Rotel Amplifier Repair.

[urbis]

I have the following amp to have a look at.

Apparently, there's really low output and crackling on one channel.

http://www.hifiengine.com/manual_library/rotel/ra-920.shtml

I don't have a schematic, but I will upload some photos and a more detailed description of the fault tomorrow.

Any common things to look out for?

Thanks!

[IanB]

Low output and crackling could be as simple as a bad joint. Check all solder joints and clean all mechanical joints/connections before looking deeper.

[daveshah]

If you register on that site, you can download the service manual containing a schematic

[urbis]

I never noticed that, thanks!

[David_AVD]

Faults like that in old hi-fi amplifiers can be a bad electro coupling capacitor, signal transistor or op-amp.

I've even seen dirty preamp-amp RCA links (on rear panel) or dirty selector / tape monitor switches do that.

Once you have the circuit it should be easy enough to trace though it, comparing against the good channel.

[retrolefty]

I have the following amp to have a look at.

Apparently, there's really low output and crackling on one channel.

http://www.hifiengine.com/manual_library/rotel/ra-920.shtml

I don't have a schematic, but I will upload some photos and a more detailed description of the fault tomorrow.

Any common things to look out for?

Thanks!

 Starting in the mid 90s for about 10 years I made quite a bit of coin buying quality vintage hi-fi equipment from thrift stores, fixing them up and selling them off, keeping my favorites along the way. People were donating their quality stereo equipment for newer 5 channel stuff.

I can tell you that low output and crackling 99% of the time is dirty/corroded switch contacts and pots. That was almost always the problem and a good cleaning with a quality contact cleaner (WD40 is not good for a lasting fix) will get you going again. Sometimes it took a couple of applications but again it almost always was what fixed all symptom problems. High end Hi-Fi equipment from the 70s was my favorite era, solid state and built like battleships. Here is a pic of one of the few keepers, Pioneer SX-1980, like 240 watts per channel RMS, when the rated per FTC wattage advertising regs.

http://1.bp.blogspot.com/-hzFF3_lrZdE/T8OriFHwY4I/AAAAAAAAA5Y/toGK1HA7FgM/s1024/SX-1980+front.JPG

http://www.classicaudio.com/value/pio/SX1980r.jpg

http://www.chrisinmotion.com/PioneerSX-1250003.jpg

[Mark Hennessy]

I can tell you that low output and crackling 99% of the time is dirty/corroded switch contacts and pots. That was almost always the problem and a good cleaning with a quality contact cleaner (WD40 is not good for a lasting fix) will get you going again. Sometimes it took a couple of applications but again it almost always was what fixed all symptom problems.

Yes, I was about to say the same. Electronic faults on these are not unheard of, but it's usually switches in my experience too.

[urbis]

What would you recommend to clean it?

Thanks everyone for the responses!

[Mark Hennessy]

I use Electrolube EML: http://www.electrolube.com/products/contact-lubricants/122/22/

There are plenty of similar products about. The key thing is it cleans the contacts, and leaves a lubricating agent behind. Use it sparingly - you don't want it all over the PCB. Repeat applications is better than a massive soaking.

Some swear by "Deoxit". I've never tried it.

[urbis]

Thanks Mark, I'll look into these.

[Smith]

Top reasons for crackling sound and low output:
1. Output relay
2. Bad solder joints
3. Bad switches/pot

My guess, just try to bypass the output relay and see what happens.

Just my experience

[urbis]

OK, so finally got some switch cleaner today and took the lid off the amp.

F601 and F602 are blown. These appear to be the two output fuses.

However, there's still a very slight noise from the speakers, I would have expected them to be silent had both fuses blown?

They're T4A, 20mm. Not sure if Maplin stock them but what would cause these to blow?

I can't see any other damage internally.

[macboy]

OK, so finally got some switch cleaner today and took the lid off the amp.

F601 and F602 are blown. These appear to be the two output fuses.

However, there's still a very slight noise from the speakers, I would have expected them to be silent had both fuses blown?

They're T4A, 20mm. Not sure if Maplin stock them but what would cause these to blow?

