TDA7295:

[mark g]

I am in the process of repairing an audio amplifier that employs a TDA7295 as
the woofer driver.
The repair of the original fault, which was causing the TDA7295 and a TDA7265
to be held
permenently muted, went fine, but having got the mute control level working
again, I find that I
have no output from the TDA7295. It seems to be shot - for reasons that are
however unclear to me
- but I would appreciate a second (or more) opinion. The levels around the
chip are as follows:
Supplies: +/- 28V,
Vin+: input signal of +/- 0.7V,
Vin-: just a dc level of -9.6V,
Vout: just a dc level of -24.8V,
Bootstrap input: just a dc level of -25.6V.
These levels remain the same regardless of the state of the MUTE input.
STANDBY is always inactive, i.e. >+3.5V. Any suggestions on how this state
could have arisen?

--
+---------------[ SERVER SIGNATURE ]-------
| posted via http://www.electrondepot.com
| Web, RSS and Social Media Interface to
| sci.electronics.repair and other groups
+------------------------------------------

 

[Phil Allison]

"mark g"

I am in the process of repairing an audio amplifier that employs a TDA7295
as
the woofer driver. The repair of the original fault, which was causing
the TDA7295 and a TDA7265
to be held permenently muted, went fine, but having got the mute control
level working
again, I find that I have no output from the TDA7295. It seems to be
shot - for reasons that are
however unclear to me
- but I would appreciate a second (or more) opinion. The levels around the
chip are as follows: Supplies: +/- 28V, Vin+:
input signal of +/- 0.7V, Vin-: just a dc level
of -9.6V, Vout: just a dc level of -24.8V, Bootstrap
input: just a dc level of -25.6V. These levels remain the same
regardless of the state of the MUTE input. STANDBY is always inactive,
i.e. >+3.5V. Any suggestions on how this state
could have arisen?

** Those voltages suggests the + supply is missing on the TDA.

Look for cracked pins, bad soldering, cracked PCB tracks etc,

Or the IC was damaged during previous trouble or service work.

If all still looks OK, stick a new one in.


.... Phil

 

[N_Cook]

I am in the process of repairing an audio amplifier that employs a TDA7295 as
the woofer driver.
The repair of the original fault, which was causing the TDA7295 and a TDA7265
to be held
permenently muted, went fine, but having got the mute control level working
again, I find that I
have no output from the TDA7295. It seems to be shot - for reasons that are
however unclear to me
- but I would appreciate a second (or more) opinion. The levels around the
chip are as follows:
Supplies: +/- 28V,
Vin+: input signal of +/- 0.7V,
Vin-: just a dc level of -9.6V,
Vout: just a dc level of -24.8V,
Bootstrap input: just a dc level of -25.6V.
These levels remain the same regardless of the state of the MUTE input.
STANDBY is always inactive, i.e. >+3.5V. Any suggestions on how this state
could have arisen?

--
+---------------[ SERVER SIGNATURE ]-------+
| posted via http://www.electrondepot.com |
| Web, RSS and Social Media Interface to |
| sci.electronics.repair and other groups |
+------------------------------------------+

What is the simple, no powering, DVM ohms measurement between , separately,
output and each supply rail ?

 

[Adrian Tuddenham]

... The levels around the
chip are as follows:

Supplies: +/- 28V, [...]
Vout: just a dc level of -24.8V,
Bootstrap input: just a dc level of -25.6V.

The word 'around' might be significant. You need to measure the levels
actually on the pins of the chip (make sure you have a probe that will
do it without risk of shorting) because it looks as though your +ve
supply isn't getting through.

 

[Phil Allison]

"Adrian Tuddenham"

mark g

The word 'around' might be significant. You need to measure the levels
actually on the pins of the chip (make sure you have a probe that will
do it without risk of shorting) because it looks as though your +ve
supply isn't getting through.

** Absolutely.

Many scope probes have a simple accessory that shrouds the point so it will
not possibly short adjacent pins on a DIL package.

BTW:

When troubleshooting, it pays to use a scope set to DC coupling first -
rather than a DVM.

Cos the scope screen shows what is really there.

And I mean a REAL scope - ie an analogue one.




.... Phil

 

[mark g]

responding to http://www.electrondepot.com/repair/tda7295-145354-.htm , mark g
wrote:
Thanks for all the above replies. I'm very grateful.

The measurements I reported were made using the pointed tip of an oscilloscope
probe
and I thought I had been on pins of the chip, but I agree, the negative
voltages do suggest
the positive supply is missing.

