What diode can I use for my AU 7700?

Last year I bought an AU 7700 from e-Bay. It has been wonderful, such a good sounding amp. Then the other day when I switched it on I heard a little buzz coming from inside and then it came on and played all day. Next day, no buzz but also no sound.

Today I opened it up. I'm not a very experienced fixer of electronics but with the guidance of a very nice book by Michael Jay Geier on fixing everything electronic I saw that one of the 2 fuses on the PCB had blown. I replaced it and the other one blew. Not very encouraging. As usual , a blown fuse means something else is wrong, typically a short. So I went down the circuit one stop away from the the fuses and saw 4 diodes. Anyway, I tested the 4 diodes and Yes! one of them is shorting.

Since it took me ages to disassemble the whole thing I decided to test a bit more, so I nominated two transistors for the job and I think managed to prove at least that they were not shorting anywhere. By the way, these were the first diodes and transistors I have ever tested, so I'm feeling quite chuffed that I actually found one that seems flaky!

Anyway, back to the real question: the flaky diode has IR4F (or maybe 1R4F) on it, and that's all. A quick search of my usual electronics shop's web site (in South Africa they're not called Radio Shack, they are Yebo Electronics) and then googling the whole world, I didn't find anything like a diode called IR4F.

I guess I'll take it in to Yebo and see if they can give me any advice, but maybe one of you AK good people will know how I can get one. I have attached a picture where you can see the two fuses and 3 good diodes and a space (just to the right of the big blue capacitor (which seems new)) where the bad diode is to be replaced.

You can use a 1n4007. Replace all 4 diodes. You should probably also replace the 2 220uF/50V caps in that circuit, one could be leaky and could have caused the diode failure.

Thanks

Thanks dr* audio - I'll definitely do that. Can I use 1n4007 for all the diodes?

Question about caps

I had a quick look at the circuit - there is only 1 220uF/50V cap. That's the big blue one in my photo. There is a 100uF/50V and 2x 100uF/35V. I wonder if someone before me replaced a 220uF/50V with the 100uF/50V?

Would it break things to replace with exactly what's there, or should I put a 220uF in place of the 100uF?

PeterTuffin said:

Thanks dr* audio - I'll definitely do that. Can I use 1n4007 for all the diodes?

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No, only for the 3+1 that the Doc advised you to replace.

PeterTuffin said:

I had a quick look at the circuit - there is only 1 220uF/50V cap. That's the big blue one in my photo. There is a 100uF/50V and 2x 100uF/35V. I wonder if someone before me replaced a 220uF/50V with the 100uF/50V?

Would it break things to replace with exactly what's there, or should I put a 220uF in place of the 100uF?

Click to expand...

I would replace them both with 220uF. You can use a higher voltage part, like 63V if you can't find a 50V part.

Hyperion said:

No, only for the 3+1 that the Doc advised you to replace.

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Thanks, Hyperion. I wasn't clear - I meant those 4 diodes (i.e. the ones Doc advised), not "ALL" of them. I have had a closer look and all 4 are labelled 1R4F, so it makes sense.

dr*audio said:

I would replace them both with 220uF. You can use a higher voltage part, like 63V if you can't find a 50V part.

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Thanks again. I have managed to get some 220uF 65V caps, so I'll replace the one 220 and the 100uF/50V with 220uF/65V.

PeterTuffin said:

Thanks again. I have managed to get some 220uF 65V caps, so I'll replace the one 220 and the 100uF/50V with 220uF/65V.

Click to expand...

So I replaced the caps and diodes (and the blown fuses). It was a bit of a mission because someone had obviously been there before me (not surprising on such an old piece of equipment) and one of the solder points was a bit of a mess. Also, the PCB connections in some places had broken and when I re-soldered, one of the diodes and one of the caps no longer connected to the rest of the circuit, and so I soldered in some connecting wires in place of the broken "printed" link.

Now the amp powers up, doesn't blow any fuses, but also doesn't do anything. The output is totally dead, not even a subtle "click" when the power comes on.

I have found the Service Manual which is wonderful. Now I'm not quite sure how to proceed. I started going from the components I have replaced down from the power supply and tested two transistors down from there. The best I could do was show that there was no short between any two of the three connectors on both transistors. My literature that I'm reading indicates that I need an oscilloscope now, and I was hoping to avoid getting one of those until the last minute, but maybe I've reached that point now.

