By JVC
Tue May 22, 2012 6:57 pm
(Warning: this is a long post!)
Recently I bought old, used Numark DM-1650 on ebay, which I kind of regretted bidding when I won, because I didn't really need it, plus old mixers generally have some issues. This Numark DM-1650 mixer had more issues than I thought.
Those Numark mixers were really popular back then. Many nightclubs had Numark mixer installed (GLi PMX series were also popular), and Numark mixers had good reputation back then.
This is the mixer:
Those were the Issues:
1. The metal shaft on Phono/Line stereo channel on the left was broken. Fortunately, I had old pots (same type of dual fader) in my junk box, and I replaced the broken knob.
2. The right channel (one the the stereo channel) of the Phono/Line stereo channel on the right were silent. (I thought op-amp was faulty, and it was because of both crossfaders, and the exactly same channel was faulty for both of two crossfaders (one for main out, other one for monitor channel selection.
3. all the pots had dusty feel, although none of the pots bleed.
4. Not really a issue, but pots for the 6 band EQ had center detent, which I found it a bit annoying. So I removed ball bearing and spring in the pot, thus removed center detent.
5. All of RCA jacks for stereo channels (2 stereo channels) were loose. I was thinking about replacing the RCA jacks, but it's just too difficult to find the perfect parts. I was able to fix the loose connection by putting small piece of paper between metal contact and plastic around it. No more loose connection!
6. the face plate was way too wide (for rack mount), each side was sticking out about 2 inches! I cut out each side with my jig saw.
(check out viewtopic.php?f=11&t=148018&start=30 )
This is back of the mixer, not many input, but AC line out is very useful.
Metal enclosure - so many screws!
There are 46 (!) screws I had to remove from the top metal chassis alone! ALL the switches and pots are attached to the top chassis.
There are so many screws used on this mixer! the main PCB board is attached to the metal chassis on top. In order to access to the PCB, I had to remove all the screws, too many screws! Every single pots and switches are attached with screws to the top chassis(!) This was nightmare for repairing. But, I think this mixer can survive from serious physical abuse...
Since this is a DJ mixer from late 80s', there are a lot of discrete parts.
No sound coming out right channel on Program 2
The main issue on this DJ mixer was that right channel of the Phono / line stereo channel (on the right) was silent. Initially, I thought it was op-amp was shot, so I replaced the suspected op-amp (it was JRC4558DD, which was higher-end op-amp that time)
After that, I found out that the main crossfader (there are two crossfaders on this mixer, one for main output, monitor select for the other) was the culprit, not the op-amp.
Before:
After:
Replacing JRC4558DD with LM4562 was a bad idea. I hear something like morse code in low volume, when phono is selected.
So, I put back JRC4558DD again and the problem is gone. (I think it has something to do with RIAA curve.) The mixer sounds good with JRC4558DD anyway, so I won't touch them. The mixer is not quiet, I do hear a bit of white noise in the background, but I don't think it is because of the op-amps.
I did replace JRC4558D chip (5 in the pic), which was used for monitor out. This one is doing fine, although I cannot tell whether the sound quality from monitor out was improved or not.
Broken knob on potentiometer
The other issue was one of the pole on slide pot was broken. I was looking for the replacement, but I realized that I can just swap out the parts. I had the same time of used pots (which was from my old Gemini mixer's crossfader) in my junk box, so I replaced it.
Slider pot dismantled:
Inside the slider pot. The metal part must touch taper (PCB board) This part is always pushing against taper, so it loses tension over time. Make sure that the metal brush always make contact to taper. Bend it if necessary:
If inside the pot looks like this, don't even bother clean up, you need a new one:
I opened & cleaned up all of those pots!
Cleaning slide potentiometers
(This is the most important step)
All the faders were in decent conditions (no bleeding) then I got it, but most of the faders had dusty feel when moving faders. So, I decided to remove (desolder from board!) ALL the slider pots (there are 13 pots!), and open them, and clean up and lube inside the pots.
Crossfaders on this mixer is interesting, it uses 4 channel (quad) slider pots (usually, a 2-channel (dual) pot is used for crossfader. Of course circuit design is different between using 4 channel pot and 2-channel pot for crossfading.)
