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-   -   Problem with Rotel RA820BX4 - distortion in one channel (http://www.diyaudio.com/forums/solid-state/225595-problem-rotel-ra820bx4-distortion-one-channel.html)

Buhl 13th December 2012 01:39 PM

Problem with Rotel RA820BX4 - distortion in one channel
 
Hi there
Fixing up an older Rotel RA 820 BX for a friend who has no stereo :/ Fixing this could be really great as it's got both MM and MC input, which is needed! :)

Found this amp on the recycling station - one channel distorts gradually when the volume is above 8-9 o clock (using 2V input) - other channel seems a better/cleaner, but seems to have some degree of distortion also - it certainly does not sound like a 30 watt well designed amp (as I cound Rotel amongst - like early NADs?)

I can find the service manual online - any ideas on what to look for? I do not have a scope - only a soldering iron, an volt/Ohm meter and hands..... :(

Start off with checking bias?

Best regards
Hans

stevensoosai2527 13th December 2012 01:48 PM

First try check the amp's output for any dc bias without any input and volume at zero. What u get?

Vostro 13th December 2012 02:05 PM

1 Attachment(s)
Rotel Advice 2.5mV, giving Iq of about 11.4mA per output tranistor.

Set up as they advice, if after setting bias, it still sounds bad,
maybe increase bias abit, see if it changes your listening experience.

Attached picture is to set offset and bias.
Regards

Buhl 13th December 2012 07:20 PM

OK - DC offset is 0,05 volt on "good" channel, and 0,7 volt on the distorted channel.... Will continiue with Bias settings....
Thnx for helping!
Best regards
Hans

jaycee 13th December 2012 10:53 PM

Try changing the input coupling capacitors, C601 and C602.
The next suspect is the input stage transistors Q603-Q606. These sometimes go noisy. You will probably find it hard to get genuine 2SA1016 transistors.

Buhl 26th December 2012 05:53 PM

Ok - so i'm back again, the check up took a couple of weeks as first my voltmeter, then small tools had misplaced themselves...somehow :)
Anyway, the good channel measures ok, allthough idle current was a little high (does it hurt - if the sink is not to hot i suspect it's ok with a little higher idle current??) - dc offset was spot on with almost unmeasurable offset.
The bad sounding channel i could not get below 0,5 volt in offset - and there were NO voltage over the bias measuring points of TP1 and TP2 - closeby resistor R643 (2watt 910 ohm) have been hot i can tell.
Any ideas?
If anyone crave a schematic i'll upload it....
Cheers!
Hans

Buhl 30th December 2012 11:50 PM

Anyone?

jaycee 31st December 2012 12:34 AM

No bias is a bad sign. R643 being hot would indicate either one of the output transistors Q631/Q633 has failed, or the driver Q625 has.

The schematic indicates DC voltages at various nodes in the circuit. It would be helpful to measure these nodes on both the good and bad channels and compare. This should give you an idea of where to look next


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