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Old 4th February 2013, 02:37 AM   #1
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Default Rotel RA-971 MK2 problem

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.
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Old 4th February 2013, 05:12 AM   #2
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Originally Posted by Martian78 View Post
........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.
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Old 4th February 2013, 05:48 AM   #3
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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?
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Old 4th February 2013, 06:30 AM   #4
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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.
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Old 4th February 2013, 07:05 AM   #5
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Originally Posted by Mooly View Post
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.
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Old 4th February 2013, 07:42 AM   #6
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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.
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Old 4th February 2013, 07:59 AM   #7
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Originally Posted by Mooly View Post
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?
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Old 4th February 2013, 10:04 AM   #8
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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
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Old 4th February 2013, 10:07 AM   #9
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Here's the switch shown in bypass position.
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Old 4th February 2013, 10:22 AM   #10
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Originally Posted by Mooly View Post
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....
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