Alps volume control mod

I added an Alps volume control to my 500c several years ago. I did this as part of a larger project to solve several problems with this series of Fishers.

With my Khorns (high sensitivity speakers) I had problems listening at low volume levels. The loudness control was just wrong and did not help. The volume control traces were worn the most at low volume. Channel imbalance was horrible and changed dramatically with the slightest control movement.

The problem was that by combining the volume control and on/off switch the carbon traces are worn down every time the unit in powered on. Also, the loudness design is antiquated and so far off as to be useless.

My solution was to move the on/off switch to the headphone jack location, change the volume control to a good Alps pot and redesign the loudness control.

The Alps blue that I used did not have enough shaft length and required an extension. I got my extension from Terry's junk drawer so I do not have a vendor. Google it, there are some out there. They will require some modification since the extension required is short and will need the "D" for the knob. I also moved the brass nut which holds on the faceplate to next pot over since the Alps has no threads like the original.

Not shown here is the one destructive change to the faceplate were I added a 1/8" headphone jack to replace the one now used by the switch.

The PC board is a prototype and is not available (just to ward off the inevitable question).

Shown is the back of the new on/off switch. I used a lamp socket for hum shielding.

I am hoping to get back to this project and incorporate an Op-amp based loudness control with a remote control volume (again the Alps version).

Very innovative Drew -- and so nicely done! Is the gray cable running into the back of the AC power switch a shielded flex channel for the actual power wires to run through?

Dave

Yes, that is a shielded, 18 gauge, 3 conductor. The shield is connected to the chassis at the lamp base only, it is unconnected at the other end (to avoid a ground loop). It is a Belden 8770 to be specific.

I wanted to preserve the original design as much as possible.

Hi Mr. Bolce!
I SURE do wish you'd identify the specifics of those brown caps (I can see that's what they are now) on that prototype board.

At least (2) of them directly behind your new Alps pot. Those are the ones!

Appear to be the same types (brand) as the ones on the IBAM prototype board I asked you about, and you basically told me not to worry about it.

Whose are they and why do you like them?

Thank you!
Phyllis

PShuff said:

Hi Mr. Bolce!
I SURE do wish you'd identify the specifics of those brown caps (I can see that's what they are now) on that prototype board.

At least (2) of them directly behind your new Alps pot. Those are the ones!

Appear to be the same types (brand) as the ones on the IBAM prototype board I asked you about, and you basically told me not to worry about it.

Whose are they and why do you like them?

Thank you!
Phyllis

Click to expand...

They are long gone and I don't remember what they were exactly. The pictures I posted are over 5 years old. The current IBAM board in that unit is of the design I have posted.

drewbolce said:

I added an Alps volume control to my 500c several years ago. I did this as part of a larger project to solve several problems with this series of Fishers.

With my Khorns (high sensitivity speakers) I had problems listening at low volume levels. The loudness control was just wrong and did not help. The volume control traces were worn the most at low volume. Channel imbalance was horrible and changed dramatically with the slightest control movement.

The problem was that by combining the volume control and on/off switch the carbon traces are worn down every time the unit in powered on. Also, the loudness design is antiquated and so far off as to be useless.

My solution was to move the on/off switch to the headphone jack location, change the volume control to a good Alps pot and redesign the loudness control.

The Alps blue that I used did not have enough shaft length and required an extension. I got my extension from Terry's junk drawer so I do not have a vendor. Google it, there are some out there. They will require some modification since the extension required is short and will need the "D" for the knob. I also moved the brass nut which holds on the faceplate to next pot over since the Alps has no threads like the original.

Not shown here is the one destructive change to the faceplate were I added a 1/8" headphone jack to replace the one now used by the switch.

The PC board is a prototype and is not available (just to ward off the inevitable question).

Shown is the back of the new on/off switch. I used a lamp socket for hum shielding.

I am hoping to get back to this project and incorporate an Op-amp based loudness control with a remote control volume (again the Alps version).

Click to expand...

Hi!
The BROWN radials in the second picture, right behind the Alps pot. :yes:
Phyllis

That sure looks nice, great work.

Does that particular Alps pot have the forth terminal that Fisher used for their loudness circuit, most of the Alps pots you see
online just have three terminals per channel although I do remember seeing one or two that had four terminals somewhere.

Those light brown caps are referred to as Silver Mica caps. That's not a brand name, but refers to their construction or make up. They are generally available in values up to about 10,000 uuF (.01 uF), and are known for their high degree of stability over time and temperature variations. You will often see them used in MPX units around the 38 kHz oscillator to help ensure stable operation with regards to frequency. On Drew's board, they are undoubtedly part of the loudness switch circuit.

Dave

BrokenHill said:

That sure looks nice, great work.

Does that particular Alps pot have the forth terminal that Fisher used for their loudness circuit, most of the Alps pots you see
online just have three terminals per channel although I do remember seeing one or two that had four terminals somewhere.

Click to expand...

Yes it did. The loudness control was a 5 position switch. The idea was to be able to alter the severity of the correction. It was based on the original design and used the loudness tap.

The one I am working on currently does not use the tap and could use one of the excellent "fake" Alps. These Chinese fakes are stepped and have small resistors in them. They are easy to "tap", you just select the appropriate resistor and run a jumper off of it.

The Opamp based design I am going to next is continuously variable via a pot.

I saw that switch and was wondering what it was for, that's neat how you used it for a 5 position loudness control.

I thought about the stepped attenuator volume control and how you described tapping off the appropriate resistor for the loudness
circuit, it does make perfect sense to me and opens up some more options for me on replacing the volume pot in these Fishers, Thanks.

Mr. Bolce,
Did you play around at all with the original value of the pot?
Thank you,
Phyllis

PShuff said:

Mr. Bolce,
Did you play around at all with the original value of the pot?
Thank you,
Phyllis

Click to expand...

No, The Alps you see is a 500k carbon trace (non-stepped) with a loudness tap. They seem to have become unobtainium. However as I noted the 500k "fake alps" (stepped attenuator) is available and can easily be tapped.

The new design loudness control uses a linear pot which is easily obtained. However you have the same issue of knob adapting as with the Alps pot. So now you have two shaft extensions and mods to deal with.