I am having trouble with a power stage. It is giving very low power and I was wondering if there might be a limiting circuit doing that. The attached file AMP1 is the schematic.
Could triac BTB24-400 be limitting the output? I thought the triac would short output to ground a open protection fuse. But no fuse is gone and the power is just reduced. Actually the voltage appllied to the driving BJTs (MJ15030&MJ15031) is bigger than the output voltage, what can be producing this? Bad triac? Thank you!
Could triac BTB24-400 be limitting the output? I thought the triac would short output to ground a open protection fuse. But no fuse is gone and the power is just reduced. Actually the voltage appllied to the driving BJTs (MJ15030&MJ15031) is bigger than the output voltage, what can be producing this? Bad triac? Thank you!
rest of circuit woould probably prove helpful. Signal picture if available would tell a lot.
My limited experience is that if the scr is triggered, fuse would definitely go. I would wonder about the rest of the circuit and the power connections to the output transistors. I think they are TO-3 transistors (metal cases) and the supply voltages go through the screws to te cases (the metal cases should have B+ and B- voltage on them. If voltage i missing, the output would go through just the drivers, making them hot or worse and power limited through the 10 ohm resistors.
My limited experience is that if the scr is triggered, fuse would definitely go. I would wonder about the rest of the circuit and the power connections to the output transistors. I think they are TO-3 transistors (metal cases) and the supply voltages go through the screws to te cases (the metal cases should have B+ and B- voltage on them. If voltage i missing, the output would go through just the drivers, making them hot or worse and power limited through the 10 ohm resistors.
without the rest of the circuit known it is almost impossible to tell
It could be a VI limiter issue which is nested in the driver area
It could be a failure or setting in the input limiter if any exists (LDR style or so )
It could also be some power supply issue in the first stages of the amp .
After a quick look at the schematic these amplifiers feature both xover and limiter i think so probably you need to verify setting and/or separate units to see where the fault comes from ...rather complicated ...start with checking voltage present first ...you need to work with a method ...
as about the triac No i think cause most of the times triac short and don't have ""about"" conditions usually its an on off thing .
You may as well remove it from there since the triac will only operate in case you have DC in the output ( only for testing procedure ) monitor offset , verify rail voltage at both sides ( often people get confused with that )
Kind regards
Sakis
It could be a VI limiter issue which is nested in the driver area
It could be a failure or setting in the input limiter if any exists (LDR style or so )
It could also be some power supply issue in the first stages of the amp .
After a quick look at the schematic these amplifiers feature both xover and limiter i think so probably you need to verify setting and/or separate units to see where the fault comes from ...rather complicated ...start with checking voltage present first ...you need to work with a method ...
as about the triac No i think cause most of the times triac short and don't have ""about"" conditions usually its an on off thing .
You may as well remove it from there since the triac will only operate in case you have DC in the output ( only for testing procedure ) monitor offset , verify rail voltage at both sides ( often people get confused with that )
Kind regards
Sakis
Thanks sregor. I attached schematics of the driver unit, preamp, mipreamp (limitter) and power source.
Basically the driving signal that the driver transistor gets is what I have in the output of each channel and also is the same I get in the emmitters of the power transistors, no amplification at all and they are all ok. The B+ and B- loos ok and all the power transitor get the expected voltage.
Basically the driving signal that the driver transistor gets is what I have in the output of each channel and also is the same I get in the emmitters of the power transistors, no amplification at all and they are all ok. The B+ and B- loos ok and all the power transitor get the expected voltage.
It may seem a stupid question but I am starting to suspect from the driver circuits, what is supposed to be seen in the base of the driving transistors? which kind of signal will produce the driver circuit to attack the base of the driving transistor in the power stage?
The signal coming from the preamplifiers coupled with the bias voltage?
The driving BJTs are not being turned on so I guess the problem might be in the driver circuits.
The signal coming from the preamplifiers coupled with the bias voltage?
The driving BJTs are not being turned on so I guess the problem might be in the driver circuits.
Just wondering, another on.
Checking B+ and B- rails voltage I realised that what is suppose to be +70V&-70V for the power transistors is actually +87V&-87V.
Also another rails suppose to be +80V&-80V is reading -99V&-99V.
These voltages come from their power supplies wich are basically a transformer, rectifier bridge and filtering caps. I measure the ripple and was acceptable so I guess the caps are ok. Could these high rails' voltages causin problems?
