I have amp with bad power in amplifier section.
I am certain it is not in the preamp stage.
I have have heard "caps" and I have heard "transistor"
I am away from home and was hoping to order parts for a weekend project.
Could somebody suggest what to replace on this amp?
I want to to order my components asap and have them waiting for me.
I'd rather solder it all in then to underestimate what I may need.
Thanks
I am certain it is not in the preamp stage.
I have have heard "caps" and I have heard "transistor"
I am away from home and was hoping to order parts for a weekend project.
Could somebody suggest what to replace on this amp?
I want to to order my components asap and have them waiting for me.
I'd rather solder it all in then to underestimate what I may need.
Thanks
This is a very old amplifier. You describe the fault(s) as 'bad power" which could refer to anything but let's assume you mean bad sound - perhaps distortion? Is it low power, lacking bass, treble etc? What sort of bad power do you mean?
In the case of old gear, capacitors eventually dry out and fail and this leads to semiconductor failure too. Faults will likely be as you already suspect but there could be collateral damage to resistors, PCB traces, connectors even. Corrosion can be a factor too so assume nothing without checking the obvious physical evidence and re-joining any connectors, if used for parts or leads.
I don't have a schematic to refer to but it will be unlikely that you will find original replacement power semis anywhere local and there is little point replacing them until the electrolytic type capacitors are replaced. When you have done this, power up with a current limiting device in the power line, such as a conventional lightbulb or variac or current limiting power suppy to prevent further damage in case other parts are taken out when the circuit operation is restored with good caps. It is easy enough to re-cap, allowing that you can use higher voltage ratings and say 50% higher capacitance ratings if necessary to obtain parts that both fit with hole spacings and the space available plus even mounting clamps if they are used. Replace them all in an old beast like this and use reasonable quality parts by Panasonic, Elna, Nippon Chemicon, Rubycon etc. Don't try to fit special grades until you find what is wrong and sort that first.
Keep us updated with progress and good luck
In the case of old gear, capacitors eventually dry out and fail and this leads to semiconductor failure too. Faults will likely be as you already suspect but there could be collateral damage to resistors, PCB traces, connectors even. Corrosion can be a factor too so assume nothing without checking the obvious physical evidence and re-joining any connectors, if used for parts or leads.
I don't have a schematic to refer to but it will be unlikely that you will find original replacement power semis anywhere local and there is little point replacing them until the electrolytic type capacitors are replaced. When you have done this, power up with a current limiting device in the power line, such as a conventional lightbulb or variac or current limiting power suppy to prevent further damage in case other parts are taken out when the circuit operation is restored with good caps. It is easy enough to re-cap, allowing that you can use higher voltage ratings and say 50% higher capacitance ratings if necessary to obtain parts that both fit with hole spacings and the space available plus even mounting clamps if they are used. Replace them all in an old beast like this and use reasonable quality parts by Panasonic, Elna, Nippon Chemicon, Rubycon etc. Don't try to fit special grades until you find what is wrong and sort that first.
Keep us updated with progress and good luck
Schematic available here.
http://akdatabase.org/AKview/albums/userpics/10004/Sansui AU-555 Schematic.pdf
http://akdatabase.org/AKview/albums/userpics/10004/Sansui AU-555 Schematic.pdf
Thanks indeed, RJM1.
Like the contemporary Sansuis, it's a Quasi- complementary, single rail power source design and output is via a big cap (C819, 820). Check for DC on the +ve output connector, as a priority - it should quickly settle to zero volts via R843,4 if it is OK. Using ground as a reference point for the -ve probe, check the power supply voltage at fuses F001,2 and also measure voltage on the +ve terminal of C819,820. Take care not to short close spaced tracks or connections. It should be close to half the supply voltage and can be a very helpful indicator in isolating output stage faults.
Note that speakers and inputs should be disconnected for this test. It has a preamp disconnect even - how good is that?
Like the contemporary Sansuis, it's a Quasi- complementary, single rail power source design and output is via a big cap (C819, 820). Check for DC on the +ve output connector, as a priority - it should quickly settle to zero volts via R843,4 if it is OK. Using ground as a reference point for the -ve probe, check the power supply voltage at fuses F001,2 and also measure voltage on the +ve terminal of C819,820. Take care not to short close spaced tracks or connections. It should be close to half the supply voltage and can be a very helpful indicator in isolating output stage faults.
Note that speakers and inputs should be disconnected for this test. It has a preamp disconnect even - how good is that?
I should have clarified that the dead channel is not completely dead but is weak and distorted.
I swapped channels VIA the jumpers and the right channel continues to be the bad one.
I was hoping there would be an easy solution like "order this, that and this".
I am thinking it is best to just replace the caps with good ones and solder in a handful of outputs.
I swapped channels VIA the jumpers and the right channel continues to be the bad one.
I was hoping there would be an easy solution like "order this, that and this".
I am thinking it is best to just replace the caps with good ones and solder in a handful of outputs.
your approach is bad ...
outputs are on-off devices either work or not
1-RTFM
2-change as many caps possible starting from the small ones on the amp section
3-clean all pots switches and trimmers in the best possible way
Additional information
Even if there was a solution like : ""change R128 and all will be fine"" then
A) usually forum members are tech guys do not like ""fast food solutions"" and see most repairs as an opportunity for a small upgrade also
B) that any way will not apply in your case since the amplifier is quite old and suffers from a number of problems eventhough only one take the amplifier down ...
As a closing argument please notice that repairs is about being methodical and follow procedure which in both you seem to fail .
