Main power supply filter capacitors in audio equipment have primarily a dual function - one is
to filter out any ripple from the bridge-rectified DC, which would otherwise
manifest in your audio as a 60Hz or higher-order harmonic hum. The second
function is almost like a set of storage batteries - once the big caps charge
up, they resist changes in voltage. So, if you're playing a particularly
demanding passage with lots of transients for example, it tends to exhaust the
capabilities of the power supply, and voltage tends to "sag". The big filters
don't like this change, and supply their own stored current to the circuit
(they 'discharge'), shoring up the power supply's voltage, so the demanding passage
may be reproduced cleanly and without distortion.
Older filter caps start to "leak", or lose their ability to hold a charge, and
since they aren't as good at keeping the supply voltage constant, distortion
tends to increase, and usually manifests as weak, or "muddy" bass response. If a
filter cap is really on the ropes, you may even start to notice hum, even with
the volume control all the way down. Bottom line, if you want a 30+ year-old
receiver to sound like it should, you need to replace those ancient filter caps.
Replacing the rest of the electrolytic capacitors in the unit will provide
further improvement, but the lion's share of benefit will be realized when you
refresh those main filters.
Replacing the main filter caps in a vintage receiver is usually a fairly straight-forward operation. Find new filters of approximately the same size, pull the old filters, and solder the new filters in. Unfortunately, on some of the larger Marantz receivers, it ain't so easy - because Marantz engineers, perhaps as a space-saving measure, decided to employ "dual-section" filter capacitors in these units. These dual section caps are actually two separate electrolytic capacitors contained in the same housing, connected by a common terminal. For whatever reason, these dual-section caps are no longer manufactured, so there are no easy drop-in replacements available. I learned about the possibility of creating custom-made replacements for these obsolete components on the AudioKarma vintage audio forum, and leveraging the expert experience and kind advice of fellow forum members, arrived at the procedure shown below.
The process basically involves removing the old filter caps, cutting them open, and stuffing them with modern capacitors that, due to modern manufacturing advances, can provide similar or greater capacitances in much smaller packages. The original filter capacitors used in the Marntz 2385 are dual section 6,800 µF components at 85 working volts DC (WVDC). Panasonic now manufactures a low profile series of electrolytic caps known as the "TUP" series, and I purchased four of the 8,200 µF, 100WVDC version (Mouser part #667-ECE-T2AP822FA, as of this writing).
As the original Marantz dual-section filters are rarer than hens' teeth, and those in this particular 2385 appeared to be in good working condition, I couldn't bring myself to destroy them in order to install the internal replacement Panasonic caps. As it happens, I had some old Elna filter caps from the previous restoration of a Pioneer SX-1250 receiver, and they just happened to be exactly the same size as the Marantz filters. Even though they utilized two screw terminals rather than the three-solder-lug design of the original filters, I decided to use them anyway. I'm very pleased with the results. Here's the procedure:
Mask the cut line on the old filter cap. You want the cut line to fall in the middle of the area covered by the clamp when the cap is installed. IMPORTANT: Since the screw terminals add about a 1/4" to the final height of the replacement, make sure the clamp is shifted down to compensate before marking your cut line (more on this below).
Cut open the old filter. A dremel tool with cutting wheel works well for this.
Cutting complete.
The disassembled filter. In this particular Elna, the capacitor substrate is held in place with a wax-type substance.
Break up the wax with a small screwdriver and remove the wax.
A view of the cleaned out can.
Drill out the rivets on one side of each terminal, then continue drilling down another 1/4". I used a 1/8" diameter drill.
Obtain some #8 pan-head sheet metal screws, and shorten them to fit the 1/4" deep holes you just drilled.
Obtain a kit containing miscellaneous crimp connectors (Walmart sells these for about $10). Screw the connectors to the drilled out terminals as shown.
Here's a shot of the original Marantz filter compared to the Elna replacement. The Panasonic TUP caps in the foreground will be installed in the Elna's can (note the Walmart crimping terminal kit in the background).
Obtain an old terminal strip (or something equivalent), cut it to size, and mount it to the top of the replacement can. This will become the common terminal for our new dual-section replacement filter.
A shot of the common terminal from the underside. A machine screw and nut were used to secure the crimp terminal; the screw was shortened so as not to protrude beyond the nut, due to space considerations.
Select four in-line connectors from your connector kit and remove the excess plastic collar from both ends of each connector.
Tape the Panasonic caps together, bottom to bottom. Obtain some good quality 18-gauge stranded wire to make your connections. Use the in-line connectors to connect the wire ends to the cap lugs.
Bend the Panasonic cap's terminals down to minimize the connection profile, then solder the other end of the in-line connector to it. You can see that the terminal in the foreground has already been soldered.
A shot of the completed core for the first replacement. Note that the plastic connector surrounds make good insulators once the cores are installed back in the old cans. Note also the drawings and schematics in the background. Check, double-check, and triple-check that you have all polarities correctly oriented. These caps can EXPLODE if you get it wrong!!
Rear connections of the completed core.
Two layers of electrical tape are added to top and bottom connections for good measure, and the wires are crimp-connected to the can's top section. Again, CONFIRM CORRECT POLARITY!
Insert the core into the empty Elna can and wrap two layers of electrical tape around the gap. Note that the gap is not completely closed, as the core is just a tad too long. This is okay, as the metal mounting clamp will cover the gap, and there is plenty of additional height available in the 2385's cabinet to accommodate the extra 1/4".
A shot of the completed replacement filter with the mounting clamp installed. Since the screw terminals protrude more than the solder lug terminals of the original filters, I've mounted the new cans a bit higher to provide connection room.
Repeat the process for the second filter. Here they are, ready for testing.
Strip the wires for the filter connections and install crimp connectors. I used two different styles to help me keep track of the proper polarity.
Connect up the new filters with jumpers and power up the receiver using either a variac or a dim-bulb tester (DBT). If you reversed a connection somewhere, the variac/DBT precaution will protect you and your receiver.
Orient the new cans correctly in the mounting clamps and install them, with the common terminals inserted into the slots of the grounding strap. Solder the commons, connect the crimped connectors to the screw terminals, CONFIRMING CORRECT POLARITY, and tighten everything up. Power back up using the variac or DBT to confirm proper operation.
Replacement caps installed.
New Marantz stickers applied. (Details are critical!)
Congratulations! You've probably just made the single most important improvement to your 2385's performance AND longevity. Some folks would advocate running your receiver at a relatively low volume for the next five to ten hours to allow the new filters to "settle in". I'm not sure this is really necessary, but after all the work you just put in, perhaps it's prudent to err on the side of caution. Enjoy your Marantz!