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Old 30th August 2013, 01:03 AM   #1
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Default What limits the power?

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!
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File Type: pdf AMP1.pdf (130.7 KB, 80 views)
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Old 30th August 2013, 03:13 AM   #2
sregor is offline sregor  United States
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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.
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Old 30th August 2013, 11:14 AM   #3
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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
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Old 30th August 2013, 11:24 AM   #4
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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.
Attached Files
File Type: pdf Driver stage.pdf (233.3 KB, 44 views)
File Type: pdf Mid. Pre.Amp.pdf (193.8 KB, 30 views)
File Type: pdf Power.pdf (162.1 KB, 35 views)
File Type: pdf Pre. Amp 1. 2.pdf (278.6 KB, 31 views)
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Old 30th August 2013, 02:05 PM   #5
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It seems likes it outputs the signal from the preamplifier applied by the driver to the driving transistors MJ15030&MJ15031. I check the voltage between collector-emitter for the driving transistor and power transistor, it is the HV rail, for some reason they are not turned on.
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Old 31st August 2013, 12:56 AM   #6
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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.
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Old 31st August 2013, 02:15 PM   #7
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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?
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Old 31st August 2013, 02:26 PM   #8
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no ...probably the amplifier is quite old and rated for 220 v and you are operating at 230-240 or your DVM has a failure
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Old 31st August 2013, 02:55 PM   #9
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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.
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Old 2nd September 2013, 06:01 PM   #10
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It seems those overvoltages in the rails are causing part of the problems mainly in the driver circuit section. Wonder how an expensive unit like this one runs on such a bad and "cheap" PSU design.
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