Kind regards
sakis
outputs are on-off devices either work or not
1-RTFM
2-change as many caps possible starting from the small ones on the amp section
3-clean all pots switches and trimmers in the best possible way
Additional information
Even if there was a solution like : ""change R128 and all will be fine"" then
A) usually forum members are tech guys do not like ""fast food solutions"" and see most repairs as an opportunity for a small upgrade also
B) that any way will not apply in your case since the amplifier is quite old and suffers from a number of problems eventhough only one take the amplifier down ...
As a closing argument please notice that repairs is about being methodical and follow procedure which in both you seem to fail .
Kind regards
sakis
I would do some more troubleshooting before you order parts. The simplest thing you could do is plug in a set of headphones and see if the distortion goes away. If it does then it means that everything is ok up to the driver transistors. Then you would have an open emitter resistor and probably a shorted output transistor (R837,839 TR809,811 in one channel R838,840, TR810,812 in the other channel. That's the first thing that I would look at since you say one channel is distorted.
You didn't say which channel was distorted so I included the location numbers for both channels.
You only have to check the distorted channel. At least you have one channel that works so you can make resistance and voltage checks between the two channels to find out where the problem is.
You didn't say which channel was distorted so I included the location numbers for both channels.
You only have to check the distorted channel. At least you have one channel that works so you can make resistance and voltage checks between the two channels to find out where the problem is.
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Yes it is didtorted even in the Headphones.
I am not sure whether the previous comment was an insult or constructive critisism.
I have no education and very little experience with this stuff. I am an automotive technician and have experience only with 12VDC circuits. I would really love to understand electronics but every time I attemp to learn it, I get overwhelmed. Does that mean I have small brain?
I am not sure whether the previous comment was an insult or constructive critisism.
I have no education and very little experience with this stuff. I am an automotive technician and have experience only with 12VDC circuits. I would really love to understand electronics but every time I attemp to learn it, I get overwhelmed. Does that mean I have small brain?
No it was a bit of arrogance to people that look for fast food solution and/or do not appreciate the simple help given . Please do not take this as a personal attack or an insult .
Take it as a helpful advice cause this was the real intention . To read the manual will take just a few minutes and to perform the advice given will take for an amateur a couple of days and may be 10-15 usd .
In between many people will run to your aid including me ... but please ...don't expect the amplifier to fix itself ! you have to do a few things like you ve been told so far .
Kind regards
sakis
Take it as a helpful advice cause this was the real intention . To read the manual will take just a few minutes and to perform the advice given will take for an amateur a couple of days and may be 10-15 usd .
In between many people will run to your aid including me ... but please ...don't expect the amplifier to fix itself ! you have to do a few things like you ve been told so far .
Kind regards
sakis
to put you more inside the picture inside your amp exist 40-50 capacitors ...in your case a few big ones that will serve power supply and another function that i called dc blocking in the output .
Even if all of them is half dead amplifier will still produce some sound ...BUT !!! if one of them in the feedback chain ( on the amplifier board ) ( a small one that may costs just less than a cent ) is leaky then the amplifier can perform like you said
you have a an amplifier that is 40 years old .... if there is a dirty speakers AB switch or a tape monitor switch you may also have similar results ...
If you change all capacitors and amplifier works ...in some while it will present some other failure related to dirt in switches or pots ...so you have to do that also ...
Even if all of them is half dead amplifier will still produce some sound ...BUT !!! if one of them in the feedback chain ( on the amplifier board ) ( a small one that may costs just less than a cent ) is leaky then the amplifier can perform like you said
you have a an amplifier that is 40 years old .... if there is a dirty speakers AB switch or a tape monitor switch you may also have similar results ...
If you change all capacitors and amplifier works ...in some while it will present some other failure related to dirt in switches or pots ...so you have to do that also ...
The OP has given us one clue - sound is weak and distorted. Sound familiar guys? Let's just get on with it and test the DC voltage at the output node (positive leg of C819 or 820, the 2,200uF output capacitors) to identify likely output stage failure.
As an automotive tech, argonrepublic, you will have a DMM capable of measuring DC up to ~100V, right? Use it to measure the voltage from ground to that point - much will be revealed by telling us. I'm afraid that whatever the manuals advise, someone still needs to assess the specifics to decide which of the possibilities is most likely in a specific case, since amplifiers are an interdependent assembly of components which show similar symptoms for many faults.
If you can test and replace, we can help, though this is not simply a free repair help centre as you probably realised over your 7 years membership. As to being overwhelmed (with probably the invisible complexity of electronic theory and design) I think that you need not be if you gain some experience, just like you and most of us had to get in our day jobs anyway. The DIY part of constructing your own project is the easiest and most rewarding way of getting experience but repair process is a good, if daunting, start too.
I guess you know this anyway so it is a good reason to apply and extend the experience you already have.
As an automotive tech, argonrepublic, you will have a DMM capable of measuring DC up to ~100V, right? Use it to measure the voltage from ground to that point - much will be revealed by telling us. I'm afraid that whatever the manuals advise, someone still needs to assess the specifics to decide which of the possibilities is most likely in a specific case, since amplifiers are an interdependent assembly of components which show similar symptoms for many faults.
If you can test and replace, we can help, though this is not simply a free repair help centre as you probably realised over your 7 years membership. As to being overwhelmed (with probably the invisible complexity of electronic theory and design) I think that you need not be if you gain some experience, just like you and most of us had to get in our day jobs anyway. The DIY part of constructing your own project is the easiest and most rewarding way of getting experience but repair process is a good, if daunting, start too.
I guess you know this anyway so it is a good reason to apply and extend the experience you already have.
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