Rotel RA-971 MK2 problem

Martian78

Member
2012-12-29 2:11 am
Hi guys, recently i have purchased Rotel RA 971 mk2 from ebay, almost mint, no scratches to the fascia nor ever repaired... But, it has 2 problems: after carefully inspected it turns out that the tone off/on rotary switch is damaged( left channel is a bit lower than right one at low level volume, when i push the volume up beyond 3 they are equal, i tried to clean it with wd-40, minimal improvement is there, but after 2-3 days its the same as it was... So i plan to replace both tone on/off rotary switch and volume control resistor, i have the spec manual, the part numbers are

4TR-2780 Rotary Switch (Tone)

C4170A17 Variable resistor (Volume)

I have tried everywhere to my google knowledge and skills, no luck of finding them, any one knows where i can purchase them online? If they are not made anymore, can anyone suggest equal if not better replacements?

And the second problem is slight hiss(white noise?) from the right channel, no buzz or humm, but just hiss, at first i thought its the way some older songs are recorded, but its there on every song i play, blues, jazz, electro.... I know its common on this amps to have slight hiss on both channels when the amp is on with volume to 0, but not when listening music. Any advice? Thank you, Bojan.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
........4TR-2780 Rotary Switch (Tone)
C4170A17 Variable resistor (Volume)
I have tried everywhere to my google knowledge and skills, no luck of finding them, any one knows where i can purchase them online? If they are not made anymore, can anyone suggest equal if not better replacements?.....
The specification for the pot. on the schematic is simply VR50KA. We can assume that means 50K Audio, or logarithmic taper. I suggest that's a specification for a standard 50K dual pot. and from the drawing, a standard 16 mm diameter body size. Check the description marked on the pot is roughly in accordance and don't worry over brands if your favourite is not available in that size.

The important features are: The pin size and spacing has to fit the PCB. It must be audio or log. taper. The shaft must also be the same pattern, fluted or plain or with flat to suit the knob. If the shaft length is too long, trim with a fine metal saw. Check the catalogues of RS components, Farnell etc. or any parts supplier offering suitable genuine products. Be aware that extra shaft washers may be needed to get correct alignment so that it fits both PCB and the front panel without stress.

However, unless the volume pot is noisy and you can't clean it properly with a suitable solvent like IPA or Alcohol, why attempt to replace it?

It is unlikely anyone holds special stocks of standard components for old Rotel or any brand amplifiers. The switch, of course, is another matter but I don't think you'll find a replacement unless you have a similar wreck as a donor of parts. The problem will be wear in the switch drive mechanism, if it does not operate correctly. You could though, manipulate the contacts of the slider switch to place it in the best position and then leave it alone, if need be.
 

Martian78

Member
2012-12-29 2:11 am
Hi Ian and thank you for your reply.

The thing with the Volume pot is that on 971 its 2 in 1, meaning the volume and speaker balance left/right are manipulated from the same knob, divided in 2 circles, left and right, so you basicaly move them together for equal sound balance and volume, its like dual mechanism inside the pot, so i think one(the left) is damaged. However, the one that is def damaged is the on/off tone control rotary switch, so when i find replacement i will first change the rotary switch, if that solves the problem, i wont touch the volume resistor, but since i must buy rotary switch i thought of buying the volume resistor as well, just in case i need it. And about the hiss, when i first inspected the amp, the bias was totaly out of balance, left channel was like 0.4 and the right 0.6, i set them equal at 0.8 as suggested in the schematics, and sound improved notably, but still at low volume the left channel is lower for about 10% than the right one, i checked again, and this time the bias was 0.7(left) and 0.78(right), so can that be simple bias trimmer malfunction, or there can be something else not alowing the bias to stay firm? Would the slight hiss come from bias not set right?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
If you mean quiescent current adjustment (bias) then its normal for it to appear unstable because its hugely temperature dependent. Always set when the amp is fully up to temperature and with no signal and no speakers connected. If thats what you meant :)

Hiss that is audible (and certainly if one channel is different to the other) isn't normal. Does it hiss with no inputs connected ? Does the hiss respond to the volume setting ? Does the hiss alter with the tone on and off switch.
 

