Rotel RB 990BX fuse issues?
Dabutcher
Posts: 2,596
Poppin some tags at the local thrift store yesterday and picked this up for $23.51 as is- no return. Hooked it up to my garage rig w the intention of powering my Monitor 10A,s. only one side works? There are four fuses inside total and one on the back. The side that does not work has a blown 6.3 AL 250v fuse. Other than replace fuse and try again. Is there anything else to look at or try? As always thanks. DarrylAttachment not found.
MIT Magnum MH-750, Monster HTS 5100MKII, Sony 77" Class - A80CJ Series - 4K UHD OLED,PS4, Def Tech 15” sub,LSIM 706c, Sunfire Signature Grand 425 x 4,Parasound hca 120, LSiM 702 x 4, Oppo 103D, SDA SRS 1.2, Pioneer Elite SC63 , Pioneer Elite BDP-05 “Why did you get married if you wanted big speakers?”
Post edited by Dabutcher on
Comments
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My rude and crude check before offering up another fuse would go like this....
Check all the big PS capacitors for stored voltage with a DC meter. Also for a good measure check each of those fuses especially on each side of any suspected blown. That's the safety first stuff.
Then look for resistance differences at the same point on each channel. Go from ground to each of the fuses and on the blown one, on each side of the fuse looking for any more than a small difference. Check each channels output devices in the same way to ground, comparing channels. Right on the exposed metal can side.
If you show a very low resistance on the one with the blown fuse then you've uncovered a blown output. This would be the expected normal failure to have taken out a rail fuse. Sometimes, when an output goes the driver goes too. If so, look into any coupling caps carefully on that blown side. If bad at all, then change all the coupling caps.
It's possible somebody got carried away with level and you'll find no low resistance numbers, you'll change the fuse and all will be good. Let's hope so! But do the easy basic checks like that before you put in another sacrificial fuse.
Good luck!
CJA so called science type proudly says... "I do realize that I would fool myself all the time, about listening conclusions and many other observations, if I did listen before buying. That’s why I don’t, I bought all of my current gear based on technical parameters alone, such as specs and measurements."
More amazing Internet Science Pink Panther wisdom..."My DAC has since been upgraded from Mark Levinson to Topping." -
Your technical skills go way beyond mine. But I do have a multimeter and will give it a go Sunday as you have specified . Do I need to worry about shock from the voltage you talk about in the capacitors? I heard the old tube TV's had stored energy that could hurt you. Thanks. DMIT Magnum MH-750, Monster HTS 5100MKII, Sony 77" Class - A80CJ Series - 4K UHD OLED,PS4, Def Tech 15” sub,LSIM 706c, Sunfire Signature Grand 425 x 4,Parasound hca 120, LSiM 702 x 4, Oppo 103D, SDA SRS 1.2, Pioneer Elite SC63 , Pioneer Elite BDP-05 “Why did you get married if you wanted big speakers?”
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Yes, you can hurt yourself! If it's not something you're comfortable with, then stop. Let someone else do a few preliminary checks for you.
A bargain is not a bargain if you hurt yourself and it's reasonable to assume it's not an emergency to get it going. Multimeters must be set and used right or you run a risk with them alone.
The purpose of checking the caps is to confirm that voltages do not exist. If those are discharged and the unit unplugged, then your safe. But you must know how to use the meter correctly.
Please be safe and error on the side of care. Find someone to assist and the unti will live for another day and so will you!
CJA so called science type proudly says... "I do realize that I would fool myself all the time, about listening conclusions and many other observations, if I did listen before buying. That’s why I don’t, I bought all of my current gear based on technical parameters alone, such as specs and measurements."
More amazing Internet Science Pink Panther wisdom..."My DAC has since been upgraded from Mark Levinson to Topping." -
Thanks for all the help. I just installed a new fuse and it works great w the NAD pre and Monitor 10A,s in the garage set up. Have a great weekend! DarrylMIT Magnum MH-750, Monster HTS 5100MKII, Sony 77" Class - A80CJ Series - 4K UHD OLED,PS4, Def Tech 15” sub,LSIM 706c, Sunfire Signature Grand 425 x 4,Parasound hca 120, LSiM 702 x 4, Oppo 103D, SDA SRS 1.2, Pioneer Elite SC63 , Pioneer Elite BDP-05 “Why did you get married if you wanted big speakers?”
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Right channel had two blown fuses. I suspect the speaker cables were shorted. I replaced them and all was ok for about a year. Now the fuses are still intact, but the right channel has lost all treble. I can still hear some bass. A starting point would be helpful.
A visual inspection showed no obvious damage. Protection circuit light is ok.
Thanks! -
Glad to hear your all good DaButcher! Your livin' the dream of many peoples cheap/broken pickups. One that's an easy fix in the end! Congrats!
johnchabin - It's easy with a previous problem to go right back to the same place. But have you checked to make sure it's the amp? Flipped R and L speakers. And seperately, flipped the source inputs? A high and low jumper on the rear of the speaker with a bad connection? Blow crossover elements? You may well have done this already, but a speaker problem would be my first guess.
Past that...is there level pots? A really bad connection like a wiper on a pot might tend to give the problem you've listed. Or the input connectors broken loose? If the output got seriously stressed before then I'd wonder if one side of a complimentary pair going bad now might not pull enough to blow a fuse but create sonic problems. If so, then I'd think of flipping output transistors between channels if they are socketed and easy to do. Otherwise, then look for high voltage drop across a resistor that doesn't match the other channel.
Was this sudden or get bad slowly over a period of time? Does it start ok before warmup? If so then careful use of freeze-it and a heat gun can help find trouble.
Electrolytic caps drying out first tend to have weak bass, not the weak highs you've mentioned. But if the amp age is towards ten years, then if servicing is necessary, then it'd be time to go through the stage coupling caps along with the power supply ones.
Come back with some details and maybe somebody will have something helpful. Also your electronics abilities.
CJA so called science type proudly says... "I do realize that I would fool myself all the time, about listening conclusions and many other observations, if I did listen before buying. That’s why I don’t, I bought all of my current gear based on technical parameters alone, such as specs and measurements."
More amazing Internet Science Pink Panther wisdom..."My DAC has since been upgraded from Mark Levinson to Topping." -
All speaker possibilities were eliminated. It's the amp.
I see a level pot for each channel. I'll check them next time I have the amp open. I had to put it back together to get it off the dining room table.
I checked the input connector. Seems ok.
I don't see any socketed transistors. All soldered, but I can handle that.
It was sudden and persists whether warm or cold.
I used to service circuit boards (input cards) for industrial PLCs. I'm comfortable with electronics and understand the hazards. I have the schematics, but there are so many transistors I'm not sure where to start. I checked them all for a dead short across collector/emitter, but all measured ok. This would be easier if I could generate a square wave and had a scope so I could compare the two channels at different points in the circuit to narrow down the bad component(s).
Thanks! -
Darn. The way you listed the trouble, it sounds most likely to have been speaker issues. It'd been easier if it had been!johnchabin wrote: »All speaker possibilities were eliminated. It's the amp.
Somehow, having electronics spread all over the table does create issues... :eek:johnchabin wrote: »I see a level pot for each channel. I'll check them next time I have the amp open. I had to put it back together to get it off the dining room table.
Make sure it's a level pot your looking at and not bias. Just sayin"...... Again, looking for the easier first. Just touch it to know if it suddenly pops back in.
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We're looking for the big output transistors here. They'll be the ones heatsinked to the sides. If soldered in, then just look to compare current draw from the working side to the troubled side first.johnchabin wrote: »I checked the input connector. Seems ok.
I don't see any socketed transistors. All soldered, but I can handle that.
An alternate to the scope and square wave could be to burn a disc with a sequence of kind of short tones. Maybe 10-20 seconds of a 100hz, 1k, 10k. Just step thru over and over. Then you can hook up a level controlled CD player to the amp and you're good to step thru stages and see where trouble begins.johnchabin wrote: »It was sudden and persists whether warm or cold.
I used to service circuit boards (input cards) for industrial PLCs. I'm comfortable with electronics and understand the hazards. I have the schematics, but there are so many transistors I'm not sure where to start. I checked them all for a dead short across collector/emitter, but all measured ok. This would be easier if I could generate a square wave and had a scope so I could compare the two channels at different points in the circuit to narrow down the bad component(s).
Thanks!
Just use your AC voltmeter looking for where you significantly show the high end dropping down in level. An average non RMS voltmeter should still be good enough to see the trouble. Again, look at the good side to determine how much the meter drops off at the higher end. Not a scope, but could be good enough to find bad rolled off audio.
I'd be looking to see if the driver stage output looks identical between channels. It should be capacitor coupled and that'd be good point to try to find. If you can prove good at that point, then you can focus on the output section. Your mention of a previous fuse blown from a possible output short has me suspecting something in the output went bad later. But in any case, you try and divide the good and the bad. Find where you're good and then not good.
Yes, the problem is the translation between the schematic and what's physical in front of you. All the parts and wires can be intimidating. Get parts identified and narrow the focus.
You may find that the headsinked sides can be unfastened and lay down without having anything removed. Ideally, any way.
Good luck doing a little looking!
CJA so called science type proudly says... "I do realize that I would fool myself all the time, about listening conclusions and many other observations, if I did listen before buying. That’s why I don’t, I bought all of my current gear based on technical parameters alone, such as specs and measurements."
More amazing Internet Science Pink Panther wisdom..."My DAC has since been upgraded from Mark Levinson to Topping."
