Hello,
I'm having kind of a strange problem with the A and B speaker terminals on a Sansui A-7.
When using system B everything is ok (both channels work) but when I switch to system A with speakers hooked up my bulb tester lights up. I've checked everything and as long as I have no speakers on either the left or the right channel on system A everything works as it should. I measured voltages and resistance without speakers on both systems and I have exactly the same values. I also checked the ON/OFF and A/B switches and they're fine.
I don't see what could be the problem here.
I could use some help with this
I'm having kind of a strange problem with the A and B speaker terminals on a Sansui A-7.
When using system B everything is ok (both channels work) but when I switch to system A with speakers hooked up my bulb tester lights up. I've checked everything and as long as I have no speakers on either the left or the right channel on system A everything works as it should. I measured voltages and resistance without speakers on both systems and I have exactly the same values. I also checked the ON/OFF and A/B switches and they're fine.
I don't see what could be the problem here.
I could use some help with this
Yes, that is exactly what it thought, that's why I'm a little lost. Actually the protection board is empty, apparently this is common in the European model. so it can't be that.
It used to work just fine but left and right channel were out of balance due to a faulty volume pot. I changed it and that is when this problem showed up.
It used to work just fine but left and right channel were out of balance due to a faulty volume pot. I changed it and that is when this problem showed up.
If this is the only difference in effect, surely you have made an error in the type of replacement pot. or the pinout. This won't fix itself so logically, you need to retrace your steps by refitting the original pot. or carefully comparing the pots and assessing whether it really was faulty. Perhaps it was not the pot. that was the problem.
I'm not very familiar with model but some earthing connections for pots. can be important.
The replacement pot is the exact same model from a new old stock and as the Speaker system B works fine I suppose the pot is installed as it should be. The speaker selector is on a different board. The wires come from the power amp section straight to the speaker ON/OFF switch, the to the A/B switch and from there directly to the speaker terminals. I have measured resistance, DC offset and AC between ground and both left and right positive terminals and I have exatly the same results on both systems.
As there is no relationship between the volume pot. and the speaker connections, you need to check the contacts and wiring of S5c and A speaker connectors, or even the speakers and leads themselves, more thoroughly. At least this is simple circuitry that can be easily tested with a DMM too. Also, you should not get any indication from the bulb tester if there is no load (speakers) connected to a working amplifier - at least that is in agreement so far. Don't forget to check for shorts to ground, chassis, other circuits too.
Free service manual download here: Sansui A-7 Manual - Master Integrated Amplifier - HiFi Engine
According the schematic diagram, S5a,b select the speakers on or off and additionally in export models, S5c,d select speaker sets A or B.
You could also perform some more tests with the bulb tester by eliminating possibilities until you arrive at only one. For example, does the bulb tester light up when the switch is operated with no speakers connected to the amplifier? Does it still light up when there is no signal either?
A last question; are you trying to drive 2 sets of speakers and are connections and leads to both sets OK?
Free service manual download here: Sansui A-7 Manual - Master Integrated Amplifier - HiFi Engine
According the schematic diagram, S5a,b select the speakers on or off and additionally in export models, S5c,d select speaker sets A or B.
You could also perform some more tests with the bulb tester by eliminating possibilities until you arrive at only one. For example, does the bulb tester light up when the switch is operated with no speakers connected to the amplifier? Does it still light up when there is no signal either?
A last question; are you trying to drive 2 sets of speakers and are connections and leads to both sets OK?
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OK so I checked for any shorts to ground and found nothing wrong. Just for info, the problem occurs on the speaker system A and not B. Now I did some other tests :
-left and right speaker on system B, speaker switch ON, System B, no problem on power up, sound on both channels.
-left and right speakers on system A (same speakers, only one set of speakers), speaker switch ON, System A, bulb shines bright when I power up.
-same as before but switch OFF before powering up, no problem. speaker switch ON, still no problem and sound on both channels.
So the problem only occurs when speakers are hooked up to System A and I power up with speaker switch "ON". It also works if I power up with speaker switch "ON" but set to system B and only after powering up switch to A.
-left and right speaker on system B, speaker switch ON, System B, no problem on power up, sound on both channels.
-left and right speakers on system A (same speakers, only one set of speakers), speaker switch ON, System A, bulb shines bright when I power up.
-same as before but switch OFF before powering up, no problem. speaker switch ON, still no problem and sound on both channels.
So the problem only occurs when speakers are hooked up to System A and I power up with speaker switch "ON". It also works if I power up with speaker switch "ON" but set to system B and only after powering up switch to A.
