What diode can I use for my AU 7700?

Thanks Mike for further advice.
Yes, my line voltage is 220.
The AC between terminals 7 & 8 is just above ~60V.
Between 7 & GND and 8 & GND is about ~30V.

I think the diode you meant is D21 - that's the one with the Anode at F07 (it's easy to see the labelling on the circuit itself).
Now comes the embarrassing part - I measured the voltage at the Cathode and if I remember it was about 16V. But while I was verifying, I slipped the probe and shorted a connection - I think between the cathode and that part of the circuit that joins one end of each of R77 and R78 and a whole slew of others. That blew the fuse F07 and now I have to stop because I have no 7A fuses and it's too late in the day to get any. As I say, very embarrassing.
Oh, and while I had the whole thing in view, I decided to remove the diode from the circuit and test it - looks good.
You had voltage on the cathode of D21 but 0V on R19. Either you didn;t make contact with R19 or you have an open trace between R19 and D21. Check it out.
 
You had voltage on the cathode of D21 but 0V on R19. Either you didn;t make contact with R19 or you have an open trace between R19 and D21. Check it out.

Thank Dr. I have re-checked everything. Cleaned the points where I was measuring. I still get 0V on either side of R19. But maybe you and I are looking at different R19s?

I have attached a photo. I hope it shows properly. The two R19 end points are on the right(the upper contact is under R37 which is on the underside of the PCB so I measured a point connected to it). The cathode of D21 is way up at the top of the board just to the left of terminal 5. There is no direct connection, not even a bad one.
 

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Thank Dr. I have re-checked everything. Cleaned the points where I was measuring. I still get 0V on either side of R19. But maybe you and I are looking at different R19s?

I have attached a photo. I hope it shows properly. The two R19 end points are on the right(the upper contact is under R37 which is on the underside of the PCB so I measured a point connected to it). The cathode of D21 is way up at the top of the board just to the left of terminal 5. There is no direct connection, not even a bad one.

Obviously your unit differs from the schematic. Wonderful. That makes it very difficult to troubleshoot remotely. What is the number on the board with the relay on it?
 
Obviously your unit differs from the schematic. Wonderful. That makes it very difficult to troubleshoot remotely. What is the number on the board with the relay on it?
The only number I can see is F2097.
The conductor side of the board looks exactly like the picture on page 11 of the service manual that I downloaded.
 
The only number I can see is F2097.
The conductor side of the board looks exactly like the picture on page 11 of the service manual that I downloaded.

I will have a look and get back to you on this. Probably not until tomorrow evening.
 
A little bit of progress

I have not managed to spend much time on this over the past few months. But I DID spend some time fiddling around the relay. I discovered that with the power on the relay never closed. So I tested the voltage across the coil - zero.

Just to make sure, I took the relay out of the circuit and tested it with a desk power supply that I have. Even though the relay has "24V DC" written on it, it still worked quite nicely at 16V. I gave it a good look over and with the power supplied the switched parts seem to have good connection.

You may have guessed that I have only a rudimentary knowledge of electronics - I know how the individual components work but as soon as you start stringing a bunch of them together to make something nice, then I start getting a bit baffled. But I looked at the circuit diagram around the relay and it seems that one thing that could cause a bad circuit would be if transistor TR29 did not close and give a nice path to ground. IN other words its base is not getting enough of a voltage. And that could be any number of things, so in the end I tested (out of circuit - took all afternoon) D11, D13 and D15 and also TR29, TR31 and TR33. They are all working according to spec.

But now I'm stumped. Everything I test, so far (either because it seems a good idea to me, or because of advice from the others who have been following this thread and my progress) seems good. The only thing is the zero voltage across R19 on the F-2097 board.

What concerns me is that this all started with a blown fuse on the F-2092 "Equalizer & Power Supply" circuit board, which went away when I replaced the diodes in the rectifier circuit (and the two capacitors). How did we end up on the other power supply board?
 
I have not managed to spend much time on this over the past few months. But I DID spend some time fiddling around the relay. I discovered that with the power on the relay never closed. So I tested the voltage across the coil - zero.
...
I meant to attach a picture of the circuit diagram around the relay, but forgot. Here it is.
 

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Here's another picture of the circuit diagram, with some annotations. The voltages were read at a volume setting of about 75% of maximum (much louder than I would ever dream of playing anything!).
 

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Please measure the voltage at D21 cathode again. Does your power light come on LED701 (?) - also please measure the voltage at C21? or is it C27? - poor quality schematic can't read what it is - a 33µF 50V component - this is the smoothed power supply for the protection circuit.

I am thinking there is a missing jumper wire from D21 to a point nearer R19, as you say there is no connection between these points... please confirm this :scratch2:
 
Please measure the voltage at D21 cathode again. Does your power light come on LED701 (?) - also please measure the voltage at C21? or is it C27? - poor quality schematic can't read what it is - a 33µF 50V component - this is the smoothed power supply for the protection circuit.

