#59 - Harman Kardon AVR 235 receiver repair

hello in this episode I have Herman cardoon AVR two three five receiver for repair this is a seven channel receiver and something is wrong with it I am not sure what exactly I believe there is no output let's have a look I plugged it in you probably can see this light is on and it powers up just fine and the next step would be to connect speakers or headphones but I want to be careful because I'm not sure what's wrong with this and I want to at least check if there is no DC on the output which can burn the speakers or headphones here is the back of the unit with a lot of connectors analog in digital inputs and outputs and seven pairs of speaker terminals so let's grab a multimeter and check for DC - 55 millivolts 55 what is 7:59 3 3 4 and 30 mall DC offset like this is normal so no problem so far and here I found a stereo connector to check the headphones output here I removed the cap from the connector and attached my probes to one of the channels and there you go no DC offset here and I'll check the other channel as well and here is the other channel no problems there and of course this test is not a guarantee just a basic test to avoid the most obvious trouble and I still would suggest not to connect expensive headphones or speakers and perhaps even use diamond load power resistors and the Scylla scope here I am probing the headphones output with my oscilloscope I switched the unit to AM receiver and the volume is fine this must make some noise but I see nothing at all here and I also tried switching to CG analog input and I attached this cable to the CD input and this must make some noise as well when I touch the input connector and nothing is going on at all let's check the pre amplifier outputs here I attached this adapter to my scope and let's check front left channel and again I'm using this cable I connect it to CD input and this chip is the left channel and as you can see nothing at all it's time to have a look inside so I took the cover off and someone was in here before me because this ribbon cable was disconnected I doubt that the unit is not working because of this perhaps this is just an indication that someone tried to trouble true that before I think so because this board is for video inputs there are several composite and s-video inputs here there is still a chance that the power let's say might be routed through this board so i can reconnect and test again but let's have a look around first here we have a massive transformer on this side is probably power supply board with another section here with the voltage regulators on the heatsink on the other side probably high voltage power supplies for the power amplifier this is mains outlets on the back this as I said is video inputs this board is 4 analog inputs this must be a fam and AM receiver and on the bottom we have seven channel power amplifier board here is a fan on this massive heat sink and all active devices are on this side and this ribbon cable is running to the front panel board I found the service manual with full schematics for this unit but I see on the board next to this connector plus and minus 15 volt power rails and marked let's start from checking this let's see i connected the ribbon cable just in case the receiver is turned on here we have ground and here we should have positive 15 volt rail and we don't have it and here we should have negative 15 and it's not there either here's our problem so perhaps we should focus our attention on the power supply and in particular on this section which has free I would think voltage regulators on this heatsink here I found this power supply board in the service manual this connector is from the transformer board and this connector is the output to the main board so we should have pin 2 and pin 4 AC 17 and a half volts coming in going through this bridge rectifier and after that we have after some filtering we have plus and minus 18 volt rails which go through 7815 and 79 15 voltage regulators providing plus and minus 15 volts reals so let's check the input and the output let's check the voltages on this connector from the transformer board to the power supply board here we have a ground point and I set the multimeter to DC for now pin 1 should be 5 volts no problem here and pin 8 also should be 5 volt standby and we have it as well so now let's switch this thing to I see and we should see on pin two and four seventeen and a half volts this is pin two and we see nothing and here is pin 4 and nothing as well so now we should check this transformer board here is the transformer board and I believe this section of the transformer provides these two seventeen and a half volt rails with this center tab going to the ground each side goes through a fusible resistor 0.47 ohms and we have just a few capacitors for filtering this side goes to pin 2 and the other side goes to pin 4 so let's check here so I believe this is the center tab and this is one side 17.4 and this is the other side 17.4 no problem right on the transformer and here we have one resistor and another resistor of course on the other side of the board and this one goes to pin 2 and here we have nothing and here on the other side of the resistor there is a trace 2 pin 4 and we have nothing as well so it seems to me that these two fusible resistors are blown so now we have a choice one approach would be to find some sort of replacement and replace them and see if the receiver would work but they can blow again if there is some other fault somewhere which results in excessive current drawn these two rails plus minus 15 volt rail and we could try driving these two rails from current limited lab supply for example to check for that and only after successful check and worried about replacement resistors let me think about this and going back to the schematic I see that besides this rectifier providing plus minus 18 volt rails there is also this rectifier providing this rail marked PU and going to pin 7 so if we provided plus minus 15 both rails from a lob supply this rail would be missing and we can provide this one too but