Troublesome TDA7293 - UK Vintage Radio Repair and Restoration Discussion Forum

ITAM805 · 24th Oct 2021, 2:30 pm

Here we go again. I have a budget "Power Dynamics PDM C805A" stereo powered mixer here that uses a pair of TDA7293. It apparently kept blowing fuses. I found one of the TDA's to have dead short between the 'standby' pin and both power rails. I replaced it and all appeared good, no shorts etc.

So I powered the unit up via a lamp limiter and variac. All was going well, display lit up, fan spinning, till increased the voltage from around 200V to 240V when the lamp lit brightly and the display went off. It appears either the replacement has popped or the 'good' chip has gone the same way as its twin

The supply rails now measures 47Ω as opposed to 0Ω the first time round.

Google has lots of threads regarding these chips going pop without much provocation but I thought I ask the experts here before I have to strip the damn thing apart again. Is there something that I should check beyond the obvious?

Mooly · 24th Oct 2021, 2:55 pm

The only things I can think of would be to ask:

1/ Is the power supply an SMPS rather than a linear type? If so then that could be be suspect (cap issues). Also SMPS may not respond well to DBT and variacs.

2/ Was the chip from a reputable authorised supplier?

Might be worth monitoring the supplies to the chip (usually split rail) with a DVM and scope to make sure they are clean and not to high.

ITAM805 · 24th Oct 2021, 3:10 pm

Thanks Mooly. It's a linear supply. Is the chip genuine, well that I can't say for certain, it's been in the parts box for 2 or 3 years but looked pukka? IIRC it came in a pack of 2, and the other went in a repair which never back so assume was good

The supply is as you might expect, a simple bridge and pair of caps but I will check as you suggest

Mooly · 24th Oct 2021, 3:19 pm

A linear supply should be fine. I guess you have to try another chip and if it is a metal tabbed version then also make sure the insulating washer is good.

There isn't normally much else really unless something upstream of the chip was causing an issue... random thoughts... hf oscillation being applied to the input of the chip and frying it.

It is worth dangling a scope on the chip output and making sure it is clean as you ramp the voltage up.

ITAM805 · 24th Oct 2021, 3:57 pm

Well, I just pulled the 'new' chip and the short is still there on the board? The bridge measures good so something else has quit, possibly the other TDA?

I'll try scoping it Mooly and see what's what

ITAM805 · 24th Oct 2021, 5:10 pm

Just found this useful video on one method of checking genuine or otherwise TDA7293's here

My replacement (that I've now pulled) measures OL and reversing the leads 4.9meg, while the chip still on the board measures 0.4Ω!

I found another sealed pack of 2 chips and both measure identical to my replacement as above

Mooly · 24th Oct 2021, 6:13 pm

If the other chip has failed then it might be worth powering up without either fitted and checking rails and input to the chips.

I'm guessing this unit has a possible hidden history with you saying:

Quote:

It apparently kept blowing fuses.

If that is the case then check basics such the PSU being configured for 230/240 volt and also give it all a good visual inspection to see if any work has been done such caps (fitted backwards etc).

ITAM805 · 24th Oct 2021, 9:29 pm

I powered the board up without chips fitted and get +/-32V on the caps, this voltage stays on the caps so no shorts anywhere.

One anomaly is pin2 (-in) on one amp I get 0.4V but 0.03V on the other? The diagram shows this as connected to the out pin(14) via a resistor (NFB?) but odd seeing as no chips are fitted

Mooly · 25th Oct 2021, 6:53 am

Assuming the actual circuit is similar to the data sheet example then you would expect to see zero volts on both inverting (-) and non inverting (+) inputs.

It is worth checking those voltages by using pin 1 of the chip as the reference ground point.

Beyond that and you would have to confirm the circuit configuration for pin 2 does match the data sheet example and if it does then you need to find out why there is a residual voltage on that pin. Leakage on the PCB might be one reason (contamination or spillage for example).

Also bear in mind that if the speaker is attached as shown then pin 14 and pin 2 (via the feedback resistor) should be tied to ground.

If there is no speaker/load attached as shown in the diagram then the pin is floating and residual charge on the feedback return cap could be the cause. That is actually the most likely reason I think.

Refugee · 25th Oct 2021, 2:31 pm

I would be tempted to do the electrolytic capacitors as it is under strong suspicion of having been oscillating.

ITAM805 · 25th Oct 2021, 5:12 pm

Mooly, thanks for the suggestions, particularly the last sentence. Refugee, many thanks, that was my suspicion but they all test good

It's strange that both chips went pop, I wonder what could have happened. Maybe they were run at high power with no speakers connected?

