Yamaha M-4: bypass volume controls

[recycler27]

I have gotten some advice from avionic on the Yamaha M-4 power amplifier. I could not figure out how to send an attachment to him by PM, so I am posting in the DIY forum. I will probably reveal my incompetence and lack of understanding of electronics in the following, so I apologize in advance.

How can I safely and effectively bypass the two little variable resistors/potentiometers in the back of this amplifier? I suspect that the pots are worn or damaged; rather than try to find replacement parts, I want to remove them from the signal path. This is a fine vintage amp, so I want the modification to do no harm and be reversible.

I looked for information and advice online, then examined the circuit diagram and the actual configuration on the amp. Attached is my best guess for doing the bypass.

View attachment M-4 bypass volume pots.pdf

 

[whoaru99]

My first thought would be to jumper where I put the little ones by the potentiometer symbols VR101 and VR102. I think that will leave everything intact, but effectively making it like the pots were set to max/wide open.

 

[avionic]

My first thought would be to jumper where I put the little ones by the potentiometer symbols VR101 and VR102.

I would agree.

Do not bridge your input to ground....As depicted in your illustration on the left hand side.

 

[recycler27]

I soldered two wires into position as specified by whoaru99. The ends of the wires are in physical contact with the volume pot pins and the circuit board traces. They are soldered in place securely.

I powered up the amplifier on the bench and everything was normal.

I put the amp into my listening system and turned it on and listened to radio and music for about fifteen minutes, and then I turned the pots off and on. The pots are still fully functional, and both speakers can be completely muted with them.

I checked for continuity with a VOM and there is continuity between the pairs of signal-carrying pins on both sides of the PCB. So I do not understand how the attenuation is still working. How would the signal get reduced and turned off if it is able to bypass the pot? :scratch2:

 

[whoaru99]

Ok...on second thought/review, since the potentiometer is still in the circuit and can shunt the signal to ground there is still the ability to cut the signal.

Given that, I'm not sure there is a way to simply jumper it without additionally removing the ability of the pot to ground the signal.

Hopefully someone will see something that doesn't involve cutting/removing.

 

[recycler27]

The little circuit board which contains the volume pots also includes a capacitor and a resistor for each channel. I do not know what function they serve, but I suppose they are there for a good reason.

I could de-solder and remove the two volume pots and save them. This would make it possible to reverse the modification. But would the amp be affected in any adverse way?

My intention is to remove the pots from the signal path to pass on a clean[er] signal to the next stage of amplification.

I do not understand circuitry electronics, but I can follow directions. Thank you.

 

[recycler27]

Apparently there is no interest in understanding or explaining this little task.

Most important: no harm has been done to the amplifier.

 

[stillspin'n]

you can remove the pots but would put a resistor the same rating as the pot across the outside legs, that would keep the same load on the rest of the circuit and would just act like the pot was turned all the way up.

 

[avionic]

I suspect that the pots are worn or damaged

What warrants your suspicions ?

 

[recycler27]

stillspin'n, that is what I have thought might be the solution, based on several threads I found on the DIYAudio site. "If you bypass the pots, the circuit still needs to "see" the same resistive load," if that is the right way of expressing it. Based on that information, what I need to know is, what kind of 30Kohm resistor, and what wattage? I notice resistors have wattage ratings. Am I looking for a shiny ceramic-look one with color bands, such as those found inside amps, or one of those rectangular ceramic ones such as those found in loudspeaker crossovers, etc.?

avionic, this amp is pretty much up to date; five years ago it was fully recapped, the output relays replaced with new, the trim pots replaced with 25-turn Bourns units, the speaker wire connectors updated. Glenn (EchoWars) did most of the work and adjustments; I did some replacements as well. That does not rule out everything else that could be wrong, but it lessens the uncertainty. BTW, Glenn is usually surrounded by a basement full of expensive world-class unobtanium electronics waiting to be repaired.

