Hi folks - don't know if i'm in the right section but i bought a technics SU-7700 integrated amp recently on ebay and it's died already and i haven't a clue what the problem is with it. It's impossible to find anyone in my part of the world who would be able to fix it and i was wondering if anyone had any ideas. I was using it this time last week and the speakers went dead - it kicked back in a few minutes later and repeated this a couple of times over the next 24 hours and then died for good. The lights in the VU METER windows stayed on and the very low hum that's audible when it's on is still there, it just doesn't seem to be delivering to the speakers. i checked what i could re fuses and connections and couldn't find anything though my knowledge is pretty basic in this dept. It did normally take a couple of minutes to warm up in the morning and had recently been making static noises when adjusting the volume though i understand this isn't unusual in old amps. I'm really fond of this amp and would hate to see it go to waste esp if the problem is something simple. Can anyone help me? Many thanks 
Hi,
I don't know anything about this particular amp., but from what you say, this could be a problem with the 'wiper' going open circuit on the vol control, especially if it is as old as you suggest.
This could explain the intermittent static noises you mention when operating the vol control, and would more than likely give rise to some hum as the input to the first stage would probably then be open-circuit.
However, I assume it is a stereo amp, and it would be unusual for the same problem to occur (at one and the same time) in both channels.
Did both channels work OK when you first tried it out, and immediately before it finally died?
Edit. I just realised you later said the hum was from the amp, not the speakers, but a slight hum from an amp is not unusual, anyway. However, if the problem is with both channels, this is significant as it is unlikely that both channels of the amp boards would die together.
You would then need to consider any 'shared' parts of the circuit more like the power supply, perhaps.
I don't know anything about this particular amp., but from what you say, this could be a problem with the 'wiper' going open circuit on the vol control, especially if it is as old as you suggest.
This could explain the intermittent static noises you mention when operating the vol control, and would more than likely give rise to some hum as the input to the first stage would probably then be open-circuit.
However, I assume it is a stereo amp, and it would be unusual for the same problem to occur (at one and the same time) in both channels.
Did both channels work OK when you first tried it out, and immediately before it finally died?
Edit. I just realised you later said the hum was from the amp, not the speakers, but a slight hum from an amp is not unusual, anyway. However, if the problem is with both channels, this is significant as it is unlikely that both channels of the amp boards would die together.
You would then need to consider any 'shared' parts of the circuit more like the power supply, perhaps.
ok thanks Bob - i'm going to have to try to get someone to have a look - i wouldn't know where to start - it's very difficult to find anyone around here who fixes electronics that are not new or under warranty - i just thought someone here would be able to point me in a def direction where i could go at it myself - many thanks to all who offered their help - it was much appreciated 
ps, if it's a dry solder problem, where would it most likely occur in this instance?
ps, if it's a dry solder problem, where would it most likely occur in this instance?
Dry solder joints can occur anywhere, there is no "more likely place". Unfortunately, any bad connection on the amplifier board can cause the symptoms you've described.
Could even be a bad coupling capacitor. It could introduce a DC offset at the output of one of the amplifiers, causing the speaker relay circuit to cut out (protection).
If you have a DVM handy you could try measuring voltages; that would be a starting point but you'd have to have a schematic and know how to read it (i.e. people here could tell you where to measure on the schematic, but you'd have to have a basic electronics background to be sure you're measuring the right point on the actual amplifier...
Cheers!
Could even be a bad coupling capacitor. It could introduce a DC offset at the output of one of the amplifiers, causing the speaker relay circuit to cut out (protection).
If you have a DVM handy you could try measuring voltages; that would be a starting point but you'd have to have a schematic and know how to read it (i.e. people here could tell you where to measure on the schematic, but you'd have to have a basic electronics background to be sure you're measuring the right point on the actual amplifier...
Cheers!
dry solder problem...
How old is the amp and how abusive u/earlier users have been?
u said sometimes it worked intermittently. Then it is definately a dry solder problem. For this u will have to access the solder side of the PCB (the board on which electronic parts are mounted).
Now u will need a soldering iron(any cheapo upto 30w), solder and a lens.
If u have good normal eyesight then bare eyes are the best.
