Rotel RMB-1075 blows left front fuse

[zacheriaj]

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.

[R_J]

Re: Rotel RMB-1075 blows left front fuse

Start by checking q414,15,16,17 for shorts, also check the .22 ohm resistors

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

Start by checking q414,15,16,17 for shorts, also check the .22 ohm resistors

Thank you. I will double check them tomorrow.

[Longbow]

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?

[zacheriaj]

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?

I'm going to say early 2000's. The power amp its self must be fine because every other channel works fine. It's just the left main that blows the fuse immediately at power on with no input or output connected. Checked for dead shorts on that channel and had nothing. Like you said, it's difficult to check the rails in circuit so I may isolate them.

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

Pictures.

[R_J]

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 fuse

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

Start by checking q414,15,16,17 for shorts, also check the .22 ohm resistors

No 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?

[zacheriaj]

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))

Can you give me contact points for this?

[R_J]

Re: Rotel RMB-1075 blows left front fuse

No 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?

The fuses supply + and - voltage to each amp section, If a fuse is blowing then that amp section is at fault. follow the circuit. I thought you said the rest of the amp was working except for the channel that blew the fuse.

[R_J]

Re: Rotel RMB-1075 blows left front fuse

Why 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?

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

Why 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?

Yes. Every other channel works. I have the new fuses on hand but I didn't want to risk damaging any other channels and also used the good fuses during testing. Also, being that the amp is getting some age to it, I decided to order the new style time delay BelFuses through Digikey. Figured i'd replace all at once as soon as I figure out the issue with channel 4

[zacheriaj]

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 fuse

I removed the left side amp board and isolated the emitters on the output transistors. I found Q414 dead short from collector to emmiter. Depending on cost, I may replace them all. I'll update when I can on the outcome.

[Longbow]

Re: Rotel RMB-1075 blows left front fuse

At 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.

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

At 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.

VR401 is fine and shows no signs of stress. I found R425 toasted but everything else is testing ok. Now I have to find the values of the output transistors so I can order some. I think i have it in the parts list though.

[R_J]

Re: Rotel RMB-1075 blows left front fuse

If 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

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

If 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

Found direct replacements here. I will test the drivers tonight as well.
http://www.digikey.com/product-searc...376376?k=A1492

[zacheriaj]

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.

[R_J]

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 wattage

[zacheriaj]

Re: Rotel RMB-1075 blows left front fuse

It'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 wattage

It is shown in this picture. I have it pulled now but can't see the middle color codes.