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Old 15th Aug 2018, 7:05 pm   #1
Reidar5
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Default Fuses keep blowing on Rotel RX400 amp

Hello,

I found this amplifier in a dumpster (Rotel rx400L). There are two 4amp fuses in it that have blown a couple of times now, after I replaced them.
Since both of them keep blowing simultaneously, I assume there's a short circuit in the transformer, and not that for example transistors in both channels are broken.

I'm a novice, and I wonder if anyone has any suggestions about what I can do to make the amplifier work again?

Regards Reidar
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Old 15th Aug 2018, 7:26 pm   #2
turretslug
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Default Re: Fuses keep blowing on Rotel rx400 amp

There are a few things that can cause this, but a classic fault is shorted diodes in secondary side rectifiers- I don't have the circuit to hand but I recall a similar fault in a Leak 3400 (badge-engineered Rotel) whose fuses were in either limb of the centre-tapped PA secondary. If you have a digital multimeter with a diode test function, this can be a convenient go/no-go in-circuit tester. This would be a "good news" sort of fault- something like shorted output transistors would be less good news.
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Old 16th Aug 2018, 11:03 pm   #3
Reidar5
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Default Re: Fuses keep blowing on Rotel RX400 amp

Hello,

thanks for your answer. I'm not able to locate the rectifiers you mentioned.
I'd be grateful, if you could locate them for me.

I send you the service manual with the wiring diagram (page 14) attached to this message.

Regards Reidar
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Old 16th Aug 2018, 11:14 pm   #4
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Default Re: Fuses keep blowing on Rotel RX400 amp

There seem to be 4 fuses. Two protecting the transformer secondaries, two protecting the speakers.

None of these would be blown by a transformer short. That leaves faults in either the power supply downstream of the transformer, or in the power amps.

Identifying which fuses blow will cut down the search area.


David
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Old 17th Aug 2018, 12:53 am   #5
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Default Re: Fuses keep blowing on Rotel RX400 amp

Quote:
Originally Posted by Reidar5 View Post

There are two 4amp fuses in it that have blown a couple of times now, after I replaced them.......
F901 & F902 are the 4A fuses protecting the transformer secondaries. The bridge rectifier D904 is to the left of the fuses as shown in the layout diagram on the last page of the manual.

As previously mentioned there are a few things that could cause these fuses to blow. Some more information would help. For example, does the amplifier perform properly before the fuses blow? How long does it take? Are there any other fault symptoms?

Alan
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Old 17th Aug 2018, 7:32 am   #6
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Default Re: Fuses keep blowing on Rotel RX400 amp

Firstly how experienced are you? Do you have a DMM (digital multimeter) and soldering iron?

Is the voltage selector set to the correct voltage in your area?

1) If you have a meter set it to DC resistance (ohms) take out F901 to F903 making sure you note what fuse goes where. Attach your DMM leads to the tfmr (transformer) side of the F901 and 902 fuse holders, note the reading and post here. Now attach your black lead to E4 of connect to the chassis, red lead to tfmr side fuse holder F903, take reading and post.

2) Next with DMM still on DC resistance put leads on fuse holder F901/2 but on the circuit side, note reading, post. Next black lead to E4/ground/chassis, red lead circuit side of F903 holder, same again. This is with fuses still out.

Fuses still out switch DMM to AC volts, repeat # 1) but turn on amp briefly and check you have AC from the tfmr. The schematic doesn't say but you'd expect around 40v for F901/2 and a lower voltage for E4/F903

You say you tested OP (output) transistors for shorts, is this so?

Lastly take some good quality pictures of the circuit board etc and post here, we might spot something.

Good luck, Andy.
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Old 17th Aug 2018, 12:52 pm   #7
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Default Re: Fuses keep blowing on Rotel RX400 amp

Reidar, have a look at the bridge rectifier (D904). It's rectangular with four 'legs'. If it's showing signs of swelling or burning then it will be the source of the fuse problem although there could be other issues too. In particular, take a look at the two large capacitors (C901 and C902) to the left of the rectifier. If they are swelling at the ends or showing evidence of discharge they will need to be replaced. You can check these things without needing a meter or switching on the receiver.

