Bad DC Offset TL072 Op-amp

[Syn-fi]

hi,
I'm new this forum but hope to add my insights and contribute in respect of Samplers and Loewe TVs

My present problem is related to a TL072 OP-Amp on an Ensoniq DP4+ FX rack. I am getting a 'Bad DC offset' message on boot up and indeed the DC is not zero across the op-amp. This is causing feedback across channel 1 and noise.

I have recapped the Alu-Cap and also swapped out two SMD 0.1uf caps, but neither of those components seems to be the issue and I wonder if it is a bad Op-Amp.

I attach a photograph of the circuit board and also the schematic.

Any pointers would be gratefully received.

[budm]

Re: Bad DC Offset TL072 Op-amp

Since the circuit is direct couple circuit, any DC present at the input of the opamp, it will be multiply by the gain setting of the op amp. How high is the DC offset at the output of the opamp? Is it positive or negative offset voltage?
Learn about offset voltage:
https://cdn.badcaps-static.com/pdfs/...edf24d297a.pdf


http://www.analog.com/en/precision-o...cts/index.html

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Hi, thanks for responding so quickly.

however, I am struggling with the pin outs of the SOIC package tl072 . I assume where the small circle, is pin 1 and possibly output 1 (ch1). I can only find datasheets for the dip version of the tl072

separately i have 15v across the op-amp which seems right.

[Khron]

Re: Bad DC Offset TL072 Op-amp

The pinouts of SOIC's (and MSOP and whatever other 8-pin opamp packages are used) are quite standardized and "mirrored" between each other The silicon die's the same anyway.

Point being, pin 1 of the DIP is the same as the pin 1 of the SOIC

Second google result: https://cdn.badcaps-static.com/pdfs/...4612b37d90.pdf
Page 4 - D package = SOIC, P/N package = PDIP (as you can read in pages 2-3)

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Ok got it, over and out, will be testing this morning.

One thing I have discovered; is if swap the tl072 over to ch3/4 I get DC on ch3 but also on ch1. Swap them back and it goes back to dc on ch1. That seems to suggests that the opamp could be bad and there could be a problem elsewhere. Unfortunately I cannot test the op amps on a breadboard.

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Ch1 on tl072 seems to be 0.9v
Ch2 is zero

[redwire]

Re: Bad DC Offset TL072 Op-amp

In the pic I don't see the TL072's pins 5-8 soldered well, can you touch that up. It's a $0.72 part so instead of swapping them, I would just replace it before the PCB tracks get damaged.
Did you notice the trimpot labelled offset? Sometimes things drift with age and you just have to recalibrate. It looks like it runs to pins 5-6.
The DC could be upstream of the op-amp, but I'm not sure what the cct looks like.

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Hi, I have already re-seated the tl072 and also cleaned the pads underneath. There is a trimpot for the offset but it does not make any difference. The troublesome tl072 is also running very hot compared to the others, too hot to touch. Prior to the op-amp there are inverted audio streams (balanced audio) and there some alu caps on the way but i have replaced those.


There is some further history to this board. It looks like someone, stateside tried to swap out a converter on the output and the blew off a transistor and caused irregularities on the voltage regulators. There was track damage near where the output converter had been removed, i have fixed all of that and I can now get audio from ch3/4 but there is noise on ch1.

[Khron]

Re: Bad DC Offset TL072 Op-amp

Running hot? Well, if it's not shorted internally, it might be oscillating. Do you happen to have / can you borrow an oscilloscope?

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

no oscilloscope at the moment but i'm on the hunt for one. I will buy some TL072 replacements anyway to narrow this down.
The fact that the TL072 is running hot suggests the problem is either with the TL072 or before rather than being anything to do with the offset control which occurs later.

[Khron]

Re: Bad DC Offset TL072 Op-amp

Is there some way you can attach / link the schematic where this opamp "fits into"? The snippet in the first post "barely" shows the opamp itself, with not much clue where its inputs are coming from (earlier than those switches).

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

here you go.

Just be reading an article regarding oscillation and this does some to be the likely problem. But i can't see that the 0.1uf ceramic caps are at fault. Could it be the film caps?

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

the entire schematic is here:

[redwire]

Re: Bad DC Offset TL072 Op-amp

Thanks for posting the schematic, that makes it much easier.
Note the dual PSU voltages involved: +/-5V and +/-15V. Can you measure those, especially after seeing someone else was in there.
*If the TL072 outputs (offset) ever goes over +/-5.7V, it will dump current into the 5V rails and get hot (because it runs off +/-15V). I'm saying a good op-amp would heat up then. It might move the 5V rail too.
Possible reasons for too much offset would be:
Damaged op-amp TL072
CMOS switch IC 74HC4053 is no good
ADC IC current output1 has too much offset

The TL072 (in-) pin2/6 should be 0V, it is a summing point for many signals. If it's even +/- 0.005V the op-amp will have many volts of offset on its output.

I hope some of this helps. I'm not sure how far you can troubleshoot with a multimeter. The ADC normally runs warm.

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Hi,
Thanks for your detailed analysis. I have swapped over the hc4053 switches and it doesn't appear that they are at fault. Equally the TL072 seems fine too, when swapped over with the other channels. Re voltages I am going to struggle to test these across the board with a multimeter. But surely if the voltages were off, that would effect the other channels and introduce DC on ch3/4.

Could the ADC be sending too much current that isn't compensated for later ? Is it worthwhile flipping out the ADC and swapping it for ch3/4

[redwire]

Re: Bad DC Offset TL072 Op-amp

The 5V powered stuff (A/D, 4053) can get damaged by the 15V powered stuff (TL072). So you have to check voltages first. If you just swap things, you can make things worse. I would check the five voltage regulators to see that they are in spec. and let us know what you have on all pins of the op-amp.

To see if I am following what you've tried: U18 (TL072) runs hot, U22 (4053) was also swapped but distorted audio on Ch.3 is still a problem (I would replace parts instead of swapping them...)
That leaves U19 and the A/D in the signal path. I would compare voltages on CIN1 and CIN2 as they should be close. The A/D has a current output to U18 to keep it zeroed, but careful the voltage on the TL072 pin 2 should never go over +/-5.7V or it will damage the A/D. So if you swap the A/D it could get damaged. Always check voltages first.

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

VR2 looks to be out


VR7 should be + 15v
In - 29.49v
out 14.82v

VR1
In - 29.49v
out 4.44v

VR2 -should be -15v
In 29.29v
Out -24.30v

VR8
In 29.28v
Out 13.63v

VR6
in - 14.36v
out 9.34v

[redwire]

Re: Bad DC Offset TL072 Op-amp

I expect different readings from the PSU, I think you used the wrong ground point with your multimeter, or there is a bad ground to the PSU:

Input you measure -29.49V:
VR1 (7805) should be +5.0V; you measure +4.44V (low, 4.85V min.)
VR7 (7815) should be +15V; you measure +14.82V; ok

Input you measure +29.28V:
VR2 (7905) should be -5.0V; you measure -24.3V (oh boy, way too much)
VR8 (7915) should be -15V; you measure +13.63V; ???

Input you measure -14.36V:
VR6 (7905) should be -5.0V; you measure +9.34V; ???

At the very least VR2 is bad, and VR8 might be feeding into VR6. Kinda scary.

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

I'll report back when i have checked the regulators properly. They seem to have two ground points because the heatsink also has a grounding

[budm]

Re: Bad DC Offset TL072 Op-amp

78xx series Center pin and the Tab are tied together, they are Ground.
79xx center pin and tab is the input, left pin is ground.