I can't see any other damage internally.

Why are you connecting speakers to an amp that you suspect is faulty? Do you like buying new speakers? At the very least, please connect your speakers inline with a DC blocking capacitor (two electrolytics, a few 100 uF or more, 63 V or higher, in series with negative sides together, in series with the speaker). This will prevent you from destroying your woofers if the amp has shorted output devices and is outputting the + or - rail. If the amp is oscillating, your tweeters will still likely die.

Those fuses are indeed the speaker output fuses. There is a 33k resistor in parallel with those fuses, I suppose to prevent the amplifier from going crazy when the fuse blows, since the amp negative feedback is after the fuse. This will allow the amp to provide a very small amount of signal to the speaker through that resistor, but it will drive itself into clipping after delivering only a tiny fraction of a W of power to the speaker. That clipping distortion is likely your "crackling".

If you are very lucky, the only problem is that someone pushed the amp too hard and blew those fuses. You can replace the fuses with smaller (lower amp) ones if you have them handy. This has the added benefit of faster protection in case of a fault. You won't be able to get maximum output, but even with a 1 A fuse you can get at least 8 W into 8 ohm.  DO NOT use higher rated fuses.

[urbis]

Hi Macboy,

I used a couple of ancient "test" speakers I have.

Picked up some identically rated fuses earlier, I'll give them a try later.

Is there anything else I should be checking before swapping the fuses and trying it?

Cheers.

[TMM]

Even if the output fuses are blown, the speaker can be driven at a low level via the fuse shunt resistor (R633/R634) or via the feedback resistor (R635/R636) going back to the input stage.

Crackling can be caused by dirty volume/tone control pots but the cause of the output fuses being blown could be more sinister. Hopefully it has just been abused a bit.

The first thing i would do is check that the output transistors (Q619, Q620, Q621, Q622) are not damaged. Use the diode check mode on a multimeter. For transistors, measuring from base to emitter and base to collector (positive probe to base for NPN, negative probe to base for PNP) should show a diode drop (0.5-0.8V), every other combination should be open circuit (or the in-circuit impedance).

If those look ok then do the same for the the driver transistors (Q615, Q616, Q617, Q618), output biasing transistors (Q613, Q614) and finally the voltage amplification stage (VAS) transistors (Q611, Q612). It's unlikely that any other transistors would be damaged unless there was a severe over-voltage on the powersupply rails.

Also check that the capacitors across the speaker output terminals (if present) are not shorted.

Replace the output fuses and power up the amp and with no signal applied and no speakers hooked up, measure the voltage drop across the output emitter resistors (R629/R630). This should be around 4mV which corresponds to normal bias current through the output stage. If it is less than 2mV let the amp warm up for a few minutes and measure it again. If it is more than 20mV shut down the amp immediately. VR601/VR602 set the output bias current, they might require cleaning and resetting. If you decide to clean them, don't rely on simply turning them back to around the same position that you found them in. Turn them so they are at maximum resistance (wiper is all the way towards R623/R624, then power up the amp and very slowly turn it the other way until you see 4mV across the output emitter resistor. Let it sit for a few minutes while monitoring the emitter voltage because the biasing current will change a little as the amplifier gets up to temperature. Make the necessary adjustment so it stabilizes at 4mV.

Next, apply a low level test signal (say 500mV peak to peak sine wave). Probe on the input side of the input caps (C601 and C602), you should see the audio signal. If you don't, then something is wrong with the opamps or the input selection switches. Probe the output side of the same caps, the signal should look exactly the same. If it doesn't then C601/C602 might be faulty.
Probe the collectors of the VAS transistors (Q611/Q612), you should see a clean amplified signal.
Next probe the input side of the output fuses. You should see the same signal that you saw at the VAS.

If that all looks good try hooking up a speaker and cross fingers

[nanofrog]

What would you recommend to clean it?

You might want to give Cramolin Contactaclean a go (previously sold as Cramolin Red).

I'm envious, as it's not available here in the US anymore (was the original Deoxit cleaner; Caig just imported the stuff in bulk, repackaged it, then slapped their own label on it).