Hope this helps.

Alan
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Old 17th Aug 2018, 7:54 pm   #8
Reidar5
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Default Re: Fuses keep blowing on Rotel RX400 amp

Hello,

and thanks for your help so far


Answer to David:

The F902 and 901 are the ones that keep blowing. The fuses blow although the A & B switches for the speakers aren’t pressed in.

Answer to Alan:

I’m not able to turn the amplifier on at all. Once I turn the power on, the fuses blow instantly. Neither capacitor nor bridge rectifier seem to have any sign of swelling, but there is small amounts of some kind of substance that seems to have leaked out of all of the capacitors at the bottom.

Answer to ‘Turretslug’:

I set the DMM to the diode-test function.
Then, I checked the bridge rectifier, and got ca. 0.002 for the positive and negative DC outputs, in both directions (black probe on + DC output, red probe on – DC output and vice versa)

Next, I touched the one probe lead to + DC output and the other probe lead to one of the AC inputs. I got 0.634 in the one and 0.654 in the other direction.

One probe lead to – DC output and the other to AC input gave 0.683 in one and 0.664 in the other direction.


In the next step I followed Andy’s instructions:

“Attach your DMM leads to the tfmr (transformer) side of the F901 and 902 fuse holders, note the reading and post here. Now attach your black lead to E4 of connect to the chassis, red lead to tfmr side fuse holder F903, take reading and post.”

Here I got ca 00.20 in all cases. (Strange number. The DMM was set to 200 ohms)

“2) Next with DMM still on DC resistance put leads on fuse holder F901/2 but on the circuit side, note reading, post. Next black lead to E4/ground/chassis, red lead circuit side of F903 holder, same again. This is with fuses still out.”

Here, I got 1 in all cases, which I believe means that my DMM measured no connection.


“Fuses still out switch DMM to AC volts, repeat # 1) but turn on amp briefly and check you have AC from the tfmr. The schematic doesn't say but you'd expect around 40v for F901/2 and a lower voltage for E4/F903”

Here I got 40 V for F901 and F902 and 12.7 V for F903/E4.

I’ll upload some pictures of the circuit board later


Reidar
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Old 17th Aug 2018, 9:19 pm   #9
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Default Re: Fuses keep blowing on Rotel RX400 amp

Unless I'm misreading your notes, there is a short-circuit between the +ve and -ve terminals of the rectifier (I'm assuming that a reading of 0002 on your DMM means zero ohms) If this is the case, the fuses will keep blowing. The next thing to check would be which component is s/c. - it could be the bridge rectifier -the only way to do this is to desolder the =Ve & -ve terminals of the rectifier, then apply your DMM, set to it's lowest Ohms range, directly across the rectifier terminals.
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Old 17th Aug 2018, 10:16 pm   #10
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Default Re: Fuses keep blowing on Rotel RX400 amp

The KBP-02 rectifier is an in-line package so you might find it easier to remove it completely for testing. I have a later Rotel RA 920AX amplifier which was originally fitted with the same rectifier. The amp blew two of them before I replaced it with a higher spec component. The KBP307 is a good bet as it has a similar flat profile and is readily available.

The substance at the base of the capacitors might just be a type of glue sometimes used for securing them in positon during manufacture. Anyway, let us know how you get on with testing the rectifier out of circuit.

Alan
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Old 18th Aug 2018, 5:00 am   #11
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Default Re: Fuses keep blowing on Rotel RX400 amp

Thanks for checking the fuse identities.

People move fuses around and fit all sorts of values, so it can be risky thinking you've identified which is which just by the value fitted.

If you can post a photo of the substance at the base of the capacitors, we can tell you whether it's glue or not. These electrolytic capacitors are liquid-filled and the liquid is highly conductive and quite corrosive. It can leak out of capacitors which are failing or have been abused.

