Hi, I am new to the forum and looking for some assistance.
I have acquired my first amplifier and speakers but was having some sound issues with the a400 on one channel where it would only work when the control volume was pushed backwards...after some reading it seemed like a bad contact and the potentiometer required cleaning.
I desoldered the potentiometer took it apart and cleaned it up. In my eagerness or inexperience I broke the brass pin holding the volume and balance control together.
I am now struggling to find a replacement. The potentiometer is dual concentric and has a loudness tap so has 8 legs instead of 6. It also has a extra leg at the back which I am not sure what this is for giving it 9 legs in total.
Any with some experience of a suitable replacement or workaround?? All help appreciated.
I have acquired my first amplifier and speakers but was having some sound issues with the a400 on one channel where it would only work when the control volume was pushed backwards...after some reading it seemed like a bad contact and the potentiometer required cleaning.
I desoldered the potentiometer took it apart and cleaned it up. In my eagerness or inexperience I broke the brass pin holding the volume and balance control together.
I am now struggling to find a replacement. The potentiometer is dual concentric and has a loudness tap so has 8 legs instead of 6. It also has a extra leg at the back which I am not sure what this is for giving it 9 legs in total.
Any with some experience of a suitable replacement or workaround?? All help appreciated.
These products are now virtually unobtanium and the original, if the balance pot was fitted at the rear of the volume pot, would have been a custom manufacture, though loudness controls were once universal features on Japanese designs.
As you found, you dont try to disassemble multi-stage pots without a good mind for small mechanical devices and a fair bit of experience. If it was damaged anyway, as it seems, it might be a good teaching aid but not if irreplaceable. Often, the metal case pots could be flushed through a small hole or gap in the case, avoiding disassembly.
The simple option is to search Ebay for ALPS RK27 series pots (that's if the large square case will fit) as these definitely are available in their somewhat standard format and spacing of connections. E.g. 300929388260. that gives you 5 mm spaced pins IIRC but you can Google a spec sheet easily enough.
The sad part is that you then only have the volume/loudness control, assuming the resistance value and audio (log) taper and shaft type (knurled or plain round) is correct. It's actually a common problem with popular amps of the era and there are many threads on all audio forums discussing this same problem. You will likely lose your balance control by fitting an off-the-shelf part but without a new replacement or a similar wreck to cannibalise, there are no options. The balance can then be set at midpoint with just 2 equal resistors since few people ever adjust it.
As you found, you dont try to disassemble multi-stage pots without a good mind for small mechanical devices and a fair bit of experience. If it was damaged anyway, as it seems, it might be a good teaching aid but not if irreplaceable. Often, the metal case pots could be flushed through a small hole or gap in the case, avoiding disassembly.
The simple option is to search Ebay for ALPS RK27 series pots (that's if the large square case will fit) as these definitely are available in their somewhat standard format and spacing of connections. E.g. 300929388260. that gives you 5 mm spaced pins IIRC but you can Google a spec sheet easily enough.
The sad part is that you then only have the volume/loudness control, assuming the resistance value and audio (log) taper and shaft type (knurled or plain round) is correct. It's actually a common problem with popular amps of the era and there are many threads on all audio forums discussing this same problem. You will likely lose your balance control by fitting an off-the-shelf part but without a new replacement or a similar wreck to cannibalise, there are no options. The balance can then be set at midpoint with just 2 equal resistors since few people ever adjust it.
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Thank Ian for your input.
Does it need to be the rk27 as I found 6 legged potentiometers locally that would fit the spacing? Also there has been some conflicting info that I have read on the type of pot. should i use a log or linear pot as supposedly a dcouple of the resistors on the board are tapering the volume.
Finally would a replacement pot just slot in missing two of the holes on the board and still work? I don't mind losing the balance as I don't use it.
Thanks Craig
Does it need to be the rk27 as I found 6 legged potentiometers locally that would fit the spacing? Also there has been some conflicting info that I have read on the type of pot. should i use a log or linear pot as supposedly a dcouple of the resistors on the board are tapering the volume.
Finally would a replacement pot just slot in missing two of the holes on the board and still work? I don't mind losing the balance as I don't use it.
Thanks Craig
IIRC:
It doesn't have a balance potentiometer. It's more of a "dual-mono" arrangement with the left and right volume pots mounted on concentric shafts with a split knob on the front (i.e. front half of the knob rotates one pot, back half of the knob rotates the other pot). Quite a bit of friction between them so they normally rorate together.
That explains the number of pins as well: 4 for the left channel volume pot and 4 for the right channel volume pot. The ninth pin is probably just to ground the metalwork for shielding purposes.
Anyway, I'd probably try to repair the existing unit. I gather the problem is a small broken metal pin. Can't you fashion a replacement?
p.s. Something I learned the hard way: Don't try to cut a sewing pin with a wire cutter - the wire cutter loses the hardness competition.
It doesn't have a balance potentiometer. It's more of a "dual-mono" arrangement with the left and right volume pots mounted on concentric shafts with a split knob on the front (i.e. front half of the knob rotates one pot, back half of the knob rotates the other pot). Quite a bit of friction between them so they normally rorate together.
That explains the number of pins as well: 4 for the left channel volume pot and 4 for the right channel volume pot. The ninth pin is probably just to ground the metalwork for shielding purposes.
Anyway, I'd probably try to repair the existing unit. I gather the problem is a small broken metal pin. Can't you fashion a replacement?
p.s. Something I learned the hard way: Don't try to cut a sewing pin with a wire cutter - the wire cutter loses the hardness competition.
I usually skip the preamplifier stage unless it's pretty well done.
Bass & Treble controls, well, they use to allow "direct" at that time
So if you aren't amused by the aestethics of that amplifier ( you don't mind )
you can take out all the pots & related circuitry and use the power stage only.
Same for the Loudness switch.
A proper preamplifier or buffer might be needed, though
Bass & Treble controls, well, they use to allow "direct" at that time
So if you aren't amused by the aestethics of that amplifier ( you don't mind )
you can take out all the pots & related circuitry and use the power stage only.
Same for the Loudness switch.
A proper preamplifier or buffer might be needed, though
We were talking about replacing an 8 legged pot. That is necessary unless you want to lose the loudness feature as well. Fine, but the 6 pins will then not match the offset centre of an 8 pin array. Look and measure - Yes, you can buy a simple stereo log. taper pot almost anywhere but I have yet to find much, apart from junk audio products, music and sound reinforcement equipment or DIY projects that they actually fit.
There are other standard grade ALPS pots in sizes down to 11 mm sq, but this is a matter of availability. RK27 are popular, high quality, low noise pots. (aka Blue Velvet) and like it says, 27mm sq. but the case can be flat on, rather than raised from the PCB. You can buy almost any value/type of them on the net so that makes them a good bet. That may include a loudness tapping, assuming it is in the right position. Check the link out for starters.
If you find something else that does fit and you can sort out the circuit to remove the loudness feature but restore the basic volume control circuits (they are interdependent), fine too.
All audio level controls need to be log. taper to approximate our hearing sensitivity but the labelling may vary as a suffix A,B or log. Note that Japanese and US codes are different to UK codes so don't be fooled by conflicting marks or advertising - read a properly presented datasheet from a reputable manufacturer. They're free and easily found on the net.
Godfrey is no doubt right about the dual shaft pot. I couldn't make it out on the assembly diagram but didn't see any balance control detail either. That simply means don't worry about it.
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