*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.

Step 1: a little diagnoses

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.
<p>I recently found my receiver blown after a party. Cracked it open, and found a burnt fuse that looked like a burned out bulb with a little soot on the side. I replaced the fuse, turned on the receiver, and saw smoke coming from one of the transistors. After reassessing, I concluded that it must be the right channel controlling transistor, as when the right channels are used, the speakers produce a loud, crackly sound. Is my assumption correct? I already ordered a replacement transistor (from China, sadly), should I order one from a more reputable place? Thank you</p>
Wrong speakers could blow the transistors if cranked up. That would result in no audio. You should check to make sure you have audio on both channels feeding the amp board. There should be a light weight shielded two conductor cable feeding the board. That cable should be the one that carries audio from the receivers preamp section into your amp. <br><br>If you fix the amp again and no audio, I'd check the feeds to the amp.
<p>Hi guys</p><p>I have an old amplifier from my Dad (a realistic STA-18), and the left channel doesn't work. I've never worked on something like this before but I tried to figure out the problem. I found a blown fuse inside the amplifier that connected to the left channel on the main amp circuit board. I used a multimeter to test the 4 transistors and found one on the left channel side that didn't give a reading, so I replaced it, and put a new fuse in. Well... limited success: I can now get a faint sound out of the left channel speaker. The fuse didn't blow, but the reading off the new transistor I put in is still a bit funny (very low- changed since installed, so maybe partially burnt right away?). I measure the voltage coming out of the two speakers now: 80 mVolts from right channel (just right I'm told) and only 4 from the left channel... which is about what we're hearing.</p><p>Any advice?</p>
<p>80mV DC offset is high. should be in the 10mV or less range. this could be from leaky caps as it is an old unit. some stupid questions.. is the part that you used an exact replacement for what was in the unit? if it was, and no fuses have blown, my next step would be to hit all the controls and switches with contact cleaner. old stereo gear that has sat for a long time can always benefit from a good cleaning of its switches and pots.</p>
Thanks for the answer!<br>So there are two transistors on the left channel, and I replaced both of them with the same type of trasnsistor- they were shorted out and the fuse was blowing. Now, the fuse doesn't blow and they're not shorted out, and there's some volume (very faint) coming out of left channel, but not much. Also...the multimeter reading on both the new transistors goes straight down to zero within a second of touching the leads (the two right channel transistors test normally with the multimeter while in-circuit.) Does this maybe mean that a capacitor or resistor isn't working on the same amp board? Nothing is discoloured/blew out that I can tell. But what/how should I test next?<br><br>As for the contact cleaner- I will try that, BUT this amp was in fairly steady use. As far as I can tell there's no gunk or dust buildup- it all seems pretty clean. Then again.. what would I know :)<br><br>Appreciate your help! This has been driving me crazy...<br>I've attached two pics- one of the whole interior of the receiver, one of a close-up of the main amp board, with the two transistors I replaced on the left side.
<p>on the two output transistors you replaced, did you put the mica spacers back in? I see a cooked bias resistor too. If the bias resistor has been replied and the mica washers are back my next stop would be dried out coupling caps. Now on the transistors, they should have been two different part numbers and you did note their placement right?</p>
<p>Ok- I put the mica spacers back and replaces both blown bias resistors- there was some discolouration on the metal next to them that I hadn't seen. The result... no sound! Unfortunately the very weak signal is gone entirely again from the left channel. Still, thanks to you I feel I'm on the right track. Any suggestion as to what I should try next?</p><p>Thanks!</p>
<p>FYI- What seems to have happened is someone (my brother...) connected the wrong speaker setup to the left channel and blew out the two resistors, transistors and the fuse on the left channel. Those things haven all been switched out now, but clearly there's something else not right/or that was blown at the same time as the other items.</p>
<p>Or is this the blown resistor you were referring to- the one with the most black discolouration (at the bottom left)- though several have a little bit of black on this side.</p>
<p>Is this the one (this is the 2nd of two, behind the two left channel transistors)- it's red, not brown, but I think it's the one you're talking about. I didn't think the black was a burn mark, just the colour marking...</p>
Thank you for this. I have a Marantz 2420 that my Dad bought in 1970. In my first floor dorm room, it could be heard on the third floor. Beautiful piece of equipment. Not knowing what I was doing, in 1999, I hooked up my speakers (8 ohm) and my friend's truck box (4 ohm). There was a strange chirp and it gave up the ghost. It's been boxed up since then, but I recently repaired a TV and thought I might have a crack at the receiver. I'm currently on step 3. The transistors failed the continuity test. Two had two way continuity and two others had none at all. With the transistors removed and a new fuse in place, it successfully powered on. There are no good local stores for electronic components, but I ordered some that should arrive by the end of the week. This beast shall live again! Pictures to follow when it is up and running.