They are all ALPS pots, but they are not high-end pots. However, considering the fact that they've survived this long (it looks it was used heavily) I guess those pots are quite durable. All of the pots are 100K audio taper, except pots for 6 band EQ, which are 100K linear taper.
Op-amps (as turned out, all the op-amps on the mixer were fine)
There are 4 of JRC4558DD chip and one JRC4558D (used for monitor headphone out), so total of 5 op-amps are in the mixer.
JRC4558DD and JRC4558D are the same chip, better batch was labeled as DD. Therefore JRC4558DD was more expensive than JRC4558D, or JRC4558. JRC4558 op-amp can be found in a lot of old guitar pedals, and old boombox, TV, etc.
Interestingly, I found out that older JRC4558DD (one that has shiny top) is sought after by DIY guitar pedal geeks (at least in Japan) my numark mixer uses the op-amp.
Summary
Basically, all I had to do was clean up and repair pots. All tapers on all the pots are in good condition, so all I needed was to clean up inside the pot, and bend the metal part (brushes) so that it touch taper. As turned out, all the circuitry was working fine. Four of the loose RCA jacks were fixed with paper.
Put it to test
Just note that it took a few weeks to fix all the issues on this mixer. To be honest, I wouldn't have bought it if I knew it would've required this much of work!
I have to say that this mixer is fun to use. 6-band EQ is powerful and useful, you can use it like isolator. This mixer is great for playing house.
12 level LED VU meter is very useful. One of the best features of this mixer.
Phono preamps on this mixer is not bad neither, it sounds good enough for me. It sounds "warm". The mixer is a bit noisy, but the noise is not high-pitch, switch-power-supply-induced noise, but more like white noise type of noise, (should I call it analog audio noise?) so it is not annoying to me.
Other great feature is that this mixer has one 120VAC jack (300W max.), which is very, very useful for setting up DJ set (you can plug in a turntable directly to this DJ mixer, for instance.). It has power supply build-in, so I don't have to worry about AC adapter.
Recently I bought old, used Numark DM-1650 on ebay, which I kind of regretted bidding when I won, because I didn't really need it, plus old mixers generally have some issues. This Numark DM-1650 mixer had more issues than I thought.
Those Numark mixers were really popular back then. Many nightclubs had Numark mixer installed (GLi PMX series were also popular), and Numark mixers had good reputation back then.
This is the mixer:
Those were the Issues:
1. The metal shaft on Phono/Line stereo channel on the left was broken. Fortunately, I had old pots (same type of dual fader) in my junk box, and I replaced the broken knob.
2. The right channel (one the the stereo channel) of the Phono/Line stereo channel on the right were silent. (I thought op-amp was faulty, and it was because of both crossfaders, and the exactly same channel was faulty for both of two crossfaders (one for main out, other one for monitor channel selection.
3. all the pots had dusty feel, although none of the pots bleed.
4. Not really a issue, but pots for the 6 band EQ had center detent, which I found it a bit annoying. So I removed ball bearing and spring in the pot, thus removed center detent.
5. All of RCA jacks for stereo channels (2 stereo channels) were loose. I was thinking about replacing the RCA jacks, but it's just too difficult to find the perfect parts. I was able to fix the loose connection by putting small piece of paper between metal contact and plastic around it. No more loose connection!
6. the face plate was way too wide (for rack mount), each side was sticking out about 2 inches! I cut out each side with my jig saw.
(check out viewtopic.php?f=11&t=148018&start=30 )
This is back of the mixer, not many input, but AC line out is very useful.
Metal enclosure - so many screws!
There are 46 (!) screws I had to remove from the top metal chassis alone! ALL the switches and pots are attached to the top chassis.
There are so many screws used on this mixer! the main PCB board is attached to the metal chassis on top. In order to access to the PCB, I had to remove all the screws, too many screws! Every single pots and switches are attached with screws to the top chassis(!) This was nightmare for repairing. But, I think this mixer can survive from serious physical abuse...
