Customer unit. The customer shorted out a couple of outputs but can't remember which. Once I got it, fuse 402 was blown. I ran some tests, and couldn't find any issue other than the simple fuse. Replaced them with the same spec and with no external connections, it blew fuses 401 and 402. Can anyone give me a good starting point for further testing? I have attached the service manual with schematics.
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Rotel RMB-1075 blows left front fuse
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Re: Rotel RMB-1075 blows left front fuse
This is a big amp. You will find lots of problems using resistance checks from the main rails to ground. Blown output stages can often take the whole power amp, so look beyond your output transistors.
Since it uses a conventional supply, you might benefit by using a variac. Fuses blow for a reason.
Interesting push-pull input circuit. What is the approximate date on this unit?Last edited by Longbow; 06-21-2016, 09:54 AM.Is it plugged in?
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by Longbow View PostThis is a big amp. You will find lots of problems using resistance checks from the main rails to ground. Blown output stages can often take the whole power amp, so look beyond your output transistors.
Since it uses a conventional supply, you might benefit by using a variac. Fuses blow for a reason.
Interesting push-pull input circuit. What is the approximate date on this unit?You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
If fuse 401 and 402 blow, you have a fault in the power amp that is connected to P4 & N4 Left front (last power amp on the last page) If you don't have a shorted output or open emiter resistor, the you could have either a shorted or leaky driver transistor in that circuit.
Those fuses supply the L.F. amp and if they blow then your fault is too much current draw through the fuse, You could try compairing resistances between the L.F. amp section and the Center amp section at the top of that page (3** component markings)
Without a variac it could be hard to troubleshoot, what I would do is slowly turn up the variac while I checked the voltage at S4B to ground, It should be 0 volts, if its higher I would would also note if its positve or negative with respect to ground, then I would know what part of the circuit had the bad bias problem.
If the outputs and the .22 res. check ok then something before them is turning the output transistors on full. and that will take the B+ (or B-) supply to ground blowing the fuseLast edited by R_J; 06-21-2016, 07:32 PM.
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostStart by checking q414,15,16,17 for shorts, also check the .22 ohm resistorsYou Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostIf fuse 401 and 402 blow, you have a fault in the power amp that is connected to P4 & N4 Left front (last power amp on the last page) If you don't have a shorted output or open emiter resistor, the you could have either a shorted or leaky driver transistor in that circuit.
Those fuses supply the L.F. amp and if they blow then your fault is too much current draw through the fuse, You could try compairing resistances between the L.F. amp section and the Center amp section at the top of that page (3** component markings))You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by zacheriaj View PostNo shorts on those at all. Resistors are all good. Is it possible that the fuse blew while testing because I don't have the other channel fuses installed?
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostWhy did you take out the other fuses? I thought they were ok and only fuse 402 was blown.
If all the other fuses are in place and 401 & 402 are left out does the amp power up and do the other channels work?Last edited by zacheriaj; 06-21-2016, 08:19 PM.You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostIf fuse 401 and 402 blow, you have a fault in the power amp that is connected to P4 & N4 Left front (last power amp on the last page) If you don't have a shorted output or open emiter resistor, the you could have either a shorted or leaky driver transistor in that circuit.
Those fuses supply the L.F. amp and if they blow then your fault is too much current draw through the fuse, You could try compairing resistances between the L.F. amp section and the Center amp section at the top of that page (3** component markings)
Without a variac it could be hard to troubleshoot, what I would do is slowly turn up the variac while I checked the voltage at S4B to ground, It should be 0 volts, if its higher I would would also note if its positve or negative with respect to ground, then I would know what part of the circuit had the bad bias problem.
If the outputs and the .22 res. check ok then something before them is turning the output transistors on full. and that will take the B+ (or B-) supply to ground blowing the fuseYou Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by Longbow View PostAt minimum, it is likely that R421 is open. Q414-17 are your outputs. I would spend some time calmly checking all the parts in that amp section including resistor values. Make sure VR401 does not show signs of burning. Good luck.Last edited by zacheriaj; 06-22-2016, 11:12 AM.You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostIf R425 was open, I would check the driver transistors also, like Q407, 408, 409 and maybe back to Q401, Q402 Digikey has the output transistors for about $4.00 each
http://www.digikey.com/product-searc...376376?k=A1492You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
I tested everything on the board. Driver transistors are ok. diodes are ok. The only other fault is the R425. Any idea how to find the value for replacement? I'm thinking it's either 33ohm or 330 ohm but no idea on the wattage.You Can't Succeed Until You're Willing to Fail
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Re: Rotel RMB-1075 blows left front fuse
It's a 33 ohm metal film, (see schematic) not seeing it I would guess 1/2 watt. measure the voltage drop accross the same resistor (any of these: R125, r126, R225, R226, R325, R326 ) in one of the working amp circuits, ohms law will give you the wattageLast edited by R_J; 06-22-2016, 10:33 PM.
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Re: Rotel RMB-1075 blows left front fuse
Originally posted by R_J View PostIt's a 33 ohm metal film, (see schematic) not seeing it I would guess either 1/2 watt or maybe 1 watt . measure the voltage drop accross the same resistor (any of these: R125, r126, R225, R226, R325, R326 ) in one of the working amp circuits, ohms law will give you the wattageLast edited by zacheriaj; 06-22-2016, 10:28 PM.You Can't Succeed Until You're Willing to Fail
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