It sounds like your transformer is OK. Rectifiers and capacitors are cheap. If it's the transistors in the power amplifiers, then that's more trouble to fix, but still worth doing.

David
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Old 18th Aug 2018, 7:19 am   #12
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Default Re: Fuses keep blowing on Rotel RX400 amp

Some odd readings on the resistance but as David sais the transformer sounds ok, so you have a short somewhere, either the rectifier or something else. Until you find the short/fault your going to go through a lot of fuses, unless you have resettable circuit breakers.

Some pics would help.

Andy.
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Old 18th Aug 2018, 9:11 am   #13
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Default Re: Fuses keep blowing on Rotel RX400 amp

I very much suspect you have one (or both) output transistor totem-pole failure: short-circuit c-e on both.

With the amplifier un-powered check the collector-emitter using the diode test position or low ohms on all four transistors Q613 to Q616 .



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Old 18th Aug 2018, 9:27 am   #14
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Default Re: Fuses keep blowing on Rotel RX400 amp

Output transistors are a common failure in audio amps. Keeping popping fuses hammers the rectifier somewhat so it should remain a suspect.

It would be good to break the power feeds from the reservoirs to the amplifiers to see if the power supply then comes up. The sequence would be to fix the power supply if it needs it, and then check the amplifiers one at a time.

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Old 18th Aug 2018, 11:24 am   #15
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Default Re: Fuses keep blowing on Rotel RX400 amp

I'd tend to agree with the diagnosis that the output transistors on one or both channels have failed short-circuit. You will have to unsolder them from the board to test properly, but a C-E short is usually blindingly obvious even in-circuit.

About four times out of five in my experience, if the output transistors of an amplifier have failed short-circuit collector to emitter, they're usually the only faulty parts. If they are trying to pass too much current (perhaps because there are too many speakers connected and being driven too hard; or if a short-circuit gets applied across the output) they tend to fail in this manner, and their demise -- or rather, the consequential sudden loss of power when the fuse does exactly what it is supposed to -- tends to protect the rest of the amplifier circuit. So it's often worth taking a punt. Get the part numbers (make appropriate substitutions if exact replacements are NLA) and if they're cheap enough, just replace them. If you are lucky, the amplifier will Just Work and continue to give Years of Trouble-Free Service.

The fifth time is ..... well, let's call it a good one for a masochist. In this case, something else is faulty and that is what is killing the output transistors. Let's just hope it doesn't come to that. Still, with a stereo amplifier, at least you usually have one working channel to compare against.
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Old 18th Aug 2018, 11:55 am   #16
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Default Re: Fuses keep blowing on Rotel RX400 amp

All very true about output transistors of course. However, looking on the bright side, Rotel amplifiers from this era are well known for the under specified 1.5A bridge rectifier so I'm hopeful that this will prove to be the only issue. You never know your luck!

Alan
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Old 20th Aug 2018, 10:45 am   #17
Reidar5
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Default Re: Fuses keep blowing on Rotel RX400 amp

Hello,

I plucked out both the rectifier and the power transistors, and it turned out that the rectifier was fine.

When I tested the four power transistors with my DMM, the readings were 0.000 in all cases, regardless of the way I tested them.
It could be that there's something wrong with my multimeter (Excel xl830l)

I've ordered four new transistors from ebay, and from what you've told me, I guess there's a good chance that this will solve the problem.

Reidar
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Old 20th Aug 2018, 11:08 am   #18
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Default Re: Fuses keep blowing on Rotel RX400 amp

Shame it's not the rectifier. Best of luck with the transistors.

Alan
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Old 20th Aug 2018, 1:08 pm   #19
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Arrow Re: Fuses keep blowing on Rotel RX400 amp

Before you fit those new output transistors, check their drivers. It is not uncommon to find that the O/P transistors and their drivers are all s/c. If the drivers are s/c and you haven't replaced them, when you fit the replacement O/P transistors and switch on, those O/P transistors will probably fail instantaneously.

Al.
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