Since VR01 is giving me low voltage and won’t raise higher than 8 is that the channel that would contain the possible short or failing component? Forgive me if I sound dumb. Measuring transistors down the line are giving me all kinds of different voltages. The DBT just flashes. Dims when turned on then flashes.
Barn find Sansui AU-7900
- Thread starter Kylek1443
- Start date
GrisPato
Super Member
Ah, ok. I will have to think about that a little. The changing of the voltage is because the current through the resistor is changing indicated by the bulb flashing.
My first thought is that your pre drivers are not in good condition but will have to analyse a little bit more.
My first thought is that your pre drivers are not in good condition but will have to analyse a little bit more.
GrisPato
Super Member
Be sure to give correct information otherwise we will get confused?
You could start with measuring the resistors R43,44,45,46, just Ohms reading, with power off.
R43 measured 0.5 ohms
R44 measured 0.5 ohms
R45 measured 0.5 ohms
R46 measured 0.5 ohms
All 4 are measuring the same. I measured them with the power off.
Attachments
BilboBaggins
Addicted Member
I believe you are measuring the resistors in circuit? You are measuring the resistance of the whole circuit, not the resistor. To measure the resistor, you need to lift a leg out of circuit so one end is open. If it measures good, put back in and solder.
Forgive me for jumping in, but I just finished reading through the thread. Interesting, and you are very lucky to be getting all this help.
One of the reasons Hyperion and GrisPato are telling you to fix the problem before replacing parts/restoring the unit, is that it is very easy to introduce new problems, and if you do a mass swap, you will have a very hard time figuring out where to start. And it is possible to have more than one problem after a mass swap, so you could theoretically replace a part that's bad, but because another part is also bad and gives you the same symptoms, and you would not know that.
By fixing it first, you would then know if you introduce any issues as you restore it. Best to do your mass swap one board at a time, and then test. That way, you will be keeping your areas of uncertainty down to a reasonable level of complexity. The other thing to keep in mind, is that you could replace with a new part, and that new part could also be dead or failing. Just one of the issues techs run into all the time.
A few rules of thumb as you go through this process;
1) never measure resistance with power on.
2) never measure voltage with power off.
3) move wires as little as possible. Moving the wire can cause the old copper inside to break, and that can give you all kinds of intermittent problems.
4) The dim bulb tester is used to get your amp to a certain minimum level of functionality. It is basically used to prevent short circuits from damaging the amplifier any more than it already is. Once the dim bulb tester indicates you have resolved the short circuits, you can then put test speakers on the unit. Always use the dim bulb tester when you may have introduced a short. ie) when you've changed parts or have had your hands inside the unit moving things. Once you are happy there is no short, you can then plug it into the wall in confidence, and the amp can go through its normal power up cycle.
5) Don't solder in shorts.
Sorry, I had to inject some humour. It is how I roll.
Forgive me for jumping in, but I just finished reading through the thread. Interesting, and you are very lucky to be getting all this help.
One of the reasons Hyperion and GrisPato are telling you to fix the problem before replacing parts/restoring the unit, is that it is very easy to introduce new problems, and if you do a mass swap, you will have a very hard time figuring out where to start. And it is possible to have more than one problem after a mass swap, so you could theoretically replace a part that's bad, but because another part is also bad and gives you the same symptoms, and you would not know that.
By fixing it first, you would then know if you introduce any issues as you restore it. Best to do your mass swap one board at a time, and then test. That way, you will be keeping your areas of uncertainty down to a reasonable level of complexity. The other thing to keep in mind, is that you could replace with a new part, and that new part could also be dead or failing. Just one of the issues techs run into all the time.
A few rules of thumb as you go through this process;
1) never measure resistance with power on.
2) never measure voltage with power off.
3) move wires as little as possible. Moving the wire can cause the old copper inside to break, and that can give you all kinds of intermittent problems.