Martian78

Member
2012-12-29 2:11 am
If you mean quiescent current adjustment (bias) then its normal for it to appear unstable because its hugely temperature dependent. Always set when the amp is fully up to temperature and with no signal and no speakers connected. If thats what you meant :)

Hiss that is audible (and certainly if one channel is different to the other) isn't normal. Does it hiss with no inputs connected ? Does the hiss respond to the volume setting ? Does the hiss alter with the tone on and off switch.

Hello Mooly and thank you for your reply.

About the bias, when i first plugged the amp the sound was horrible, dull, not dynamic, like 50-15 000 Hz, not like 5-100 000 as it should be, with left channel 25% lower than right. After i adjusted the bias( under same conditions you mentioned) and at the same time cleaned the tone rotary switch, the sound improved almost as it should be, and the channel balance was fine but there was that hiss from the right channel.... I turned off the amp and went to bed, next morning i turned it on, and to my surprise the left channel went lower again( not drastic as it was the 1st time), and that hiss on the right channel was still there. When i turn on just the amp, no input signal(cd player turned off), there is slight hiss from both channels, its the same no matter how up i turn the volume( or turn on the cd player), when i put some music, at low levels i can't hear it, but when i crank it up from 10 o'clock further the hiss is there, most notably on song intros or quiet acoustical music, it sounds like those old blues/jazz recordings from 100 years ago, like you can hear the background of the room it was recorded in, or a bit like casette recording, its not buzzing or humming, just annoying hiss sound that shouldnt be there... like the hiss you hear when you put empty sea shell close to the ear.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
All the problems do point to something physical going amiss... the tone switch and/or faulty rotary tone controls. I suspect something is going open circuit somewhere either internally in the controls or due to damage or maybe even poor soldering. Check/examine and resolder all the joints for the switches and controls.The tone control is an "active" type with an opamp and this is the sort of problem that would give the symptoms you describe.

The bias. Just set as per the manual and forget. It won't alter the sound in the way you describe. Something else is going on for that.
 

Martian78

Member
2012-12-29 2:11 am
All the problems do point to something physical going amiss... the tone switch and/or faulty rotary tone controls. I suspect something is going open circuit somewhere either internally in the controls or due to damage or maybe even poor soldering. Check/examine and resolder all the joints for the switches and controls.The tone control is an "active" type with an opamp and this is the sort of problem that would give the symptoms you describe.

The bias. Just set as per the manual and forget. It won't alter the sound in the way you describe. Something else is going on for that.

First of all a big thank you for taking time to help me.

I will do exactly as you proposed and since im at work in the office at the moment, i will post the results later. I was thinking, since i dont use tone controls at all when listening music, can i just bypass the tone control switch and solder the board as it is without tone control( set to tone off)? Would i make any damage or worsen the sound? Is this amp made to be used with tone control switch, or it is bypassable? And if the answer is yes, how exactly should i do it?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
You could bybass the tone stage but its still nice to confirm (and maybe fix) the original issue. Drys are a definite possibilty. Its also often possible remove and open up switches to clean them properly... squirting with something often doesn't last. The same applies to pots to. Many (not all) can be opened and cleaned and the wiper bent slightly both to retension and move it to a new part of the track.

To by bypass the tone controls you need the circuit in front of you. The switch is in the bypass position as shown and so you solder across pins 1 and 3, 2 and 4, 11 and 7 and 12 and 8 but check that first with a meter... you go cross eyed looking at these things on a screen :)
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Here's the switch shown in bypass position.
 

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You could bybass the tone stage but its still nice to confirm (and maybe fix) the original issue. Drys are a definite possibilty. Its also often possible remove and open up switches to clean them properly... squirting with something often doesn't last. The same applies to pots to. Many (not all) can be opened and cleaned and the wiper bent slightly both to retension and move it to a new part of the track.