Since this is a DJ mixer from late 80s', there are a lot of discrete parts.
No sound coming out right channel on Program 2
The main issue on this DJ mixer was that right channel of the Phono / line stereo channel (on the right) was silent. Initially, I thought it was op-amp was shot, so I replaced the suspected op-amp (it was JRC4558DD, which was higher-end op-amp that time)
After that, I found out that the main crossfader (there are two crossfaders on this mixer, one for main output, monitor select for the other) was the culprit, not the op-amp.
Before:
After:
Replacing JRC4558DD with LM4562 was a bad idea. I hear something like morse code in low volume, when phono is selected.
So, I put back JRC4558DD again and the problem is gone. (I think it has something to do with RIAA curve.) The mixer sounds good with JRC4558DD anyway, so I won't touch them. The mixer is not quiet, I do hear a bit of white noise in the background, but I don't think it is because of the op-amps.
I did replace JRC4558D chip (5 in the pic), which was used for monitor out. This one is doing fine, although I cannot tell whether the sound quality from monitor out was improved or not.
Broken knob on potentiometer
The other issue was one of the pole on slide pot was broken. I was looking for the replacement, but I realized that I can just swap out the parts. I had the same time of used pots (which was from my old Gemini mixer's crossfader) in my junk box, so I replaced it.
Slider pot dismantled:
Inside the slider pot. The metal part must touch taper (PCB board) This part is always pushing against taper, so it loses tension over time. Make sure that the metal brush always make contact to taper. Bend it if necessary:
If inside the pot looks like this, don't even bother clean up, you need a new one:
I opened & cleaned up all of those pots!
Cleaning slide potentiometers
(This is the most important step)
All the faders were in decent conditions (no bleeding) then I got it, but most of the faders had dusty feel when moving faders. So, I decided to remove (desolder from board!) ALL the slider pots (there are 13 pots!), and open them, and clean up and lube inside the pots.
Crossfaders on this mixer is interesting, it uses 4 channel (quad) slider pots (usually, a 2-channel (dual) pot is used for crossfader. Of course circuit design is different between using 4 channel pot and 2-channel pot for crossfading.)
They are all ALPS pots, but they are not high-end pots. However, considering the fact that they've survived this long (it looks it was used heavily) I guess those pots are quite durable. All of the pots are 100K audio taper, except pots for 6 band EQ, which are 100K linear taper.
Op-amps (as turned out, all the op-amps on the mixer were fine)
There are 4 of JRC4558DD chip and one JRC4558D (used for monitor headphone out), so total of 5 op-amps are in the mixer.
JRC4558DD and JRC4558D are the same chip, better batch was labeled as DD. Therefore JRC4558DD was more expensive than JRC4558D, or JRC4558. JRC4558 op-amp can be found in a lot of old guitar pedals, and old boombox, TV, etc.
Interestingly, I found out that older JRC4558DD (one that has shiny top) is sought after by DIY guitar pedal geeks (at least in Japan) my numark mixer uses the op-amp.
Summary
Basically, all I had to do was clean up and repair pots. All tapers on all the pots are in good condition, so all I needed was to clean up inside the pot, and bend the metal part (brushes) so that it touch taper. As turned out, all the circuitry was working fine. Four of the loose RCA jacks were fixed with paper.
Put it to test
Just note that it took a few weeks to fix all the issues on this mixer. To be honest, I wouldn't have bought it if I knew it would've required this much of work!
I have to say that this mixer is fun to use. 6-band EQ is powerful and useful, you can use it like isolator. This mixer is great for playing house.
12 level LED VU meter is very useful. One of the best features of this mixer.
Phono preamps on this mixer is not bad neither, it sounds good enough for me. It sounds "warm". The mixer is a bit noisy, but the noise is not high-pitch, switch-power-supply-induced noise, but more like white noise type of noise, (should I call it analog audio noise?) so it is not annoying to me.
Other great feature is that this mixer has one 120VAC jack (300W max.), which is very, very useful for setting up DJ set (you can plug in a turntable directly to this DJ mixer, for instance.). It has power supply build-in, so I don't have to worry about AC adapter.