You haven't answered the question about the problem remaining (i.e. the bulb tester remaining bright) with no speakers connected nor whether it remains bright when no signal is connected to the amplifier. That would tell us what type of overload is occurring.
The only reason I can see from the schematic (yes, the switch connections look confused) that the bulb would light on A but not B connections, was if the switch contacts, the solder joints, wires or connectors were indeed different in a way that you have not determined yet.
If the bulb tester only lights when system A speakers are selected, the switch must be suspect as it works OK when the speakers are switched on and B is selected, right? But you say that by turning the speaker switch (which is in series with the A-B switch) off first, then on after selecting A speakers, everything works and presumably, the bulb no longer shines bright either. That all points to switch malfunction.
These appear to be simple, mechanical "make-or-break" sliding contact switches, so this can only occur if the contacts are burnt, resulting in unreliable contact or by shorting (perhaps only briefly when operated) to ground. If that is so, you need to check that switch out thoroughly by disassembly if possible.
I doubt you could replace the switch so it will need care and patience to do this without losing components like tiny springs, rivets etc. The option is to get used to this problem as the amplifier is quite old at 35 years now.
The only reason I can see from the schematic (yes, the switch connections look confused) that the bulb would light on A but not B connections, was if the switch contacts, the solder joints, wires or connectors were indeed different in a way that you have not determined yet.
If the bulb tester only lights when system A speakers are selected, the switch must be suspect as it works OK when the speakers are switched on and B is selected, right? But you say that by turning the speaker switch (which is in series with the A-B switch) off first, then on after selecting A speakers, everything works and presumably, the bulb no longer shines bright either. That all points to switch malfunction.
These appear to be simple, mechanical "make-or-break" sliding contact switches, so this can only occur if the contacts are burnt, resulting in unreliable contact or by shorting (perhaps only briefly when operated) to ground. If that is so, you need to check that switch out thoroughly by disassembly if possible.
I doubt you could replace the switch so it will need care and patience to do this without losing components like tiny springs, rivets etc. The option is to get used to this problem as the amplifier is quite old at 35 years now.
Yes sorry I forgot to answer that part. Having no signal or operating the two switches with no speakers shows no problems at all.
Concerning the second part, I can power up the amp (with speaker on system A) with those combinations without the bulb shining bright : Speaker switch ON + system B.
Speaker switch OFF + system B
If I power up the amp with Speaker switch ON + System A the bulb stays lit.
But When the amp is already turned on, I can do what I want with the two switched and everything work fine. So the Speaker ON/OFF switch or A/B switch both are fine.
It's kind of hard to explain, I'm doing my best but I don't know if what I'm saying is very clear.
Concerning the second part, I can power up the amp (with speaker on system A) with those combinations without the bulb shining bright : Speaker switch ON + system B.
Speaker switch OFF + system B
If I power up the amp with Speaker switch ON + System A the bulb stays lit.
But When the amp is already turned on, I can do what I want with the two switched and everything work fine. So the Speaker ON/OFF switch or A/B switch both are fine.
It's kind of hard to explain, I'm doing my best but I don't know if what I'm saying is very clear.
I could only see two directions
One the switch is faulty messing up signal with ground which also include a weird case of failure : there is some type of switches that accommodate the pins of the switch with pcb material there is others that are plastics It happened before in Sansui switches that the pcb material if have a crack will actually drink and preserve some of the amount of spray that you used to to clean it .
That amount of spray will be enough to create problems of even that type if concentrated on the pins of the switch but only when the circuit is powered .
( this is to point you that a DVM is not always enough to trace a short There is a possibility that short appears only when power is applied in the circuit ,,,Notice that for feature reference also . )
Similar to that some models feature "emission control" capacitors and that will be some small ceramic capacitors located in the binding post ( separated per A and B speakers ) much after any protection if existing .
Seen only one case in a Marantz that this capacitor was working up to a certain point of power and then presented a complete short ....Lower the power and capacitor will work again Weird ?
Kind regards
Sakis
One the switch is faulty messing up signal with ground which also include a weird case of failure : there is some type of switches that accommodate the pins of the switch with pcb material there is others that are plastics It happened before in Sansui switches that the pcb material if have a crack will actually drink and preserve some of the amount of spray that you used to to clean it .
That amount of spray will be enough to create problems of even that type if concentrated on the pins of the switch but only when the circuit is powered .
( this is to point you that a DVM is not always enough to trace a short There is a possibility that short appears only when power is applied in the circuit ,,,Notice that for feature reference also . )
Similar to that some models feature "emission control" capacitors and that will be some small ceramic capacitors located in the binding post ( separated per A and B speakers ) much after any protection if existing .
