TDA7294 AMP Thermal cut out. HARTKE AC5

[ziggy007]

Oksolid state brothers! I’ve got a Hartke acr5 that stops working after a bit. Originally went for about 5min then shuts down. Freeze spray on the main OP amp (TDA7294) and she comes good. Replace said amp and same thing. Now it uses more freeze spray to get it going and it dies off real quick. These opamps have a thermal shut down cct so it’s doing what it should. Thing is, it’s not that hot. I can get the heat sink freezing and it still doesn’t come on.
Anyone with experience with these TDA7294s. A related circuit issue? Something has to be triggering this thing to shut down. I’m out of ideas

Cheers Gav.

[shortcircuit]

What are your voltages pins 7 & 13, 8 & 15 ?? connections ?? Hartke was good to me when seeking schematics.

[Enzo]

And how much current does the amp draw from the mains at idle?

Disconnect the speaker, does it still act this way? Connect a different speaker, same question?

[olddawg]

What are you using for a load?

[ziggy007]

Hi guys,
All voltages at +/- 30 volts at 7,8,13,15. Stable with fault present as well.
Amp draws .167 at start then stabilises at .090A drops tp .070 when fault kicks in.
Using the onboard speakers as the load. Does it with speaker disconnected.

[shortcircuit]

Thing is, it’s not that hot.

TDA7294 has a short circuit protection also ,maybe it's not a thermal issue ( at the op amp )maybe another component is heating up and failing, do some testing on components . Have you tried running off a bulb limiter ??

[ziggy007]

Yep. Has to be something external unless it’s two bad IC’S. Tomorrow’s problem when I get up! 👍🏼

[ziggy007]

Ok, I’m scratching my head now. I read that these ICs were sensitive to a dirty power supply. So I scoped the +/- voltage rails and saw a slight sawtooth of approx 100mv ( on 30V) whilst amp working. To my surprise as it shut down, i.e the fault appears, the sawtooth went to a smooth DC. Cause or effect? Is that because there is less load on the power supply?

Cheers Gav

[J M Fahey]

The small sawtooth is ripple and is normal, more or less of it depending on current consumption.

That it disappears when your amp "fails" clearly shows consumption dissappears.

2 possibilities:

1) your amp is getting into mute/standby for whatever reason to be determined

2) some track/connector/wire/solder pad in the path opens and chipamp is not longer connected to supply, so pulling no current.

In principle measure votages at all IC pins and write them down, then as soon as it cuts off measure again, write them down, and post both sets here.

IF it were a single chipamp doing that, I might think a cracked leg , but two in a row places blame somewhere else.

Some thermal protection might also be turning it OFF, but since you say it stays stone cold, I should discard that.

Any other ideas are welcome, of course, I run out of them.

[Enzo]

The TDA has a mute circuit, so explore that.

[ziggy007]

The mute circuit works as it should as far as I can figure. There is 5v on pin 9 and 10 which goes to zero when phones or tuner are activated enabling the mute. It fails to 50mv or so when the fault kicks in. There is 24vac to the voltage supply diode on the mute circuit and 13vDc downstream when working. This is 5v after the first 10k dropper resistor. The fault drops this to 10vdc and 50mV respectively. Checked the caps checked the zenner all seem ok.
Perplexed to say the least

[J M Fahey]

There is 5v on pin 9 and 10 which goes to zero when phones or tuner are activated enabling the mute. It fails to 50mv or so when the fault kicks in.

Then it IS being muted.
0V and 50mV is exact same for all practical means.

Not sure why you suspect a bad/fake chipamp when it is working *perfectly*, exactly as datasheet says.

Think why Q3 is clamping 5V (Enable) down to 50mV (Mute) instead.
It´s controlled from the Microprocessor (MCU).

2 possibilities:

1) your amp is getting into mute/standby for whatever reason to be determined

So this seems to be the case.

There is 5v on pin 9 and 10 which goes to zero when phones or tuner are activated enabling the mute.

So ... are you using the phones or tuner?
maybe the MCU "thinks" you do.

If you find it annoying you may add a switch to kill the Mute circuit manually.

[ziggy007]

Thx Juan.
Yep I’m sure it’s the mute cct but I can’t work out why it works for 5 min then just dies. I originally thought it was the IC amp because freeze spray bought it back to life. Not now. Finding it hard to troubleshoot because of the SMD and it’s all pcb. Very hard to just “disconnect” a component and see what happens.

[J M Fahey]

Ok, try this, which is still doable on an SMT board: locate R148 (100k) and short across it with a U bent piece of naked wire, just a single strand will do and helps later removal.

This will kill the mute signal coming from the microprocessor.

If it works (I guess it will) you can leave as is, just remember it won´t mute while tuning or headphoning.
No big deal.

If you want to restore manual control, add a thin wire from R148/R144 junction to an added switch which grounds it at will.

We are not really solving the original problem , but making amplifier usable, not a bad compromise.

--------------------------------------------------------
EDIT: MAYBE it´s not a crazy MCU but a defective "Headphone sensor" which both Mutes and Standbys the amp through Q4

To test this theory short Q4 BE.

Post results.

Also upload the full ACR5 schematic, it may help a future Member with Preamp problems.

And maybe a gutshot or two, one showing the board you are working on, another of speaker and Tweeter .

[ziggy007]

Thanks again mate. That’s so obvious, shorting out the base/emitter junction. It was staring me in the face. I was trying to work out how to get those transistors out of the cct. Thank you.
It’s a brand new amp. So I’ll see what they want to do. (Loooong story)😊.
Won’t get a chance until tomorrow. Will post all the relevant stuff.
👍🏼👍🏼