Does anyone have any suggestions? I really do want to get this amp working again, it is a really wonderful piece of sound equipment.

You don't need an oscilloscope at the moment, you need to start by checking all the fuses - remove them and check them with a multimeter on Ω range. Then check all the power supply voltages - refer to the schematic for what they should be, any wild variations - note them and report back. Measure the voltages at the power supply and where they eventually end up at the various PCB's, note any discrepancies - that should keep you quiet for a while

Make 110% sure that ALL traces are OK!

Where others have walked … be AWARE….
Do you get the power light??
After being REAL careful about checking your work….
( and it wouldn't be a bad idea to get a DBT built and in circuit )

Measure the points on the marked up drawing.

Mike

Thanks Hyperion and Vigman. I do indeed get a power on light. Thanks for the detail of where to test. I'm waiting for a new multi meter. My previous (inexpensive) one is slowly dying. There's an expression in Afrikaans "Goedkoop is deurkoop" which translates as "cheap is expensive". But I'll start with the cheapo.

vigman said:

Where others have walked … be AWARE….
Do you get the power light??
After being REAL careful about checking your work….
( and it wouldn't be a bad idea to get a DBT built and in circuit )

Measure the points on the marked up drawing.

Mike

Click to expand...

Hi Mike

I got a bit of time this weekend to follow your advice. I measured the voltages as you suggested (after studying the circuit diagram vs the actual circuit for ages before I could work out what was what - in spite of Sansui labelling the PCB very nicely). The voltages at terminals 5 and 6 were around 41V (one was + the other -, I forgot to write them down but I'm guessing the 41 is OK).

There WAS a voltage at the purple point at L01 and L02 - L01 0.024 and L02 0.008. But those seem sort of small and arbitrary.

I also followed Hyperion's suggestion and looked at the trouble shooting tips in the manual and measured the voltages at terminals 24 and 25 of F-2092. These were -22.6V and +21.56V instead of the -25V and + 24V that they should have been, but these don't seem too off. I also measured the voltages at what I guessed were the collectors of the power transformers and they were all around 41V (+and -).

So it seems to me maybe the very low voltages at L01 and L02 point at something? Do you have any advice for me (by the way, I REALLY appreciate your help with this, otherwise I'm not sure exactly what I would do)?

(For the record, I have made sure my signal input is working (it's my nice Nokia Lumia phone) and the output - ear phones and also speakers), and that I'm choosing the correct input selection (although I'm baffled by the TAPE PLAYER selector from which I've removed the knob - it seems to have one more possible position than there are labels on the face; so I've tried that in a couple of places near the middle). All those things seem correct. I would be really embarrassed, but happy, if I did nothing more and connected it all up and it then decided to work).

Thanks again for you help.
Peter

P.S. Excuse my ignorance - what is a DBT?
Peter

All those voltages look good. Check the voltage on both sides of R19 in the protection circuit.

dr*audio said:

All those voltages look good. Check the voltage on both sides of R19 in the protection circuit.

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Thanks Dr Audio. I'm not sure which is the protection circuit, so I measured both sides of R19 in

1. the Equalizer & Power Supply PCB (F-2092): On the R29 side: 0.16V; on the R18 side: 22.72V
2. the Driver & Power Supply PCB (F-2097): Both sides 0V.

I'm talking about the Driver & Power Supply PCB (F-2097): Both sides 0V is wrong. You have no power for the relay or drive circuit. Check F07 and make sure it isn't blown. If it's ok, measure the AC voltage between pin 8 of this board and ground. Should be around 29V.

F07 is OK, and the AC voltage at pin 8 is around 29V.

So I guess something else has blown?

When I saw the move Apollo 13 there was a point when dejavu kicked in and I was reminded of how me trying to sort out a software error with a remote user was like what the Mission Control people were doing (back in the day when we couldn't just view a remote screen like you can today). Now I feel like the roles are reversed, and you guys out there are Mission Control. Thanks for all the help (again).

Back up a bit more ( for my clarity ) ..
Confirm your line voltage I assume 220.
What is the AC measurement between the two purples on 7 & 8 ( Should be ~ 58.X V)
THEN verify between GND and 7 then GND & 8 ( ~ 28 ea )
Next the Cathode ( ( D-11? )) Print's kinda Low rez)) to GND Hint the Anode is tied to F07 out…
Report back please

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.