Toshiba SA7150 No Sound Left Channel

so what do we have to repair today today we have this big Toshiba si 7150 problem with this unit this time only sounds out of the right channel so let's tear into this thing and see what's going on I've had to cheer it before I had this unit a year or two ago for another problem and it's back that's the problem with old electronics is you can get married to these things very easy but this was a paying customer so let's take a look and see what's going on with this big brute today get clumps off the app so we need to determine whether our problem is in the preamplifier section or if it's one of the power apps okay you've got two separate amplifiers on this left channel and this one's the right channel on the back of the unit there's a little bridging strap these are the plugs here that connects to pre out to the power amp in so if you unplug them you'll have no sound so I'm just going to put my scope on to the left channel and we'll see whether we have in the audio going into the amplifier when I turn up the volume here there's the left channel so we do have sound going in as you can see on the scope so I know the problem is there for something in the power amplifier so let's just see whether we've got any power going through the amplifier at all make sure that we got signal getting into it and do we have anything going through the amplifier maybe maybe the problem is in the speaker switching maybe we have a fuse is blown if this unit has speaker fuses I don't know because I haven't opened the bottom yet well that's first of all see whether we've got any power so let's just take poke around in the amplifier and see whether we have any any audio signals that none of the others looks like we have audio here turn my game down a bit so we have on the left channel you can see we have signal on our drivers so um it just might be the test point here for outputs so we have tech we have outputs here these are our test ones so that tells me that the the actual audio amplifier probably is working it's either a protection circuit may be a bad relay or possibly possibly there our speaker fuses on this unit so let's get the bottom off it now because they're going to be the bottom I don't see any I don't see any fuses on the top side so if there's any speaker fuses they're gonna be on the bottom so let's get the bottom off this unit and just take a look and see whether we're gonna be lucky and it's going to be something simple like blown fuse and that would happen either if we're very well protected because they were a high power amplifier so they did generally protect the circuitry from things like a speaker wire shorting at high volume they usually did protect the output stages with fuses of some type so let's get the bottom off and see whether the problem is as simple as a blown speaker fuse clicking at the bottom of this unit there are two fuses but these are going to be main AC and DC fuses I don't see any speaking fuses here I'm sure the unit has them somewhere they may be on the main boards here's our outputs from our transistors here they go back onto the main board it also has protection relay that kicks the speakers in when the voltage stabilizes so like for example when I turn it on you'll hear a click I want to click there you hear another click in a second that's the real a speaker relay kicking in so typically how that protection circuit works is if there's any DC voltage that allows the voltages to stabilize before it actually turns on the speakers if there was a Fault in one of the amplifiers that was causing in DC voltage to appear on the speaker terminals it would kill the sound completely I'm still convinced that there probably is it probably is a fuse in here somewhere but where they've hidden it I choose a relay on there's a relay on the board as well so might be down here this thing this brute weighs a ton doing this thing probably 70 pounds 80 pounds there is a relay down here this will be the speaker relayed this is another reading up on this one so maybe if maybe we have a bad relay just see I can hear that really clicking they don't hear this one so it may even have a bad day yeah the other relay is ticking in this one's not this really is not kicking him down here so the relay on the left board is not turning on so I have to investigate a bit find out why it's not turning on it's in production protection mode for some reason do we have a DC offset or we have a bad relay or something in that protection circuit is keeping the left channel from initializing turning on the relay so work on the the amp out of the circuit I'm going to disconnect the wires it's connecting the board to the power supply so we've got a red I mean a yellow I'll blue black and a white wire that have to be connected and then we're able to unplug the actual amplifier board the screws back in so that I don't lose them as you'll notice on this unit it's two completely separate these they're what's called a mono block design so we have two filter capacitors here for the right channel its own rectifier block here's the rectifier block for the left channel on here are two filter capacitors for the left channel so now that I've unplugged and disconnected all the wires I can now lift