I ΄ve recently bought for cheap a Rotel RA-980BX integrated amplifier. The amp was in a very bad state inside and looked abused and very dirty. Looked like it was played in a beach bar. Many of the capacitors had been rotten as well as many resistors and diodes. I ΄ve replaced all of them one by one with quality replacements. Critical parts as the rectifiers, the power transistors and the main caps looked OK. So when I tried to power up the amp the fuses blown imeediately so I decided to start using a braker type fuse cos I knew that this would go very far. The tranformer gives 42volt across red - black and 42v across orange -black and 84v between red - orange. I ΄ve checked the rectifiers of the board and the seem to be working fine. I ΄ve found that one of the 250v capacitor c905 was faulty and replaced but not luck. I΄ve noticed that there must be a great amount of surge or a big short cause the 10amp bracker drops imediately on both suplies ac1 ac2. I΄ve checked all of the power transistors by desoldering them off the board and they looked fine. I tried to power up the amp with the transistors unconnected and guess what the braker went off again. I also noticed that the R623 and R624 went blown. I don΄t know what else to look for or what is the proper checking procedure so I am going to need your help.I really need this thing to start working again..
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Assuming you've already done whatever damage can be done by simply plugging in and turning on the power with inadequate protection, it can't hurt much more to run routine tests, starting off with a dim bulb tester (DBT). This is just a plain, ~60W incandescent lightbulb wired in series with the mains power. Google it for literally dozens of home-brew illustrations of how to build a simple device like this, aka "lightbulb limiter". Some suggestions are obviously flaky, dangerous or overdone, others sensible and safe (yes, 110VAC does kill!). Bear in mind that most suggestions are by and for guitar players with tube amplifiers drawing relatively high current even idling. These typically need big old 150-250W bulbs which are way too much for this solid-state amp.
Unfortunately, a circuit breaker only saves fuses here, not the amplifier. Significant fault currents in the amplifier could appear as quite small and normal on the primary side of the power transformer. However, if you have a DBT inline, the bulb will always be limiting to some degree, from nothing at all to just glowing and then full brilliance. This actively limits the transformer primary current to the bulb's rating and if you can get one, a 40-60W bulb is appropriate for a small stereo amplifier and usually not enough to allow further damage.
Then you can simply check for DC voltage at the output (speaker) terminals. As long as the output protection relay has closed and you have any DC voltage greater than say, 100mV there, you do have a serious problem and most likely with the output stage. It typically means shorted power transistors if not also collateral damage to the driver and voltage amplifier stages too.
Unfortunately, a circuit breaker only saves fuses here, not the amplifier. Significant fault currents in the amplifier could appear as quite small and normal on the primary side of the power transformer. However, if you have a DBT inline, the bulb will always be limiting to some degree, from nothing at all to just glowing and then full brilliance. This actively limits the transformer primary current to the bulb's rating and if you can get one, a 40-60W bulb is appropriate for a small stereo amplifier and usually not enough to allow further damage.
Then you can simply check for DC voltage at the output (speaker) terminals. As long as the output protection relay has closed and you have any DC voltage greater than say, 100mV there, you do have a serious problem and most likely with the output stage. It typically means shorted power transistors if not also collateral damage to the driver and voltage amplifier stages too.
As I said ...the state this amp was found was not promissing at all and I was not expecting to go so far.. The power transistors are desoldered from the board and is still tripping the braker. For clarification the braker replaces the fuses. All the transistors are checking out as well as the rectifiers unless they are developing a short when current gets throught them. The PT is my primary suspect. Can any one guide me throught how to check it. The only thing that I havent check yet is the protection circuit. I will do the lamp test. I am not an expert in electronics but as a mechanical engineer I wont let go. I have to identify the fault
Here's a simple way to test for a transformer short, as those are not always so obvious with a simple ohm meter test.
Blown fuses indicate a short or very low impedance.
Its usually outputs gone but can also be power supply rectifiers or smoothing caps gone. Try powering it up with amp pcb disconnected and just the supply running.
You said the outputs looked ok ? You cant always tell by looking if they are gone. Look up transistor diode check to see how to test them.
If they are mosfets then usually they go short between any of the terminals.
this is exactly what I thought it would be. very obvious short that i cannot discover. I have disconnected all the power transistors diodes from the board but It still burning fuses. I desolder the rectifiers from the board and tested them with that same PT and they give me stable dc current. The only thing that I haven't tested yet are the smoothing capacitors. I also would like to identify where the supply ends so I would isolate and test (looks dificult). Power transistors are simple NPN PNP type and no mosfet so I guess there is no trick to them to Identify a shorted one. Next thing I am going to check is the PT. Unfortunately I can not find info about output figures for it.
Don't bother with all that crap and neon bulbs.
SIMPLY DISSCONNECT the secondary wires of the transformer making sure they don't touch one another or anything, and then run power (through a fuse of course) to the primary.
As I previously mentioned, it's a fast, easy way to troubleshoot the damn thing.
If the fuse blows, the transformer is BAD.
OR, if you use a 60W lamp in series with the primary, it will glow brightly.
A dim glow means good.
Hi!! Thank you for your contribution here!! I kept desoldering and meassuring the components on the board and I find nothing obvious whrong with them. I΄ve also measured the resistance of the primary winding (the 220v winding) and it gives me 6.2Ohms . The secondary (the one giving power to the board) is giving 0.5Ohms lead to lead and lead to the ct 0,32Ohms each. Is that normal? Should the secondary winding be of a higher resistance_?
NO.
Those are typical readings.
Do the power test that I previously mentioned, to confirm that the transformer is good.
Resistance readings cannot determine that.
Those are typical readings.
Do the power test that I previously mentioned, to confirm that the transformer is good.
Resistance readings cannot determine that.