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Rotel RA320. No 0mv bias in one channel?
Rotel RA320. No 0mv bias in one channel?
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Old 30th October 2017, 08:39 PM   #1
brittney is offline brittney  Ireland
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Default Rotel RA320. No 0mv bias in one channel?

Hi,

I have an old Rotel RA320 which distorts in the right channel when the volume is above 3 or 4 (Swaping L&R speakers the distortion stays on the right and also is present when using a different set of speakers, so I dont think it is a speaker issue).

The service manual says to have ~15mv across the bias adjust points (across R629 and R630 in the circuit diagram). The left is fine, but the right reads 0mv even when turning the trimpot fully from one side to the other. The trimpot appears to change resistance when rotating it. What is likely to cause this difference?

I have measured voltage across most points comparing the left and right and cannot find any noticeable differences. All power transistors have similar V across them. No noticeable damage to caps or scorch marks. However it is possible this amp has been mistreated as the glass quickblow was blown when I picked the amp up.

Any input on what the problem may be is appreciated.

Thanks in advance, if more details etc is needed let me know

Photo of the circuit diagram - it was split across numerous pages in the manual so printing and taping together was the easiest way to see it all.

Rotel RA320. No 0mv bias in one channel?
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Old 31st October 2017, 12:29 AM   #2
motronix is offline motronix  Israel
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First go see if R628,630 is ok and it's still 0.47R
then R624 if it's still 4.7R

Last edited by motronix; 31st October 2017 at 12:36 AM.
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Old 31st October 2017, 12:52 AM   #3
Andrew Eckhardt is offline Andrew Eckhardt  United States
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R623/624 is a potential weak link. This amp is so simple it would be worth measuring all resistors and (if you don't find anything there) transistor junctions in the bad channel with no power applied.
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Old 31st October 2017, 01:20 AM   #4
rayma is online now rayma  United States
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Quote:
Originally Posted by Andrew Eckhardt View Post
R623/624 is a potential weak link. This amp is so simple it would be worth measuring all resistors
and (if you don't find anything there) transistor junctions in the bad channel with no power applied.
Also look for open traces and bad solder joints with an ohm meter.
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Old 31st October 2017, 10:47 PM   #5
brittney is offline brittney  Ireland
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Thanks for the suggestions, I'll check across those resistors. For that kind of testing is measuring which still on the circuit board ok of should they be removed?
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Old 1st November 2017, 10:34 AM   #6
motronix is offline motronix  Israel
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you can test it all still on the circuit board.
i am 99% sure that the problem is the emitter resistors
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Old 1st November 2017, 08:10 PM   #7
brittney is offline brittney  Ireland
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Quote:
Originally Posted by motronix View Post
First go see if R628,630 is ok and it's still 0.47R
then R624 if it's still 4.7R
Checked these resistors with a multimeter, R624 was pretty close to 4.7R. R628/630 were a bit off at 0.6 or 0.7R but potentially down to error with the dmm. Equivalent readings on the "good" sounding side gave similar readings.

Could go ahead and replace these resistors anyway to see if it helps?
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Old 1st November 2017, 10:37 PM   #8
motronix is offline motronix  Israel
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it seems like the resistors are OK. try see if the soldering is weak or corrosion PCB.
also check the idle current trimmer may be no contact with the trimmer wiper. if all good you need to start test the transistors. all of them!
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Old 5th November 2017, 06:02 PM   #9
brittney is offline brittney  Ireland
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Quote:
Originally Posted by motronix View Post
it seems like the resistors are OK. try see if the soldering is weak or corrosion PCB.
also check the idle current trimmer may be no contact with the trimmer wiper. if all good you need to start test the transistors. all of them!
So checked for broken traces and bad solder, everything seems ok there. Trim pot seems to fine also.

With the amp powered, no speaker attached and volume at 0 I started measuring voltage across components comparing one side to the other.

At R619 and the corresponding R620 voltage was 0.57 and 0.557V
After Q609/608 voltage across the 330ohm resistor differed. One the left R625 was 0.64 and on the right voltage across R626 was 0.26.

Would this indicate an issue with one of the non power transistors? Q606/608?
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Old 6th November 2017, 01:25 AM   #10
Ian Finch is offline Ian Finch  Australia
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Location: Coffs Harbour, NSW
Before you can be sure the resistors are OK, you have to have a bias voltage to begin with. The bias voltages are provided here by the voltage drop across multiple diode arrays D601 and 602. These are the notorious SV04 which, along with similar types, are prone to failure. This leads to loss of bias control and possibly the problem you are having with no bias at all.

Measure and compare the voltage drop across these devices and if it isn't close to 4 x silicon diode drops or about 2.6V, you should have the problem identified. Fixing it is not so simple, as these are long obsolete and as usual, fakes or poor copies seem to make that route a worse problem. There are several sources of replacements, including NTE but it's still risky. 4 x IN4148 or similar ordinary signal diodes, closely wired in series in a tiny side-by-side pattern, should make a good substitute if located in the same position as the SV04s. They appear to be located at the back of the pcb, a long way from the heatsink. That's not ideal, but then, many design features of old amplifiers like this weren't either.
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