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November 2014
Motorola DCP-501 All-In-One Home Theater Receiver Mods
Part 1
Article By
Jeff Poth
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The Motorola DCP-501 is an all-in-one integrated home theater receiver which was released in the early 2000s with a hefty price and a lot of functionality. Unfortunately, this unit lacks video connectivity beyond S-Video and had challenges getting situated as a cable box for use with various providers; resulting in a commercial flop of the first order. Add to that an extremely large and rather ugly chassis and this was a product destined for the dust heap, however despite the challenges the sound quality of this unit is surprisingly good. It's quite well made and utilizes a Class D variant of amplifier technology known as Tripath as well as a nice transformer and generally good-quality parts and design. Because of the commercial failure these units began showing up in 2006 or so extremely inexpensively on eBay at about $150 shipped. I wound up purchasing a number of these to give away as good sounding gifts as well as a spare to (eventually) modify. I've more than gotten my money's worth over the years, with my bedroom systems powered by a Motorola for quite some time. Fast forward nearly a decade and I had two units, one sitting on a shelf with a glitch and one pulling duty in my current bedroom rig, driving the B20 TLs and my dipole surrounds. See Pass The Bofu Pioneer One I gave my father is still working nicely for him and still
sounds good running vintage Polk Monitor 7 speakers. I hung onto the glitchy amp
because I suspected that the amp section was perfectly fine, and it was the
control circuitry that was problematic. The long intended modification involved
yanking out all the preamp circuitry and running the nicely made Tripath
amplifier section and its associated linear power supply as a multi-channel
power amplifier (five channels). It's also possible to upgrade the power supply,
swap out the input capacitors on the amp section for better sounding film caps
(replacing low quality electrolytic caps), use better connectors, and increase
the output current capability by loosening an overzealous over-current
protection scheme (one can also use a bigger power supply as appropriate). The first phase is opening the beastie up and removing all the unwanted circuitry. You'll begin this on the outside of the chassis by removing all screws on the chassis holding the top and sides in place. There are several on each side, and a number around the perimeter of the back of the unit. Lift the lid off and you'll see why this is quite an endeavor- metal sub-enclosures, multiple stacked boards, and other goodies to slow disassembly down. Begin removing all screws to anything besides the power supply and amp circuits, as you'll want to start with the cable box circuitry (connected to the coaxial cable connectors). Take your time and track where each piece is connected- there will often be external screws for the connectors and internal screws holding the circuit boards and sub-chassis components. Be sure to go slowly and get all screws out, then remove the wiring harnesses, and pull each board and case component out. Some parts are interlocking but a little bit of patience will get you there. Each removal, starting at the top-rear and moving down and forward will expose new circuits to work on. The front panel was tough for me and I wound up using more elbow grease than screwdriver, but I think it can be done more elegantly than my glass-cracking version.
The power transformer and A/C input cable are connected to the switch-mode supply, with a relay connecting the two (input A/C to transformer primary winding). You'll have to remove the connections to these two components and add a proper fuse into the case- the linear supply for the power amplifier section was connected with a 7A Slow-Blow fuse, so I attached a fuse holder to the bottom of the chassis to replace and wired up the power primary wiring.
Once you're down to the basics, you'll want to scrounge any bits of the removed boards you wish to retain. I suggest yanking the ICs/heat-sinks and their mounting hardware off the switch-mode supply (left side of amp as viewed from the front). I de-soldered the first one, but it was easier to wiggle the second off with pliers, breaking the traces. Some of the back panels RCAs from the main board were also salvaged to fill in some of the gaps in the rear panel. These required the use of a Soldapult to remove- always fun to break out the tool with the best name ever. After you've saved everything of value, to you dispose of the boards and other undesirable pieces at an e-recycler. Three main pieces remain from the original amp: the transformer, the power supply board, and the amplifier board. These are along the rear and the right-hand side of the chassis, leaving you with a relatively large empty space in the front of the case. In the future, this space will be a greatly useful spot for various things like DSP, volume controls, or other slick projects. In this article I will document getting the amp running as a stand-alone five channel power amplifier with direct inputs to the amplifier section. All connections you'll need to make are on the input wiring header- this has input and ground connections for each of the five amplifier channels, and three additional pins for power/mute/fault control. In order to power on the amplifier and implement a standby switch there is a 5V supply that needs to be connected to two pins on the input header- Speak-On and P-Mute. This supply is available at the corner of the amp board closest to the power supply, there's a clearly labeled jumper that's easy to access. Since it's a control voltage its' low amperage and a simple hookup wire can be run to the switch then back to the header connections. I mounted mine on the front panel immediately next to the amp board. There's also a protect pin that you can leave disconnected. A simple SPST On/Off configuration on the connection from 5V to Speak-On and P-Mute will do the job. To access the 5V supply and make any other changes to the main boards you'll need to remove the Tripath board. Remove the screws for the spring terminals, the screws coupling the heat-sink to the main chassis, and the internal shield/divider panel immediately next to the amplifier board. This panel is a little bit of a bear, but it's really tough to get the amp board out without removing it. Look for the locking notches, don't forget to remove additional screws on the panel and get things removed carefully. Before physically lifting the amp and divider panels out, you'll also need to disconnect the connectors between the power supply and amp board. It's very self-explanatory, three cables all with connectors right on the PS board immediately next to the amp divider panel. It is really important to use the standby switch, as there is a report of the amp outputting a 50V DC pop on the outputs when unplugged (switching at the mains input). If you live in a location with frequent power outages, this may also be an issue. I'd suggest taking care to put the amp in standby (which mutes the outputs via relays) when not in use. I haven't experienced this shutoff pop, but it would be consistent with one power supply rail shutting off before the other- one reason for using bipolar supplies in amplifiers (+V,0V,-V) is to allow the amp to operate without a servo or output capacitor, and if one shuts off before the other, you can get full DC voltage. Keep in mind that many amplifier power supplies have a sequenced or controlled startup, but we removed the control logic along with the preamp section and switch mode power supply and thus there's only the 5V control voltage and mains input switching.
