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Old 30th March 2006, 05:13 AM   #1
Clipped is offline Clipped  Thailand
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Default How do you check opamps?

in a previous post i was trying to fix two of my orion 225 hcca's (thanx for all the help by the way-got one them to work again)...but the other one still has low output...

what ive done since was checked/replaced all :

1.bipolars...fets....hi speed diodes-ok
2.changed all drive transistors-A06...A56...2n5539's...all of them
3.changed all caps big and small
4.checked all the mylars-ok
5.changed all diodes,regular and zeners
6.rewound a new transformer, getting 21.XX volts at the secondaries
7.replaced all the tantalums

jezus christ i just realized how much ive done to find the problem and still to no avail....ahhhhhhhhhhhhhhh


still the same problem....'extremely' low output...

only things left to check are the opamps and the 100 million resistors left on the board as well as the pot....

i really do think that an opamp is bad, one of them is mounted unevenly (sloped) from the factory..the opamp is a 2N5532N with a big 'S' before the part number...know what manufacturer this is?

so my question is..."How do you check Opamps" in or out of the board....and what is a suitable 'drop in ' replacement...cuz im not capable of putting in a different opamp and compensating for it...

thanx
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Old 30th March 2006, 06:02 AM   #2
lndm is offline lndm  Australia
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Extremely low output accompanied with a little too much background hiss is one possible outcome when an op amp is damaged, I've done it to a couple of 5532's before, doesn't mean that has to be the problem though.

If it were me, I would replace the op amp(s) before trying to replace all the resistors (for sure). The 5532 is a dual op amp which is already compensated for a certain gain (can't remember how much, not important here). Many duals are factory compensated due to not enough pins in the package (the 5534 single is not comp).

Use any brand, and if using something else, make sure it is pin compatible and can handle the same standing current. Some options are the TL072 low noise or the LM833 audio pre. I would stick with a 5532.
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Old 30th March 2006, 06:45 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
can you change the pot easily?
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regards Andrew T.
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Old 30th March 2006, 06:50 AM   #4
Clipped is offline Clipped  Thailand
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ive found a few 5532's but with different suffixes such as
2NE5532- 'AD'..'S'..'P'... can any of these be used?

opamps dont look like they are going to be fun at all to remove...
anyway to check them in the board? if not, how about out of the board?


thanx
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Old 30th March 2006, 07:39 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
you can do a few checks while the opamps are on board.

Using a DMM set to voltage;- check the voltage on the supply pins.
Check the DC voltages on the input and output pins are close to zero offset from ground and that they are approximately mid way between supply rails. Ground might not be car chassis ground.
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Old 30th March 2006, 09:31 AM   #6
lndm is offline lndm  Australia
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Quote:
Originally posted by Clipped
'AD'..'S'..'P'... can any of these be used?
There are minor differences (revisions), but you could use any. They have slightly different noise figures etc. etc.
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Old 30th March 2006, 11:31 AM   #7
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It's possible but unlikely that an op-amp will fail in a manner that causes low output. It's more likely that failure will result in a distorted output or DC offset.

What happens if you pull the two 2n5639 muting transistors from the circuit?
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Old 1st April 2006, 03:15 PM   #8
Clipped is offline Clipped  Thailand
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thanx for the help fellas, kinda slow to reply...been busy hooking up systems lately...

Andrew-I'll check the Opamps and let you know what i come up with...where do i put the negative lead of the meter?

i can change the pot, not easily but i can do it...only thing is, the new pot wouldnt be the same brand, and im not sure of the value.

Perry-if i pull the muting transistors from the board can i safely power up the amp? i've changed them already but still the same low output.

i'll pull them and power up if you verify that i wont fry anything...but, if i already replaced them and still no progress...wouldnt that mean that thats not the problem?

Indm- thanks for setting my mind at ease, ive been freaking out and thinking the 'p' means that it is a 'p' channel...

if i can fix this thing, i'll probably be able to fix anything...
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Old 1st April 2006, 03:57 PM   #9
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To test the opamp you have there, short both inputs to ground and check for 0 output. If it's bad, you will probably have a substansial DC voltage at the output.

When you remove it use chem-wik desoldering braid and suck all of the solder out of the row of pins and holes at once. I always spend 20 cents to install a socket after that, and plug in the new one.
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Old 1st April 2006, 10:03 PM   #10
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It's safe to power up the amp with the muting transistors removed.

The muting transistors may not be the problem. Something driving them could be defective causing the audio to remain muted. If the amp plays at normal volume with them removed, you know that all of the op-amps, drivers and output transistors are OK. Then you will have only about 10 components to work with to find the problem.
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