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Old 21st November 2006, 03:40 PM   #1
Ovation is offline Ovation  United Kingdom
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Default C-Audio RA2000 Protection LED

Hi All,
I'm trying to repair a C-Audio RA2000 amp for a friend of mine. I've had it sat around for months so I can't remember what he told me was wrong, but when I turn it on the 'Protect' LED lights immediately.

I have not tried it with speakers attached yet, but I believe it should only protect itself is the poles are short or have too low impedance...

Does anyone have any schematics and/or advice about fixing this amp. I'm trying to work out what can trigger the protect circuit.

So far I've checked the Rectifiers and other Diodes, and have also gone through all the FETs - As far as I can tell they are all okay (Although the two poles of each rectifier connected across the transformer read short [as expected] and I can't easily take them out to test).

Each channel has a group of four 110v 4700uF caps on their supply (which I believe are for smoothing?), and every one of these caps is bulging at the top. I suspect a need to replace these but I'm not convinced that they're the cause of this exact fault. At approx 10 per cap I don't want to rush into replacing these yet!!!

Any advice, or diagrams, or explanations of the principles surrounding protection circuits will be much appreciated.

Many Thanks
Jonathan
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Old 21st November 2006, 03:48 PM   #2
Netlist is offline Netlist  Belgium
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Do you have a global picture of the interior?

-=Hugo=-
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Old 21st November 2006, 03:56 PM   #3
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Default repair

If the amp powers up check the dc on the power rails you might not have to buy 110 volt caps might be lower voltage saving money,strange that all have bulged this is an indicator of too much heat drying the electrolite innards out and expanding case,normally result of shorted output stage,the outputs and drivers will have to be tested for shorts.

Age does not help hifi the first power up can do damage ,years ago i had a rotel ra5000 got from tip for 2 pounds,full of cobwebs anyway powered it up fireworks,absolute monster of an amp got a service manual after testing would have cost a fortune to fix shame that i did not know then how rare they are and scrapped it
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Old 21st November 2006, 03:58 PM   #4
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Default repair

Apologies seen you have done tests good luck with schematic
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Old 21st November 2006, 04:09 PM   #5
Ovation is offline Ovation  United Kingdom
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Hi Hugo,
No I don't but I can probably take one and post it later on...

Just to describe the most basic layout:

REAR
--^^-^^------XXXX----
|..............................| <- Here is loads of lower voltage stuff
-------------------------------- <-- Heatsink (two thermal cut-out switches attached)
| FFFFFFFFFFFF |
| FFFFFFFFFFFF |
-------------------------------- <-- Heatsink
| OO.....------.....OO |
| OOB /..T..\ BOO |
|.....R...\ __/..sss.. |
--------------------------------
FRONT

Legend:
T = Toroidal Transformer
O = Capacitor
B = Bridge Rectifier
F = FETs (Don't actually know how many from memory!)
R = Relay
sss = Soft-Start Resistor
(^^ and XX are Binding Posts and XLRs)

I know my ASCII diagram is rubbish, so I'll get some photos ASAP as well, but perhaps just seeing the locations helps?

Thanks
Jonathan
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Old 22nd November 2006, 05:15 PM   #6
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I recall from older c-audio gear that small value electrolytic capacitors needed to be changed in the frontend. There were also some very tricky 22k resistors that got bad. In those with K135/J50 mosfets, I found several bad mosfets, you have to take them out to properly measure them. The output relay capacitor went bad also. It's all very vague and from memory. That's about all I know, you can of course still post a picture but the real help would be the schematic, which I don't have.

/Hugo
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Old 22nd November 2006, 11:01 PM   #7
Ovation is offline Ovation  United Kingdom
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Hi Again Hugo,
I've been working on this amp today again. It must be similar to the ones you remember as the FETs are indeed J50 and K135... I've desoldered one leg of each device and tested it and they all appear to be fine - no shorts..

When I apply power the protect light comes on instantly - None of the relays switch. I've also noticed that the VU meters for both channels light up to the second mark (So Green '-40' and Amber '-20' are illuminated). I'm wondering if there's something happend to the input stage that's causing this - Given that it's both channels I have to suspect power as I believe that's about the only thing they have in common.

I still haven't got round to taking any photos yet.
Theres a pair of 16v 2200uF Electrolytics at the front of the amp that I'm a little wary of. They connect between the AC output which drives the FETs and the AC output which gets send to power the input stage.

Also, I previosly mentioned the 110v 4700uF caps which are all bulging - What's the chance that these are actually causing the problem? As much are they've bulged they all give pretty consistant readings out of the board.

Any other advice would be great because I'm struggling to find anything that has failed in an obvious way.

N.B. I've not gone hunting for 22k resistors yet - Will do that tomorrow.

Thanks
Jonathan
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Old 23rd November 2006, 06:24 AM   #8
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Jonathan,

At least we need some decent pictures (caps). That will hopefully give an overall impression of the state of the amp. Some clear photos of the solder side would be nice too.

/Hugo
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Old 23rd November 2006, 09:23 PM   #9
Ovation is offline Ovation  United Kingdom
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Okay - I've taken a whole load of pics of this amp... Hopefully some of these may help a bit.

Have a look at:
http://dynamicdiscos.co.uk/AMP_Pics/

You can click on pics.html if you want which will load all the pics in a dumb page, and clicking on a picture will load the high resolution version.

Please let me know if you have any advice.


NOTE: The silver writing and arrows on the large Capacitors were just so I could make sure I put them back in the right places... (FL = Front Left , BL = Back Left)...


Thanks
Jonathan
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Old 24th November 2006, 06:55 AM   #10
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Jonathan,
That's indeed the layout I remember. I'm in a hurry but as a starter, tighten all nuts and bolts from the output mosfets. The big caps are most likely good. You will notice that the bulging is nothing more then the plastic cover that expanded to give the caps a more female look. Push the cover and you'll find a hollow space underneath. Removed it and you'll see a perfect flat capacitor top.
What about the main voltage rails B+ and B- ? Are they present and more or less equal in value?

I'll have a more detailed look at the pictures later, I suspect a fault in the power supply as both channels seem to be equally affected. Also, try to find some uneven voltage measurements in the circuit.

Edit: Asking for the obvious here but you did check all fuses did you?

-=Hugo=-
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