Hello everybody,
I'm repairing one power amp CREST LA 901....
Mainly the problem it's with the clip indicator of B channel that it's turn on all the time.....
I found a problem with driver stage STK 3152, this IC has the output for B channel clipped at +vcc.
I removed the IC but tested alone with the datasheet circuit test it looks like good but in the driver circuit don't work fine... 😕
The power stage are ok, I tested them with the A channel that work fine...
Did someone find a problem like this with that IC?
Thanks in advance
I'm repairing one power amp CREST LA 901....
Mainly the problem it's with the clip indicator of B channel that it's turn on all the time.....
I found a problem with driver stage STK 3152, this IC has the output for B channel clipped at +vcc.
I removed the IC but tested alone with the datasheet circuit test it looks like good but in the driver circuit don't work fine... 😕
The power stage are ok, I tested them with the A channel that work fine...
Did someone find a problem like this with that IC?
Thanks in advance
hello,
you could try swapping the good known driver ic from channel A to channel b to verify, sometimes semiconductor passes the bench test but fails when inserted to the ckt. Reminds me of a proamp that I had service once, It keeps on blowing the rail fuse and the output transistor, It turns out that the crowbar protection ckt is the culprit.
I have some crest schemes I can email it to you if you don't have one to make your life easier 😀
Best regards,
audiomachines
you could try swapping the good known driver ic from channel A to channel b to verify, sometimes semiconductor passes the bench test but fails when inserted to the ckt. Reminds me of a proamp that I had service once, It keeps on blowing the rail fuse and the output transistor, It turns out that the crowbar protection ckt is the culprit.
I have some crest schemes I can email it to you if you don't have one to make your life easier 😀
Best regards,
audiomachines
Hi guys,
thanks for replay....
Yesterday I received the STK spare part, I changed the IC and the circuit it works better..... I mean, I don't have a clip led on but sometimes I can see some little flash from the clip led.
Probably the broken STK it was not the cause but the effect of other problems... This evening I'll try to test the ring control loop and understand where it's introduced the spike.
I can see this flash at the startup, after few seconds and then it looks like good, I supplied the circuits with external power supply (in this way I can have a current control) but with the correct voltage.
I have a little doubt about the first op-amp , it's a pretty strange circuit OPA 627, strange because it's supplyed with +/- 22v .... this op-amp it's a pretty worm... I dont know this op-amp but I have a little doubt about.
Some advice?
Thanks in advance
thanks for replay....
Yesterday I received the STK spare part, I changed the IC and the circuit it works better..... I mean, I don't have a clip led on but sometimes I can see some little flash from the clip led.
Probably the broken STK it was not the cause but the effect of other problems... This evening I'll try to test the ring control loop and understand where it's introduced the spike.
I can see this flash at the startup, after few seconds and then it looks like good, I supplied the circuits with external power supply (in this way I can have a current control) but with the correct voltage.
I have a little doubt about the first op-amp , it's a pretty strange circuit OPA 627, strange because it's supplyed with +/- 22v .... this op-amp it's a pretty worm... I dont know this op-amp but I have a little doubt about.
Some advice?
Thanks in advance
those op-amps are probably custom built to crest specifications. The "regular" opa627 has only a maximum +/-18 Vcc.
it is normal for the clip led to illuminate a few seconds at power up that's the turn on thump that triggers the clip detect ckt. That's why we have those turn on delay relay.
It is normal to see the clip led flash a little during loud passages or loud transient. But if it flashes at a modest volume level then something is wrong with the clip detect ckt.
best regards,
audiomachines
it is normal for the clip led to illuminate a few seconds at power up that's the turn on thump that triggers the clip detect ckt. That's why we have those turn on delay relay.
It is normal to see the clip led flash a little during loud passages or loud transient. But if it flashes at a modest volume level then something is wrong with the clip detect ckt.
best regards,
audiomachines
Thanks for fast replay,
the clip led it's turn on just on the B channel at the startup....
The B channel was the faulty channel at the beginning of reparation.
The strange thing it's I can see sometimes the flash from the clip B led but without the signal and without the load...
the clip led it's turn on just on the B channel at the startup....
The B channel was the faulty channel at the beginning of reparation.
The strange thing it's I can see sometimes the flash from the clip B led but without the signal and without the load...
check for cold or cracked solder joints on the whole amp module and the power supply board/filter caps ( especially if this amp has been on the road)
mute the amp by turning the volume down.( by the way is the amp in stereo mode?)
if you had a scope you could monitor the output to see if the triggering of the clip led on channel B is cause by a large transient/noise from
the output of the amp or from the clip detect ckt. itself.
or you could hook up a cheap speaker and AC coupled it with a large electrolytic just to see if the channel B 'pops' when the clip LED flashes.
check for loose connectors
mute the amp by turning the volume down.( by the way is the amp in stereo mode?)
if you had a scope you could monitor the output to see if the triggering of the clip led on channel B is cause by a large transient/noise from
the output of the amp or from the clip detect ckt. itself.
or you could hook up a cheap speaker and AC coupled it with a large electrolytic just to see if the channel B 'pops' when the clip LED flashes.
check for loose connectors
Thanks AUDIOMACHINES,
yesterderday I tested again the power amp... I can see at the output some noise.
I have this noise in the ring control loop but I can't understand where it's introduced.
Sorry because I told you the op-amp it was op 627.... The op-amp inside it's OPA2604.
The op-amp signal it looks like good, without noise , the STK supply it's clean, the channel A works fine, the output modules work fine and all the capacitors and resistors in the control ring are ok...
It should be my workbench??? 😕 .........
I'm joke....
Did you have the schematics? If you're interested I can send you a copy...
Thanks!
yesterderday I tested again the power amp... I can see at the output some noise.
I have this noise in the ring control loop but I can't understand where it's introduced.
Sorry because I told you the op-amp it was op 627.... The op-amp inside it's OPA2604.
The op-amp signal it looks like good, without noise , the STK supply it's clean, the channel A works fine, the output modules work fine and all the capacitors and resistors in the control ring are ok...
It should be my workbench??? 😕 .........
I'm joke....
Did you have the schematics? If you're interested I can send you a copy...
Thanks!
Hi everybody,
I found what is the problem with a little noise at the ouput stage.... It's a problem with insulation at the pins 1 & 2 of the STK driver... I mean, these pins are very sensible at the external noise, I tried to clean and made a better insulation with this pins and I solve the problem.
With these gain all very invisible, little dirty on the board could be the causes of very strange noise at the output...
I found what is the problem with a little noise at the ouput stage.... It's a problem with insulation at the pins 1 & 2 of the STK driver... I mean, these pins are very sensible at the external noise, I tried to clean and made a better insulation with this pins and I solve the problem.
With these gain all very invisible, little dirty on the board could be the causes of very strange noise at the output...
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