Available schematics for the Marshall MG50CFX amp?

[plugbark]

Hi everyone!

I replaced the blown TDA7295, so that I make some measurements.

The input signal is a 440 Hz sine wave with an amplitude of 200 mVrms.

The screenshots below show the output signal (Loudspeaker Out) based on different gain, volume and master settings :

1. Gain: 6, Vol: 8, Master: 2


2. Gain: 7, Vol: 8, Master: 2


3. Gain: 9, Vol: 8, Master: 2


4. Associated FFT (Setting n°3)


The critical level is about 20 Vrms. The replacement of TDA7295 did not change the sound compared to before. It's probably something else that causes this problem because it's not normal that the signal gets distorted this way.

What do you think about that?

Thank you :-)

[nickb]

Have you measured the power rails to the TDA chip? If that is OK an you probe the input to it somewhere?

EDIT: I was looking at the schematic in his thread that was meant to me the MG50CFX but I see it is actually the MG100FX so I guess it's the wrong one.

[J M Fahey]

I think (and I hinted that at earlier) that you do NOT have any problem, at all.

I asked

Do you get a distorted signal at all levels? (send us an image) or you can get, say, 5 or 6 V RMS clean and it istorts at , say, 15 or 20V RMS?

and although you did not answer my question, at least you stated that:

The critical level is about 20 Vrms.

and proceeded to show 3 screen captures:
1) showing just reaching the edge of clipping and 20.2V RMS
2) showing visible clipping on the top wave side and reading 23.2V RMS
3) showing both top and bottom clip`pingand showing 26.8V RMS.

I repeat my question: what seems to be the problem, ? because I see none.

[nickb]

Juan has a point 20Vrms into 8 ohms is 50 Watts. I would have expected more symmetry in the clipping which is why I asked about the power rails. Do the test again into a load and lets see what you get.

[plugbark]

Hi,

My bad... I made a mistake by showing you these snapshots because they do not show the problem at all. Indeed, they rather show the normal behaviour of the amplifier.

To give you a better idea of what I ear, I recorded the sound generated by the loudspeaker with a microphone. I get that sound with the following setting : Gain at 3, Volume at 4 and Master at 1; which is pretty reasonable in terms of sound pressure!

https://drive.google.com/file/d/0B1S...ew?usp=sharing

Here is the associated snapshot:


So, to answer J M Fahey's question, the signal gets distorted at very low levels (about 500 mVrms).

[plugbark]

Hi nickb,

I measured the voltage of each pin of the "box header" connector on which is connected the TDA7295:

Pin #1: 0V (non-inverting input?)
Pin #2: Input signal (inverting input)
Pin #3: 4.12V (I don't know yet what is this signal)
Pin #4, 5: 0V
Pin #6, 7, 8: -37.7V (Negative power rail)
Pin #9, 10, 11: 37.3V (Positive power rail)
Pin #12, 13, 14: Output signal

The voltage of the power rails seems to be good.

[Jazz P Bass]

http://music-electronics-forum.com/a...295-pinout.jpg

Please remeasure according to the IC pinout.

Pin 14 should be the only output pin.

[nickb]

I guess pin 3 is the mute function. Well, there isn't an issue with the power supplies causing the asymmetry. If you look at the data sheet you'll see the upper output transistor is an N-channel FET. This needs a few volts from gate to source to work and that would account for it. It's a non-issue.

Anyway the low level distortion is a problem. Try to narrow down where the problem might be, here are some suggestions.

Make 100% sure the digital FX are off.
Have you tried a different speaker? I have heard almost exactly this noise before and that was the problem.
Try going in through CD/MP3 input
Try headphones. This output may be after the power amp in which case the test won't tell us anything new.
Lastly, while playing through it, listen to the output of each op amp in turn using a signal tracer ( a second amp should do) to see where the problem shows up.

[plugbark]

I tested all the things you suggested me and as you mentioned, the problem comes from the loudspeaker!

Yet I have tried a different speaker before but the funny thing is that my Fender amp's speaker generated enough sound pressure to vibrate the defective one! Since they were close enough, I concluded all this time that the problem didn't come from the speaker. What a big mistake!

I put my hand on the speaker to dampen vibration, and distortions disappeared. So it was a mechanical issue rather than an electronic one.

Anyway, thank you all for taking time to help me, it's really kind of you :-)

I wish you all a good day/night!

[J M Fahey]

Just for future readers:
1) the problem was not solved, mainly because it was never there
2) we had a communication problem, voltages posted did not match pinout AT ALL, pins were counted in a different sequence so they are meaningless
3) the guitar signal waveform shows NO distortion or clipping.
4) the amp only clipped beyond 20V RMS , which is the rated output voltage/power (into 8 ohms).
5) the gain structure was set up to distort at low volume anyway:

Gain at 3, Volume at 4 and Master at 1

[nickb]

Just for future readers:
1) the problem was not solved, mainly because it was never there
2) we had a communication problem, voltages posted did not match pinout AT ALL, pins were counted in a different sequence so they are meaningless
3) the guitar signal waveform shows NO distortion or clipping.
4) the amp only clipped beyond 20V RMS , which is the rated output voltage/power (into 8 ohms).
5) the gain structure was set up to distort at low volume anyway:

Not quite sure where you are coming from. Maybe I missed something?

1) The problem was solved - it was the speaker.
2) The OP clearly stated that the voltages were measured on the power amp header not the chip so you can't say he got the pinout wrong. It wasn't the most helpful place but I think it served the purpose.
3 & 4) Are the same point. It is unusual to see asymmetrical clipping on a SS PP amplifier plus I would have expected more than 20Vrms ( 28.2pk) with 37V rails and no load. It deserved looking into to be sure IMHO. I'd still like to see what you get into a dummy load.
5) I don't think the gain wasn't turned up enough to actually make it distort as the clip was clean. Also the distortion could be heard only as the signal faded away.

[plugbark]

Just to be clear, I redid the measurements directly on the TDA7295 pins this time.

1. Stand-by Gnd: 0V
2. Inverting input: 100 mVrms at 1 KHz
3. Non inverting input: Same signal than 2
4. SVR: 0V
5. N.C.
6. Bootstrap: cf. snapshot 1
7. +Vs (signal): 36.8V
8. -Vs (signal): -37.1V
9. Stand-by: 4.04V
10. Mute: 4.04V
11. N.C.
12. Must be N.C. but I got a signal (cf. snapshot 2)
13. +Vs (power): 37V
14. Out: cf. snapshot 3
15. -Vs (power): -37.5V

Snapshot 1: Bootstrap signal


Snapshot 2: Pin #12


Snapshot 3: Out