AU-919 blowing main fuse instantly

Sylph

New Member
Hi, long time lurker and finally got a chance to post for the first time.

I've recently acquired an AU-919 with known bad caps that were in need of a replacement so I decided to go through and make it a summer project to restore it, everything has been going smoothly except for when I got around to putting it all back together.

The amp worked fine before the recap but it is now blowing the main fuse as soon as the power switch is switched on. None of the relays actuate, it is an instant fuse blow.

I am not specialized in diagnosing these problems, therefore I come to you and your experience, I have a general knowledge of electronics, but it is more of a hobby to me.

My current assumption is that something is shorting to the case or to ground perhaps? Beats me, what do you think? :scratch2:

Here is the fuse that is blowing up:
ycHU76b.png
 
You have one or more capacitors in back to front, a trapped (and thus shorted wire), or wiring incorrectly reconnected.

If you have removed the main smoothing capacitors it is very easy to re-install them the wrong way round.

I would disconnect the output transistors from the lugs on the PSU board (pairs of red and blue wires - on some models pairs of yellow and grey as well), Recheck ALL of your soldered wiring and plug on connectors for positioning - from your carefully taken digital camera pictures of the 'before' locations of everything, and then try it again - this time using a Dim Bulb Tester.

I have to say I am really surprised that you did not use a DBT at the first power up. :nono:

BTW:
Welcome to AK, and especially the Exclusively Sansui Forum ;)
 
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Welcome Sylph,
This will be a tough one to unravel.

First I would suggest using a load lamp to power up your amp, to limit further damage while sorting this out. Don't connect the Amp directly to the Mains without this.

If could be any number of causes which is blowing the fuse on the AU-919, short circuits between tracks, capacitors wrong way around, interconnecting board wires in the wrong position and so on.
As the Fuse is blowing almost immediately, I would check you have reconnected wiring from the transformer to the F2845 board. I hope you have taken pictures before you started dismantling to help you when you put it back together.
As the fuse feeds the main transformers, first step will be to isolate the output from the transformer to the F2844 and F2845 Power Supply Circuit Boards.
I would suggest before starting to dismantle, clear close up pictures of F2928, F2844 and F2845 would be essential. Also, snap of the Power Selector strip/block would be useful.
Also, on which boards did you change the capacitors on, were there any other components you changed.

Where are you located ?, it may be easier to find a member on this forum near you to resolve this for you.
 
Alright thanks for your input guys, I'll look into everything again, it seems like even though you triple check, you can miss something.

I'm a resident of Quebec, Trois-Rivieres, I don't know anyone in the area that might have it's ways around those integrateds.

The main filter caps were restuffed with pairs of Panasonic THAs 8200uf. (16400uf in each cans) I checked them to see if they have been hooked up the wrong way around but everything seems fine, I wrote their wiring order directly on the board to remember in case I was to lose the pictures for it. I know their hook up order is:

- + + -
+ - - +

As for the board f-2845, all the caps are in the correct orientation. I'll look into board f-2844 since I know there is a silkscreen error on the PCB (atleast in my version) and I might've completely forgotten about it when recapping the board.

Wish me luck, going back in, this time with my trusty DBT. :yes:
 
Me....but it's a bit of a hike. Go back over your main cap job, sounds like something is backward which will cause the fuse to blow- DBT is your best friend. You should disconnect one cap at a time till the DBT behaves.
 
Make sure the solid wire ribbon cables didn't break during the service. I find you have to desolder them to prevent this, as they can take only a small amount of flexing. Check ground continuity between all the boards and chassis. Contrary to what everybody else does, I never use a DBT, but always use a Variac. I bring the line up to about 30 VAC, which is rarely enough to turn on relays or fully operate the unit, won't blow the fuse unless the AC has a short, but plenty to see if the supplies are coming up properly- not shorted and correct polarity. I can also see if the output section is conducting or not. Not is generally a good thing!

Check across the line with an ohmmeter. You should have a couple ohms or so. Check both sides of the line to the chassis. Shouldn't see a thing. With all caps discharged, check resistance from each supply to ground/chassis. They should charge up slowly. It's all about clues to get you to the right section having the problem.

If you have a backwards PS cap, and the odds are pretty good, be sure to check the associated rectifiers.
 
The main filter caps were restuffed with pairs of Panasonic THAs 8200uf. (16400uf in each cans) I checked them to see if they have been hooked up the wrong way around but everything seems fine, I wrote their wiring order directly on the board to remember in case I was to lose the pictures for it. I know their hook up order is:

- + + -
+ - - +

:
this has me wondering :scratch2:
any chance of a picture or 2 ?
 
Alright thanks for your input guys, I'll look into everything again, it seems like even though you triple check, you can miss something.

