Rotel RA-02 troubleshooting. Only left channel working correctly

[StewartH83]

Hi all,

I have a Rotel RA-02 here. It had an issue with the selector switch lighting up all channels and no sound from any input. I managed to sort that after replacing some faulty overheated resistors in the power supply, likely caused by cold solder joints on the two 1000uF 35v caps before them.

I have the selector switch working as normal now yet only getting full sound through the left channel, the right only very faint at 12 o clock volume.

Headphones has full sound in both channels, all inputs have the same issue. I have used the preamp outputs to my Arcam Alpha 8 power amp and the issue is the same, ruling out the Rotel's power amp stage I guess.

I have no scope and have been checking voltages all over and seeing nothing odd, matching the schematics.

The faulty channels does go slightly higher when I turn the tone controls On, which I find strange. I've contact cleaned all switches and connections to no avail. All relays are clicking.

Any guidance on where to check would be much appreciated.

Service manual is here:
Rotel RA-02 - Manual - Stereo Integrated Amplifier - HiFi Engine

 

[rayma]

Are you sure that the headphones work in both channels? Doesn't figure.
Look at the IC501, or the power supplies to it, check pins #1 and #28).

 

[StewartH83]

Thank you for your reply rayma.

I just checked again with the headphones, when balance pot is centre the right channel is on yet significantly lower like through speakers. When I turn the pot full to the right I get nothing at all. Never come across that before.

I checked IC501 again, I get +/- 15.3v there. I get the - 18.4v before R922 and R921. 15.2v before C922 and C923.

 

[rayma]

Ok, first try Tape In as an input (for any line source you have), does that work ok?
Set the Tape Monitor switch to Tape for this test.

 

[sgrossklass]

"One channel missing/low in speakers but OK in headphones" often indicates bad output (protection) relay contact. I would double-check that; also probe around for other issues with continuity from amp output to speaker terminals. A bad solder joint on a terminal wouldn't be too exotic.

 

[StewartH83]

Thanks sgrossklass, I have suspected the output relays. I've not had them out and cleaned them yet as it's the same on both on both A&B so I was unsure really.

R743 after the selector relay has no voltage before it on any input. R644 before the relay was replaced as it was faulty. I have 40v upto the relay and it clicks when input is changed.

I will remove the 3 serviceable relays in the morning, can only benefit anyway. I've reflowed many joints and none look bad now

 

[jaycee]

There is no "headphone amplifier" in the RA-02 - it is simply the power amplifier with some series resistors. So that pretty much means the relays have to be the problem

 

[Ian Finch]

You can have enough output for headphones even when output transistors have failed. Accordingly, I wouldn't assume that the muted amplifier is OK even if you can hear full headphone volume.

Did you do the basics like checking for DC of more than about 50 mV at the power amplifier output nodes? That's at fuses F881 or F882 as best I can read the schematic at the moment.

 

[StewartH83]

Thanks for the replies.

The right channel on the headphones is low aswell. When the balance is centred its lower on the right channel yet when the balance is turned all to the right only it goes off completely. I didn't check properly the first time. I have cleaned the balance pot again well.

F602 is measuring 24.2mV and F601 is 35.5mV. All output transistors measure very similar on both channels in circuit. The emitter resistors all measure 0.8 ohm in circuit which I thought was high as they are 0.22 ohm.

I presumed going through the preamp outputs and still having a low channel shows the issue is in its preamp sections?

 

[StewartH83]

I removed RY901 and cleaned the contacts. One was much dirtier than the other. I tried the headphones and both channels seemed to be the same loudness now, yet still get nothing when the balance pot is all the way to the right, which confuses me. I tried the right speaker outputs, still lower on the right channel.

This has me thinking its an output issue why the right channel is lower and maybe I shorted two solder joints together when I reflowed the balance switch.

My next plan is to check the solder joints on the balance switch. Remove both the output relays and clean their contacts and check continuaty in the output.

Thanks for all the help so far. I usually clean or replace relays regardless and I'm disappointed I didn't this time.

 

[Ian Finch]

..... The emitter resistors all measure 0.8 ohm in circuit which I thought was high as they are 0.22 ohm......

Hmm, are you trying to measure resistance when the circuit is still powered? If so, that's likely the reason you can't make any more sense of your readings than I can. Otherwise, the meter battery could be a bit tired and not up to the current needed for low resistances ?

Anyway, to measure resistance, the test current needs to be sourced from the meter only, or you would be reading the net current in the circuit you make when you put the probes across the resistor and that will be anybody's guess. For accuracy, you could unsolder one leg of each resistor and thus ensure other circuit resistances weren't interfering but that shouldn't be necessary at such a low resistance. The accuracy of the resistors is usually specified at no better than 5% so being concerned about that isn't going to achieve much when wirewound types will either work like any piece of resistance wire or blow like a fuse if the there is a really serious fault .

Just power off, no speakers and wait until there is zero DC voltage at the points you want to test for resistance. Otherwise, lift one emitter resistor leg, though I don't like this idea as it just stresses the copper foil bond to the circuit board and often leads to ugly damage unless your skills are good. If you have a choice, don't do it - I'd be inclined to just assume the resistors are fine if they read less than the marked value plus say, 10%.

 

[StewartH83]

Thanks for the great information in your reply Ian.

I usually measure resistors with the unit off yet I've been a bit sloppy on this amp, I'll check again tomorrow to make sure.

Maybe due to a long week I feel. I'll have some rest for a day or two and refresh as I've had my head in audio gear everyday for weeks lol.

 

[Ian Finch]

I've missed something that should have been obvious here. I think it was mentioned earlier that the meter leads have their own resistance which may already be in the region of 0.3 ohms for each lead but that can vary with the cross-sectional area of the copper in the leads and their length. You have 2, effectively in series with the resistor under test and their total resistance has to be subtracted from the reading.

Are the suggested values realistic? Well, 0.3 * 2 = 0.6 ohms. Add that to the resistor value of 0.22 and bingo! You get 0.82 ohms just by coincidence. Still, you can see why lead resistance can't be ignored in low value measurements. The leads supplied with low cost DVMs may be any convenient length and some are really too long and lightweight for this job. Very short and thick would be best but we can always compensate for errors that we can also measure.

There are fancy LCR bridge instruments with 4-wire measuring jigs if we really need to get serious about accuracy but if you measure just the resistance of the probe leads by shorting the probe tips together, you should have the total lead and connector resistance right there on the meter, with sufficient accuracy to then measure and calculate the resistor value alone.

 

[StewartH83]

Yep, your spot on Ian

My cheap prope wires split last week and I temporarily attached some gold plated pins used for speaker wires, while I wait for new probe wires and alligator clip wires to be delivered.

I thought while attaching the pins that the resistance needs checking again yet forgot all about it.

I just measured the pins together, 0.6 ohms. The emitter resistors are showing 0.8 ohms in circuit with the unit off. As you said it's best to lift a leg yet risk lifting the tracks so I'm thinking they should be fine.

Lesson learnt lol. Always check resistance of the leads and rest when I get tired

Thanks for your thoughts, probably saved me a lot of work and head scratching lol