Sansui AU 7900 Recap

[Nuel]

I am in the process of recapping a Sansui AU 7900. It works fine, but it was pretty beat up. Not sure if the sound is as expected plus there is also a cold solder somewhere in the main board. So thought I'd replace the usual prone to fail components (Caps, VD1212, fuse resistors) with new ones and run a re-solder at suspected points.
Something weird is that, the Fuses F601 and F602 had been replaced with a wire. Not sure if that was how it was or someone was in there before. Can any one help? Also I would be happy to hear anything that I must be careful about and anything that needs to be replaced besides those mentioned.
-Thank you.

 

[911s55]

Have a look at the inputs on the horizontal rear panel. Those solder points can crack and separate due to the cord stress. My 7700 has a couple.

 

[Nuel]

Have a look at the inputs on the horizontal rear panel. Those solder points can crack and separate due to the cord stress. My 7700 has a couple.

Thanks @911s55
It crackles only when I touch C12 on the Amplifier board. Looking underneath figured someone had broken a few copper traces near by and did some work to reconnect broken joints. So that needs to be re-done.
Regarding the question about the fuses, there seems to be more to it. The thermistor/posistor PTH487A had been bypassed with a pair of wires. May be it blew before??? Looking to find out what the original unit looks like.
Appreciate any info.
-Thank you.

 

[Robisme]

Here is a pic from the web.

Rob

 

[Nuel]

Thanks @Robisme . I came across this thread that suggests, in 110V units there are only 2 fuses (instead of 4). So wasn't sure.
http://audiokarma.org/forums/index....wer-supply-driver-board-problem.686600/page-2

Now it seems like the fuses had been removed by someone. I'll include 2 fuses and start the work. Thank for your help.

 

[Nuel]

Hello AKers, relatively starter here when compared to the members of this forum. I've been dormant and reading a lot of threads to learn about audio units and this is my first recap/rebuild project. I will post the progress as it goes and share what I had done. Please excuse me if I ask not so elegant questions and feel free to point out if there are any suggestions.
I have this AU 7900 that works fine, but the sound did not appear as good. There is a 'loud pop' sound when toggling the Tone Selector to Tone & Filter. I've cleaned all the contacts very well. One of the speaker terminals has between 60 - 85 mV. And there are some cold solder joints that need to be addressed.

Tone Control Board
So that is the first board I worked on. Instead of removing the the Tone control and Filter boards that are sandwitched, I let the amplifier vertically and made enough space to access the components.

Before:



After:


WIMA for C01, C02, C11, C12
Nichicon MUSE ES Series (Bi-Polar) for C09,C10
Nichicon FG for C17, C18
NIchicon PW for C15, C16

Filter Board:
Before: (except for C33,34. started before the photoshoot)



After:

Nichicon MUSE ES for C33, C34.
WIMA for C11, C12, C13 and C14.

Put them all back together. DBT tested positive. Played something and the sound came through speakers. Have been running for about 8 hours now and everything seemed okay, all tone controls work fine. Here are some findings:
1. When increasing the volume, right channel starts first and left catches up slowly and they match at about 9 o'clock position. I injected audio directly through AMP MAIN IN the sound is equal. So it is from the pre-amp stage. Is there any quick diagnosis for this? While I can live with this, I am wondering if there is anything I can do.
2. The loud pop when away mostly. Now it is more of a feeble click from the speakers. Several threads said this could be normal. Any thoughts?

Thanks in advance to everyone.

 

[Nuel]

The amplifier board gave me one hell of a time. The cold solder problem wouldn't go away even after re-flowing the joints 3 times, cleaning up a lot of sansui glue on the component side and cleaning the relay contacts. After about 6 weeks of battle it finally came to light. The board was cracked a little, breaking a few copper traces. The thermistor/posistor PTH487A connections hence were bypassed by someone (as I mentioned in my previous post) as a result. However the crack went further breaking more traces over time. It was hard to find as there was a chunk of solder on the two parallel traces to the right.



Relief after solving this puzzle, so went ahead with replacing components, added the missing fuses and here is the main amp board after.



Bias current was set to 18mV and is very stable.
The Equalizer board (with input ports) luckily didn't have any cold solder problems. Gave it new caps too, put them all in place and it plays beautifully. A lot of threads in this forum had great info and helped me a immensely during this process. Thanks to all the contributors.

Now onto the cosmetics.

 

[stereofun]

Good catch on the crack

 

[Subchannel]

Anyone have the value on the BP Cap´s C09/ C10? Previous owner replaced them with 10uF polarized, and the value is not in the SM.
Thanks!