Rotel RKB-8100 Amplifier going into protection mode

[redoxy]

Unit goes into protection mode within a minute after turning on.
I get sound for a few second before protection mode.

The unit seems to be made up of "two identical halves", which I think will help during troubleshooting.

I tried finding a technical/service manual online but so far by Googling has returned no results.

I think I narrowed it down to Amp 1 in the first picture (pardon my ignorance if that is not an Amp). There are some burnt Chip resistors (shown in picture 2 right image). The big one (R49) is easy to tell from Amp 2 but the two small ones (R52 and R28) I cannot tell.

I also noticed two transistors were bad. Hard to get a picture of it as it they are hidden on the board.

Some questions for the Pros here:

1. A search on Digi-key for the big chip resistor showed it will be a 0.05 Ohms resistor, but I saw multiple Watt ratings available. How can I determine the wattage to buy without access to service manual?

2. ow do I determine the values of the small chip resistors the purchase a replacement?

3. the small chip resistors, how do I determine their value in order to get replacements.

4. I noticed that on Amp 1, the solder on one of the surface mount ceramic capacitors (C34) is joined to the middle pin of a nearby transistor whereas that is not the case on Amp 2. Could that be a reason for the damage, and should I desolder and resolder so they are no longer joined?

5. Are there anything else that will likely get damaged along with the ones that are visibly burned that I should be checking and replacing while at it?

6. This might be a stretch but is there a possibility of disabling one side and still have the other side working? I do not need all of the channels so I would consider doing that if possible.

Happy to answer any questions and upload additional pictures.

Full disclosure: I am not a pro by any stretch of the word, but I have basic knowledge and very good at follow instructions. I have decent soldering and component replacement skills.

Thanks in advance

[tmaster21]

Re: Rotel RKB-8100 Amplifier going into protection mode

Maybe i'm wrong,but to me, this is 2 power supplies. And one is obvius burn. Maiybe the amp gets without current with only one psu?

[redoxy]

Re: Rotel RKB-8100 Amplifier going into protection mode

Thanks for the response tmaster21.
Now that I think about it, you are probably right that those are power supplies.

I believe the unit goes through a start-up check process to determine whether to go into protection mode. I'm not exactly sure what it checks but as you are mentioned, it could include current from both power supplies.

I want to start by replacing the burnt resistors and bad transistors.
Any advice on how I can determine the wattage of those chip resistors?
Or is there a rule of thumb about resistor wattage that I can use as a guide for selecting one without actually knowing what the wattage is?

I checked the resistance of the two small burnt chip resistors on the good unit and I get 0.9 kOhm on both. I am assuming they will be a 1kOhm resistors.

[CapLeaker]

Re: Rotel RKB-8100 Amplifier going into protection mode

If you measure something on board, all depends on the circuit. So when in doubt, take it out! Remeasure. And yes, it is a PSU.

As for the SMD resistor sizes, google something like: smd resistor size chart
Datasheets will have the sizes printed in them. The size determent's the wattage.

As for the protection mode, it measures voltage and heat for sure. I guess you need a service manual. Sometimes you get lucky calling the manufacturer, it's worth a shot. The worst thing that they can say is NO!

What are the part numbers on the transistors?

[redoxy]

Re: Rotel RKB-8100 Amplifier going into protection mode

Thanks, CapLeaker for the reply.

I did not want to take out the SMD resistors on the good power supply in case I damage anything given how tiny those parts are.

I will give the manufacturer a call as suggested, maybe I will get lucky.
I will also do a Google search for the size chart for the resistor wattage.

While on that topic, I have been trying to read around the resistor wattage. I came across this quote "the power rating of a resistor is the maximum amount of power that the resistor can withstand". Will I be correct to infer that using a higher power rating will not result in any damage since it will be able to withstand if lower power passes through it? That is, if I am unable to conclusively determine the wattage then I will use one with a higher wattage than the standard 1/4 and 1/2 W just to be safe.

The bad transistors are identical with part numbers - P17NK40ZFP. I have added a picture as I took them out.
Shows on Mouser as STP17NK40ZFP

[CapLeaker]

Re: Rotel RKB-8100 Amplifier going into protection mode

The STP17NK40ZFP are n channel mosfets. Since this device is shorted all three pins, you have to check all the components that drive the gate of the mosfet. If something is wrong with the gate drive and is stuck on, all the parts you put in are going to blow again.
Be smart, make pictures, take notes on lots of measurements from the good and the bad PSU, order every part you need twice, fix that what you know is wrong, double-check your work, take more measurements and pray you got it all before powering the unit up.
Good luck!

[redoxy]

Re: Rotel RKB-8100 Amplifier going into protection mode

Thanks again CapLeaker.
I will do exactly as suggested. I guess this is where having two identical power supplies will come handy. I can compare values between the two.

After removing the two small burnt chip resistors, I noticed the damaged wasn't just to the resistors. There is damage to the PCB traces connecting the resistors. I have to take it to a professional to fix the traces as it is beyond my capabilities, and I do not want to cause more damage.

[CapLeaker]

Re: Rotel RKB-8100 Amplifier going into protection mode

Damaged traces are an easy fix. There a tons of videos that explain how to do it. But most of them do have a microscope, which makes repairs much easier.

[redoxy]

Re: Rotel RKB-8100 Amplifier going into protection mode

I think this is where the Pros and noobs like me differ . I came to the conclusion to have a Pro fix the damaged traces after watching some of the repair videos on damage traces. I'm sure a Pro will watch the same videos and go, piece of cake

Anyway, I wanted to thank you again for the input and advice. I took it to a Pro in the electronics teaching lab/repair shop on campus where I work. He was adamant on not trusting measurements in-circuit, so I agreed to him removing the two small resistors on the good power supply to measure the values and sure enough, they were different. One was 1k Ohm and the other was 10k Ohms. Looking back, it makes sense that I was measuring 0.9k Ohms on both.

He fixed the traces, replaced the burnt resistors and the bad MOSFETs.
Took it home and did a comparison of values between the two power supplies and fortunately, they were all comparable.

I put the power supplies into the amplifier connected every and rechecked everything. Turned it on and viola, its back up and running again.

I must say I lucked out on having two identical power supplies in the amplifier since contacting the manufacturer as well as their authorized service centre for a service manual or just the values of the burnt resistors yielded no results.

[CapLeaker]

Re: Rotel RKB-8100 Amplifier going into protection mode

Good job, at least you got it working again! Another one saved!