Restoring a 37 Year Old Soundcraftsmen DX4000 Preamp

I recently restored my Soundcraftsmen PM860 amplifier, and was offered a deal on a Soundcraftsmen DX4000 preamp that wasn't quite working. I couldn't find any reviews from the audio press, but plenty were found in some of the audiophile forums, most saying good things. I decided the DX4000 might go well with the PM860, so I bought it.

Front panel of the DX4000, not mine- this one is in a little better shape.

Here's the rear panel of my DX4000. Remember when audio gear had "convenience outlets" on the back?  Those were the days- the days before the marketing people decided audiophiles should buy $1k power cords! Note- no gold plating anywhere!

This preamp has no tone controls, but has three loop I/Os for connecting different signal processors such as equalizers, and two tape loops with switches to dub from one to another. I have never seen a preamp with this much switching before. It also has a headphone amplifier with two 1/4" headphone jacks, one that cuts output to the power amplifier and one that doesn't. One other interesting thing this preamp includes is an output inverter that allows you to connect two stereo power amps in bridged mono mode.

My new preamp had a few problems that were immediately obvious. The top cover had a couple rust spots and bubbling paint. It has a bunch of ganged pushbutton switches that all use the same rectangular button caps, 15 in all. A few of the caps were missing and a few of the remaining ones were cracked. The power-on LED was dead. There was no audio passing through the preamp, except that turning the balance pot made a lot of scratchy noise at the output.

The good news is that I measured the power supply voltages and found the + and -15V regulators working. I also checked all the diodes and found them to be OK. The muting relays were also working properly. There were no burnt parts or exploded caps on the PCB, so I figured worst case I'd need to replace the electrolytic caps (this thing was made in '87) and the opamps (4x RC4136, still readily and cheaply available). There is a MM phono preamp board that has some discrete transistors, but I wasn't too worried about those.

You can access the full size DX4000/4200 schematic diagrams here.

Input switching, one channel shown. See the diagram below to see just how crazy this is.

Tracing signal path from "digital" input on the upper left through output (follow the green line from the upper left to the lower right), the unbuffered input signal passes through 13 sets of switch contacts! That's probably why you don't see this sort of thing done much. The signal passes through 5 switch contacts (blue line) just to get to the tape outputs.

The DX4000 schematic diagram. The phono preamp is at the top, power supply lower left, and line/headphone amp lower right. The DX4000 phono preamp does not include the cartridge matching switches or the op-amp buffer stage.

Power supply schematic with power off. The yellow switches short the power-on LED and the 47 uF cap (red). When power is switched on, the yellow switches open and the LED turns on, and the 47 uF cap (red) charges slowly through the relay coils (about 1k Ohms) and the 2.2M resistor. Once the voltage on that cap gets high enough -it takes about 4 seconds- the transistors (green) switch on, shorting the 2.2M resistor, allowing more current through the relay coils which switches them, connecting the preamp signal to the power amp. This delay prevents turn-on transients from causing the speakers to thump if the power amp is turned on before the preamp. Note: The 17V connections actually sit at 22.8V.

I did some additional testing and found that some audio went through the preamp when I wiggled the input selector buttons on the front panel. I examined the switches closely and found that the solder joints to the PCB were cracked. The single-sided PCB has oversized, unplated holes, and small area pads for the switch pins so you have to really flood the connections with a lot of solder to ensure that it bridges the gaps. Once I resoldered the pins the input switch worked fine. I resoldered all the other switches on the PCB- quite a job, given the number of switches.

Why was the LED dead? Hmmm. The power-on LED is powered via the +/-17V rails (measured +/-22.8V) through the 10k resistor that drops the voltage and limits current through the LED. If the LED has 1.5V across it (typical for red LEDs), the resistor is dropping 44V, which means there should be about 4.4 mA going through the LED. That shouldn't kill the LED. Maybe just an early failure. It happens...

Yikes! Wirewrap connections were used at the I/O jacks and on the PCB. Most of those wires were stuffed under the PCB- I pulled them out so I could inspect the underside of the PCB. Like the PM860, there is no silk-screen layer indicating part numbers or values on the PCB. Soundcraftsmen didn't believe in keeping connections short or using shielded cable! Was wire wrap really cheaper than soldering?

Side note: to me, this preamp looks like the kind of electronics projects I did when I was in high school. My web searches indicate that the DX4000 cost $499 when it was new in 1988. Adcom's GFP565 preamp from around the same era sold for $800 new. I realize that's a significant price difference, but compare the photo above to the photo below. Which looks more serviceable? Which looks less likely to require service? Which looks like it was designed and assembled by professionals? There really is no comparison. This is similar to the difference between the Soundcraftsmen PM860 and Krell KAV-300i amplifiers I recently recapped. Sometimes it is worth the extra money that some items cost, even if the specs are essentially the same, and even if you can't hear a difference between the items being compared.

Adcom GFP565 preamp, sold at the same time as the DX4000. Today you can buy the Soundcraftsmen DX4000 on ebay for $150-470 depending on condition. You can get the Adcom GFP565 for $250-500. I know which I would rather have, just based on the build quality.

Underside of the PCB, not much to see here, except for the dozens of switch contacts that had to be resoldered.