Now look in the area where power devices, generally the ones that are mounted on heatsink are and inspect the solder points at the point where component leads come out.
If u find there is an inner circle on any point, then it is a crack that is responsible for disconnecting the lead. Just solder such points.
One soldering hint...after u solder, lift the iron vertically upward. This prevents any solder bridge formation to the nearby solder point.
Gajanan Phadte
How old is the amp and how abusive u/earlier users have been?
u said sometimes it worked intermittently. Then it is definately a dry solder problem. For this u will have to access the solder side of the PCB (the board on which electronic parts are mounted).
Now u will need a soldering iron(any cheapo upto 30w), solder and a lens.
If u have good normal eyesight then bare eyes are the best.
Now look in the area where power devices, generally the ones that are mounted on heatsink are and inspect the solder points at the point where component leads come out.
If u find there is an inner circle on any point, then it is a crack that is responsible for disconnecting the lead. Just solder such points.
One soldering hint...after u solder, lift the iron vertically upward. This prevents any solder bridge formation to the nearby solder point.
Gajanan Phadte
Never give up hope!
Hi madmadra,
It is unfortunate that diagnosing such problems at a distance is not easy, and perhaps that you are not very experienced and probably without any test equipment.
However, all is not necessarily lost here, and we still might be able to help you identify the problem area. Whether we can suggest how to fix it is another matter, but this cannot even be considered until the area which is faulty has been identified.
You clearly know what has happened here, and that helps a lot, and you obviously know a little about the matter as you mention fuses etc., and if you don't know who to take it to for repair, and are willing to still be involved with solving this, we might just get lucky.
It is much harder if you are presented with a dead amp (say if it was not working at all when you received it) and there is no known 'history' or sequence of events, to assist the repairer with his thoughts.
As others have attempted to do, we need to apply some logic to crack this, and that is why I asked about the immediate 'history' before the unit finally died.
You see with any stereo gear, it is most unlikely that both channels would die at precisely the same time as a result of problems with the individual amplifier circuits, or components on these boards, or even the interconnections here. Unless, of course, there was an obvious blow-up, which you don't mention.
The fact that the fault was intermittent for a while before it finally died, will provide some clues, too.
If you can provide specific answers to the following questions, this will be a great help.
1) Did the amp play satisfactorily through *both* channels at any time since you had it?
2) When you noticed the intermittent problems, did this affect both channels (i.e. no sound at all, or the noises you mentioned) at the same time?
3) Did you ever notice a time since it was in your posession when only one channel was playing OK, but there was a problem with the other?
The answers to the above should indicate if it is likely to be a problem on the amplifier circuit boards or maybe (and I think more likely) with some 'shared' part of the circuitry.
You say that there are some lights and hum, so it has not entirely died, but it is beginning to look like a power supply problem, or maybe output protection (if there is one), as these are areas which are 'shared' by both channels.
Accordingly, when there is a problem in these areas, it affects *both* channels at the same time, usually.
If you don't wish to pursue this, I am sure everyone will understand, but I for one am quite willing to spend more time on this if it would be of any further help.
Regards,
Edit. Looking back at your original post, I am puzzled by what you say about a couple of minutes to warm up at switch on. Even with valve gear (which from your pic, this isn't) this would be unusual, in my experience. What happened, and what did you hear, during this first few minutes?
Hi madmadra,
It is unfortunate that diagnosing such problems at a distance is not easy, and perhaps that you are not very experienced and probably without any test equipment.
However, all is not necessarily lost here, and we still might be able to help you identify the problem area. Whether we can suggest how to fix it is another matter, but this cannot even be considered until the area which is faulty has been identified.
You clearly know what has happened here, and that helps a lot, and you obviously know a little about the matter as you mention fuses etc., and if you don't know who to take it to for repair, and are willing to still be involved with solving this, we might just get lucky.
It is much harder if you are presented with a dead amp (say if it was not working at all when you received it) and there is no known 'history' or sequence of events, to assist the repairer with his thoughts.
As others have attempted to do, we need to apply some logic to crack this, and that is why I asked about the immediate 'history' before the unit finally died.
You see with any stereo gear, it is most unlikely that both channels would die at precisely the same time as a result of problems with the individual amplifier circuits, or components on these boards, or even the interconnections here. Unless, of course, there was an obvious blow-up, which you don't mention.
