Rotel RX-950 AX Stereo Receiver blowing speaker fuses, (6 amp fast blow)...

hdstonemeist

hdstonemeist

Active Member
So, I picked up this Rotel RX-950 AX, MKII, and the person said he was one day adjusting a control, (he didn't say which one), and it made a loud "popping" noise and the sound went completely out. (I can only gather that the pop was the two 6 amp fast blow fuses blowing out.) He said he had no idea how to check it but he didn't want it to go to the dump, and, well, I ended up with it. When I got it, I saw that the fuses were blown and replaced them. At that time, left side worked but the right side didn't work.

I found a "cold solder" on a small transistor, (@ Q617, which is a 2SD600K), that is close to the output transistors that are lined up in a row, (4 - 2SD896, [2 each side] and 4 - 2SB776, [2 each side]). I fixed the cold solder and now it is back to just blowing fuses again. Only the 6 amp fast blow, (F901 & F902), not the 5 amp fuses up close to the output side, (F601 & F602). I have checked over the boards and do not see any obvious "blown" capacitors anywhere. I looked for any more cold solders and nothing.

I do not see any obvious "over heating" anywhere either, (ie: burns on either top or bottom of boards). It does not look like anyone has replaced and/or fixed anything, (but looks can be deceiving as I thought that one of the capacitors was not polarized, but it was, it was just hard to see).

Before, I was able to take readings, as the fuses were holding out at that time, but now that the fuses keep blowing, and I can't take any readings with my multi-meter.

Before I fixed the cold solder @ Q617, the readings I was able to take of the 4 - 2SD896, (2 each side) and 4 - 2SB776, (2 each side) are as follows: With it powered on and the setting on the diode function, I got these readings: (Left side) #1 (776)= 231, #2 (776)=230, #3 (896)=214, #4 (896)=207, (Right side), #5 (896)=164, #6 (896)=159, #7 (776)=149, #8 (776)=140. With the power off and with the dial set to 20M on the ohms, I got these readings: (Left side) #1=2.90, #2=2.92, #3=2.54, #4=2.55, (Right side) #5=1.71, #6=1.70, #7=0.73, #8=0.70. (Note: The numbering starts from left to right with the picture attached. They are located @ Q623 to Q630. I think the ones on the left are for the left side and on the right are for the right side) (I used an Innova 3300 multi-meter to test.)

As I said, now I can't test at all with the fuses blowing, (at least I am not sure what to check anyway). I am by no means experienced with electronics. I have no idea what terms mean. BUT, if you spell it out what you are wanting me to do, I am smart enough to follow directions. I do have a service manual that has the schematics, but, for the most part, it is Greek to me. I can find locations and such, but as I said, have no idea what the terms mean. I do know how to do a recap as I have do so a couple of times previously. Any help with this would be greatly appreciated.
 

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Greetings;

Blowing fuses is a major issue. First is clean that board up. Discharge mains caps with a large 5w resistor or even solder a couple leads to a 60watt light bulb and discharge. get some q-tips and mild gen cleaner or windex on a qtip etc. no residue etc.. Straighten up the components get a better eye on it. i.e. I see two driver transistors touching on the middle right. not good.

I don't know what that big glob on the heat sink is. we don't know if the 'guy' adjusted the bias. So you need to read up on that. there are two VR pots in the pic. They may be bad or good.

Cold solder joints.. you caught one.. probably more. I'd pull all the driver transistors and test them as well as the output transistors. I'd replace all the electrolytic caps in view and in the power supply. the large mains caps maybe service ready but most likely replace them. Test any diodes.. you can do that in circuit power off, too.

You just have to dig in and test parts.. we all do it. Rotel is good name should come out well.

Give us a picture of the power supply section.. you'll need to check all there too... as it all starts from there.

xmas time is a little slow for responses and you have some work ahead of you. (btw try to edit your posts a little better for readings you take or what you have done in paragraphs and lists instead of making sentences.. much easier to read for us to understand.
 
Binkman said:
Greetings;

Blowing fuses is a major issue. First is clean that board up. Discharge mains caps with a large 5w resistor or even solder a couple leads to a 60watt light bulb and discharge. get some q-tips and mild gen cleaner or windex on a qtip etc. no residue etc.. Straighten up the components get a better eye on it. i.e. I see two driver transistors touching on the middle right. not good.

I don't know what that big glob on the heat sink is. we don't know if the 'guy' adjusted the bias. So you need to read up on that. there are two VR pots in the pic. They may be bad or good.

Cold solder joints.. you caught one.. probably more. I'd pull all the driver transistors and test them as well as the output transistors. I'd replace all the electrolytic caps in view and in the power supply. the large mains caps maybe service ready but most likely replace them. Test any diodes.. you can do that in circuit power off, too.

You just have to dig in and test parts.. we all do it. Rotel is good name should come out well.

Give us a picture of the power supply section.. you'll need to check all there too... as it all starts from there.

xmas time is a little slow for responses and you have some work ahead of you. (btw try to edit your posts a little better for readings you take or what you have done in paragraphs and lists instead of making sentences.. much easier to read for us to understand.
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Great! That gives me a place to start! I will do some checking and see what turns up while I wait for my order for caps to come in. I apologize, I didn't think about the formatting issue when writing the first time. I went back edited it, hope that it makes more sense now.
 
actually some readings are off but not crazy. Serious about starting in power supply, too. You mentioned caps on order.. hmm how detailed a cap job? all of it? best test the above mentioned see what fails and causes the blow. 6 amp fuses are more than 4amp and blowing. got a dead short somewhere and maybe lost some. No testing till you check out board level stuff. we all do it. timely but we do it.
 
Binkman said:
actually some readings are off but not crazy. Serious about starting in power supply, too. You mentioned caps on order.. hmm how detailed a cap job? all of it? best test the above mentioned see what fails and causes the blow. 6 amp fuses are more than 4amp and blowing. got a dead short somewhere and maybe lost some. No testing till you check out board level stuff. we all do it. timely but we do it.
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Yes sir, I was planning to replace all the caps. I know it will take time to replace them all, but don't want to do a half way fix up job. I want to use this for myself, but even if I wasn't, I would want to do it right. Do it right the first time. While I had it apart, I was planning to clean up the board, but, yes, I did notice it was really dirty. May be part of the problem. Ok, I will let you know what happens.
 
i think you are going to find one or more amp board and outputs transistors have gone short circuit . or the bridge rectifier is bad
p.s the blowing fuses arent speaker fuses , they are the rail fuses for both channels
 
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If fuses are blowing look for shorts in components, look back through similar threads to see some of the likely culprits, Happy Christmas to all.
 
OK, so I am back after doing a complete recap on the system. ALL brand new capacitors.

The only cold solder I found was the one mentioned previously and I checked the board over several times just to make sure I wasn't missing any other possible cold solders.

I also made sure that nothing was touching anywhere as it was pointed out that some of the transistors might be possibly touching. I replaced the bridge rectifier. It wasn't bad but it had readings that showed it might be going bad, so better safe than sorry. I replaced it with the one the schematics called for.

On to performance.......nothing changed. I plugged it in, let it sit for a bit so the new capacitors could charge before pressing the power button. The very second that I pressed the power button, the two fuses that were blowing previously, blew again. (Not sure if it means anything, but the fuse in F901 seems to blow out "harder" than F902. What I mean by that is the burn inside the fuse is darker in F901 than F902.) As you can see from the picture, the face plate still comes on and displays the radio station, but no sound, obviously.

While ordering parts, I found out that both 2SD896 and 2SB776 were not available from Mouser. It seems that they are no longer produced. I messaged them with the data sheet on both of them and asked if they sold anything compatible, and the parts they suggested were no where near compatible, from what I could tell. It was suggested that my initial reading of those were not too far off, so I am not sure they are really in need of replacing.

Basically, I am back at square one. I am really stumped and don't know what to do next. Any assistance would be greatly appreciated.
 

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hdstonemeist said:
person said he was one day adjusting a control, (he didn't say which one), and it made a loud "popping" noise and the sound went completely out.
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Here's my guess... It was the bias he adjusted and when the power transistors shorted the fuse blew. New fuses do not unshort the power transistors. New caps do not unshort transistors. You need to check for shorted power transistors and other parts. If the bias is turned up too high, new transistors will blow again. Many people suggest using a dim bulb tester to limit damage during startup after a repair.
 
redk9258 said:
Here's my guess... It was the bias he adjusted and when the power transistors shorted the fuse blew. New fuses do not unshort the power transistors. New caps do not unshort transistors. You need to check for shorted power transistors and other parts. If the bias is turned up too high, new transistors will blow again. Many people suggest using a dim bulb tester to limit damage during startup after a repair.
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I may have been a bit confusing when I said he was adjusting a control. The controls that he was talking about were on the front panel on the outside, as in volume, radio to turntable, that kind of control. Not something on the inside. He did know enough to check for blown fuses, and did so, but when that was not a fix, he said he has no idea about doing a recap or looking elsewhere and decided to sell it instead of junking it. I just happened to be the poor schmuck that bought it.

As I mentioned, there was MOSFET the was cold soldered on one of its three pins. I guess I need to take it back apart, unsolder all the MOSFET's and test them for shorts, starting with that one. Are those what you are saying I need to check out?
 
hdstonemeist said:
I may have been a bit confusing when I said he was adjusting a control. The controls that he was talking about were on the front panel on the outside, as in volume, radio to turntable, that kind of control. Not something on the inside. He did know enough to check for blown fuses, and did so, but when that was not a fix, he said he has no idea about doing a recap or looking elsewhere and decided to sell it instead of junking it. I just happened to be the poor schmuck that bought it.

As I mentioned, there was MOSFET the was cold soldered on one of its three pins. I guess I need to take it back apart, unsolder all the MOSFET's and test them for shorts, starting with that one. Are those what you are saying I need to check out?
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which mosfet ?
 
petehall347 said:
which mosfet ?
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It was the one located at Q617. It is a 2SD600K. (I hope that I am calling it by the correct term. If not, please let me know.) I guess the data sheet says it is a BJT, power transistor, made by Sanyo, (so I am probably not calling it the correct term). It was/should be the Base terminal part that was cold soldered.
 
petehall347 said:
q617 isnt a mosfet .. its a bjt .. NPN it is in the bias circuit .
https://english.electronica-pt.com/components-cross-reference?ref=2SD600
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Ok, well, like I said, I have no idea what things are called, (or what I'm really doing), so I guess I'm kinda dangerous. I know what I have been shown, but...

Anyway, I did some cross referencing with the link that you provided, (petehall347, THANK YOU!!!), and it seems that most all the BJT's, along with their alternate parts, are obsolete. Not sure what to do now except hope when I check them out that they are not bad. I was able to find 4 specific BJT's that were not obsolete, (one of which was the one that had the cold solder), and I ordered replacements for those.

I looked into the suggestions that redk9258 put out and I didn't think the first one would work as it was way over the specs. The second set, 2SC4467 / 2SA1694 Sanken looked close to the original parts, as far as specs, so they should be suitable replacements for the 2SD896 and 2SB776's. I ordered them from DigiKey, as they were not too expensive. I do not see a reason, (unless someone thinks otherwise, becauseI will take new readings when everything is apart again), then I won't replace them, but at least I think I have a suitable replacement on the way if needed. Not sure what to do about ordering the other BJT's that have only obsolete replacement parts, but I guess I will have to deal with that when I pull it back apart. I will check each one to make sure they are not shorted out. If I find one, hopefully, someone will have a suggestion about replacing them. At least the one that was cold soldered is one of the parts that I can get, so if it is bad because of the cold solder problem, then I can replace it.

Just asking, as I don't know, but can I, if needed, replace an obsolete BJT with one that is greater than the one that is called for. I know that it is acceptable, if you have no other way, to replace a capacitor with the next higher rated one, (IE: 10v 100uf with say an 15v 100uf)? In other words, if it calls for a 55v can I replace it with a 60v? I know that with capacitors you can't go lower, but you can go the next rating up, just don't know if it would apply with BJT's also.

As always, thank you to everyone that has assisted. It has been much appreciated. If anyone has any further suggestion, please, don't hesitate to say something. As I said, as long as I get detailed "directions/instructions", I can do what you suggest.
 
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