the setup gets even trickier and besides this board is quite hard to reach if I wanted to solder wires on the back of the board or to this connector this is a bit difficult to do so I'm not sure this is the best way to proceed here I found these two resistors in the list of components are 104 and 105 you see the resistor and fuse 0.47 ohm one watt and J means 5% tolerance I found a couple of point three three ohm resistors of the same size this is the closest match I have right now and I hope there should be a key for a test and by the way I'll look how these resistors are standing above the board this is intentional longer legs work as it seems to some extent unfortunately these have slightly shorter legs but they have lower resistance as well so they won't heat up as much all right I replace the resistors so let's turn this thing on and see if they blow again oops and they did I saw some smoke look at this I'm not sure you can see it very well on camera but this capacitor has a burn mark on it right here at the top and this is the one which is right across these two AC lines here this now you should be able to see the burn mark check this out my LCR meter is going crazy with a negative value and that means our device and the test is not a capacitor anymore so let's switch to resistance and sure enough we have a dead short about nine millions here I found a replacement capacitor no problem at all hundred and four nanofarads I am checking the resistors and it seems to me that one of them has died this is the one on top and it seems to be open the second one here seems okay point seven that's because of some offset in the meter let's look at the schematic again just to clarify what happened this capacitor was shorted so we had short circuit right across this section of the transformer and this time one of the resistors failed and the other one survived I managed to find one more point three three ohm resistor now let's put this one in I replaced the resistor let's check now this is the center tab which is ground and this is one side and this is the other side and now it looks right now let's check the D cereals after the regulator's here is my ground and here we should have positive 15 and we only have 9 volts and here we should have negative 15 and we have minus 10 and a half this does not look right I disconnected this ribbon cable and now 15 volt rails are fine and I also noticed that these two resistors were running hot before and now they are not even warm but this disconnected so it seems to me that there is some other fault on this board and that's why probably this was disconnected before someone figured this out and didn't care about the video inputs and perhaps you need worked fine so let's check here I am proving the headphones output again and I am touching the CD input and there you have something here I attached this old single speaker I have for testing purposes and I don't want to play any copyrighted music that's why I attached this a quark Griffon synthesizer instead no problem we have our output so now let's see if I can figure out what's wrong with that video input board after some poking around and looking at the schematics I believe that there is nothing wrong here at all except that this marking on the board plus and minus 15 volts is misleading this world indeed has two rails plus and minus 15 volts and these two resistors are here to pass these two rails to the video board and on the schematic the input here is marked plus and minus eight volts and these two rails go directly to these two voltage regulators and this one is 7805 and this one is 17 905 so they provide plus and minus 5 volt rails and these resistors are used just to drop a few volts before these regulators so they don't have to drop all 10 volts on them here is the schematic for the audio input board and here we have these two resistors from plus and minus 15 volt rail going directly to the video board connector on the positive side we have 15 ohm - what resistor and on the negative side 33 ohm resistor and in fact in this unit I see that on the positive side 27 own resistor is populated instead let's do a little sanity check on this 27 own resistor we have about 4 point 6 volts drop and how we divide 4.6 by 27 and that means about 170 Millions and if we multiply this by 4.6 this means about 0.8 watts which is not too bad yes they are running hot but these are 2 watt resistors it's not a problem and to check that the video board is working I connected this composite monitor output to my monitor and when I'm making selections here this info also appears on the monitor I don't really like the idea of leaving these point 33 ohm resistors in place instead of point 47 first of all they have lower value and that means they can handle more current before they blow and that means less protection and the second reason is that I replaced the one which failed but the other one also experienced this current shock and might be damaged to some extent and can potentially fail at any moment so I shopped around and in a local electronic store I found these 0.51 resistors also one what 5% and the marked RS f1b and I found datasheet for them and they are free improved resistors unfortunately the datasheet does not explicitly say they are fusible resistors and I'm not entirely sure this is the same thing I think that fusible means some extra guarantees about the time it takes to blow and such but I believe this should be good enough and point 51 instead of point 47 means 51 divided by forty seven about eight and a half percent higher value and the original resistors have 5% tolerance so this should be good enough the actual value is about point 5 ohms and this meter should be around 1% accurate in this range and let's check the second one about the same perfect the receiver is back together job done thanks for watching and don't forget to subscribe to see more repairs and other projects bye

Channel: FeedbackLoop

Views: 71,493

Rating: 4.8540878 out of 5

Keywords: #diy electronics, electronics repair

Id: JS9_3qPIn6Y

Length: 23min 3sec (1383 seconds)

Published: Tue Nov 14 2017