the amplifier block right out of the chassis and basically this has to be done in order to separate the driver board so that I can check and see why this relay is not working so now we've got the board out I can start to test components on here maybe we have a component open maybe we even had the bad read it could be a bad coil we'll figure that out but now that I've got the board out I can actually work on the separate so those we run some tests on the driver board here I just disconnected the relay coil because I want to make sure the coils not open till just going to measure across the coil here and I've got 600 ohms which is for a 24 volt coil relay well 600 olds will be considered right about where it's supposed to be we've also got a diode across here which is okay that's that's to its testing finance to absorb any kick back from the relay windy and energizes yeah there's a resistor here it's a 1k ohm resistor and then measuring nine hundred and seventy-three ohms so we know that that's okay as well it goes back into this transistor array it's an IC they call it Q 803 and basically what it is is it's just an array of transistors and this is the switching for the relay not liking the looks of this because this part if it is bad just probably going to be hard to get a handle get a hold of its a TA seven three one seven P and if that turns out to be where the fault is we may be running into some trouble trying to get parts for something that was made back in the 1970s but we'll just run some tests on this thing and see what the problem possibly could be okay I'm just checking some resistor readings here we've got a 22 K resistor here it's our six zero eight it looks like 22 K resistor I put my meter across here and I'm measuring one point three mega homes I'll show you on the meter display here so this is 22 kg red red oranges of color and as you can see 1.4 mega ohms here's another 22 K right next to it you know see it's measuring 21.92 so this resistor is open let's change this resistor see if it fixes the problem on this amplifier look at the old resistor out now we'll just measure it out of the circuit should be open here we go my resistor is open so that resistor shot and looking at the circuitry it's right here it applies this looks to be B minus supply it's a dropping resistor to take the B minus supply into this transistor array so one of a couple things could have happened either the transistor array itself shorted and that's why that resistor burned up or the resistor just could have got tired and failed and that's what I'm hoping the problem is that this resistor just got tired and you know it's not Burt right up there's no burn marks or anything on it so I think it just probably got tired and went open so I got a new one here we're going to install the new one so the new resistor is installed we're just going to solder it back down to the board here clip off the excessive leads there we go new resistors installed here and we're going to now reassemble the board install it back in the unit and see if it works so we reattach the four screws to attach the driver board back to the heatsink a gain nice amplifier design this is what I like about working on these old units even though here sometimes seem like you're getting married to them that's because people who have these old units they don't want to part with them you know it's worth to to someone who owned something like this they know how good the quality of this was compared to a lot of new products today and they don't want to park with their old equipment and I can't blame them I've got an old Sansui myself that I'm hanging on to it for exactly the same reasons I know that I would never get one anywhere near the quality today so I've been told in my collection of vintage receivers I've got an old Sansui 9090 which I'm going to do it I've littered a service video on at some point there's not much wrong with mine controls need to be cleaned it hasn't been used in many years so I'm going to do a video on that at some point I just don't have a time frame as to what I'm doing going to do it but at some point I will do one and you guys will see it because that like this is a beautiful amplifier a beautiful receiver and it's about the same size the same vintage same power it's just a great unit let me get the heatsink bolted back in and then we'll hook up all the wires ok the unit is remounted we put our plugs in here for our output transistors we have two plugs we have the speaker output here which is the white wire this is the output terminal for the amplifier so we'll reattach and try not to drop the screw I should find a shorter handle screwdriver for this using one that's like two feet long it's kind of overkill but my other one is in my other workshop okay we have our power wires be sure to match three the right polarity yellow to yellow blue to blue otherwise you're going to have some major smoke when you apply power to it so there's the yellow wire these big capacitors they have big screw down lugs I've talked about quality this is the second to none and then the final wire here is the blue wire that will complete the work on the bottom we still have some plugs to put it on the top the plug in the input and plug in the the