Bringing New Life to an Old Classic Marantz Stereo Receiver With a Class D Amp Board.

*this instructable takes for granted that you can solder, understand basic electronics, and have fixed electronic gear before.*

so you acquired an old stereo receiver but it has issues. there are many things that can go wrong with an old stereo receiver but one of the common issues can be blown outputs. sometimes the cost of parts is prohibitive, making it not worth the effort of trying to fix a piece back to its original design.

what to do? junk it? NO..

this instructable is intended to be an option for those that have a piece not worth fixing. vintage stereo gear can be valuable, even  broken so don't just dive in till you know what you have. you may regret your actions later if by chance you had something quite valuable, even if broken. finally, this instructable is focused on transistorized stereo gear from the 70's and 80's. old tube gear is a totally different animal and not covered here. throughout this instructable an effort will be made at trying to solve some of the common ailments of old stereo gear. there's always a chance you might get lucky and what ails your unit is covered here and you are able to fix it instead of mod it.

with that out of the way, let's see what's up with this old marantz.

time to hook it up and see what happens.

first thing, inspect the power cord. any signs of damage to the cord where you may be exposed to bare wire are a bad thing and need to be addressed before applying power. if cord is all good let's move on to powering it up.

what can happen? smoke, fire, sparks, or nothing at all. find somewhere safe to work in case something goes VERY wrong with your stereo. turn the volume knob down all the way, plug the power cord in, hit the power switch and watch.

smoke = bad
sparks = bad
fire = very bad
does nothing = not so bad
powered up and seems to be acting normal = doing good

if you got smoke, fire, or sparks, we are done here and that is beyond the scope of this instructable. your unit may have a catastrophic failure but may still be valuable to someone for parts. don't just trash it. put it up on craigs or something and let somebody else take a crack at it if it's beyond your abilities.

before we proceed keep in mind there are dangerous voltages inside old stereo gear. you can die if you touch the wrong thing inside an old stereo while its plugged in. if you are unfamiliar with proper safety procedures, you should seek the help of someone who is trained in working with high voltages.

so nothing blew up. good!

did the unit power up and the lights came on? yes? then hook some speakers to it and let's see what happens. for this step it is HIGHLY recommended you use a pair of sacrificial speakers. in other words, speakers that you know work but wont be heartbroken if they get damaged. some electrical faults in a stereo unit can result in smoked speakers.

no lights? unit looks DOA?
check the fuse. if its blown, replace it with a fuse of the same rating. a higher rated fuse can result in fire and more damage if there is a hard short somewhere in the unit.

a word about blown fuses.
a clean break like the one shown could be from a light fault, the amp being over driven, or too much stuff was plugged into its accessory outlets if it has them. if the fuse has brown or black tinge in the glass like a burnt light bulb, there may be a dead short in the unit so expect the next fuse you put in to go out with a flash. just a warning.

things get complicated here. if you score high on troubleshooting geek points you should have no trouble following me. if you've never fixed anything before, get a geek buddy to give you a hand.

constant blowing fuses are the result of a short somewhere. you need to track down that short. common causes are shorted power supply capacitors, shorted diodes in the power supply often caused by failed capacitors, or shorted audio output devices.

if your stereo uses large metal cased transistors that can be removed like those pictured, you may be in luck. take a sheet of paper and make a diagram of which transistors are in which location on the heatsink. the same part number that came out of a socket needs to go back into that socket. some transistor sockets are held in place by the same screws that hold the transistor down. it's like a sandwich with the aluminum heatsink between the transistor and the socket. you may have to use a finger or screwdriver to get socket in place when re-inserting transistors.

unscrew them one by one. take an ohmmeter, set it to continuity and measure from each leg to the case of the transistor, note findings then redo measurement except this time flip the polarity of the leads. a good result will be continuity to only one transistor lead in only one direction. continuity in both directions or to more than one transistor lead indicates a failed transistor and could be the cause of your blowing fuse.

