Seeking Soundcraftsmen DX4000 preamp op amp upgrade experience

[Racingh11]

Thanks. I spread a thin layer of solder paste on each pad, it doesn't take much. Then laid it on a solder hot-plate until it melts in. It can also be done with hot air, but makes it trickier because the parts tend to blow around. Then I install the PBC on the terminal pins set into a circuit prototype-breadboard to hold them straight, and solder them with a standard soldering iron. I didn't notice how dirty this one is from sitting on the bench until I looked at the photo. I'll try to clean it up and take one with better detail.

I'm at least a couple hours north of you, and will assemble these if wanted. Private message me if interested.

 

[Racingh11]

The cleaned up pics. Scrubbed it with prep-sol and a toothbrush.

 

[Jimbo40]

The cleaned up pics. Scrubbed it with prep-sol and a toothbrush.

Now your just showin off!

 

[Racingh11]

Well.. I still see a little fabric hair stuck to it, if that makes you feel better.

 

[mrehorst]

Well.. I still see a little fabric hair stuck to it, if that makes you feel better.

Hey Racingh11, any chance I can get 4 of those PCBs from you? I just got a DX4000 and I'm in the process of fixing some of the problems.

The opamps are all powered by the unregulated 17V rails (which are actually 22V rails, dropped to 17.4 with 220 Ohm resistors), even though the opamps are rated for a maximum +/- 18V. There's no margin for line voltage variations, etc. The -15V regulator is good for 1.5A but the +15 regulator is only good for 250 mA (why would they do something like that?). I'm replacing that regulator with a higher current capable part, and then wiring the opamps to the regulated 15V rails.

The two 470 uF bypass caps for the opamps are both located near the power supply, even though the opamps are far away. I've never built an opamp circuit that didn't have bypass caps (electrolytic or tantalum and ceramic) on each rail at each chip. I'll be adding some, maybe on the bottom of the PCB.

9 of the 15 button caps are missing or cracked and I haven't been able to find a replacement so I am going to 3D print them.

I just finished restoring my PM860 that I bought new in the early 80s. What a nightmare! The main power supply filter caps were rated for 75V even though the rails sit at 72V. It's a wonder they never blew up. I replaced those caps with some Kemet 13000 uF 100V parts rated for 18000 hours at 85C. I might have to replace them again in 100 years. I replaced all the other electrolytics with Nichicons, too.

I'm not overly impressed with SCs engineering. They skimped in so many ways- no silk screen layers on the PCBs identifying components, components operating at the edge of their specs, unmarked wires running everywhere. It takes a few hours of prep work tracing connections and identifying parts before you can even start on fixing anything. Ugh!

 

[Racingh11]

Yes, I'll PM you. Soundcraftsmen was somewhat secretive since they invented many of the circuits they used. Not all schematics are complete either. The goal was also to produce products that performed as well as more expensive brands. I've never seen cracked buttons. That's very odd. Those are the same kemet caps I use. Put a 14 awg power cord on the PM860 and you'll gain around 30 watts clipping power per channel. The wires are common during the wire-wrap era. It is actually a supperior connection, but more cluttered and slower to service. The later Pro Control stuff had board to board connections or flat harness.

Fun fact; Paul Rolfes, the lead Soundcraftsmen engineer and patent holder, went to college in Wisconsin, Marquette University and graduated at the top of his engineering class in just 3½ years

 

[Racingh11]

I finished this today. CX4200. It's like an SP4002, except it adds CX record processing. And the SP4002, is basically a combination of a SP4000 and an RP2215 equalizer. And also adds the phono Cartridge-Match system that the DX4000 and DX4100 do not have. I did not add the opamp adapters to it. I replaced them with new Texas Instruments 4136. I have not found the 'upgrades' to actually test better in circuit. Only the datasheet specs are better than original. The boards give more options for opamps, but I don't believe it to be an audio upgrade at the point.

 

[Racingh11]

Another thing with those adapter boards; the pin size does not seem to fit all sockets. Some units they slide in nice, others will spread the terminals open and they won't be good for going back to the original ICs if desired. There are probably different pins that could be soldered in there. These are just standard header pins.

 

[eizner23]

Fun fact; Paul Rolfes, the lead Soundcraftsmen engineer and patent holder, went to college in Wisconsin, Marquette University and graduated at the top of his engineering class in just 3½ years

Right down the road from me, very interesting tidbit. my first attempt to using your PCB was unsuccessful, I hand soldered the chip in backwards. I now have paste to correctly bake the chips, I just havnt gotten around to it.

I did measure distortion using the newer version of the stock Ti opamps after a recap and power supply rebuild. It was right on the factory spec of 0.01%

 

[Racingh11]

It should be interesting to see what the difference is. I haven't messed with those opamps in a while. I think the limits of distortion and noise are inherent to the board layout and design, more than any particular opamp design. What you find there, may determine what I use in my own designs. I'm working on an amp balancing box at the moment. Basically something that would get a stereo input from a preamp and distribute it to several amps. It has several different outputs and filters and bridging options. It has 9x quad opamp chips in the design. I'll start a thread for it after I build one.