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I Broke My AU-7900

Sriskie

Sriskie

Well-Known Member
A guy I work with picked this up at a thrift store and asked me to have a look at it. I adjusted the bias, checked the DC offset (around 11mv.), and cleaned the pots. The only issues I found were a crackling sound through the speakers when powered off and a bent midrange control shaft. I fell in love and bought it from him for 3 times what he paid for it before attempting these repairs in case something went wrong.

I bent the shaft straight and cleaned the relay. I have since been told to never clean relays, live and learn, I guess. I didn't test it after, I just put it away for a couple of months until I finished getting my recroom home theater set up. I wanted to use it in the same cabinet for a turntable set up with a pair of JBL 4311Bs.

When I finally got it hooked up, sure enough, no sound.:tears: Pulled it back out and opened it up. The 1a fuse in F603 was blown. it blew again when I replaced it. The LED comes on and the relay clicks, bias is still within spec, just nothing at the speaker terminals. I don't know what went wrong but I figured the first step was to replace the relay. Thanks to you guys a new relay was found and I installed it this morning. I used my DBT this time to power it up...it stayed bright so I have a short somewhere. The relay doesn't click now, and the fuse didn't blow, I guess due to the DBT?

I have a sevice manual but only a slight understanding of how to read a schematic. Shorts are usually a result of bad transistors, right? I could start pulling and testing them but if you guys can point me in the right direction, I would really appreciate it.:ntwrthy:
 
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First step is to restore the power supply board. Replace the fusible resistors and the VD-1212 diodes.

Check the voltages.
 
does the led still light up ?
voltages at L01 L02 ? should be a few mv.s
voltages anode and cathode d613 ?
voltages reference ground and DC
 
Good to hear from you again Petehall347!

All tests performed using a DBT. Ground attached to chassis.

LED lights up

L01 and L02 measure in mv. but I can't get consistant results.

D613 Anode-7.15v Cathode-7.20v

R71 showes signs of being hot at one time. Plastic tubing on the legs has blackened an looks a little melted.
 

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get voltages r602 r603 r609 .. 2 voltages for each one .i can work out which end .
is that really negative voltages at d613 ?
r71 should be 4.7 ohms
 
r602 7.25v/7.58v
r603 -12.68v/-12.65v
r609 7.27v/7.29v

d613 are not negative. Sorry, I'll be more concious of where I put -.

I measured 4.5 ohms on r71 and r72.
 
make sure f603 and f604 are intact and connecting well .
and what wattage dim bulb are you using ?
if you could pull fuses f601 f602 f603 and f604 and measure AC voltages pins 14 15 16 17 ..
put fuses back in and measure voltages again
 
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f603 and f604 are still intact measured from the fuse holders.

I'm using a 60 watt bulb.

f601 and f602 are just a piece of wire, no fuse holders.

I'm not sure how to measure AC voltages. Do I connect one of the leads to the power cord where it comes into the amp?
 
For f603/604 measure from 14 and 15 pin, you should see 70v ac. Have you tested all your diodes d603-608 to make sure they are all good, a blown one would take a fuse out instantly.
 
To test those diodes, I need to pull them, right? They are 10D-2s, I have suitable replacements pn. 1N5393-E3/54. If I need to pull them, I'll replace them.

Do I understand correctly where to hook up the test leads?
 
I would test them in circuit, if one reads off then pull it.

As for understanding where to hook up the test leads- see the first part of my post.
 
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Okay, so 1 lead on each orange wire, 1 on each purple too? What about the last 2, orange and brown?
 
d608 .508v/ .OL
d606 .000v/ .000v
d607 .506v/ .OL
d605 .000v/ .000v
d604 .470v/ .996v
d603 .470v/ .996v
d613 .326v/ .326v
 
Sriskie said:
d608 .508v/ .OL
d606 .000v/ .000v
d607 .506v/ .OL
d605 .000v/ .000v
d604 .470v/ .996v
d603 .470v/ .996v
d613 .326v/ .326v
Click to expand...

605 and 606 may be reading strange due to their configuration in the circuit, i'd pull on leg of each and retest- 613 is probably due to it being connected to a transistor.
 
No, with f603 & f604 pulled I get 80v on pins 14 & 15, 71v on pins 16 & 17. Are pis 18 & 19 DC?

DBT is dimming now.
 
18 and 19 would be DC as they come out of the rectifier bridge. Since you measured 80v that means the transformer is good. Since the unit isn't in front of me, do pin's 10 and 11 pull out from the board? I'm going to pm you my phone number, call me and we will save a lot of time.
 
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I pulled all of the diodes listed above. d606 was shorted .000v both ways. I'll replace them all. What do I need to do after that?
 
Nothing- replace them and fire it back up again on the DB, I imagine it will work now unless something else is fried.
 
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