I'm a resident of Quebec, Trois-Rivieres, I don't know anyone in the area that might have it's ways around those integrateds.

The main filter caps were restuffed with pairs of Panasonic THAs 8200uf. (16400uf in each cans) I checked them to see if they have been hooked up the wrong way around but everything seems fine, I wrote their wiring order directly on the board to remember in case I was to lose the pictures for it. I know their hook up order is:

- + + -
+ - - +

As for the board f-2845, all the caps are in the correct orientation. I'll look into board f-2844 since I know there is a silkscreen error on the PCB (atleast in my version) and I might've completely forgotten about it when recapping the board.

Wish me luck, going back in, this time with my trusty DBT. :yes:

I can confirm that if you did make this 'error' (referring to C07 & C08 on the F2844) it won't blow fuses. I know this because I made this error with the one I am reconditioning now. After referring to the schematic, I have corrected the error on this one, but even before that it powered up just fine - on the DBT and then with the 'full beans' connected to the mains. ;)

And even as I was changing the electrolytics I re-installed the first new one in accordance with the silk screening, I said a few choice anglo-saxon words to myself - and then put it, and its opposite number in backwards (correct) :D
 
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Thanks for the heads up, I rechecked the caps you were referring to but in the end I have them soldered correctly.

As for the pictures, I try to have some up tonight, right now I'll lay off of it a bit. Will resume the investigation later tonight or possibly tomorrow, will keep you guys updated on the situation. :D
 
I have to thank you very much for reminding me of this one ;)

I was vaguely aware of it and looked at it without knowing what the real issue was precisely, and decided there was no problem. Well, there isn't a problem on the schematic! - as you say the error is with the silk screening. :yes:

I have one here that I am half way through - please let us know if I/we can be of any assistance. We have at least 2 picture resources, and several members who are experienced with these, so we should be able to get this sorted out between us ;)
 
Hyperion, are you saying the schematic is indeed correct? :saywhat:

I thought there was an error with the silk screening AND the schematic. But now I am unsure as to what is really the real way to mounting the caps. Here is how they are currently mounted on my board:

78AAd7J.png


I believe this is how they originally were, although I am now uncertain, I know the schematic bares to differ with my current setup. Is this the schematic you have access to or could I be using a wrong version?

pwab3GP.png


Edit:

Conrad, you may be onto something with the ribbon cables, I found two that had broken legs, it almost looks like they become brittle over time. Will go back over every single wires and check continuity again. It went completely under my scope, the legs break right inside the solder holes not allowing you to see it. I sure hope this is all it was, the board in question was that one with the audio jacks at the back with the phono inputs. I also unsoldered the red and blue wires to the output transistors, I can power up the amp on the DBT without them being hooked up?
 
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Hyperion, are you saying the schematic is indeed correct? :saywhat:

I thought there was an error with the silk screening AND the schematic. But now I am unsure as to what is really the real way to mounting the caps. Here is how they are currently mounted on my board:

78AAd7J.png

Remember that you are looking at the underside of the PCB in the SM view. Your capacitors (C07 & C08 in pic above) are correct to schematic, but back to front according to the silk screening - which is how they should be IMO.
 
Could be the angle of the picture image - the Electrolytic at the top right corner looks a little stressed(?) (Top bulging?)

John, it maybe worth posting a few pix of PCB F-2844 of the AU-919 you are currently working on (or from your previous AU-919 restoration if you have any) - providing it is the same board revision as Sylph's etc - It might help to confirm component orientation?
 
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Yes, I was thinking of doing that, every picture of the 'before' status I have seen for these capacitors - puts them how Sylph has them now - so he is correct IMO.

The examples I have referred to:-
scottrt's photobucket restoration pics.
My own AU-919.
The one I am restoring now.

All of the above 'before pictures' show the capacitors installed transposed referring to the silk screening on the board. It is my belief that the schematic is correct, the positive of the +ve rail capacitor going to what I think will be the 'more positive' voltage, and the negative of the -ve rail capacitor going to the 'more negative' voltage respectively.

I think I'll take some voltage readings tonight just to confirm, as specified they are only 10V capacitors - so there can't be much voltage across them.

EDIT: Update
Those capacitors have slightly over and slightly under 10V across them - polarity confirmed as correct with them reversed compared to the silk screening.
(and at 10V working voltage - they are underrated too !)
 
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Exactly. :)

This is one of Scott's pictures, (hope you don't mind Scott). Where you can clearly see the orientation of C07 & C08 on the F-2844 PCB in a 'before' condition - in other words, assumed to be correctly orientated from the factory. (which is reversed compared to the PCB silk screening).

 
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That is how it looked on my AU919 before recapping. If that may help.

En passant, bienvenue sur le site Sylph (de Lévis).

Best luck!

Helios
 

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