The fact that the fault was intermittent for a while before it finally died, will provide some clues, too.
If you can provide specific answers to the following questions, this will be a great help.
1) Did the amp play satisfactorily through *both* channels at any time since you had it?
2) When you noticed the intermittent problems, did this affect both channels (i.e. no sound at all, or the noises you mentioned) at the same time?
3) Did you ever notice a time since it was in your posession when only one channel was playing OK, but there was a problem with the other?
The answers to the above should indicate if it is likely to be a problem on the amplifier circuit boards or maybe (and I think more likely) with some 'shared' part of the circuitry.
You say that there are some lights and hum, so it has not entirely died, but it is beginning to look like a power supply problem, or maybe output protection (if there is one), as these are areas which are 'shared' by both channels.
Accordingly, when there is a problem in these areas, it affects *both* channels at the same time, usually.
If you don't wish to pursue this, I am sure everyone will understand, but I for one am quite willing to spend more time on this if it would be of any further help.
Regards,
Edit. Looking back at your original post, I am puzzled by what you say about a couple of minutes to warm up at switch on. Even with valve gear (which from your pic, this isn't) this would be unusual, in my experience. What happened, and what did you hear, during this first few minutes?
howdy Bob, thanks for the reply - I had 2 pairs of speakers set up, one pair on remote and the other on main without any problem so i assume both channels were working all along - all 4 speakers went dead and came back twice before going for the last time. As for taking time to "warm up" i never actually heard anything out of the speakers while it was doing this, it usually took a couple of minutes though sometimes up to 4 or 5 - and then the sound would suddenly appear - i thought it was all part of the vintage experience! initially i switched it off every nite but had started leaving it on standby some nites because of this delay so perhaps if dry solder problems are made worse by heat this would seem to be a strong possibility? i just got the last few posts and haven't had time to look closely at the ones re soldering parts on the board - i'll have a look and see if i understand enough to attempt this and get back if i need any help identifying the right locations - cheers Bob and all
howdy Bob, thanks for the reply - I had 2 pairs of speakers set up, one pair on remote and the other on main without any problem so i assume both channels were working all along - all 4 speakers went dead and came back twice before going for the last time. As for taking time to "warm up" i never actually heard anything out of the speakers while it was doing this, it usually took a couple of minutes though sometimes up to 4 or 5 - and then the sound would suddenly appear - i thought it was all part of the vintage experience! initially i switched it off every nite but had started leaving it on standby some nites because of this delay so perhaps if dry solder problems are made worse by heat this would seem to be a strong possibility? i just got the last few posts and haven't had time to look closely at the ones re soldering parts on the board - i'll have a look and see if i understand enough to attempt this and get back if i need any help identifying the right locations - cheers Bob and all
Hi again - had a look at the board and i'm not sure what area i should be focusing on - the earlier post which said ...........
"Your amp is wired so look at those wires coming from and on the heatsink mounted components"
mentions a specific area though i don't know what the heatsink is - i see a large black cube shape up to the top left of the board with a smaller brass coloured component on top which looks like some kind of transformer? where is it in relation to this? - i see a lot of wires on this side of the board though everything seems to be connected ok - i uploaded a pic earlier and will try to upload a bigger one now and maybe you can narrow down the area and what i should be looking for - cheers
"Your amp is wired so look at those wires coming from and on the heatsink mounted components"
mentions a specific area though i don't know what the heatsink is - i see a large black cube shape up to the top left of the board with a smaller brass coloured component on top which looks like some kind of transformer? where is it in relation to this? - i see a lot of wires on this side of the board though everything seems to be connected ok - i uploaded a pic earlier and will try to upload a bigger one now and maybe you can narrow down the area and what i should be looking for - cheers
Attachments
I just can't help myself - I have to comment.
First it is NOT definitely bad solder joints, it could be, but it is dangerous to think in definites when trying to fix electronics, you could spend lots of time barking up the wrong tree.
If both channels go out at the same time it has to be something that is common to both channels OR something that effects something common to both channels - a good guess would be a speaker relay circuit - I think I can make out a relay (a little hard to tell from the photo) next to the main power supply caps on the left. One could check to see if both channels were working and the relay was turned off with a scope probe to the correct spot, but I'm guessing you do not have a scope...