when removing transistors make sure you keep all the hardware in order so you can reassemble exactly as it came out. there may be a thin mica washer that goes up against the flat part of the transistor and insulating washers for the hardware. they MUST go back in on reassembly. the white grease on the transistors is similar to whats used on CPU heatsinks and must be there. don't wipe it off.

if you have shorted transistors, there's a good likelihood your stereo may be saveable with just a new set of replacement output transistors. time to look at cost of new transistors versus modding the unit you have. modding will kill its collector value. if you do install new transistors, make sure and get a small tube of heatsink grease as the new transistors will have none on them.

if you do find shorted transistors go ahead and remove all the output transistors, make sure the back side of the sockets which may be loose and flopping around aren't shorting against anything, and power back up with a good fuse. if your unit comes to life you may be lucky and only need a set of output transistors. this is very crude diagnoses and there may be other faults but you have a good likelihood that your stereo quit working due to abuse, hence the blown transistors.

some equipment brands soldered the wires to the transistors. in this case the diagnosis method is the same but you have to unsolder the wires going to the transistors at the transistors themselves or at the board. make sure you note colors and position of colors. do NOT overheat the legs on the transistors. you can ruin an otherwise good transistor with too much heat.

if you opt to try and replace the old transistors, buy replacements only from a reputable vendor. the auction site is flooded with chinese counterfeit transistors so be careful where you spend your money.

sometime in the 80's, chip amps caught on and many manufacturers started using them. they look like whats in the picture. its basically a large chip bolted to a heatsink. often times there is an STK part number on it. this is basically an amp on a chip. it was a cost cutting measure.

theres no way to easily test these. unsolder the whole chip, make sure you didnt bridge any solder pads, power amp up and see if fuse keeps blowing. fuse stops blowing? probably a shorted STK chip. fuse keeps blowing? other issue which we will address.

stereo gear with STK's in them may prove to be harder to mod. keep that in mind. a great many STK part numbers are still available but price varies drastically. be careful of chinese counterfeits. buy only from a reputable vendor.

still blowing fuses?

a failed power supply could be the culprit. old electrolytic caps can short out and take out the diodes in the power supply. look for bloated or leaking capacitors.

to check the power supply diodes you need to disconect one leg of the device from the circuit and check for continuity in diode test. there should be continuity in only one direction. if your amp uses a rectifier bridge, you will have to desolder the whole device to test it. it should be marked with a pinout.

blown diodes and bad caps go hand in hand.

if everything so far checks out and your still popping fuses it may be time to consider walking away from that project. you could have a bad power transformer which is basically the heart of the stereo. no power = no musical love.

with sacrificial speakers connected, power up and let's see what you get.

unit plays fine = seller didnt bother testing things? score!
unit plays only on one channel or on neither channel = you could have a blown channel (maybe bad STK or output transistors), dirty controls, missing preamp jumper, or "tape monitor" button is pushed in.
unit plays but makes static noises = electronic fault or dirty controls. if using FM make sure you have an antenna connected. having no antenna will get you a noisy signal.

before condemning a unit to have a bad amp section make sure the controls are set right. if it has a tuner, set it to FM, muting off, on mono (since you will likely just be using a random wire as an antenna), and work the "tape mon" button a few times. could just be a dirty control.

look at the back, is there a set of RCA jacks labeled "pre out" and "amp in" or something to that effect? some receivers had the ability to be split from their amp sections via those jacks on the back. if your unit has those jacks, run an rca jumper from the pre out to the amp in and see if it doesn't come back to life. many a receiver has been junked because it was just missing those jumpers.

if unit has static crashes and random crackles try getting in there with contact cleaner and spraying out all the switches. give them a shot of contact cleaner and then work the switch.

the goal here is to try to bring it back to life. the last option is to mod it.


