Rotel RA320. No 0mv bias in one channel?

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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.

An externally hosted image should be here but it was not working when we last tested it.
 
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?
 
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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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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Yip, I did initially suspect the diodes as other threads on the internet about old Rotels mentioned these as being problematic. I will remeasure the voltage drop across them again tomorrow but I do recall both being similar, around 2.4V maybe. The are actually mounted (glued) on the heatsink between the power transistors.

However, comparing to the image you attached and others online both SV04s look a bit fried on my Rotel board....
They are just about visible on this heatsink in the picture below.
An externally hosted image should be here but it was not working when we last tested it.
 
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Have any of the transistors (drivers and outputs) been replaced for modern versions ?

The four diode array gives around 2.5 volts bias voltage and that is marginal for turning on four B-E junctions.

Non original semiconductors could give rise to this.
 
Have any of the transistors (drivers and outputs) been replaced for modern versions ?

The four diode array gives around 2.5 volts bias voltage and that is marginal for turning on four B-E junctions.

Non original semiconductors could give rise to this.

Hi,

Checked the transistors - all are original, at least the are the same as whats stated in the circuit diagram. The diodes were glued to the heatsink but have come unstuck. Would this cause incorrect voltage across the diode?

As the SV04 diodes are no longer available I will try with 4 signal diodes in series in its place to see if boosting the bias voltage helps
 
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As you already have 2.5V across the SV04s, its unlikely they are faulty or need replacing but if the glue has failed, then bias won't track with temperature as it should - which is not good but probably not like the problem of no bias at all. Though bias would be higher on average, it can't easily skyrocket into thermal runaway as some Vbe multiplier circuits can.

As a suggestion, measure the voltages of Vbias (the actual forward bias voltage applied to the output stage) by measuring across the bases of Q607 & 609. Compare this voltage with the measurement across Q608 & 610 bases. The expected voltages at the output transistor bases are marked on the schematic and I guess this where the disparity shows up.

p.s. I believe there are hard setting thermal compounds out there that should be good for re-fixing the diodes. A freshly roughened aluminium surface with a little clamping force until firm is best - the compound will be a tad pricey though.
 
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Something else to consider, a purely low bias condition would not give rise to the fault of badly distorted sound at higher levels. Something else is going on here. There are only four transistors and a handful of resistors in the output stage.
 
As you already have 2.5V across the SV04s, its unlikely they are faulty or need replacing but if the glue has failed, then bias won't track with temperature as it should - which is not good but probably not like the problem of no bias at all. Though bias would be higher on average, it can't easily skyrocket into thermal runaway as some Vbe multiplier circuits can.

As a suggestion, measure the voltages of Vbias (the actual forward bias voltage applied to the output stage) by measuring across the bases of Q607 & 609. Compare this voltage with the measurement across Q608 & 610 bases. The expected voltages at the output transistor bases are marked on the schematic and I guess this where the disparity shows up.

p.s. I believe there are hard setting thermal compounds out there that should be good for re-fixing the diodes. A freshly roughened aluminium surface with a little clamping force until firm is best - the compound will be a tad pricey though.

Cheers, Just measured voltage for Base of q608 to q610 and got a value of 1.96v. Similarly measurement on the left side was 1.84v. Is this difference significant?

The voltages marked on the circuit diagram, what are these in relation to. For example the the 0.8v at Q607. Is this across the transistor, Base to emitter?
 
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Always measure the B-E voltage by reading across the junction. A silicon junction such as these will begin to conduct at around 550 to 600mv and will clamp (will not go higher than) at around 800mv.

With your bias preset turned to give maximum voltage (the 2.4 volts) between the bases of the two drivers, measure the B-E voltage of all four devices and report back with the results.
 
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Like this. Voltages quoted in service manuals can often be wide of the mark and are subject to interpretation. When dealing with problems such as this the important quantity is the voltage across the junction.

Measuring from ground to a given point in the circuit is how the voltages in the manual are given but these will alter significantly with temperature and from one amplifier to another. The actual supply voltage will also reflect in the overall reading. Also a DC offset would skew the result.
 

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