Anyhow, I fitted the pair of new IC's and touch wood etc, the amp appears to be working without issue, the heatsink is stone cold under medium load, no sign of oscillation and no dc on the outputs.

Mooly · 25th Oct 2021, 5:58 pm

Pleased to hear that is all seems to be working now new chips are fitted. All you can do is soak test it now.

24th Oct 2021, 2:30 pm	#1
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Troublesome TDA7293 Here we go again. I have a budget "Power Dynamics PDM C805A" stereo powered mixer here that uses a pair of TDA7293. It apparently kept blowing fuses. I found one of the TDA's to have dead short between the 'standby' pin and both power rails. I replaced it and all appeared good, no shorts etc. So I powered the unit up via a lamp limiter and variac. All was going well, display lit up, fan spinning, till increased the voltage from around 200V to 240V when the lamp lit brightly and the display went off. It appears either the replacement has popped or the 'good' chip has gone the same way as its twin The supply rails now measures 47Ω as opposed to 0Ω the first time round. Google has lots of threads regarding these chips going pop without much provocation but I thought I ask the experts here before I have to strip the damn thing apart again. Is there something that I should check beyond the obvious?

24th Oct 2021, 2:55 pm	#2
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Troublesome TDA7293 The only things I can think of would be to ask: 1/ Is the power supply an SMPS rather than a linear type? If so then that could be be suspect (cap issues). Also SMPS may not respond well to DBT and variacs. 2/ Was the chip from a reputable authorised supplier? Might be worth monitoring the supplies to the chip (usually split rail) with a DVM and scope to make sure they are clean and not to high.

24th Oct 2021, 3:10 pm	#3
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Troublesome TDA7293 Thanks Mooly. It's a linear supply. Is the chip genuine, well that I can't say for certain, it's been in the parts box for 2 or 3 years but looked pukka? IIRC it came in a pack of 2, and the other went in a repair which never back so assume was good The supply is as you might expect, a simple bridge and pair of caps but I will check as you suggest

24th Oct 2021, 3:19 pm	#4
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Troublesome TDA7293 A linear supply should be fine. I guess you have to try another chip and if it is a metal tabbed version then also make sure the insulating washer is good. There isn't normally much else really unless something upstream of the chip was causing an issue... random thoughts... hf oscillation being applied to the input of the chip and frying it. It is worth dangling a scope on the chip output and making sure it is clean as you ramp the voltage up.

24th Oct 2021, 3:57 pm	#5
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Troublesome TDA7293 Well, I just pulled the 'new' chip and the short is still there on the board? The bridge measures good so something else has quit, possibly the other TDA? I'll try scoping it Mooly and see what's what

24th Oct 2021, 5:10 pm	#6
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Troublesome TDA7293 Just found this useful video on one method of checking genuine or otherwise TDA7293's here My replacement (that I've now pulled) measures OL and reversing the leads 4.9meg, while the chip still on the board measures 0.4Ω! I found another sealed pack of 2 chips and both measure identical to my replacement as above

25th Oct 2021, 6:53 am	#9
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Troublesome TDA7293 Assuming the actual circuit is similar to the data sheet example then you would expect to see zero volts on both inverting (-) and non inverting (+) inputs. It is worth checking those voltages by using pin 1 of the chip as the reference ground point. Beyond that and you would have to confirm the circuit configuration for pin 2 does match the data sheet example and if it does then you need to find out why there is a residual voltage on that pin. Leakage on the PCB might be one reason (contamination or spillage for example). Also bear in mind that if the speaker is attached as shown then pin 14 and pin 2 (via the feedback resistor) should be tied to ground. If there is no speaker/load attached as shown in the diagram then the pin is floating and residual charge on the feedback return cap could be the cause. That is actually the most likely reason I think.

25th Oct 2021, 2:31 pm	#10
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Troublesome TDA7293 I would be tempted to do the electrolytic capacitors as it is under strong suspicion of having been oscillating.

25th Oct 2021, 5:12 pm	#11
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Troublesome TDA7293 Mooly, thanks for the suggestions, particularly the last sentence. Refugee, many thanks, that was my suspicion but they all test good It's strange that both chips went pop, I wonder what could have happened. Maybe they were run at high power with no speakers connected? Anyhow, I fitted the pair of new IC's and touch wood etc, the amp appears to be working without issue, the heatsink is stone cold under medium load, no sign of oscillation and no dc on the outputs.

25th Oct 2021, 5:58 pm	#12
Mooly Octode Join Date: May 2011 Location: Lancashire, UK. Posts: 1,351	Re: Troublesome TDA7293 Pleased to hear that is all seems to be working now new chips are fitted. All you can do is soak test it now.