The amp's volume and channel balance started "wandering" at initial start and warm-up, where the output volume would stray to one channel or the other and back, especially at low volume, for the first five minutes or so. Once it warmed up, it was fine. It had been about five years since the pots were deoxited, and apparently just turning them back and forth was not enough maintenance for the contacts. Then one channel cut out, and when I sprayed some Caig DeOxit into the two pots it got back full functionality, but still a little bit of that wandering signal at startup. At that point, I thought, maybe I should replace the pots or bypass them, and that was when I contacted you, based on your many previous posts in regard to the M-4 and expertise in Yamaha amps of that vintage.

 

[Binkman]

Hey.. just an observation on 'Warm up'? If you don't play often? The amp needs some time and some input at low vol. even off the speakers. caps are recharging and circuits are balancing etc.. re tweaking pots.. as long as they feel good and a little this way or the other and you hear a change in tone.. is good.

 

[avionic]

stillspin'n, that is what I have thought might be the solution, based on several threads I found on the DIYAudio site. "If you bypass the pots, the circuit still needs to "see" the same resistive load," if that is the right way of expressing it. Based on that information, what I need to know is, what kind of 30Kohm resistor, and what wattage? I notice resistors have wattage ratings. Am I looking for a shiny ceramic-look one with color bands, such as those found inside amps, or one of those rectangular ceramic ones such as those found in loudspeaker crossovers, etc.?

avionic, this amp is pretty much up to date; five years ago it was fully recapped, the output relays replaced with new, the trim pots replaced with 25-turn Bourns units, the speaker wire connectors updated. Glenn (EchoWars) did most of the work and adjustments; I did some replacements as well. That does not rule out everything else that could be wrong, but it lessens the uncertainty. BTW, Glenn is usually surrounded by a basement full of expensive world-class unobtanium electronics waiting to be repaired.

The amp's volume and channel balance started "wandering" at initial start and warm-up, where the output volume would stray to one channel or the other and back, especially at low volume, for the first five minutes or so. Once it warmed up, it was fine. It had been about five years since the pots were deoxited, and apparently just turning them back and forth was not enough maintenance for the contacts. Then one channel cut out, and when I sprayed some Caig DeOxit into the two pots it got back full functionality, but still a little bit of that wandering signal at startup. At that point, I thought, maybe I should replace the pots or bypass them, and that was when I contacted you, based on your many previous posts in regard to the M-4 and expertise in Yamaha amps of that vintage.

Are you sure its the amp and not your pre-amp ?

 

[comish4]

M4

Are you using a Yamaha pre amp in your set up.

 

[Chip Chester]

I would think that, if you do a fresh cleaning and then turn them all the way up, they'll be "out of the way" enough for further testing. If you want the belt-and-suspenders approach, solder a wire between the non-ground side of the pot and the wiper.

I spent most of my tech time on pro electronics (mixing consoles, etc.) and don't recall seeing many gain stage trimpots wired that way. Yours has the input signal wired to the wiper, and the output taken from the high side of the pot... making the non-variable resistance value "applied" to the input of the next stage.

I've usually seen the output of a stage wired to the high side of the pot, so that the total (non-variable) resistance is applied to the output of the previous stage, so it sees a pretty consistent resistance to ground. The wiper would then be connected to the input of the next stage. But I don't know your amp, and haven't been inside the guts of nearly as much home gear as pro gear, so maybe that's the way its usually done. (Or that's the basis of the Yamaha Natural Sound...)

It's unlikely the circuit designer did that for no reason, though...

But it would be instructional to see if your image/balance issue remains when using a different amp entirely, as folks have mentioned above. It's pretty unlikely that pots are going to drift in a real noticeable way during warm-up.
You might check for things like varying DC superimposed on the signal during warm-up -- the classic symptom of excess DC on volume controls is "scratchiness" when operated, but depending on the function of the caps on that board, the result could present differently.

Chip

 

[whoaru99]

solder a wire between the non-ground side of the pot and the wiper.

That's what was suggested and done early on but the OP doesn't like it because turning the pot can still ground the signal.

You could just leave it turned to the high side with the jumper but seems the desire is to have it totally isolated, for lack of better terms. I can't see how that would be done without more than just jumpering.

If the amp has been reworked as said, I'm inclined to concur with Avionic to ensure the problem isn't actually the preamp.