4) The dim bulb tester is used to get your amp to a certain minimum level of functionality. It is basically used to prevent short circuits from damaging the amplifier any more than it already is. Once the dim bulb tester indicates you have resolved the short circuits, you can then put test speakers on the unit. Always use the dim bulb tester when you may have introduced a short. ie) when you've changed parts or have had your hands inside the unit moving things. Once you are happy there is no short, you can then plug it into the wall in confidence, and the amp can go through its normal power up cycle.
5) Don't solder in shorts.
Sorry, I had to inject some humour. It is how I roll.
GrisPato
Super Member
If you feel op to it measure Tr12-14 in place DMM on diode mode. You should get OL one way, other way around 0.6 volt ( this can be off a little ) lower then 0.5 conducting would be bad.
Also measure the output emitter resistors. They should measure around 0.5 ohm, be sure to putt the DMM in the lowest range (200 ohm) and test leads together first to see what is there resistance to deduct this from your measurements.
Then the VR resistors, use the other channel to compare. Check where they are now ( resistance wise) and put them back to that value before you power on.
Also measure the output emitter resistors. They should measure around 0.5 ohm, be sure to putt the DMM in the lowest range (200 ohm) and test leads together first to see what is there resistance to deduct this from your measurements.
Then the VR resistors, use the other channel to compare. Check where they are now ( resistance wise) and put them back to that value before you power on.
GrisPato
Super Member
I did advise to lift a leg if the measurement was lower then expected. Then I could see in the picture he had the DMM on 20k range, so 0.5 k is close enough.
Thanks for the support
.I have a lot to go over here. You all have been incredibly helpful and I can’t even begin to express my gratitude. To bilbo, nothing has been changed on the board besides the power and OP transistors ( which changed the DBT from automatically lighting bright to now flashing). Obviously those needed to be changed? I will test R43-46 with a pin out and let you guys know the results. I’m a little overwhelmed with the whole project. So forgive me for being slow.I did advise to lift a leg if the measurement was lower then expected. Then I could see in the picture he had the DMM on 20k range, so 0.5 k is close enough.
Thanks for the support
Thank you for your input. All help is welcomed. One of the problems with this board is that it’s so old and degraded that pulling old parts in some places literally pulls the leads up from the bottom of the pcb when you de solder. This thing is ROUGH and that’s not exaggerating. We have made progress though. Going from the DBT automatically lighting as bright as can be to it flashing, IS progress, right?I believe you are measuring the resistors in circuit? You are measuring the resistance of the whole circuit, not the resistor. To measure the resistor, you need to lift a leg out of circuit so one end is open. If it measures good, put back in and solder.
Forgive me for jumping in, but I just finished reading through the thread. Interesting, and you are very lucky to be getting all this help.
One of the reasons Hyperion and GrisPato are telling you to fix the problem before replacing parts/restoring the unit, is that it is very easy to introduce new problems, and if you do a mass swap, you will have a very hard time figuring out where to start. And it is possible to have more than one problem after a mass swap, so you could theoretically replace a part that's bad, but because another part is also bad and gives you the same symptoms, and you would not know that.
By fixing it first, you would then know if you introduce any issues as you restore it. Best to do your mass swap one board at a time, and then test. That way, you will be keeping your areas of uncertainty down to a reasonable level of complexity. The other thing to keep in mind, is that you could replace with a new part, and that new part could also be dead or failing. Just one of the issues techs run into all the time.
A few rules of thumb as you go through this process;
1) never measure resistance with power on.
2) never measure voltage with power off.
3) move wires as little as possible. Moving the wire can cause the old copper inside to break, and that can give you all kinds of intermittent problems.
4) The dim bulb tester is used to get your amp to a certain minimum level of functionality. It is basically used to prevent short circuits from damaging the amplifier any more than it already is. Once the dim bulb tester indicates you have resolved the short circuits, you can then put test speakers on the unit. Always use the dim bulb tester when you may have introduced a short. ie) when you've changed parts or have had your hands inside the unit moving things. Once you are happy there is no short, you can then plug it into the wall in confidence, and the amp can go through its normal power up cycle.