To by bypass the tone controls you need the circuit in front of you. The switch is in the bypass position as shown and so you solder across pins 1 and 3, 2 and 4, 11 and 7 and 12 and 8 but check that first with a meter... you go cross eyed looking at these things on a screen :)

I will take out the tone control switch and clean it properly, the last thing that has come up to my mind about left channel slightly lower is that when i gently turn up the volume knob from 0 to say 0.1, 0.2, 0.3, the first channel that gets the sound is the right one, then after i go up a bit more the left one is coming alive ( remember that on RA-971/ 971 mk2 the volume knob is also left/right balance knob, there is no separate left/right balance switch as in most of the amps) and the sound is generaly as it should be even when i crank it up, no distortion, both channels seem to work even, just that hiss in the background....
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Channel mismatching can be an issue with some volume controls, the two gangs don't track equally. Only cure for that is a replacement with a good quality part. If your source components put out a lot of voltage then an attenuator can worth trying. It means you have to rotate the volume control more and so it gets past the first non linear part.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I will take out the tone control switch and clean it properly,....
This switch (judging from the drawing) is not going to be easy to remove. Both the steel cover and the insulating switch frame holding the contacts, are likely soldered at multiple points. I would not attempt this unless you have a solder sucker or fluxed solder braid, to remove the solder completely. Otherwise, the PCB will be damaged if you try to force it out whilst heating with the iron. Even so, It may still be necessary to remove the front panel in order to release the switch.

In another thread, where there were similar problems with these Rotel rotary action slide switches, the fault was in the mechanism, which can no longer push the slider contacts to align with the fixed ones. Unless you can somehow repair the mechanism, I think the best fix will be to bypass it, as Mooly described.
 

Martian78

Member
2012-12-29 2:11 am
This switch (judging from the drawing) is not going to be easy to remove. Both the steel cover and the insulating switch frame holding the contacts, are likely soldered at multiple points. I would not attempt this unless you have a solder sucker or fluxed solder braid, to remove the solder completely. Otherwise, the PCB will be damaged if you try to force it out whilst heating with the iron. Even so, It may still be necessary to remove the front panel in order to release the switch.

In another thread, where there were similar problems with these Rotel rotary action slide switches, the fault was in the mechanism, which can no longer push the slider contacts to align with the fixed ones. Unless you can somehow repair the mechanism, I think the best fix will be to bypass it, as Mooly described.

Thanks for the tip Ian, as a matter of fact i dont have solder sucker, but one of my friends has one, so im gonna "borrow" it from him. As soon as have some results(good or bad) i will update here, maybe this tread will help other people with same/similiar problem. In the mean time, feel free to write anything that you might come up with this issue, any help means a lot to me. Thanks guys :).
 

Martian78

Member
2012-12-29 2:11 am
Well replacing the electrolytic capacitors was indeed on my mind, i just hope i will solve the problem with those 2 parts already ordered. And yes, Rotel is one of my preffered budget amps, boldish looks and dynamic open sound, unlike Marantz, which is not exactly my cup of tea, too muddy and unexciting in my opinion and taste.... Do you think this Rotel can be paired up with Arcam cd 7,7se,8? I own Marantz cd 6000 ose, but im going to sell it as it is too bright/harsh in the upper end, and too digital if i may add. The speakers are B&W 602 S2. Cheers
 
Dear Martian 78 -
I have a Rotel RX-970BX which uses the same C-4170A17 50KA x 2 Volume Pot as yours. Mine has a problem where one channel cuts out, which I think is due to the split design volume / balance control. I used to be able to cure this by wiggling the knob or rotating the affected channel part of the knob until that channel came back it. Contact cleaner works for a limited period but the problem always comes back.

The only other temporary remedy is to switch on/off my aged Sony Receiver (which is connected just for use as a tuner ) - the noise generated by the on/off switch of the Sony presumably causes a sufficient spike to temporarily jump across the dirty/damaged contacts in the Rotel volume pots and restore the channel. That's my layman's theory anyway!

I have an electronic engineer friend who is prepared to repair my amp should I be able to locate the appropriate replacement part. Rotel in the UK no longer hold parts for this model, as of 2007. So I wondered where you obtained yours from or was it a generic part you were going to install ? - (did you ever install it or did your rotary tone control remedy all your problems ?)

Incidentally - mine has since new always had slightly unmatched channels as you report, one channel kicks in earlier from 0. It is so slight I never bothered to do anything about it.

Best Regards

Simon.