I bought a C-80, M-80, T-80 stack from a guy who told me I'd need someone really good to have a look at the M-80 because one channel was intermittent, sometimes low and scratchy. He'd had the amp to a couple techs and they never could find anything wrong with it....and they were right. The problem never had been in the amp, it was a cracked/cold solder joint on the preamp output connection.

 

[Chip Chester]

Yup. I was only recommending it in order to complete further testing, as noted in the previous sentence.

I don't think the pots themselves are drifting during warm-up, so it's my belief the OP needs to look elsewhere.

Chip

 

[stoutblock]

aren't these carbon wafer type pots which should not use deoxit?

I am thinking to do a similar mod on one of the speaker set outputs on my B-2. Not sure it is needed but would like to simply have less in the circuit.

 

[recycler27]

Are you sure its the amp and not your pre-amp ?

I have been rotating various electronics. An AR LS-3 line stage, a Yamaha C-4, a B&K Pro10MC Sonata, the preamp section of a Yamaha CA-810 or a Sony TA-F6B. [A lot of this musical-chairs dance was in order to try the sound of different phono sections.] And trying a different power amp for comparison as well. What I have described with the M-4 was a characteristic of the M-4.

Are you using a Yamaha pre amp in your set up.

The C-4, which is due for at least another DeOxit treatment; and the CA-810, which has a very reliable preamp section.

Right now, the M-4 with the wires installed [as shown in previous photo], and the pots turned to maximum output, is working very well. [This is what Chip Chester predicted in his post.] If there is any volume drift during initial warm-up, it is negligible. I am still thinking of a volume control bypass--as long as it does not affect the fine sound quality of the amp. So I would like whatever I do to be a reversible procedure.

It is interesting to read Chip Chester's thoughts about how the signal passes through that part of the amp. Although the M-4 looks less complicated than the other Yamaha electronics I have looked into, Yamaha seemed to design complicated circuitry with a lot more parts, compared to their peers at the time. Bottom line, a lot of their stuff sounds really good and works really well, IMHO.

Thank y'all for your input.

 

[recycler27]

Follow-up and outcome of this thread

I have removed the volume pots and replaced them with 30Kohm 0.25w resistors using the same positions on the PCB that are shown on the schematic. I left the blue jumper wires (see previous photo) in the same position they were in.

I splurged on 1% tolerance metal film resistors for 14 cents--a whole nickel more than ceramics.

I had to find a new way of mounting the little PCB board, which was originally attached to the rear panel by means of nuts on the potentiometers. [EDIT: Actually, the nuts attached to a bracket which then was screwed to the back panel.] So I drilled a hole in the neighboring PCB (the one which is attached vertically to the heatsink) and used an existing hole in the little PCB to bolt the two boards together with a nut, bolt and lock washer combination. This maintains the little board in nearly the same position in the amp without stressing any wires.

Bottom line, this mod solved my issues with the wandering volume/balance at startup and I don't have to keep deoxiting those old pots. The amp possibly sounds better than ever. I have listened to it for about thirty hours and everything is fine.

Thanks especially to stillspin'n for helping me get past some obvious parts I could not understand. The obvious things can be the hardest of all, because of false assumptions.

 

[PeteDW]

I have gotten some advice from avionic on the Yamaha M-4 power amplifier. I could not figure out how to send an attachment to him by PM, so I am posting in the DIY forum. I will probably reveal my incompetence and lack of understanding of electronics in the following, so I apologize in advance.

How can I safely and effectively bypass the two little variable resistors/potentiometers in the back of this amplifier? I suspect that the pots are worn or damaged; rather than try to find replacement parts, I want to remove them from the signal path. This is a fine vintage amp, so I want the modification to do no harm and be reversible.

I looked for information and advice online, then examined the circuit diagram and the actual configuration on the amp. Attached is my best guess for doing the bypass.

View attachment 563782

I hate to interrupt, but I am actually looking for a place I can purchase the two variable resistors/potentiates for my Amp, does anyone have a site can visit?
again I just stumbled upon this sight searching for said parts...Please forgive any interruption I may have caused
thank you Pete