 

so you have dead channels you can't / don't want to fix or you have crackling noises that don't go away with switch cleaning? its time to do some further tests to see if your unit is worth modding to an aftermarket amp board.

if you are fortunate enough to have a receiver that has "pre out", this step will be made drastically easy for you. run an rca cable from the "pre out" jacks to a known good amp and see what you get. if it doesn't have preamp outputs, you need to dig into the unit.

somewhere feeding the amp section of your receiver, is an audio line. this line carries the line level audio from the preamp section (tone controls, volume control, balance control) and sends the preamp output to the audio amp section input. on some units, like the marantz at the start of this instructable, there is an actual cable that does this. on other units, it may be a trace on a circuit board.

if it's a cable, it's usually one three conductor cable. it supplies ground, left audio, right audio. sometimes pc board traces are also laid out in this format. on large receivers, you may have separate left and right amp sections which means separate left and right audio lines. you must find this line and separate it from the amp board.

take that line that's coming FROM the preamp section of the receiver and wire it up to an rca cable and plug that into a known good amp. if all is well, you should be able to hear the audio coming from the stereo you are going to mod and all its controls should work like balance, tone, volume, etc. the audio should be free of crackling, excessive hiss, or loud hum. you are essentially using the old receiver as a preamp.

if everything works as intended by using the preamp out of the receiver then you're ready to proceed. if things aren't working as they are supposed to, you may have made an error somewhere or there's something else wrong with the old receiver. go back and re-check things.

if you have nice clean audio coming out of the preamp section, you are ready to proceed with the mod. if you have random static crashes or other issues, you need to re-check your work. if your work is fine, you have other problems in that receiver and it may be time to reconsider proceeding.

let's say all is hunky dory and you have good audio coming out of the preamp line on your project receiver. great! time to plan and dig in.

1) how much space you got?
do you have an amp board in mind to use? will it fit?
2) how are you going to mount the new amp board?
3) how will you power the new amp board?

space is not an issue with most old receivers. mounting is. your amp board needs to be mounted in a mechanically sound fashion. you don't want it shaking loose at some point. this calls for nuts and bolts. no double sided tape rigging here please.

power for your amp can be done in different ways. you can use an SMPS (switched mode power supply), you can add a transformer and build another supply on the chassis of the receiver if space permits, or you can use what was running to old amp section as the basis for your new supply to run the new amp board.

goods and bads..

SMPS
goods - standalone unit, plug and play, you can get the voltage you need right from the get go, very compact when compared to a conventional standalone supply.
bads - can induce radio noise, specially if you intend to use the AM tuner section on your modded receiver.

building a separate conventional supply
goods - no RF noise, less stuff to go wrong in it.
bads - big and bulky, space may be an issue.

using the old power supply section to run your new amp board
goods - most of it is already there
bads - 99% chance the voltages are all wrong requiring some creative voltage regulation


for my build i went with an SMPS. since i was using a small 15wpc class d amp board, it didn't take much to run it. i used JB weld to epoxy the PSU in place and a tie wrap to hold it tight while the epoxy dried. the leads that power the little PSU were soldered to the back of the switched accessory outlet on the receiver. when the receiver is powered on, so is the amp board.

this receiver was originally rated at 20wpc so 15wpc is not too far from the original design. i could have easily gone the other way though and dropped a 100wpc board into the area that was originally occupied by the old amp board. the class d boards are quite small compared to their regular class a/b counterparts and require little or no heatsink in some cases.

this isn't a political rant, i'm referring to common ground leads feeding the speakers.

here is the one place you might have to give up a feature or two of the original receiver's design. headphones typically use three conductors, ground, left audio, right audio. the speaker selector switch on many stereo receivers is typically designed to work with this common ground setup.

all the class d amp boards i have seen so far advise against wiring for common ground on the speakers. this can actually damage the amp. because of this you may find yourself having to remove the original wiring that feeds the speaker terminals on the receiver, removing the jumpers between the ground terminals if any are present, and wiring the amp board straight to the speaker jacks on the receiver.

end result?
no more working headphone jack in most cases and the speaker select switches on the front of the unit will no longer work. a few high end receivers had a separate dedicated headphone amp but not many. i consider it a small price to pay.