I am thinking there is a missing jumper wire from D21 to a point nearer R19, as you say there is no connection between these points... please confirm this :scratch2:
I measured the voltage at D21 cathode - it is 13V (DC) and 16V~. The only 33µF capacitor according to the Sansui parts list for that board is C27, and that's connected to the cathode of D21, so its voltage is the same.

I wish there was a missing jumper that I could (not) see - but when I check the printed schematic against the actual board (which is very nicely marked on the non-conducting side), all of the jumper wires are there, and seem well connected.

By the way, in the interim (since you and I last spoke) I also looked at the trouble-shooting guide of the manual, and it told me that if the relay isn't closing then it can be anyone of TR01 - TR28. I have tested them all (out of circuit), as well as D11, D13, D21, D25 and D15. I did this with the diode test part of my multimetre. They are all OK. And I'm a few shades greyer.
 
Manual BIAS setting on an AU-7700

This is the text of a PM I have just sent to Peter.

He is asking me about BIAS setting on his AU-7700, after getting it going with assistance from Mike (Vigman). - well done Mike. :thmbsp:

Hi Peter,

I usually reckon myself to be familiar enough with a piece of gear to give detailed advice if I have worked on one - I have never worked on a AU-7700.

However I'll give it a shot.

The bias procedure in the manual asks you to measure for 45mA +/-10 mA by disconnecting a red wire (the power feed to one channel) and measuring the current, and adjusting accordingly. This is a good way to blow your meter fuse, even if you select a higher current range on your meter, and then change down to the measurement range you need, say 0-100mA - lets not do it that way.

It says adjust to 45mA this is ALL the current pulled by the output stage transistors (even the drivers). To do the measurement the better way (by measuring a Voltage - NOT the current) across the emitter resistors you have to do some mental gymnastics.

To get just the current for the output transistors take away 5mA (a guess) - 45-5 = 40mA - this is the current through BOTH pairs of output transistors for one channel. 40mA/2 = 20mA - this is the current through ONE pair of output transistors and now gives you a value for the voltage you will see dropped across the emitter resistors of ONE pair of output transistors, like this.

summing emitter resistor values. (as you will measure across both).
0.47Ω +0.47Ω = 0.94Ω

V/R = I therefore I x R = V

so, 0.020 x 0.94 = 0.0188V = 18.8 mV - so measuring across BOTH emitter resistors for ONE pair of output transistors you will see 18.8mV for a bias current of 20mA per transistor pair, PER channel. = 40mA PER channel.

So that is what you need to do.

Turn BOTH bias trimmers to MAXIMUM resistance, clip your multimeter probes onto one pair of output transistor emitter resistors (connection will be to each power transistor emitter connection), measure the voltage and adjust for ~19mV - do exactly the same with the other channel - and report back.

Take careful note of my instructions, and ask if something isn't clear to you.

Good luck,

Regards,

John

If anyone else has any input - please feel free to add.
 
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Thank you, Mike and John for your help.

By the way, what Vigman suggested I do is look at the capacitors C27 and C23, and after I replaced those my relay closed. What a lovely sound!

John:
I love the idea of measuring voltage rather than current. I did try the current measurement as per the manual, and it's a very messy business. Then - my DMM is auto-ranging so I don't think I blew anything but on the other hand I could not even get a reading. Maybe my DMM skills are bad.

My electronics vocabulary is very basic, and so I'm assuming the "pairs of output transistors" are TR23/TR27 and TR24/TR28.

Turn BOTH bias trimmers to MINIMUM resistance, clip your multimeter probes onto one pair of output transistor emitter resistors (connection will be to each power transistor emitter connection), measure the voltage and adjust for ~19mV - do exactly the same with the other channel - and report back.

Firstly, is there a convention about bias trimmers as to which direction to turn for maximum or minimum resistance? Otherwise I will need to take one out of circuit and measure it.

Then, you mention "output transistor emitter resistors" and "power transistor emitter connection" - are the terms "output transistor" and "power transistor" interchangeable? I want to get this right! :worried: :)

But if I look at the circuit diagram (page 11 of the manual), I can't see any resistors between the emitters of TR23 and TR27 on the one hand and TR24 and TR28 on the other. Well, not directly anyway. Maybe I've got it all screwed up, sorry. :dunno:
 
Thank you, Mike and John for your help.

By the way, what Vigman suggested I do is look at the capacitors C27 and C23, and after I replaced those my relay closed. What a lovely sound!

John:
I love the idea of measuring voltage rather than current. I did try the current measurement as per the manual, and it's a very messy business. Then - my DMM is auto-ranging so I don't think I blew anything but on the other hand I could not even get a reading. Maybe my DMM skills are bad.

Sounds like your meter fuse is blown.

My electronics vocabulary is very basic, and so I'm assuming the "pairs of output transistors" are TR23/TR27 and TR24/TR28.

I think so (I have no schematic at present to check this with) there are 2 'pairs' per channel, I am suggesting that you check the right hand pair of emitter resistors in each channel.


