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Old 30th June 2010, 03:42 AM   #1
tooch is offline tooch  Australia
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Default Rotel RA-920AX High DC offset

Hey guys, a few weeks back I scored a whole Rotel component setup for free. The amp is a Rotel RA-920AX. When I first got it, one of the inputs didn't work properly, and the amp sounded rubbish, so I did some investigating.
I found that one of the fuses just after the secondaries on the transformer was blown, and blew straight away when replaced. I traced the fault to a faulty bridge rectifier, which I replaced and the fuse no longer blew. The amp sounds better but still sounds a bit shoddy, so I checked the DC offset, and it's right up around 88mV on both channels!?!

There is no DC offset adjustment on this amp, only bias.

What's likely to be the cause of this high offset? I realised that I'd set the bias current way too high a few days ago as I had the multimeter set to the wrong scale Click the image to open in full size.
Could I have ruined the transistors? The amp works fine, but honestly sounds crap.

So far it owes me nothing, and is a good learning project as I'm a complete noob to amplifiers Click the image to open in full size.. It's in excellent physical condition so I wouldn't mind spending a few dollars fixing it up.
Click the image to open in full size.

Last edited by tooch; 30th June 2010 at 04:00 AM.
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Old 30th June 2010, 10:08 AM   #2
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what might cause some offset is leacky electrolytics ( small leakadge will or can do that ) but it can be also cause by a number of other reasons

it could be wise to tatake samples of electrolytics to measure and if you find any probs start replacing them all ..

on the other hand if amplifier has been expeosed to humid or thermal stress there is also a chance that cheap carbon resistors have drifetd from the original value ....

Generally in an old machine there is a procedure and a number of things to test and measure to bring back to life and real performance

Given the oportunity you may also upgrade some parts inside to gain better sound and stability ....

here is a tutorial about amplifiers vintage amplifier repair/upgrade manual

then again agood start will be a schematic that from a first look seems to be hard to find

regards sakis
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Old 30th June 2010, 11:20 AM   #3
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http://www.bwgroup-support.com/downl...A-920AX-TM.pdf
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Old 30th June 2010, 11:26 AM   #4
tooch is offline tooch  Australia
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Thank you both!

I was thinking about replacing all the electrolytics, there aren't very many in this amp so it won't cost me much to do.

I believe all the resistors are metal type already, should I replace them anyway?

Thanks for your help!

Simon
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Old 30th June 2010, 11:38 AM   #5
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thanks for the service manual others might find usefull also ...

wonderfull simple amp ....realy has the chance for wonderfull performance

regards sakis
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Old 30th June 2010, 11:47 AM   #6
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Quote:
Originally Posted by tooch View Post
The amp works fine, but honestly sounds crap.

So far it owes me nothing, and is a good learning project as I'm a complete noob to amplifiers Click the image to open in full size.. It's in excellent physical condition so I wouldn't mind spending a few dollars fixing it up.
If you've got patience and a few hours, you could really make it into something a bit special by fixing the grounding. But this is not a job for the faint hearted - if you'd like I can give you some pointers where the grounding is messed up. Rewiring the grounds is not expensive, just fiddly and time-consuming, but in my own experience where I've done this the sonic benefits are worth the time and effort.
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Old 30th June 2010, 11:59 AM   #7
tooch is offline tooch  Australia
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Quote:
Originally Posted by abraxalito View Post
If you've got patience and a few hours, you could really make it into something a bit special by fixing the grounding. But this is not a job for the faint hearted - if you'd like I can give you some pointers where the grounding is messed up. Rewiring the grounds is not expensive, just fiddly and time-consuming, but in my own experience where I've done this the sonic benefits are worth the time and effort.

I'd be very interested in doing this if you have the time to tell me!

I need a new project so this would be a good one
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Old 30th June 2010, 09:54 PM   #8
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I have the same amp (also given it along with a RDC-975 which I have already fixed the draw on) and the same issues with the DC. So far I have replaced 8 of the caps and noticed a little change but for me the mV are still around 95 & 70. I will be replace the remainder soon when they arrive along with the differential pairs. Hopefully all goes well, but I would also be very interested in fixing the grounding.

interested to see how well you get on tooch, this amp seams like it is worth the effort. I have had a listen to mine and was very pleased and that was before I stared repair
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Old 30th June 2010, 11:18 PM   #9
jaycee is offline jaycee  United Kingdom
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Looks identical to the circuit of my RA-930AX. I'd be wondering why the bridge rectifier fried.

It might be worth checking the regulators for the opamps, and the opamps themselves - it's possible that the faulty bridge rec has caused some damage.
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Old 30th June 2010, 11:36 PM   #10
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Quote:
Originally Posted by tooch View Post
I'd be very interested in doing this if you have the time to tell me!
OK then that's a deal Your side of the bargain is to report back on how it changes the sound!

Quote:
I need a new project so this would be a good one
I'll need a little time to print out the layout and schematic from the kindly provided service manual, annotate them and scan them back in. Or perhaps I could find a way to do it electronically. Stay tuned!
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