Rotel RX403 - Noisy Output

[jamesperrett]

I'm working on a Rotel RX403 receiver. I've fixed one issue which was a hum on the signal from the preamp which was fixed by replacing a leaking capacitor. However, the other fault has me scratching my head a bit more. I've attached a picture of what I think is the relevant part of the circuit diagram.

The right output is passing audio but it is noisy and the noise is generated in the power amp stage. Looking at voltages also shows that the output is at 11V. Looking on the 'scope, the noise appears on the collector of Q606 and is propagated through Q610 to the output transistor and then to the speaker output. Thinking that Q606 was faulty I replaced it with a new BC546B but the problem remains.

The voltages on Q606 on the faulty channel are
C 10.1V
B -28.0V
E -28.6V

while the same transistor on the good channel gives
C -1.13V
B -28.0V
E -28.6V

Since Q606 appears OK, the resistors measure as expected and the other transistors appear to be working I'm wondering if the issue is D602? This is shown as 4 diodes in series in the circuit symbol, the parts list shows the type number as SV5002 and it is described as a temperature compensator but I can't find any more information about this part. Originally it was glued to the main heatsink but in both channels this part has become detached from the heatsink.

Does anyone have any more information about the SV5002 or perhaps a suggestion of a suitable replacement? Am I looking for the problem in the right place?

Thanks in advance for any ideas.

[Jez1234]

What is the voltage at the top of SV5002 compared to other channel?

Is output centre point at 0V (or very near. +/-50mV say)?

Does it work ie play music in spite of the noise?

Are the input LTP OK and voltages correct?

[jamesperrett]

I've just realised that I didn't make a note of the voltages around Q608 although I did check that R618 and R620 showed the right value (I also noticed that R619 in the other channel had gone 50% high). I did check that there was no noise visible on the scope on the base and collector of the high side driver and output transistor.

The output centre point (which is the line running through the junction of D604 and R626 and back to R614) is at 11V.

It does seem to play music - though I've only used headphones and haven't tested it on load.

The input LTP seem OK apart from the voltage at the base of Q604 which is at 8.7V instead of the 0V that I see on the good channel.

I'll check the voltages around Q608 and R620 and post them later tomorrow.

[Jez1234]

Check C615, 616 input capacitors (whichever channel). The voltage on the far side of them is 11.5V so if S/C or very leaky the 11V on the output would be about right.

Lift one end of the cap and if the 11V offset at output disappears then Bob's yer uncle.

[Radio Wrangler]

Leaky currents tend to be noisy currents, so looking around for DC errors can often clear up noise problems, and if there's anything off DC-wise, it needs sorting out, anyway. So it's always worth doing the DC first.

David

[jamesperrett]

The base of Q602 which is connected to C616 is 0V. The emitter is 0.54V while the collector is -27.9V. I did lift the end of C616 when I was first trying to work out where the problem was.

I've got 12.4V at the top of the SV5002 on the bad channel and 1.2V on the good channel. There's 10.1V at the bottom on the bad channel and -0.9V on the good channel.

It is also interesting that there is 22.6V at the bottom of R618 on the bad channel whereas there is 18.2V at the bottom of the same resistor in the good channel which implies that there is more current flowing through it in the good channel.

[Jez1234]

The difference between top and bottom of the SV5002 seems correct then (4 x diode drop). Assuming all resistors are OK (has R606 56K gone way high?) I'd try changing the input LTP. The schematic calls them 2SC921 which every reference I have and on line search says is a UHF transistor in a pill type package.... which obviously these are not. 2SC also means NPN and these are PNP so I reckon there was a error in the schematic! BC560, BC416, BC214 should all be suitable along with many others such as 2SA970 etc etc. Change both together and preferably for a matched pair but don't sweat it on that.

The high gain of the voltage amp transistor Q606 means even a tiny difference in base voltage (it is a largely transimpedance [current input] stage) of a few mV will be enough to cause what you are measuring at the collector and its load.

[jamesperrett]

Thanks - I'll give that a try and report back. They are 2SA921 in the parts list. The only PNP transistors I can find here are BC327s which I bought for one of my son's projects so I've just ordered some BC560s from Langrex. However, I'll check the resistors first before replacing the transistors because I know one of the carbon composition resistors in the other channel has gone high.

[Jez1234]

BC327's will be fine if they have high gain and good enough to use to see if it it is those transistors faulty even if not. A device marked just BC327 can have a huge range of gain. For applications where it's a factor there are versions with a suffix -16, -25 and -40 of higher gain which will be just the same part but selected into gain bands. The -25 and -40 variants would be great for the job. You could use a transistor tester/Hfe tester on many DMM's to select some >200 gain and matched to within say 10% ones from the BC327's you have. High gain and good matching mainly reduce the output offset voltage (and can make a bit of an improvement to THD) but even if you had only 2 examples and they were gain 120 and 160 then that would no doubt be "close enough for jazz".

Edit: Watch out for the pin outs! The Japanese 2SA's will likely have different pin-out to the BC devices so you will have to do some judicious bending and maybe insulating of the leads on the BC devices.

[jamesperrett]

It is fixed!

Thanks to Jez's suggestion that I look around the long tailed pair and David's suggestion to check the DC conditions I decided to check resistors again. Almost immediately I spotted that R606 (which is connected to the emitters of the LTP) looked a bit corroded - probably due to the leaky capacitor that I replaced to fix the hum problem. A quick check showed that it was open circuit. I probably hadn't suspected it before because the voltage on the emitters of the LTP looked correct at 0.54V. Replacing the resistor brought that voltage up to about 0.58V, fixed the noise problem and brought the output voltage down to zero.

Many thanks for all your help.

[Jez1234]

Ah the specific resistor I asked you to re-check as it would give those symptoms if way high or O/C I love it when a plan comes together. I'm glad it worked out for you.