Firstly, is there a convention about bias trimmers as to which direction to turn for maximum or minimum resistance? Otherwise I will need to take one out of circuit and measure it.

No there isn't, some designs need trimmer resistance set for max, and some set for min as an initial condition when setting BIAS.
You dont need to remove the trimmer - just measure the 'in-circuit' resistance of the trimmer, turn the adjuster so it reads the full trimmer resistance. This is very important - if you get it round the wrong way and set it for minimum resistance you will probably toast your output transistors.

Then, you mention "output transistor emitter resistors" and "power transistor emitter connection" - are the terms "output transistor" and "power transistor" interchangeable? I want to get this right! :worried: :)

Here, I am trying to clarify for you the exact point you need to measure at, I.O.W. the junction of the emitter and the emitter resistor in each case. Sorry for using 2 terms for the same thing - 'power transistor' = 'output transistor' in this case.

But if I look at the circuit diagram (page 11 of the manual), I can't see any resistors between the emitters of TR23 and TR27 on the one hand and TR24 and TR28 on the other. Well, not directly anyway. Maybe I've got it all screwed up, sorry. :dunno:

Well, I can see it would be easier if I posted a marked diagram, but I don't have access to the schematic at present.


If anyone else has any input - please feel free to add.
 
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I thought about it and your V = IR calculations and looked at the diagram again and I now think I know which pairs of emitters to test.

Murphy is wreaking havoc with me. My "nice" DMM has indeed blown something and I can't read amps at all and no low-voltages. The 40V between the power transistors' collectors and ground shows up, but the voltages across the output transistors' emitters are showing as zero. But my "broken" DMM (its "select" button is non-functional) measures DC voltages and seems to cover the range that we're looking for - it says both left and right channels have 0.5mV across those resistors. (The bias trimmers are totally different, though, and I'm not sure I am getting a good reading there - the one says it is "0L" and the other is about 590Ω)

But the voltages across the output transistor emitter resistors - are they DC or AC? I'm assuming DC but you said I must adjust for "~19mV" and some people use "~" for AC and also for "approximately". Which one is it this time?
 
I thought about it and your V = IR calculations and looked at the diagram again and I now think I know which pairs of emitters to test.

Murphy is wreaking havoc with me. My "nice" DMM has indeed blown something and I can't read amps at all and no low-voltages. The 40V between the power transistors' collectors and ground shows up, but the voltages across the output transistors' emitters are showing as zero. But my "broken" DMM (its "select" button is non-functional) measures DC voltages and seems to cover the range that we're looking for - it says both left and right channels have 0.5mV across those resistors. (The bias trimmers are totally different, though, and I'm not sure I am getting a good reading there - the one says it is "0L" and the other is about 590Ω)

Both of the trimmers should read the full trimmer resistance if they are set correctly, 'OL' could mean the trimmer is faulty. But it is important to measure how the circuit 'sees' the resistance of these trimmers. To measure them, connect your probes to components that are connected directly to the trimmers - that way you can be sure your measurement is correct. (This is of critical importance, you MUST understand this step correctly).

But the voltages across the output transistor emitter resistors - are they DC or AC? I'm assuming DC but you said I must adjust for "~19mV" and some people use "~" for AC and also for "approximately". Which one is it this time?

Sorry, I used the symbol to mean "about" 19mV, and it will be DC - 'no signal' conditions = a DC measurement.

I don't think you will be able to use your multimeter in that state, you can't trust it - it needs a new fuse. :sigh:
 
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Both of the trimmers should measure 0 ohms, if they are set correctly, 590Ω is no good and 'OL' could mean the trimmer is faulty. But it is important to measure how the circuit 'sees' the resistance of these trimmers. To measure them, connect your probes to components that are connected directly to the trimmers - that way you can be sure your measurement is correct. (This is of critical importance, you MUST understand this step correctly).



Sorry, I used the symbol to mean "about" 19mV, and it will be DC - 'no signal' conditions = a DC measurement.

I don't think you will be able to use your multimeter in that state, you can't trust it - it needs a new fuse. :sigh:
I measured and I measured again. I think I did approximately what you suggest about how the circuit sees the resistance. I think I measured the resistance between the correct two terminals. One of the trimmers measured 1K regardless of which extreme it was at. I twisted it back and forth a few times until at the extreme at which the other trimmer read a fraction of an ohm, this second one read about 10 ohms - not zero but better than 1K.

I thought I was good to test the voltages across the emitter resistors. But when I powered on, the part of the chassis on which the power transistors are mounted got very, very hot. I suppose that's what you mean by "frying". So obviously the trimmers were at the wrong extreme.

I have set them now to about half way between the two extremes. They measure between 500 and 700 ohms (can't remember exactly). Before I started adjusting them they were at around 250-300. At that time the "bias voltage" across the emitter resistors was very low - about 1mV rather than the 19mV that we want.

I don't suppose I can do any worse now by powering up and trying again, I've already maybe overheated everything and will need to replace the power transistors.
 
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