au-7700 that needs a little love

mcintosh-nd

mcintosh-nd

New Member
Hello all. I just picked up a au-7700 last weekend at a gw store. I pluged it in the other night and it took awhile for the speaker relay to kick in, finally it did and I played it for about 4 hours with my Tu-555 tuner with no hicups at all. Well last night I could not get any sound out of her so I opened it up and started looking at it with the schematic. So far I can see that the
f05 fuse is blown and blows with a replacement fuse when turned on sometimes?? As far as the speaker relay goes I dont know if it is in some kind of protection mode or what not as I tried to adjust the bias as the service manual states for the style that has no quick acting fuses as ithis unit does not, but that was a no go..??? Any thoughts would be great or ideas to the problem(s) and get me going the right way. Thanks
 
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If it's blowing a fuse, you have trouble. It is certainly in protection mode if the speaker relay won't kick in. You need to check the DC offset at a point prior to the relay. There should be a coil with a large resistors in parallel for each channel output. Measure it there. If you have significant voltage, check the output transistors.

- Pete
 
Thanks for the reply Pete, I did some more checking. It was blowing the fuse because it was a 3a, all I cound find last night. I can not find a 7a which it calls for locally so I have a 5a in there now and it has not blown. I belive I actually blew the fuse when I was poking around with my meter. I think the coils you are refering to are lo1 and lo2 on my schematic?? I measured dc mv at the 3w resistors coming off output transistors and set vo3 and vo4 so the are around 45v, Is this correct??? The relay still will not pull in. I noticed on the transistor by the relay tr29 the base has .63v and the collector has about 16v if I put the 16v on the emiter as how it should work the relay pulls in fine. I pulled the transistor and it checked good with my meter and also tried another npn with close specs with no luck. Any more ideas???? It has got me puzzled.
 
What you are measuring there is rail voltage. The output transistors regulate that voltage to drive your speakers. It's not the correct point to measure DC offset. If you have been turning the trimmer pots, stop. You can do real damage if you turn the bias up too high. Is the heat sink getting hot?

It's very unlikely there is a problem with the protection circuit. 9 time out of 10 it's doing it's job protecting the speakers from excessive voltage. You really need to check the DC offset to determine what's going on.

To check the DC offset you need to connect to the point I described earlier. The coils should be just before the protection relay contacts. You should be able to trace a direct connection between those coils and the relay contacts. The audio output goes through those coils, the relay contacts, speaker switch and then the speaker terminals. There should be a pair of these coil/resistor packs, one for each channel.

If you are not sure, take a picture of the area around the protection relay and post it. I'll circle the correct point.

- Pete
 
Thanks Pete, no I have not been turnning the pots. I marked were they are and got nervous!!! Yes it gets warm pretty fast. I was checking the temp with my temp gun and it was around 85*F with no load around the output transistors, not good I would guess. Here is a pic of the area I believe you are talking about. Thanks Again, Dan

DSC_0973.jpg
 
You can see the two coils just to the right of the relay. There are two large resistor mounted vertically on the right side of the coils. Measure the voltage between the exposed leads and chassis ground. If it's over 100 or so mV the protection circuit will prevent the relay from energizing.

85 degrees is warm but not scary hot. You do have to check bias current but I'm not sure how Sansui recommends measuring it on this amp. They probably have you remove a fuse and measure it directly. Do you have a service manual?

- Pete
 
Well I measured and I get 15mv on one channel and 17mv on the other. I do have the full service manual and tried to check the bias as xoaphexox posted above from the service manual. I have the model with out the fuses fo1,fo2. I removed a red lead from the large cap and put my meter in series set to dc ma. I could see nothing close to 45ma I believe that both channels were about 1.5ma or so on my meter. I also checked my fuse and swaped my meter probe on my meter so that is not an issue. Thanks again for both your help
 
xoaphexox said:
Here ya go!
Click to expand...

Thanks, I'll link it into the Bias/offset sticky.

15mv and 17mv won't trip the protection circuit so that's not your problem. At 1.5 mA the heat sink should be stone cold. Don't know what to tell you at this point. The service manual will give you points to check voltages. Start with the amplifier driver circuit and then the protection circuit. You have your work cut out for you.

- Pete
 
Well just wanted to give an update. It is ALIVE!!! Had to pull my hair for awhile but finnaly got it going again. What I found was one of the PNP output drivers was shorted. I recommend always REMOVING these to check as I checked them all in and they seemed fine haha. Well I removed them all yesterday and guess what I found. Replace the PNP's and click there she was, adjusted the offset and the bias to specs and all has been good. Runs nice and cool playing it with my TU-555 and some bookshelf speakers. If it keeps it up might have to disconnect the two mc2205's and see how it sounds in the main system. Thanks again for all your help and I am shure I will need it again.

Edit: Oh also while I was in there I did a recap also figured it could't hurt!!
 
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Congratulations. It always feels good when they come to life. Replacing the electrolytic caps is always a good idea. Now it will give your mac a run for it's money.

- Pete
 
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