In addition to connecting the 5V supply to the header connections via the standby switch, I also inserted a six-terminal strip on the dividing panel, close to the input header. This allows easy configuration of preamp components when I decide to fill up all that extra chassis space. The six terminals allow for five channels of inputs and a single ground from the input wiring. All grounds reportedly need to be connected together, though I didn't try channel-independent grounding. To keep things simple, I connected the input grounds from the header together then ran a jumper to the chassis ground, which is available via a braided wire connection to a ring terminal on the heat-sink. I treated the input wiring the same way- kept the shields from the coaxial input wire separate then joined them all together at the terminal board and ran a single jumper to the chassis ground.
After doing some quick testing using a throwaway interconnect cable as input wiring the amp was up and running. Since it was working and the chassis open, this was a good opportunity to make my first improvement to the main board- replacing the awful electrolytic input capacitors with better quality units. They are 2.2uF, and only need a low voltage handling. Film-type capacitors generally have high voltage specification and the higher the voltage, the thicker the film required and the larger the capacitor. Fortunately I had some nice small 2.2uF 63V metallized films on hand from Phillips which fit in pretty well. Getting at the underside of the board requires the removal of the screws coupling the heat-sink to the circuit board- you probably don't want to mess with the MOSFET mounting, just the standoffs.
Gently bend the MOSFET leads in order to pull the board away from the heat-sink to access the underside of the circuit for removal of the original input caps. You could remove the MOSFETs from the heat-sink but this method worked well enough for me. You'll have to figure a way to get whatever caps you choose in place, my preference is to keep the layout close to stock if possible to help limit noise pickup with the class D and its associated high frequency switching, so I used the fixed lead on of the original input hole on the amplifier circuit side. Because even the smallest film cap is dramatically bigger than an equal value electrolytic, I had to run a jumper from the amp board to the other end of the capacitor and really shoehorn even my very modestly sized caps in place. It is important to observe that the caps aren't directly connected to the input header- they have 100kOhm series resistors ahead of them which you'll need to be sure not to bypass if you want the amp to be reasonably easy to drive. If you use large input caps, you may have to run twisted pairs from the original cap locations to wherever you decide to mount the caps- there's not a ton of room around the Tripath board. Be careful to route your wires well if you take this route- possibly even use shielded Twinax cable as the longer the leads the more prone to RFI pickup they'll be.
In the pictures there's also shown a burned 5W 20R Radial wire-wound resistor. Both surround channels needed these replaced on mine, as the cases had cracked. The resistive element was still intact but it appears that they overheated or were cycled too many times, and damaged the sand casing. I used two 10W in series. Notes: The replaced power
resistors could be 7.5 or even as much as 10W rated, but as I recall they were
5. Please check the thermal handling spec on them before replacing any. Also,
please take care to insulate your leads where appropriate, and take care to be
fully unplugged and discharged before working on the board. The caps can be
particularly problematic as the board locations are so close to the Tripath
heat-sinks, I used Q-Dope polystyrene and Sugru moldable silicone to
secure/insulate things. Also, be sure to insulate any primary power input wiring
connections, and anywhere in your input wiring that's likely to be a shorting
risk. This is a fairly simple project for the experienced builder but for a
novice it might be challenging to do any new skills like de-soldering or
figuring out the chassis disassembly. This is the time, also, to take a crack at bypassing the over-current resistors if so desired. I didn't, but some of these guys did and I must give credit to the DIYaudio crew for trailblazing this project. I generally use high efficiency speakers so didn't need additional current, and tend to err on the side of caution anyway. If one wanted to use this to drive subs or 4 Ohm loads, the mod would be well-recommended, as well as possibly some additional beefing up of the power supply. After getting the modified board installed, I had to decide whether to use the stock board-mount connectors for the input, or go higher-end. I went for the latter, with some nice gold RCA inputs and premium coaxial cable. Coax works well for a switch mode amp due to good shielding which will be important if I later insert DSP or other noise sources into the chassis. I had to modify the washers somewhat to fit the stock holes, but new holes could be drilled, or any number of other input solutions. I kept the stock spring terminals- I actually like this type of connector, when used with the right kind of wire- either tinned leads or raw wire (keeping in mind that bare copper will oxidize and if stranded strands may break), or optionally braided gold-plated flexible pin connectors. "Regular" pin connectors are not recommended. You can use binding posts if desired; some single-mounting models could probably fit reasonably well in the RCA cutouts.
So in broad strokes I have: Now that it's buttoned back up with better input caps and good
input connections, it is hooked up in my rig, with two channels driving the
tweeters and two on the woofers, my OSWG tweeters and BD15 woofers in the curved
bass cabinets per below. Because of this arrangement, it has a hearty power reserve
(200 WPC) and the transistors are pushing less current than in the single-module
UCD amplifier that's my reference. I won't comment overmuch on the sound, other
than to say it's excellent- nice, low noise, dynamic, engaging, and an open top
end. I will cover a little more on Tripath with my next article on this amp, but
suffice to say that this is an excellent implementation of a high quality Class
D modulation scheme (Tripath called this "Class T" but that's more marketing
than a true novel architecture). |
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