Cool! Good to hear you may save it. Dont forget to replace the thermal grease and put any insulating spacers back.
So.... New transistors installed, the unit comes to life. I start with all EQs centered, high and low filters and loudness off, volume all the way down, speakers connected to main, main on, remote off. Bringing the volume up slowly everything sounds fine. Tested balance and filters, all good. After a couple minutes there was some crackling noise from the right side speaker. I turned it off, and switched the speakers to the remote channel to test it, and also to identify if the crackling was a receiver or speaker problem. No sound, and right away a small puff of smoke from the right side (volume knob side). I shut it off, and reconnected to main, hoping the problem was limited to the remote channel. No such luck. Smoke, and one resistor almost immediately red hot (R727). Further inspection shows R716 and R728 are scorched. I think they were the first two puffs of smoke and R727 was about to do the same. The fuse did not blow. I think (wish/hope/pray) that the resistors started to cook off but hadn't failed so badly to burn the fuse or fry anything else.<br>Also came across this anomaly: above R728, the board has holes and symbols for a capacitor (C713) but there's no sign of it having been there. It is present on the schematics as a 220pf 300V capacitor. Any ideas on what is cooking my resistors and what's up with this mystery capacitor?
<p>i just noticed this post. did you ever revive it? marantz was know for overbuilding their gear. my next step in your mission would have been to check the power supply for shorted diodes before replacing burnt resistors.</p>
I have a ceramic non-conductive thermal compound for CPUs from Radio Shack that I planned on using. Should that work?
<p>my old JVC does not amplify - i.e. i can hear sound by very very softly. what is most probable to have burnt out and how can i locate/replace it?</p>
<p>This is absolute blasphemous to take the nice warm Marantz sound and turn it into class D......god kills 10 kittens every time a Superscope era Marantz is converted to class D....</p>
Well, being that I dislike cats, I'm modding s marantz model 9 next.<br>
Add images link not working for me, let me know if I can email you
Pertaining to my last post, closeups of burned resistors and empty capacitor slot.
Yep. That will work fine.<br>
The only problem with this instructable is that the sound quality of a D-class amp is no rival to an original AB-class Marantz power amp. Your receiver will sound louder, indeed, but poorer.
id have to respectfully disagree with you. i not only tinker with old stereo gear but also collect vintage tube audio gear. audio is a very subjective hobby and ultimately one should buy what sounds good to them.<br><br>i have found some lousy class d boards but have also found a great many excellent sounding ones. in particular all the boards i have gotten from Sure Electronics in the 15wpc class and above have sounded great.<br><br>were class d falls short is when it clips. it goes from beautiful sound to cats fighting on a chalk board really fast. its very important one does not drive class d amps into clipping.<br><br>the mod that was done on this marantz is fully reversible. if you keep your old parts, there's no reason somebody cant go and undo what you did, repair or replace the original amp board, and wire it all back up the way it was.
Sure, there's no questioning on the subjective part. But Class D amps, even the better ones, have its issues - as all types os amps have, even single-ended class A tube amps. <br> <br>D's are not good with very high audio frequencies (God damn you, Nyquist!) and they tend to turn things on the bass side a little muddy - just like tube amps, even the better ones, 'cause the damping factor is too low. <br> <br>This is no opinion, this is something tried and tested on the road - ask a PA owner or tecnician and he/she will share his/her disgust about Class D amps with you. They still love AB or H for a reason. <br> <br>Anyway, of course there are marvellous D amps out there, and if the instructable was about some cheap 80's stereo I'd totally support the &quot;transformation&quot;. But it was a Marantz. It deserved some respect.
Again, it's not a critique on your instructable, because many will find it inspiring and useful - and it is! <br> <br>It was just a comment about the receiver being a Marantz, not the average low cost receiver. <br>
from that angle it looks like the cals are buldging
I owned a similar model although mine was Quadraphonic. My unit was rated at 20 watts per channel output. I burnt one of the channels out in it and it was impossible to dig into it to change the transistor due to how it was made. The thing was a total rats nest! It was OK back when I used it but I've had a lot of stereos since so it wasn't worth repairing to me. Not many consumer electronics really are. I did scrap it for salvageable parts though.
wow.. that would have been an excellent unit to drop a Sure Electronics 4x100wpc board in! i'm familiar with the quad versions of the marantz line in this instructable and they are really well suited for this mod.
Really Inspiring. I am thinking of restoring my 11 years old DAPIC Amplifier! :)

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