What I forgot to mention was that the circuit actually worked for a few
minutes.
Prior to, and after that brief period, the symptoms were the same, i.e. as
described earlier.
It does rather suggest a bad joint, doesn't it, but I couldn't sort it out.

Anyway, I'll investigate again and let you know.

 

[Jamie]

responding to http://www.electrondepot.com/repair/tda7295-145354-.htm ,
mark g
wrote:
Thanks for all the above replies. I'm very grateful.
The measurements I reported were made using the pointed tip of an
oscilloscope
probe and I thought I had been on pins of the chip, but I agree, the
negative
voltages do suggest the positive supply is missing.

What I forgot to mention was that the circuit actually worked for a few
minutes. Prior to, and after that brief period, the symptoms were the
same, i.e. as
described earlier. It does rather suggest a bad joint, doesn't it, but I
couldn't sort it out.

Anyway, I'll investigate again and let you know.

maybe you have latch up.?

Some chips have problems if you over drive the input past the rails,
they latch up and remain that way until you reset them or some how over
come it.

It appears that it has MOS technology in it, so that could be the
case..

A good test would be to just power it up but don't inject anything in
it. See how long it'll sit idle.

If it seems that is the case, you can put diodes from the rails to the
input that is being driven to prevent the input from exceeding the rails.

Jamie

 

[Phil Allison]

"Gareth Magennis"

Mark, I have seen lots of these TDA powered speakers, mostly studio
monitors, with the "conductive glue" problem.

At the risk of repeating myself, have a good look at the PCB, particularly
between the pins of the TDA ICs. If there is any kind of chemical
coating, yellow or black, then this really is very likely to be your
problem.
It starts off as component adhesive, but over time melts and runs
everywhere, eventually becoming brittle and conductive, causing erratic
and illogical behaviour.

** The yellow glue problem is at least 30 years old.

Roland " Cube " series amps had problems with it and they were made in
Japan.

VCRs had problems it in spades as do many SMPS too.

Class D amps used in high end line array boxes made by dB Technologies have
it splashed about.

The stuff does NOT melt and move around, it simply goes dark with heat and
time and becomes conductive and corrosive to copper and tin.

Long as it has no contact with metal, it hardly matters.

Now, the colossally stupid Chinese spread it about on PCBs like jam on
toast.

They have the exact same philosophy on these matters as Alfred E. Neuman.

http://mikophoto.net/wordpress/wp-content/uploads/2011/01/Alfred-E.-Neuman.jpg




.... Phil

 

[[email protected]]

"Adrian Tuddenham"


** Absolutely.

Many scope probes have a simple accessory that shrouds the point so it will
not possibly short adjacent pins on a DIL package.

BTW:

When troubleshooting, it pays to use a scope set to DC coupling first -
rather than a DVM.

Cos the scope screen shows what is really there.

And I mean a REAL scope - ie an analogue one.




... Phil

Been there, done that, didn't get the tee shirt. The picture on a 23"
LCD TV was badly distorted. All voltage out of the power supply were
spot on, as were the outputs of the LDO regulators on the main board
EXCEPT for one in the area of the digital video output. The 3.3 volt
output read 3.1. Checking it with a scope showed a clean input and
nearly half a volt of noise on the output. In this case the fault was
a pair of 100µF electrolytics on the output.

PlainBill

 

[Phil Allison]

"Gareth Magennis"

"Phil Allison"


I assumed that because it was SO spread around on PCB's where there was no
need for it (on the pins of TDA IC's for example) that it must have
migrated there.
It beggars belief that anyone would do such a thing.

** In early Roland Cube amps, the reverb drive IC was a SIL power amp -
with yellow glue all over the pins to stop them from breaking under
vibration.

One Alesis powered monitor speaker has the stuff all over a toroidal choke
in the SMPS, to hold it firmly to the PCB. That choke gets hot and the glue
eats right through the enamel - so you have to rewind the damn thing
completely.

The stuff I see in modern equipment is usually black and goes brittle.
Other stuff I have seen, which may be older (?), was more yellow/brown.

** The same glue is pale yellow when fresh, turns light brown with time and
heat and finally black like burnt toast.

This looks like the stuff.

http://www.alibaba.com/product-gs/482344048/One_component_solvent_resistant_flame_retardant.html

Comes in a convenient sauce bottle.



.... Phil

 

[[email protected]]

Plus and minus supplies OK right ?

Negative voltage on the minus input right ?
Not negative voltage on the plus input right ?
Negative voltage on the output right ?

If it was an OP AMP it would be bad.

It IS an OP AMP.

Also, since manufacturers are not in the habit of using split supplies without DC coupling, the woofer is either open or the fuses would blow or the IC would probably blow a hole out the case.