Seen only one case in a Marantz that this capacitor was working up to a certain point of power and then presented a complete short ....Lower the power and capacitor will work again Weird ?
Kind regards
Sakis
I've opened the two switches and looked carefully for cracks or moist but nothing in there that seems out of place.
Also I checked and there are no "emission control" capacitors as you say.
But I did notice something, the output of the power amp is connected to the speaker selector board with a nap of three wires. When I unplug those and turn on the amp wit speaker switch ON, and system A active with speakers, there is no problem.
Also if I turn the amp off with speaker switch ON, system A active and speakers connected and turn it back ON immediately (about 1 sec, no time for the filter caps to discharge) there isn't any problem either.
On the same board as the speaker selector, there is also the phone output and the control board for the two led power meters, could the problem come from that part somewhere ?
Also I checked and there are no "emission control" capacitors as you say.
But I did notice something, the output of the power amp is connected to the speaker selector board with a nap of three wires. When I unplug those and turn on the amp wit speaker switch ON, and system A active with speakers, there is no problem.
Also if I turn the amp off with speaker switch ON, system A active and speakers connected and turn it back ON immediately (about 1 sec, no time for the filter caps to discharge) there isn't any problem either.
On the same board as the speaker selector, there is also the phone output and the control board for the two led power meters, could the problem come from that part somewhere ?
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It cant' be seen from the service manual what the connection arrangements are but there could be 6 leads in total. I assume the 3 together are R+,L+ and common(-) from the power amplifier board. The other being headphone(-) and perhaps 2 more leads connecting to the outputs to the power level meter. In any case, if you removed the power input connection, you also removed the load problem, which is still there - perhaps even on the indicator board?
If you look at the schematic (there are 3 to choose from, A5,7,9) the headphone socket L+ and R+ contacts are always connected via 220R resistors direct to either power amplifier output. The connections should simply be open circuit when the phone plug is removed so there should be no issue there as there is no internal switch for disconnecting the speakers as you find on later models.
See also the simplified block diagram 1.2 on page 4 of the manual. Much will be clarified there, assuming this amplifier is unmodified.
Good that you checked the switches. It removes suspicion of a problem there. Next, I would look at the connections to the indicator board, though it has 2k2 resistors in series there, making any faults unlikely to show as a load on the power amplifiers.
If you look at the schematic (there are 3 to choose from, A5,7,9) the headphone socket L+ and R+ contacts are always connected via 220R resistors direct to either power amplifier output. The connections should simply be open circuit when the phone plug is removed so there should be no issue there as there is no internal switch for disconnecting the speakers as you find on later models.
See also the simplified block diagram 1.2 on page 4 of the manual. Much will be clarified there, assuming this amplifier is unmodified.
Good that you checked the switches. It removes suspicion of a problem there. Next, I would look at the connections to the indicator board, though it has 2k2 resistors in series there, making any faults unlikely to show as a load on the power amplifiers.
Yes that seems to be it, the first group of three is L+, G, R+ and the second set of three is G, and the two others supply power to the two separate power meters. When I unplug the first set the load problem is gone, and of course the signal doesn't go to the power meters but when I plug back the first and unplug the second problem is back.
The only way the selector board is connected to the indicator board is through these 2.2K resistors and although it was unlikely I unsoldered the both and no changes.
I hope you're not getting out of ideas because I am !
The only way the selector board is connected to the indicator board is through these 2.2K resistors and although it was unlikely I unsoldered the both and no changes.
I hope you're not getting out of ideas because I am !
Perhaps there is no need for new ideas. Let's take a backward step and consider that if the speakers are connected and there is signal, the bulb tester should be glowing (pulsing actually) if you can hear music at a moderate level. I would be concerned about the bulb power rating being too high if it did not do this.
When you have sound and some amount of power, DC offset is OK, there is some level of steady bias etc; there is no more need to have the bulb limiter fitted as it will compress and distort the audio and probably result in false bias settings.
I suggest that if you don't have a glowing bulb, regardless of the speaker switch settings, you won't have much sound either and that's how it should be. The question is, does the bulb ever glow when there is no sound through the speakers? If not, there is most likely no problem at all.
When you have sound and some amount of power, DC offset is OK, there is some level of steady bias etc; there is no more need to have the bulb limiter fitted as it will compress and distort the audio and probably result in false bias settings.