5) Don't solder in shorts.
Sorry, I had to inject some humour. It is how I roll.
Do you agree, after reading the thread, that the issue is 100% with the amplifier board (Sansui F-2596) ? I love your humor btw lol. It eases me with this GD headache of an amp haha. I just know the AK community are the best. That’s why these forums are here. To teach, to learn, and to save these things from the dump. I shutter at the units that been tossed due to ignorance. Thank you so much for your help. I’ll do anything I can to fix it. Can it be done? Our only limits are my stupidity. I’m at the mercy of all you experts.I believe you are measuring the resistors in circuit? You are measuring the resistance of the whole circuit, not the resistor. To measure the resistor, you need to lift a leg out of circuit so one end is open. If it measures good, put back in and solder.
Forgive me for jumping in, but I just finished reading through the thread. Interesting, and you are very lucky to be getting all this help.
One of the reasons Hyperion and GrisPato are telling you to fix the problem before replacing parts/restoring the unit, is that it is very easy to introduce new problems, and if you do a mass swap, you will have a very hard time figuring out where to start. And it is possible to have more than one problem after a mass swap, so you could theoretically replace a part that's bad, but because another part is also bad and gives you the same symptoms, and you would not know that.
By fixing it first, you would then know if you introduce any issues as you restore it. Best to do your mass swap one board at a time, and then test. That way, you will be keeping your areas of uncertainty down to a reasonable level of complexity. The other thing to keep in mind, is that you could replace with a new part, and that new part could also be dead or failing. Just one of the issues techs run into all the time.
A few rules of thumb as you go through this process;
1) never measure resistance with power on.
2) never measure voltage with power off.
3) move wires as little as possible. Moving the wire can cause the old copper inside to break, and that can give you all kinds of intermittent problems.
4) The dim bulb tester is used to get your amp to a certain minimum level of functionality. It is basically used to prevent short circuits from damaging the amplifier any more than it already is. Once the dim bulb tester indicates you have resolved the short circuits, you can then put test speakers on the unit. Always use the dim bulb tester when you may have introduced a short. ie) when you've changed parts or have had your hands inside the unit moving things. Once you are happy there is no short, you can then plug it into the wall in confidence, and the amp can go through its normal power up cycle.
5) Don't solder in shorts.
Sorry, I had to inject some humour. It is how I roll.
GrisPato
Super Member
This is not necessary, as you can read you had the range of the DMM set to kOhm, so 0.5 kOhm is 500 ohm, should be 560 so this is close enough.
I made a video and put it on my YouTube channel if you’d be interested in seeing what it does when you connect it to the DBT. Hopefully everything uploaded okay. I just thought maybe you guys would want to see first hand what’s happening. https://youtube.com/shorts/4DTGKfwnNM4?feature=share
What are your thoughts? Where should we go next? A blinking current in the DBT is better than a solid lit bulb?
Thank you for telling me this. I definitely did not want to unsolder leads and put them back in. Honestly when I touch this board I want to only touch it to be replacing things that need to be replaced. The PCB is in rough shape. It’s usable, but not pulling parts definitely helps.
GrisPato
Super Member
I have to say, that is just not a dead short, but could be close.
Just another checklist:
Finding the faulty component is done by exclusion.
Just another checklist:
This has to be done. And to be sure measure the outputs from case to heatsink to be sure they are well isolated.
Finding the faulty component is done by exclusion.
Hypothetical question. If a mica is cracked on a power transistor on the heatsink, could this cause the problem? I think it’s a possibility that I cracked a mica on the PT when I did a replacement. I need to redo it. But could that be something? I don’t think it’s touching. But still.
GrisPato
Super Member
It could for sure. Yep, go and replace it!
Did you put new ones in between?