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.

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.

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.

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.

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.

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.

Here ya go.
Side-by-side of the test circuit and the original pcb... the green wire on the original is a repair.
Thanks for the encouragement J M!

Ok,
The prototype works great. I only tested it for a very short period of time as it was sloppy and had no heat sink, I didn't want to push my luck. Also, it was somewhat of a hack as some caps were seriously underrated and most resistors were not perfect values. If anything, I suppose this proves the rest of the amp's circuitry is ok and the problem is somewhere on the power amp pcb.

I will order new, proper parts and build a new replacement power amp pcb. The original is too beat from multiple repair attempts.
Since the original pcb was a separate board mounted to the heat sink all by itself, I won't have to hack the power supply pcb or the heat sink.

After sufficiently testing the repaired amp I'll let you guys know how it all held up.

Thanx teemuk!
First, i will breadboard the power amp as i drew it and plug it into JB1's associated jumper.
If the amp works, i know i have a bad chip amp or some other component on the pcb.
If not, i'll have to dig deeper into the other boards.
I feel like i can make some progress now.

Thanks guys,
Got the Limiter built. Revised the sketch (attached). Tested the circuit. No glowing bulb, no DC on the output. Supply voltage good. Good logic level on pins 9/10. Sound thru speaker is still weak and very distorted.

What else can I test? Thinking about prototyping a test circuit with the new chip amps rather then just going ahead and soldering one in and risking losing it...again. Wish these things fit in a breadboard! How do you guys prototype a circuit with these things?

Anyway, here is the schematic I came up with. All of the 20k resistors are in a smd. That, and a few other devices block my view of the board, so it was difficult to trace. Hope its right.

To hell with caution i suppose, seeing as how the moderators are not stopping me.
Yes, its a line 6 Spider IV 75. I have checked and re-checked the schematic i came up with against the actual pcb and this is the final outcome. I will give another check right now...*sigh*. Not too proud to admit i can make mistakes...already admitted to frying the first replacement chip stupidly. But, can you at least tell me why you think there are mistakes as that will help me narrow down where to look?
Thanks,
JBro

OK...Thanx for the explanation on the rules.

There are 3 more boards..Power supply, DSP, User interface.(pre-amp on DSP board)
Jumper JB1 connects to the PS board.
The DSP has 10 wires jumping to it from the PS board.
There is a headphone out and aux in (both stereo) on the DSP board.

The amp seems to work because I get a good sound thru the Hphones (with guitar or mp3 thru aux). Just a really degenerate sig thru the speaker.
I have to admit, i replaced the 7293 once and had the amp working great.
Then, like an idiot, i sparked the heat sink tab of the 7293 on the chasis (GND).
I then replaced the 7293 and the amp did not work... I am hoping the second 7293 was just bad (got more on order). But just in case, while i am waiting for the new one, I wanted to get a jump on replacing the entire board if needed.

Also, i may want some advice on building a test circuit for the power amp plugged into JB1 and i'm not sure what exactly is going on there with all the apparent inputs.

Anyway here is the Power Amp PCB.

TDA7293 supply voltage bypass caps

I have a guitar amp that I am fixing for myself from a manufacturer that does not make repair info public...no schematics, no parts. So i reverse engineered the power amp stage but am reluctant to post it in case i break the rules of the forum. So instead i will describe a circuit in the stage and ask for some advice on it.

The stage is based on a single TDA7293. The amp is rated 75 watts thru a single 12", 8 Ohm speaker.
It uses +48V for +Vs at pins 7 & 13
It uses -48V for -Vs at pins 8 & 15
Following Fig. 1 'Typical Application and Test Circuit' in the datasheet, the values for the supply voltage bypass caps are as follows:
C6, C8 - supply voltage bypass for both +Vs and -Vs = 47 uF
C7, C9 - supply voltage bypass for both +Vs and -Vs = ?? uF

C7, C9 are surface mount and appear to be beyond the range of my multimeter, which goes to 20uF AND i am not sure if i can get an accurate reading of the caps IN CIRCUIT (same as datasheet).

My question is, is it possible to guess at the appropriate value of C7, C9 based only on the values of +Vs,-Vs,C6,C8 and the values shown in the datasheet?
If so, how would i determine these values? Can anyone tell me what they should be and why?

Thanks,
JBro