Checking B+ and B- rails voltage I realised that what is suppose to be +70V&-70V for the power transistors is actually +87V&-87V.
Also another rails suppose to be +80V&-80V is reading -99V&-99V.
These voltages come from their power supplies wich are basically a transformer, rectifier bridge and filtering caps. I measure the ripple and was acceptable so I guess the caps are ok. Could these high rails' voltages causin problems?
Good to know. Actually the amplifier is rated for 235Vac and the DVM is fine. I wonder how come it differs so much from the rated values even a 25% 
Still wondering why the power stage is not working. All the comps are fine and the driver BJTs are getting signal on their bases. Suspecting of some grounding problems.
Still wondering why the power stage is not working. All the comps are fine and the driver BJTs are getting signal on their bases. Suspecting of some grounding problems.
....all units produced work with these types of power supply ....
The question will be how do you know the original voltage ? do you have a schematic ? does this refer to the specific product ?
Many manufacturer use one type of pcb for many models of the same amplifier for a series of power amplifier a common pcb can be used since the only thing that change is the rail voltage and the number of outputs plus a few other minor things .
Power supply do not ""fail"" like that ..there is no chance that the psu was meant to be for 87+87 volts and now for some failure reason is 99+99 this is wrong .
Yet again it could me more possible that the schematic or the pcb markings refer to another model of the same amplifier .
Still you are approaching the problem with the wrong method and measure and this begins to be a waste of time ....
The question will be how do you know the original voltage ? do you have a schematic ? does this refer to the specific product ?
Many manufacturer use one type of pcb for many models of the same amplifier for a series of power amplifier a common pcb can be used since the only thing that change is the rail voltage and the number of outputs plus a few other minor things .
Power supply do not ""fail"" like that ..there is no chance that the psu was meant to be for 87+87 volts and now for some failure reason is 99+99 this is wrong .
Yet again it could me more possible that the schematic or the pcb markings refer to another model of the same amplifier .
Still you are approaching the problem with the wrong method and measure and this begins to be a waste of time ....
The DC voltages printed on the power schematic (sheet 10/10) are complete garbage. They show input 75VAC from TRFR1,2 but DC rails of only 70V? Obviously this is nonsense without errata or explanatory notes but even so, that is bad drafting procedure. If the transformer AC supply is indeed 75V, this should result in +/- 105VDC rails, part loaded.
You should check what you read for sensibility and do something useful by measuring (since your DVM is accurate, you say
) the voltages at the points where it is printed incorrectly. Print off copies of the files and use a red pen or whatever to remind you what is real and what is just wrong. Perhaps then you will be in a position to make judgements about what may be wrong with the amplifier.Problem solved. Some semiconductors of the drivers got deteriorated and fail as more power was required. Everything working perfectly now.
Sakis, please do not be arrogant and believe yours is the only method to fix something. I just start checking from the power supply to the rest of the unit and guess what? My method was successful again, perfect aproach and waste of time may be for you.
Ian, u'r right but correct me if I'm wrong but transformer output wont be 105V if loaded, that 1.4 rule applys to unloaded transformation, doesn't it? Thanks.
Sakis, please do not be arrogant and believe yours is the only method to fix something. I just start checking from the power supply to the rest of the unit and guess what? My method was successful again, perfect aproach and waste of time may be for you.
Ian, u'r right but correct me if I'm wrong but transformer output wont be 105V if loaded, that 1.4 rule applys to unloaded transformation, doesn't it? Thanks.
The regulation of a transformer (the % of voltage drop from the specified AC secondary voltage) is part of the transformer's specification, even if not printed on the label. A typical transformer starts off, unloaded, at about 6% above the rated voltage, but should fall to the specified voltage at ~ 10% of the rated current loading. A toroidal transformer then has good regulation as low as 5% at 1kVA but ~ 10% with 160VA size. Size does matter for the further voltage drop expected by the time full current loading is reached. Thus, the transformers' regulations would be quoted as 5% and 10%. Most "good ol" E-I transformer equivalents are not so good.
It shouldn't take long to realise that if you fit a huge transformer in there, the voltage drop under load will then be small because the max. current demand comes nowhere near the rating and regulation now available to power that circuit. This is actually not a good situation for designs in which transformers are sized to protect the output stage from extremes by virtue of the voltage drop, as much as keep on powering safely.
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