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Old 13th November 2007, 06:46 AM   #1
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Default Help needed on testing a Rotel RX-400

I purchased a Rotel RX-400 receiver - when I plug it in, nothing happens, no lights, no output. So, I lifted the covers and started poking around with my DMM. What I have found so far :

(a) The power cord is fine because the 120 VAC is coming through into the power supply

(b) The transformer looks OK because I measured the output and found it in the range of 12 VAC. I am assuming this is OK.

Now, I am stuck - I have no idea what to look at next. The AC fuses are fine (measured the resistance across each of them and they are not open).

I have the schematic that I am happy to email to anyone who volunteers to help me understand what items to test next

Thanks
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Old 13th November 2007, 07:49 AM   #2
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Default For sure you will have a lot of folks helping you...but,


if you try to motivate us will be easier.

Post the schematic, or better, the power part of the schematic...post images, pictures from the amplifier/equipment insides.

Doing that will be easier, as you will atract more people because of the pictures, because of the schematics and because your friendly behavior.

We are fanatics, that kind of guys that cannot watch an amplifier alike normal people...we want to open, to inspect inside...so...give us this kind gift that will produce for you a nice image and will result in fast return related the things you ask for.

I cannot speak in the place of others...but in my perception, the way you entered is:

Do for me
Help me
Teach me

And that will provide schematic (the gift for us) to the one will help you.... can be made in better way..reason why my suggestion to you

and you gave not "food for fanatics"....no picture, no detail.... you know...honey for bees...bond for dogs...those things.

Here is my suggestion to you.... give some...you will receive enormous return.

Images can be jpeg, gif, pdf files and many others you will see during the attachment the instructions...limit is 1000 by 1000 pixels of data and cannot be bigger than 100K

regards,

Carlos
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Power supply; ripple, filter, noise, stability and the use of capacitance multiplier... Portuguese and English; https://www.youtube.com/watch?v=sSm0ku1eIgg
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Old 13th November 2007, 10:35 PM   #3
gni is offline gni  United States
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+/- 12 VAC rectified and ready for use should give a little
more than 30wpc. Seems reasonable.

DC voltage after the rectifier?
DC offset?
Bias current?

Let's see the schematic and some pics. . .they would be helpful.
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Old 5th August 2013, 03:54 PM   #4
sonidos is offline sonidos  United States
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I'll probably revive this thread a bit. My cousin has this model and it stopped working. So I've opened it up now and looks like it definitely needs all electrolytic caps replaced.

I made some mods to it when he and I were both young dudes. Awful job, IMO! It was probably a mod that took out the light and meters. I think the amp portion of the circuit is fine. But I will report back on my actual results.
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Old 6th August 2013, 03:27 PM   #5
gni is offline gni  United States
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I don't even remember this thread. . .I will have to go back and read it. Some pics would be great. . . .most older SS and tub equipment is repairable. Too much new equipment is using the high temp no lead solder. junk.
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Old 6th August 2013, 03:35 PM   #6
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I'll definitely supply some links and pics here. I was googling for info on the receiver and found this old thread.

Receiver manual is circa 1980, so probably bears lots of traits of '70s.

My cousin has put me on the spot to get this running as his parents are celebrating their 50th at the end of the month and he wants the receiver to use at the small gathering to supply music.

I just thought it would be a good idea to update this thread in case someone has one or runs into one and would like to restore it.
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Old 8th August 2013, 11:27 PM   #7
gni is offline gni  United States
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Sounds good. I would have a backup amp just in case this one can't be brought back to life.
I 'repaired' my mother and father inlaw's Teac Costco home theater amp earlier this spring.
The output transistors were failing and causing the amp to think it was overheating -- thus
putting it into protect mode. I pulled all components in the amp section, tapped off the
headphone IC and ran that into my working Rotel amp. Works great as a preamp only and
the inlaws don't need to learn a new remote control. The switched AC turns on the Rotel amp.
Some fixes are beyond me, but there is always another bailing wire and duct tape fix-it.
(insert banjo music here)
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Old 14th August 2013, 04:38 PM   #8
sonidos is offline sonidos  United States
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gni -

Yes, his brother has a system he can use, but I actually finished the cap replacement this weekend and no smoke. What a relief!

But the left channel kept cutting out and then disappeared altogether. I traced it down to the litz wire I used to run to the speaker output terminals from the output transistors. At the time I did the mods, I had a Radio Shack pencil iron and I had to solder the connections to those copper Edison posts. The problem is cold/weak solder joints and with litz wire, to boot!

Our local computer store carries Caig Labs cleaner for carbon pots so this receiver should be ready for prime time by end of this week!
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Old 16th August 2013, 02:40 AM   #9
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Sounds like you are well on the way to a working amp. Didn't know you had made previous mods. . . .what were they?
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Old 16th August 2013, 03:18 AM   #10
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My cousin didn't like the little spring clips at the back for attaching the speaker wire. I removed the panel of clips and cut out some plexiglas to mount Edison Price Music posts.

I removed all the original speaker wire and replaced it with Litz wire. IIRC, Cardas.

I also couldn't find filter caps to replace the originals (4700 uF 25V caps). So I made a small pc board to accept 6800 uF snap in caps along with MIT Wondercaps mounted over the location for the old caps. It was a monster with wires running here and there. Just awful!!! hahahaha

This go around I was able to source 6800 uF 35V caps that fit perfectly. I need to mention that the rails run at about 23.8V which is awfully close to the 25V limit of the original caps. So I feel it's better to at least go with 35V caps here.

The original had Panasonic HD caps. I found an old datasheet on the net and at some point, Panasonic recommended that they be replaced with their FC version. The 63V versions had a lead space separation that matched quite a few and went directly on the board.

There is one bipolar cap and I replaced it with a bipolar green Muse cap.
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