power supply these are the power inputs from the power supply for the output stage here and we've still got the power inputs for the preamp stage which is fed on the top so it's going to make sure that these are nice and tight turning it over and plug it in on the top side we have our input our sorry this is our meter connector and we have one more on the bottom I forgot to I've got the audio input this is the meter this is the power connector for the bowl to twirl the driver board or for the for the for the driver board we have one more listing away the tongue we have one more connector to go in on the bottom here and that's this one down here you guys can't see what I'm doing wait one more connector to go in is this one right down here this is the input connector which is the audio input to the board it just plugs in just like that now the moment of proof and test this thing let's see if it works let's get it get my main power cord and plug it in I do not have this plugged into my isolated supply because this sucker draws way too much power and it'll blow the fuse when it furs on so that suckers plugged directly in okay volume down everything ready to go power on let's listen for the clicks and do we have music stupid me what did I do I have no sound on my left channel what did I do think about it give you a little hint I disconnected the relay I unsalted the relay for testing the quail don't I didn't connect it so now I have to it I have to get in there and Reese alter that that really but I can get in there without having to pull the board so let me do that and we'll test it okay so though just gonna unplug the power from here so that there's no power coming into this board at all and we'll just get the solver here now we get it down here and we'll just resold her that okay let's do it okay now make sure there's nothing here that's gonna short-circuit like solder any reconnect the power supply there let me get it back in well with the truth I'll turn this around so you guys can see both audio meters on the front here what you'll know you'll probably hear the stereo sound anyway because I do have two speakers here but power on and there we go Beauty huh I was supposed to happen my speakers were creating a little bit too much vibration on the shelf and just knocked some stuff down okay this unit is fixed let's get it back together so we'll just reattach the bottom of this unit and then I can phone the guy that owns it and give him the good news that his it's big old Toshiba receiver lives to see another day I think I've said it before but I'll say it again they don't build them like this anymore nothing these days is built to last the old the old vintage gears it's so nice as so nice to work on because you can take things out with them the amplifier modules you can just take the separate module out work out on the bench it's all discrete transistors immune system it doesn't get much more serviceable than this it's like working on some of the old VCRs from the 19 late 1970s early 1980s VCRs when they got into the 90s starting to become less and less serviceable with more and more plastic parts but the old ones going way back we're very serviceable I've got some old Betamax machines that I'm going to dig up out of my storage in it and we'll take those apart and service them for future videos because they always tend to attract a few viewers but let me get the top on this thing we'll give it one final test and then get it out of here the top cover goes on and then the back cover have actually there's a tab on any of here that the slides into like that they go one last look at the Toshiba model sa 7151 channel was dead there we go we're back in business it was an open 22 K resistor and hang on to this for the guy that owns it because he's certainly gonna want to what was wrong with it there we go little half watt 22 K resistor that resistor supplied the negative bias voltage to the speaker protection I see that's part of the circuit that measures if there's any DC offset voltage present in the output before it turns on the speakers so they have a positive and a negative supply basically what they do is they take a sample of the output voltage and look for a DC component and do a comparator so the transistor array which looks like a tie see little flat pack but that should desert transit to our rate it just it's basically just a it's like a bridge type network not a bridge rectifier but like a Wheatstone bridge type circuit using resistors it measures for any current and any offset voltage present and speaker terminals and if there's any DC voltage it will not turn off the relay until and what it does when you first power the unit up normally when you turn on a big amplifier like this there are some fluctuations on the outputs as the voltage is stabilized there will be DC component present once the voltage is stabilized returns to zero when that circuit determines its safety on speakers it energizes the relay and you have sound thanks for watching we'll catch you in the next one real soon bye for now

Channel: 12voltvids

Views: 97,327

Rating: 4.9099097 out of 5

Keywords: sa7150, sound, dead, toshiba, stereo, receiver

Id: S9szPQJcfe8

Length: 22min 37sec (1357 seconds)

Published: Sun Oct 11 2015