Bad DC Offset TL072 Op-amp

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

Hi guys,

Finally fixed this. I took it to an electronics repair place but even they gave up. I then recovered the unit about 2 weeks ago and in between times i have learned a great deal more about D/A circuits and Op-Amps. Anyway it turns out that the OpAMp on input 1, a 4565 was faulty and then two further Op-Amps TL072s near the A/D end of the circuit, which feed back into the Analog part of the circuit were faulty too.

I only started looking further up and down the audio change after i checked the DC offset circuitry around the TL072s which i thought were originally the problem. Not looking beyond this was my mistake as i was convinced that the 'DC offset' message related to the DC Offset Trimmer circuit around the TL072.

If anyone wants further detail do let me know.

[ben7]

Re: Bad DC Offset TL072 Op-amp

Hi guys,

Finally fixed this. I took it to an electronics repair place but even they gave up. I then recovered the unit about 2 weeks ago and in between times i have learned a great deal more about D/A circuits and Op-Amps. Anyway it turns out that the OpAMp on input 1, a 4565 was faulty and then two further Op-Amps TL072s near the A/D end of the circuit, which feed back into the Analog part of the circuit were faulty too.

I only started looking further up and down the audio change after i checked the DC offset circuitry around the TL072s which i thought were originally the problem. Not looking beyond this was my mistake as i was convinced that the 'DC offset' message related to the DC Offset Trimmer circuit around the TL072.

If anyone wants further detail do let me know.

Good job!

Indeed, many times things that get hot, burn out, etc... are not the source of the problem, only a symptom/casualty.

[Longbow]

Re: Bad DC Offset TL072 Op-amp

Anyway it turns out that the OpAMp on input 1, a 4565 was faulty and then two further Op-Amps TL072s near the A/D end of the circuit, which feed back into the Analog part of the circuit were faulty too. If anyone wants further detail do let me know.

I'd like the details please. You are saying that 3 op amps were faulty? Were these 3 different IC's or one IC with 4 op amps?

Generally, the term d.c. offset refers to the voltage that appears at the output terminal of an op amp when the input terminals are shorted. It is really a measure of the quality of the part. Offset can be externally nulled or it can be ignored depending upon circuit requirements. Inexpensive op amps might have a millivolt or so d.c. offset. Expensive units can be tweaked to microvolt levels.

On the other hand, if you see near rail voltages on the output termainal, it is telling you that the part is either shorted, or missing its negative feedback loop, or else has incorrect input voltages. In other words, a perfectly functioning op amp will output a high d.c. voltage if it doesn't have the correct inputs. It just does what it is told to do. So, what actually caused the failure?

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

hi,
i have fixed more of these DP4+s since.... around 10-15 now

Now i have a better understanding of the cause(s) of failure.

So, as the last poster pointed out failure of an op-amp is unlikely - he's right. I fixed the unit by remaking shorted traces with the heat gun now i'm much wiser and knowledgeable i also armed with an analog scope.

What seems to be happening on the DP4+ is a mixture of electrolytic discharge from capacitors, particularly those near the 5 voltage regulators being right next to the D/A stage, with the 5+ve rail doing a lot of work as it drops from 30v.

That discharge eats up traces... it creates shorts or increases resistance etc. If the op/amps near the D/A stage get a short particularly from those 22uf caps which the audio runs through the op amp directly from the D/A that off-setting.

You can see evidence of this on most boards with dried black ooze on traces..and if you leave a board on for a while you will get 'fish-oil' smell...you can even localize a bad cap this way by scenting it out.

Also, separately the 35khz sample/hold chop signal is a 'vie' that runs in the PCB and is prone to failure particularly all the way to a 4053 that goes to ch1 op-amp. If that 35khz sample/hold gets broken expect to see the Bad D/C offset message. This 35khz distributes to the A/D stage as well as the D/A stage - its the same chop signal. Hence Bad DC offset can be an indication of a problem at the A/D stage or the D/stage.

To solve this you need to trace signal paths right across the circuit board with a scope and you need to re-cap almost every electrolytic...they are leaky. Replace the 3.3uf on the Logic board too because they can cause 'waves' of ground noise in the D/A stage.

Consider better cooling, venting to prevent and mitigate against further failure. Add a DC fan and ensure you do not stack the DP4 on another piece of hot gear.

[Diggers]

Re: Bad DC Offset TL072 Op-amp

hi,
i have fixed more of these DP4+s since.... around 10-15 now

Now i have a better understanding of the cause(s) of failure.

Hey

I just joined the forum in pursuit of help or advice fixing this same Bad DC offset error in my DP4 non-plus... I have tried and tried but cant find the schematic anywhere..... can you please help either fixing it or advising me how to do that? I'm only down the road from Manchester

[Syn-fi]

Re: Bad DC Offset TL072 Op-amp

I'm still getting contacted about this many years down the line.

Another failure is a Harris marked chip 74HC04 Hex inverter SMD (U30) which sends S/H (sample and hold) to the TDA DAC. Tell tale sign will be a lack of a 35khz chop (the DP4 samples at 35khz). The encapsulate seems to be disintegrating and eating traces...i removed one of these, because of aberrant S/H and there was black ooze underneath.

In this scenario you might have a Bad DC offset and red-lined LED on say 3/4 but not 1/2 inputs. The TDA DAC has only 2 outputs but you will see a higher voltage on one AOL or AOR or waveform with this issue.

Bad DC Offset does not mean the problem is near the analog chop circuitry near the CX20018 (ADC x2) it can occur much farther up the signal chain and you have to start at the beginning the S/H and traces it around the board...the S/H goes to 4053s near the Chop circuitry but is also distributed to the DAC.

All input lines redlined and bad dc offset is probably a power rail

One pair red-lined is something more complicated such as I outline above with the S/H signal.

Hope this helps other comers

[eeling]

Re: Bad DC Offset TL072 Op-amp

Thank you for sharing all of this Syn-fi. Incredible diagnostic skills on display!

I have a DP4+ with the following issues:
'Bad DC offset' warning
Ch. 2 failed test - no signal from input, green LED on
Ch. 3 green LED on, faintly audible noise

I'm really hoping to be able to do a repair, someone has definitely been inside the unit before me and replaced a lot of the caps, though it's hard to tell which exactly. Some work has also been done to tidy and reconnect degraded traces around the analog i/o.

I've also noticed C99 is missing in the unit.

I'm going to start reflowing much of the analog board, checking continuity on traces and taking some multimeter measurements based on what I've read here so far. I don't own a scope so I might be a bit limited in what I can do. Yet to order any replacement parts.

Any further suggestions?

Do you do these repairs professionally?

Hope you see this! Cheers!