What jumped out at me from the photo is the apparent discoloration on the circuit board to the right and center right which could possibly be water damage. If so you need some knowledge, a schematic and lots of patience to repair. The power transformer also appears to have some water damage (though I would expect this to be purely cosmetic)
I have repaired water damaged sets but it can be very difficult, and very time consuming.
Two other comments: The outputs are mounted on the heatsinks, and probably in sockets and so would not have cold solder joints. And many times in amps that have lost both channels it can be a bad power supply (effects both channels), many times it was poor solder joints on a too warm transistor regulator (heat is the enemy).
But I would worry about the possible water damage first.
Bill
PS good luck
First it is NOT definitely bad solder joints, it could be, but it is dangerous to think in definites when trying to fix electronics, you could spend lots of time barking up the wrong tree.
If both channels go out at the same time it has to be something that is common to both channels OR something that effects something common to both channels - a good guess would be a speaker relay circuit - I think I can make out a relay (a little hard to tell from the photo) next to the main power supply caps on the left. One could check to see if both channels were working and the relay was turned off with a scope probe to the correct spot, but I'm guessing you do not have a scope...
What jumped out at me from the photo is the apparent discoloration on the circuit board to the right and center right which could possibly be water damage. If so you need some knowledge, a schematic and lots of patience to repair. The power transformer also appears to have some water damage (though I would expect this to be purely cosmetic)
I have repaired water damaged sets but it can be very difficult, and very time consuming.
Two other comments: The outputs are mounted on the heatsinks, and probably in sockets and so would not have cold solder joints. And many times in amps that have lost both channels it can be a bad power supply (effects both channels), many times it was poor solder joints on a too warm transistor regulator (heat is the enemy).
But I would worry about the possible water damage first.
Bill
PS good luck
What things are: (from your pic which is really too fuzzy to go in to great detail)
top left, large black object with copper is the power transformer
to the right of the transformer - the object with fins is the heatsink which has the power or output transistors mouted on it along with the bias transistor or doides and possibly the driver transistors
all the wires snaking around to right of the heatsink are probably from the input jacks to the preamp section
below the transformer the two large black cylinders are the main power supply capacitors
immediately to the right of the capacitors is I believe a relay (in a clear plastic case) probably the speaker relay. You should be able to hear a mechanical click from this a short while after turning on the unit, and the sound will start to come from the speakers at the same time.
the white rectangles further right from the relay (two are almost exactly in the center of the picture are almost certainly the emitter resistors of the output transistors. This is a useful check point for measuring dc offset and output signal of the poweramps if the relay is not turning on - you need at least a meter, and preferably a 'scope.
And if I'm right about that water damage you're probably going to need a schematic to fix it. Of course maybe the photo is just fooling me and the board is just dirty.
Bill
Also remember the preamp could be failing and causing no sound, dirty volume control, and tape monitor switch are common problems. But usually dirty switchs and controls do not usually effect both channels at the precisely the same time.
good luck
top left, large black object with copper is the power transformer
to the right of the transformer - the object with fins is the heatsink which has the power or output transistors mouted on it along with the bias transistor or doides and possibly the driver transistors
all the wires snaking around to right of the heatsink are probably from the input jacks to the preamp section
below the transformer the two large black cylinders are the main power supply capacitors
immediately to the right of the capacitors is I believe a relay (in a clear plastic case) probably the speaker relay. You should be able to hear a mechanical click from this a short while after turning on the unit, and the sound will start to come from the speakers at the same time.
the white rectangles further right from the relay (two are almost exactly in the center of the picture are almost certainly the emitter resistors of the output transistors. This is a useful check point for measuring dc offset and output signal of the poweramps if the relay is not turning on - you need at least a meter, and preferably a 'scope.
And if I'm right about that water damage you're probably going to need a schematic to fix it. Of course maybe the photo is just fooling me and the board is just dirty.
Bill
Also remember the preamp could be failing and causing no sound, dirty volume control, and tape monitor switch are common problems. But usually dirty switchs and controls do not usually effect both channels at the precisely the same time.
good luck
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