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Rotel RA1412 blowing fuses

Dazaa

Dazaa

Super Member
Got my hands on this beautiful totl integrated, but it has a few issues. Initially the left channel was very quiet, cleaned the switches and it works fine with headphones. The right channel is totally dead, both fuses blown, new ones blew instantly. Pulled the 4 2SD426 outputs and 3 test fine, one is showing continuity in all the places it shouldn't so I assume it's dead. I'm guessing that simply swapping in a new output transistor won't solve the issue. Should I pull the amplifier board out and start testing all the other transistors? The service manual says that if the fuses are blown it could be shorted caps (no problem since it will all be recapped) shorted rectifiers (I assume these are 4 large diodes on the output stage?) or a faulty output transistor (which is obviously true). How should I continue this repair? Pulling and testing the rectifier and other transistors on the output stage?

Any advice appreciated.

Edit, looking at the schematic, the fuses and rectifiers are actually on the power supply board for the right channel.
 
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I would test most everything you listed but in circuit. And if a rectifier is blown, I would for sure check all of the transistors.
In my Nikko Alpha II, I had a fuse blown and it turned out to be 3 higher wattage resistors (not the emitter ones surprisingly), a lone small transistor by them, and a diode on the rectifier for that channel.
 
Well, the diode test function on my meter is showing continuity on all 4 diodes in BOTH directions, there is also a much smaller diode which is showing the same. Surely all of them haven't failed? I'm guessing this is just the effect of checking them in circuit? I will pull them out later today and see how they test out of circuit.
 
I powered it up using a dbt and the bulb is staying bright even with all the outputs and output fuses removed. Do I need to bypass the relay in order to do this properly or is the bulb lighting up a sure sign of something seriously wrong? Seems odd as it powered up fine yesterday without the dbt...
 
exactly which fuses are out ? if supply fuses then they are before the rectifiers
is the voltage for the unit set correctly ?
 
petehall347 said:
exactly which fuses are out ? if supply fuses then they are before the rectifiers
is the voltage for the unit set correctly ?
Click to expand...

All 4 T8 fuses, which I assume are the supply fuses, f901, 902, 903, 904 on the schematic. Voltage is defo set correctly.

Edit, f903 and 904 of the right channel are the ones which are blowing, but I powered it up on the dbt with all removed and still had the lamp glowing brightly.
 
it should only have the 5v connected then .. i haven't figured out what LA is yet .
what lamp is in your dbt ?
 
there is a 5 volt tap off the transformer called LA . not sure what it powers .
try a higher wattage lamp .. 60 watt or maybe 100w if you have one .
40w is maybe not enough as there are 2 transformers .
you could also check ACV coming out of the transformer .. one probe to chassis the other to each fuse holder . see if they are all equal .
 
Will try a different bulb later today. Would be nice if we could still get 100w bulbs in the shops these days. Do you suggest powering up without the dbt to check the voltages at the fuse holders?
 
good news, bigger bulb dims on power up. Bad news, when plugging it in directly there was a nasty pop from the working channel ps board, closer inspection shows the board has cracked in the past and been repaired, will need to pull it out and see what's going on, I didn't leave it on long enough for the relay to click on.
 
Ok so I removed the fuses from the damaged board and powered up again on the dbt, relay clicked on and the fuses on the previously faulty channel stayed intact and the bulb continued to be dimly lit, this is all whilst the one faulty output transistor has been removed and the other 3 are in place. Is this a good sign?
 
you should have removed both output transistors for that channel .but doesn't matter if fuses removed .
relay clicking in is a good sign .
 
Ok here is some more good news. I powered up again on the dbt, one fuse (the holder which appeared to have sparked) removed on the good channel, both fuses in on the bad channel. Relay clicks on, hooked up a source and some headphones and we have sound on both channels, both meters working, although it wasn't very loud until the volume was at max, I assume the missing fuse and transistor were to blame for this.

Would it be safe to assume I have no more shorted components to worry about? Will it simply be a case of replace the transistor, solve the odd sparking issue and then deal with the bias, dc offset ect?
 
get the power supply fixed first .
then replace the pair of outputs on the bad channel and check/set bias and offset .
you shouldn't have run speakers without outputs because the drivers cant supply the current on their own . its lucky you didn't blow them . or smoke some resistors .
 
I only hooked up headphones with one output transistor removed, I read that this is ok to do for a short period of time but will result in lower power output. Do you suggest replacing the pair of output transistors rather than just the single faulty one?
 
I checked the offset on the speaker terminals and the left channel is showing about 20mv and the right is showing nothing, neither is there any sound from the right channel despite it working fine on the headphone output and the vu meter displaying correctly.

Could the missing transistor be the cause or should I start checking the pre drivers and emitter resistors? Seems odd that anything else could be shorted as it doesn't blow fuses nor show any problems with the dbt...
 
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