tl082

[Chrisr3521]

hi, just a quick question and please ignore my ignorance as i'm qiute new at this!
with the op-amp (tl082) is it normal for the output voltage to rise slowly when the amp i just idling? if it is, is it due to the rise in temp?

thanks
chris

 

[lineup]

you are talking about the dc-output voltage?
what level is this voltage?
how long time before it settles?

 

[Chrisr3521]

sorry yes the d.c output level, and it dosen't settle it just keeps slowly rising looking at the schematics for this amp it says the voltage must be below 100mv however like i said it just keeps rising(takes about 3 hours to rise 40mv) there is a polyester cap that connects pin2 which is the inverting input A to pin 1 which is the output voltage could this be leaking causing it to rise?

thanks

 

[Jensen DK]

It would be nice to have some more information, otherwise it is a pure guesswork what is happening.

Could you please post a schematic?

 

[lineup]

this sounds not right. for hours.
5-10 minutes or less might be, but ....

and 40mV DC-offset sound a bit high, but depends how high gain you have
typically TL07X and TL08X has got 3 mV input offset (max 6 mV)
so with a gain 10, we can expect like 30 mV (max 60 mV)

 

[Chrisr3521]

hi, sorry heres the schematic

 

[Jensen DK]

Thanks for the schematic.

Unfortunatly the resolution is so low, that the components can´t be identifyed.

Is it possible for you to post again in a higher resolution ?

 

[Elvee]

For an isolated TLO82, this wouldn't be normal. But in schematic such as yours, it could have a role like DC-servoing, and in this case, the variation simply indicates the circuit does its job and adapts to the conditions, differential T°s and the like.
With a better picture, we could be more specific.

 

[SY]

If it's the one on the lower left, it's a servo integrator and the slow drift is not at all unusual- it's an indication that the servo is doing its job.

 

[Chrisr3521]

hi, after a fustrating few trys to find the cause of this, i still hav'nt, and it is now outputing around 900mv! the only thing i have found is that there is about 300mv going to the case and the power supply to the power amp varies 100's of mv at a time, i think these are probaly related, also i was wondering if ic101 (which is a shunt reg) on the schematic could cause these problems with the tl082?

any help would be greatly appreciated. i'll upload the service manual if thats any help?


thanks
chris

 

[unclejed613]

it's difficult to read the schematic, but the cap should have a resistor across it, and from what i can see it doesn't. tl082's have picoamps of input bias current, and over the period of several hours into a low leakage cap, can cause substantial offsets if there's no discharge path for the cap. in theory, an integrator (op amp with cap in the feedback path) consists of just an op amp, cap and an input resistor. but in reality, there should be a resistor in parallel with the cap. that's one possible reason for your problem, with a second possibility closely related. the op amp could have a problem in the input stage where more input offset current is charging the cap than is there normally. this would be the result of damaged FETs in the op amp, and so replacing the op amp is the answer here. the op amp normally has a very high PSRR, so variations in the supply rails shouldn't be an issue, unless again there's a problem with the op amp.