Bypassing tone switch on Rotel RA-931 and RA-930AX

lars667 · 9th June 2012, 10:59 AM

Hi there.

I've got two amps with the problem that one of the channels drops out/crackles after playing for a while. This goes for all inputs and both speakers and headphones. I've read that I should try bypassing the tone bypass switch circuit by doing this (quote from another member here, don't remember who):

Quote:

For the left channel : cut J62 and J69. Link from the north end of J69 to the north end of J62.

For the right : cut J66 and J37. Link from J66 E to J37 N.

Now, I've located all the jumpers apart from the J69, which seems to be gone on both of my boards. It looks like it should be behind/underneath C515, but there's nothing. Should I then just make a jumper between the hole slightly north of C515 to J62N?

I'd appreciate any input, as I can't figure it out for sure. I have found schematics for the circuit boards af004111a (ra-931) and af004111 (ra-930ax), however my ra-931 uses af004111-b.

Please have a look at the schematics here if you would be so kind:
RA-930AX and RA-931

Cheers.

sakis · 9th June 2012, 11:47 AM

From your sayings i guess someone with low experience in things like that ...but i might be wrong .

It could be wise to clean the all amplifier first switches and pots before start messing with it ...

Tone switch might also be bypassed in other ways but still i recon that you start from there ....

lars667 · 9th June 2012, 11:55 AM

Yes, I've cleaned all switches etc. earlier on the RA-931, which I will try to fix first. I have earlier built a small guitar amplifier, a couple of effect pedals etc., but only from write-ups. I lack a bit in the theoretical department, and am not very experienced no.

I will be changing power amp electrolytics etc. too, but I reckon those should not affect the sound of the headphone output. Therefore I've concluded that something's wrong in the preamp section, and have heard of this tone switch being a weakness.

I'd be grateful for any writeups on how to bypass the tone circuit on this particular amp, but will try the first method on the ra-930ax, since I've got two amps anyway.

carlmart · 9th June 2012, 01:06 PM

Bypassing a tone section is relatively easy and it's already provided in the Rotel, with the S3 tone switch.

You can choose going through the Bass and Treble stage or not. Maybe that switch became noisy.

You may try a new bypass, directly wiring from before C515 to before C601, where you would have to cut the pcb track. Same thing on the other channel.

lars667 · 9th June 2012, 08:51 PM

Hm, my point was to bypass the tone bypass, so to speak, as to always include the tone section.

However, the mod I mentioned didn't seem to work, or at least didn't solve my problem. After the mod, I've got sound on the left channel, but no sound on the right.

lars667 · 9th June 2012, 09:20 PM

I was wrong. Actually, the amp works on both channels now, however the tone controls are out of the circuit. And when I look on the original advice, that is exactly what the mod did. Not exactly sure if that's what I wanted, though.

lars667 · 10th June 2012, 03:39 PM

I put the tone controls back into the circuit, and now the RA-931 works. I also cleaned the pots more thoroughly. It actually seems to be working, and has been working without problems for a couple of hours.

Also, I haven't had the problem recurring on the RA-930AX, which I only cleaned. Haven't tested that long yet, though. Difficult to know for sure with intermittent faults like this!

I will report how it goes.

9th June 2012, 11:47 AM	#2
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	From your sayings i guess someone with low experience in things like that ...but i might be wrong . It could be wise to clean the all amplifier first switches and pots before start messing with it ... Tone switch might also be bypassed in other ways but still i recon that you start from there .... __________________ SERVICE ΕΝΙΣΧΥΤΩΝ ΚΑΙ ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

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9th June 2012, 11:55 AM	#3
lars667 diyAudio Member Join Date: Nov 2007	Yes, I've cleaned all switches etc. earlier on the RA-931, which I will try to fix first. I have earlier built a small guitar amplifier, a couple of effect pedals etc., but only from write-ups. I lack a bit in the theoretical department, and am not very experienced no. I will be changing power amp electrolytics etc. too, but I reckon those should not affect the sound of the headphone output. Therefore I've concluded that something's wrong in the preamp section, and have heard of this tone switch being a weakness. I'd be grateful for any writeups on how to bypass the tone circuit on this particular amp, but will try the first method on the ra-930ax, since I've got two amps anyway.

9th June 2012, 01:06 PM	#4
carlmart diyAudio Member Join Date: Mar 2001 Location: Brazil	Bypassing a tone section is relatively easy and it's already provided in the Rotel, with the S3 tone switch. You can choose going through the Bass and Treble stage or not. Maybe that switch became noisy. You may try a new bypass, directly wiring from before C515 to before C601, where you would have to cut the pcb track. Same thing on the other channel.

9th June 2012, 08:51 PM	#5
lars667 diyAudio Member Join Date: Nov 2007	Hm, my point was to bypass the tone bypass, so to speak, as to always include the tone section. However, the mod I mentioned didn't seem to work, or at least didn't solve my problem. After the mod, I've got sound on the left channel, but no sound on the right.

9th June 2012, 09:20 PM	#6
lars667 diyAudio Member Join Date: Nov 2007	I was wrong. Actually, the amp works on both channels now, however the tone controls are out of the circuit. And when I look on the original advice, that is exactly what the mod did. Not exactly sure if that's what I wanted, though.

10th June 2012, 03:39 PM	#7
lars667 diyAudio Member Join Date: Nov 2007	I put the tone controls back into the circuit, and now the RA-931 works. I also cleaned the pots more thoroughly. It actually seems to be working, and has been working without problems for a couple of hours. Also, I haven't had the problem recurring on the RA-930AX, which I only cleaned. Haven't tested that long yet, though. Difficult to know for sure with intermittent faults like this! I will report how it goes.