I suggest that if you don't have a glowing bulb, regardless of the speaker switch settings, you won't have much sound either and that's how it should be. The question is, does the bulb ever glow when there is no sound through the speakers? If not, there is most likely no problem at all.
Let's recap, there is a flash when I power up the amp, which corresponds to the charging of the two filter caps. After that initial draw of current the bulb glows just very slightly and when I put speakers and a signal the bulb "pulses" depending on the amount of power needed for the speakers. This is when everything is OK. When I turn on the amp with speaker on System A and this system is active, I have he initial flash but the the bulb doesn't get back to the slight glow, instead it keeps glowing very bright.
OK, it is as you said initially but it makes no sense if the glow due to current draw is not reflected in the amplifier output. Use your meter to check for unexpected DC output voltages both when there is and is not a constant glow on the same A speaker set and in both L & R channels.
This is a DC servo controlled amplifier so I would not expect more than a few millivolts DC offset. You won't be able to measure oscillation without an oscilloscope but I have seen technicians use an AM pocket radio to detect the EMR of oscillation, when the amplifier is switched on/off. You could try that but there are no guarantees any particular radio will be sensitive enough at the frequency tuned. A small radio placed close to the power amplifier output stage would be best.
As there is no audio output but a steady glow when A is selected, we have to consider that either L or R channel power amplifier is oscillating at some supersonic frequency or there is another circuit malfunction that is induced by either an open output circuit or the speakers themselves (only with the A connections though).
Oscillation or other amplifier faults would probably occur only in one channel. Check which by disconnecting L or R only connections from the amplifier to the speakers or speaker switch board. Whilst the speakers are effectively disconnected, try shorting either L or R amplifiers to ground with a 47-100R resistor and then try connecting the speaker again to see if the bulb lights up or dims again after the power-up. The plan here is to stop the glow by other means than using the speaker switches. You may have to just fiddle about with this but it should not be harmful as long as you don't directly short the amplifiers, as there are no protection circuits.
Regarding other malfunctions, there are a number of small caps in the power amplifiers that could have failed and caused erratic operation of the DC servo or bias and so causing high dissipation until interrupted. I could not say without taking comparative measurements and checking the temperature changes of components what would cause this.
This is a DC servo controlled amplifier so I would not expect more than a few millivolts DC offset. You won't be able to measure oscillation without an oscilloscope but I have seen technicians use an AM pocket radio to detect the EMR of oscillation, when the amplifier is switched on/off. You could try that but there are no guarantees any particular radio will be sensitive enough at the frequency tuned. A small radio placed close to the power amplifier output stage would be best.
As there is no audio output but a steady glow when A is selected, we have to consider that either L or R channel power amplifier is oscillating at some supersonic frequency or there is another circuit malfunction that is induced by either an open output circuit or the speakers themselves (only with the A connections though).
Oscillation or other amplifier faults would probably occur only in one channel. Check which by disconnecting L or R only connections from the amplifier to the speakers or speaker switch board. Whilst the speakers are effectively disconnected, try shorting either L or R amplifiers to ground with a 47-100R resistor and then try connecting the speaker again to see if the bulb lights up or dims again after the power-up. The plan here is to stop the glow by other means than using the speaker switches. You may have to just fiddle about with this but it should not be harmful as long as you don't directly short the amplifiers, as there are no protection circuits.
Regarding other malfunctions, there are a number of small caps in the power amplifiers that could have failed and caused erratic operation of the DC servo or bias and so causing high dissipation until interrupted. I could not say without taking comparative measurements and checking the temperature changes of components what would cause this.
But isn't it so that either way the bulb tester will always slightly glow, whether there is or isn't any output ? That is what I have been observing with all the other amplifiers I Fixed. After the initial flash there is just a steady glow because of the amplifiers idling current.
Now actually something just changed, whatever channel is selected, if it has speakers on it when the amp is turned on I have a flash (charging of the filter caps) then the bulb stay lit for a few seconds and then gets back to normal. when I measure voltages across the speaker terminals (with speaker) of the system selected on power up, I get 3v DC and then gradually it goes down and stabilises around 3mV of DC. After the glow is back to "normal" and there is only 3mV dc on output I have sound on L and R channels of both systems.
Now actually something just changed, whatever channel is selected, if it has speakers on it when the amp is turned on I have a flash (charging of the filter caps) then the bulb stay lit for a few seconds and then gets back to normal. when I measure voltages across the speaker terminals (with speaker) of the system selected on power up, I get 3v DC and then gradually it goes down and stabilises around 3mV of DC. After the glow is back to "normal" and there is only 3mV dc on output I have sound on L and R channels of both systems.
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