I need to test the circuit I built on its own. I do not know how to do that yet.