Parts circled in green are electrolytic caps. Opamp ICs are circled in red. The two blue caps near the center are nonpolar electrolytic coupling caps. The two gray things in the upper right corner are muting relays. The 470uF bypass caps for the opamps are located near the power supply on the left, far from the opamps that are all located to the right. Hmmm.

Recapped phono preamp board, electrolytic caps circled in red. I replaced the two orange, 0.39uF, electrolytic input coupling caps with film caps (white caps in blue circles). The two red caps circled in blue are film caps that replaced four electrolytics wired as nonpolar parts.

This is the active circuit schematic for the DX4000. The phono preamp does not have the cartridge matching switches or the phono gain stage shown. Electrolytic caps are marked in yellow. Only one channel is shown. There are some differences between the schematic of the phono preamp section (upper left) and the parts on my PCBs. 

Modification

For some reason, the designer chose to power only the phono preamp board from the regulated +/- 15V rails, and the op-amps from unregulated +/- 17V (schematic designation). These op-amps, like most, are specced at +/-15V operation (data sheet here), with absolute maximum of +/- 18V.  The actual voltage on the "17V" rails is 22.8V. There are 220 Ohm dropping resistors between the op-amp power connections and the 17V rails that will drop that voltage a bit. I measured +/-17.4V at the opamps which (in my opinion) is too close to the 18V spec limit. 

I saw a similar thing in the PM860 amp where the main power supply filter caps were rated for 75V and there was about 72V on the rails. That's not a lot of margin. Let's say the line voltage was a little higher than normal, or there was some momentary surge on the power line. Where are those voltages going to go? What's going to happen to those caps and op-amps? Why on earth would a sensible engineer do this?

I considered connecting the op-amps to the +/- 15V regulated rails, but thought there could be some problem with putting the opamps on the same 15V rails with the phono preamp, so I decided to add a second dual 15V regulator specifically to power the op-amps. 

I installed a 15V regulator module that uses LM317T and LM337T regulator chips with a few external parts to provide regulated +/- 15V from input voltages over +/-18V or so. There will be plenty of headroom to maintain regulation because the module is powered by the +/- 22.8V that is present on the 17V rails. Each op-amp IC uses 6 mA at idle, and there are 4 of them, so 24 mA nominal load for the regulators (which squares with the measured voltage drops across the 220 Ohm resistors). That's more than enough to meet the regulator's minimal output current requirement of 10 mA.

The modification is simple. Take out the two 220 Ohm dropping resistors that sit between the 17V (actually 22.8V) rails and the op-amps and replace them with the new 15V regulators. The regulator ground connects to the preamp ground at the ground wirewrap stake. I used a drop of hot-melt glue to hold the regulator module down on the preamp PCB. 

The two 220 Ohm resistors (red) get removed, and the 15V regulator module replaces them. 

New power supply filter caps (big ones circled in red on the left)- 4x 2200 uF @ 35V, and new +/- 15V regulator module (green circle) to power the opamps. The regulator board is held in place with a drop of hot-melt glue. The regulators simply replaced the 220 Ohm dropping resistors. The white wire from the regulator board is the ground connection for the regulators and connects to the preamp ground wirewrap stake. The regulator chips on the new module don't need heatsinks as they are minimally loaded by the op-amps, even when driving headphones. Note- the original regulators are in place and operational- they supply +/-15V to the phono preamp board.

New electrolytic caps and the +/-15V regulator board that was added to power the op-amps. The regulators replace the 220 Ohm dropping resistors that were used to drop the 22.8V down to 17.4V.

The RC4136 quad op-amps used have reasonably good specs, but the pinout isn't typical of most quad op-amps. Some audiophiles would prefer to use better, lower noise, wider bandwidth parts. You can buy little plug-in adapter boards that allow you to use more modern, higher spec op-amps, but that would add another $100 to the cost of restoring this preamp. You'd be changing the op-amps, but there's still the lack of bypass caps, the funky wiring, and all those switch contacts to go through. I doubt changing the op-amps is going to result in improved sound quality when you're starting from such a marginal design.

What About the Buttons?

I searched page after page of switch cap listings at Digikey and Mouser and could not find a same-size replacement for the missing and cracked switch caps. I decided to 3D print them.

I measured one of the un-cracked buttons and the posts on the switches and came up with this design in about 30 seconds:

Back side of the 3D printable button cap. The cutout in the center fits tightly over the switch post. I printed these using TPU filament so it would flex a bit and grip the switch post tightly. They probably won't work if you print with a hard filament like PLA, ABS, or PETG.

The button is slightly tapered like the originals. The original caps had concave tops but I went with a flat surface for the sake of print quality. I printed test buttons with the fronts and backs on the printer's bed. In the end I went with the front-up prints to get a smooth surface on the visible and touchable part of the button.

One of the printed button caps. I used TPU filament and it grips the post on the switch tightly and will never crack like the original button caps. This and the the other 14 were printed in 0.15 mm layers. I printed a set in green and another in orange to see which I preferred.

This is what it looks like with all the printed button caps installed:

I went with a red LED so it would be clearly visible among the green button caps. I added orange caps to the input selector switches.

If you need to print buttons like this you can DL the fusion360 file here or just grab the STL file here. TPU tends to be hairy and blobby stuff, so plan on spending a few minutes cleaning them up with a wire clipper after printing. 

Does it Work?

After about an hour of testing all the I/O paths, I can report that everything is working fine. There's no noise from either the volume or balance pots. Music sounds clear and undistorted.