TDA7396 causing this tube amp guy some problems!

[jeeptechfred]

Hey guys, I may have bit off more than I can chew with out some help. I know this is a tube amp forum but 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

[Cliff Schecht]

Hola and welcome!

Going over everything pin by pin..

Pin 2 should have good signal. Means that everything before the output stage is most likely working as it should.

Pin 1 is capacitively coupled to ground as to allow it's DC bias conditions to be met. The little bit of signal you are seeing here is due to the action of the differential input pair and is nothing to worry about.

Pin 4: From what you are telling us, there is a fault. I think the output stage of the part itself is blown. Pins 5 and 7 form a floating fully differential output stage that allows one to achieve twice the voltage seen by the speaker (more output for a given supply voltage). Notice that the speaker is not attached to ground, this is a dead giveaway. Now remove any load from pins 5 and 7 and remeasure pin 6. I bet it still reads low, meaning a transistor in the output stage or somewhere along the line blew.

Have you tried putting in a new TDA7396 already?

[jeeptechfred]

Thanks! I have not yet replaced the chip amp. i generally like to have some evidence prompting me to replace something, not that substituting a known good part offends me in any way, more because I want to learn the most in situations like these. when you said to remeasure pin 6, did you mean pin 4? in a good chip with no faults what would that usually read?
Thanks!

[Cliff Schecht]

Yes I meant pin 4, sorry! They don't say specifically in the datasheet but a non-fault condition will probably read near whatever VREF is tied to. There is a resistor coming off of pin 4 and if you measure the side not connected to the IC, you'll know what VREF is set at.

[jeeptechfred]

Thanks for your help so far, I feel like I have a direction to go now. I am going to get after this tonight.

[JazzGuitarGimp]

The output at pin 4 is open collector. It can only pull a signal low. Thus a pull up resistor must be used in order to see a 'high' signal at pin 4. According to the data sheet, the reference voltage can be anything between +5V and +Vs (pins 3 and 9). Keep in mind pin 4 is for monitoring purposes only; if the chip is used in an amplifier design that doesn't have a need for this signal, then the designer will not have placed a pull up resistor in the circuit. So the first thing you need to ascertain, before expecting a viable voltage reading at pin 4, is whether or not there us a pull up resistor on the board to begin with. Then find out what voltage the Vref end if the resistor is tied to, so you know what the voltage will be when pin 4 is high. 0.6V will be considered a low signal, regardless of what Vref is (since the datasheet says Vref should be a minimum if 5V). But, if there us no pull up resistor, pin 4 will always read as a low.

[jeeptechfred]

Turns out as Jazz Guitar suggested, pin 4 was not even set up for use. Thanks a lot for the explaination relating to that pin and how to understand the data sheet in referance to it.

I pulled pins 5 and 7 out of the board and measured them to ground. I found 11 ohms from one of them to ground and 68 to ground on the other. those readings make me think that the chip is shorted. I ordered a new chip and I plan to measure it prior to install to see if it is indeed shorted.

Thanks guys for all of your suggestions so far.

[JazzGuitarGimp]

I pulled pins 5 and 7 out of the board and measured them to ground. I found 11 ohms from one of them to ground and 68 to ground on the other. those readings make me think that the chip is shorted. I ordered a new chip and I plan to measure it prior to install to see if it is indeed shorted.

Had pin 4 been set up for use, it most certainly would have been low. The chip is shorted as your measurements suggest. Assuming there is nothing else wrong with the board, a new chip should put you back in business.

[jeeptechfred]

Today I measured the new chip amp from pin 5 to the ground pin and pin 7 to the ground pin. In both situations I got a measurement of several kohms. I removed the old chip amp. I then rechecked the inputs and outputs on the board. All checked okay so I installed the chip and it worked correctly. Thanks for your help!!

Problem solved

[Cliff Schecht]