TDA7396 trouble

[jeeptechfred]

TDA7396.pdfHey guys, I may have bit off more than I can chew without some help. I am hoping someone here has some wisdom to help me along.
I am attempting to repair a radio shack PA amplifier using the tda7396 chip amp with no output. I do not have a schematic or currently a model number.
I have a good audio signal at pin 2 and a tiny signal on pin 1 (assuming neg. feedback)
I have 18v on pins 3 and 9.
I checked pins 7 and 5. they have good connections to the output transformer. I disconnected the transformer and put an 8ohm load on thinking maybe it was shorted. I did not find any shorts to ground on pins 5 and 7.
I have 9v on pin 8 (standby).
I have .9v on pin 11 (mute) I can not tell from the data sheet if that is normal or not.
I checked pin 4 voltage, it is .6v. The data sheet seems to say that if the self diagnostics pin (4) is showing a fault it will be low. Is .6v low or high? I have a good ground connection on pin 6.
My limited experiance with these is that when then fail they usually short and blow the main fuse. this is not the case.
Befor I replace the chip out of desperation, I am hoping somebody can help me decipher the data sheet and give me some ideas on other things to check. Thanks, Fred

[km6xz]

Welcome to the forum.
This chip has a lot of protection functions and it can mask major faults by shutting down over current conditions. If you are sure there is no short at the output terminals, measuring the resistance to ground with power off, from each output pin. The Standby voltage is OK but the Clip Detector on pin 4 is not, indicating a protect mode. When a high distortion(>.5%) is detected, it goes low. If an output, either one, is shorted or is seeing a <2 ohm load, it goes low and if there is any internal problem with excess current or offset which triggers the protection circuit to bias off the output.
Watch the voltage on pin 4 when you turn the amp on to see if there is an initial pulse of higher voltage. You should be able to see it with a scope but maybe a DMM will not be fast enough to catch it.
If there is low resistance measured to ground on either output pin, the odds are high that one, but not both, the chip output devices are shorted. It is a bridged output so be sure neither side of the speaker pins are grounded by the circuit or your test instrument.

[jeeptechfred]

Km6xz, Thanks for taking time to look at my post. I will measure pins 5 and 7 tonight for a short to ground. If one of them are shorted I'm assuming they will pretty much be very low resistance, like one sees with a typical shorted transistor right? also, what does pin 4 look like in a healthy amp?
It sounds like, from the datasheet, that an open output can also cause chip to shut down. Did I read that correctly?

Thanks again, Fred

[km6xz]

Pin 4 is pulled high by a pull-up resistor and in a safe mode, should be high Z so whatever voltage that resistor is connected to as a source but at least +5vdc.

It is a pretty easy chip to use, stable, lots of protection for shorts and overload, no turn on thump and 43 watts and mid 30s from a car battery.

[Jazz P Bass]

It appears to me that the IC is in mute.

Pin 11 (edit out 9) should go higher than .9 volts.

Use the circuit diagram from the datasheet.

Typically, manufacturers will follow it.

http://www.st.com/web/en/resource/te...CD00000164.pdf

Edit: I refered to pin 9 when it should read pin 11

[jeeptechfred]

I see.... the chip will ground pin 4 if it sees a problem, causeing the vref to be viewed as a low voltage instead of a "typical" 5v. Thanks for that explaination!
Jazz P, I tested capacitor CM and resistor RS and thay both appear to be okay. Possibly a faulty chip would put out a low voltage? looks to me that the chip provides the mute voltage unlike some of the other chip amps I have had some experiance with. I cant recall the part number of the chip in a marshall amp I worked on, it requiered 3.5v on the mute pin to unmute it. What do you think?

Thanks, Fred

[Jazz P Bass]

On page 3 of the TDA7396 datasheet, the Mute conditions are stated.

1.5V Mute On
3.5 V Mute Off.

I would try to follow where the mute circuit conditions are derived from.

And the TDA7293 is what you are thinking of, Marshall wise.

[g1]

It sounds like, from the datasheet, that an open output can also cause chip to shut down. Did I read that correctly?

I got the same impression from the datasheet (page 4, functional description, pin 4).
Not sure if it is relevant here, but thought I would mention it with regard to troubleshooting these chips with no load.

[jeeptechfred]

Jazz Bass, When I look at the data sheet I see those voltages listed for standby for sure, but am failing to see those voltages published for the mute function. The mute data seems way over my head. (-:

The Marshall Chip number sounds right on!

[Jazz P Bass]

Sorry for the confusion.

Yes, those voltages apply to the Standby pin. (which you are seeing a 0.9V reading)

The Mute function is purely for diagnostics.

It will tell you the amp is muted.

If you look at the pin with a scope & measure the timing of it, that may give an indication of why it is muted.

In your case, with Standby low, I would fully expect the Mute pin to be low.

[jeeptechfred]

Actually the standby pin is high, 9volts. The mute pin appears to be low, .9v (decimal point). I was worried when I wrote up the original post that would be missed. thanks for your help. I hope to get some time looking at this again tonight. I will report any findings.

[Jazz P Bass]

Ooops.
Missed that.
If standby is good, a load is connected & no signal is present it should unmute.
I would slap in a new chip.

[jeeptechfred]

tonight I pulled pins 5 7 out of the board and found 11 ohm to ground from pin5 and 68 ohms to ground frond pin7. I think the chip is shorted. I ordered a replacement and will try that. thanks all!

[km6xz]

You had the proper reasoning from the beginning, just needed some clarification as to how the protection circuit worked.
It is really a well done protection circuit, which drops power to the output if there is a short, no burned pc board, no drama, just shuts down and stays down. When the CD pin is low, and stays low, all other control pins become irrelevant.

The chip is defective but the protection continued to work.

Because of how good the protection is, makes the chip great for consumer products but terrible for guitar amp application.

[jeeptechfred]

The chip makes me think of some personal car audio project ideas (: Thanks for all the help everybody, I will update yall as soon as I can.