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.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.
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.
The only number I can see is F2097.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.
I meant to attach a picture of the circuit diagram around the relay, but forgot. Here it is.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 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.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:
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.
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.
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.
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 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.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: