Sansui 7070 issues

YingTK

New Member
Hi,

I've just bought a 7070 off craigslist... without... testing... it... as seller says it's working fine, and met me at a gas station. The cosmetic of the 7070 is pretty good, so I did not suspect anything too off with it...

My mistake. Went back home and plugged it in. First thing I noticed was non of the lights came on. Then I found the power output(from the meter) on left channel is higher than that of the right one without any input connected to the 7070. I followed the service manual and found the DC voltage of the left channel was way off. Adjusted it back to ~0mV according to SM then moved on to bias current, but I could not get a reading with my craftsman 82141 multimeter. The meter kept reading 0.

After 30 minutes trial of no success, I decided to try connecting the 7070 with my test speaker. As I plugged in my test speaker to the 7070, I can hear a hum from the speaker as soon as I switch the speaker selection on. The hum goes louder when I crank the volume on. I tried unplugging the plug and plug it in after switching polarity, and the hum gets quieter, but still there, still varies with volume. I was able to get sound from AM and it did not sound distorted (well, not sure how accurate I'm hearing, since I'm using a spare speaker which cost nothing, But at this point, I wouldn't connect anything better to the 7070)

Questions,

1. With all the lights off, does it means it should be more of the problem with the board? I found a previous thread about re soldering the board and cleaning some fuse holder. I did clean all the fuse and holders on F-2625, as I didn't know which one is was, but that didn't help.

2. Will I be able to get sound from the receiver if there is no bias current at all? Or I should probably borrow another multimeter?

3. From what I've read, if the hum varies with volume, it should be from the preamp section, correct? I've also read some posts about grounding issue, and changing polarity of the power plug did helped a little with the hum, so I'm confused. (of course, I am hoping it was just a grounding issue as that may be easier to fix)

Well, I do hope these are going to be fixable by myself (not experienced with electronic works though). I certainly did pay way more than a parts unit, and I hope I won't have to spend a whole lot getting it to work properly.

This is my first Sansui receiver. I've been a fan with vintage, and I have used, and am still using vintage setup in my system. I do hope I can at least get to know the sound characteristic of Sansui!
 
Welcome to the Sansui forum, I am sure the experts will be along shortly.
Check the fuse in your meter, easy to blow one.
Read the threads about the venerable 7070 receiver, there's a lot of info out there. You might add your location to your profile to assist other members in knowing where you live, maybe someone could drop by to help.
Casey
 
Hi nosirrah,

Thank you for your input. I've actually spent a whole day reading on the internet before posting(with most info coming from AK :)), as I was hoping I could find the solution to my 7070 from previous threads. Of course, I'll keep on reading while waiting for help here :)

As for the fuse, I've suspected it was the problem last night, but it "looked" fine, until just now when I took it out and test continuity... Well, I'm ashamed I did not do this in the first place. I thought when the fuse blew, the meter would just quite working... Well, heading for a new fuse now!

Also, thanks for the tip, I'll go ahead and add my location, although it might be hard to find someone around as it is pretty rural here.
 
No expert here - but just some pointers. Glad you already got through the fuses - make sure you checked them all.

Regarding the 0 mA reading on the bias - I'll bet that either one of following are to blame:

a) You did not change your meter from dc reading to mA
b) Your "amp" fuse inside the meter is toast.

The bias is pretty important - if you turned and twisted the pot already with the "zero' reading, chances are you could have left it in a "high" position which would quickly overheat your outputs. Get the meter working first, and reset the bias pot to what would be a halfway in between, then power up and be ready to adjust it.

With bias and dc dialed in - all fuses checked, it is time to clean all switches and turn knobs. There are plenty of tutorials if you search for "deox"

From this point make an assessment of what is working or not and report back and we can go from there.
cheers,
 
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No expert here - but just some pointers. Glad you already got through the fuses - make sure you checked them all.

Regarding the 0 mA reading on the bias - I'll bet that either one of following are to blame:

a) You did not change your meter from dc reading to mA
b) Your "amp" fuse inside the meter is toast.

The bias is pretty important - if you turned and twisted the pot already with the "zero' reading, chances are you could have left it in a "high" position which would quickly overheat your outputs. Get the meter working first, and reset the bias pot to what would be a halfway in between, then power up and be ready to adjust it.

With bias and dc dialed in - all fuses checked, it is time to clean all switches and turn knobs. There are plenty of tutorials if you search for "deox"

From this point make an assessment of what is working or not and report back and we can go from there.
cheers,

Sorry for reporting back this late. I was distracted by a newly received problematic Yamaha A-1.

I had removed all the fuses and test them, turns out one of them, although "looked" good, was actually burned. Since I'm buying fuses anyway, I bought every fuse I could find in local rural king, true value, and Radio shack, and replaced them with new ones.

Also, most importantly... I had the fuse in my multimeter replaced as well. I originally brought one from a friend, but believe it or not, his got a burned fuse as well! Took me a while to finally found the fuse in local RS, and to get the meter back to work. Finally, I was able to adjust the bias current. Turns out for some reason, the bias current was turned extremely low (glad it's not the other way around :) ).

Had everything adjusted, and decided to let it sing first without bothering with the lamp problem for now... Hooked it up to my Vienna Acoustics Haydn Grand... Wow, the sound stage is huge compared to my Yamaha C70- MX55 combination! The 7070 is now singing while I'm writing, and I'm really enjoying the wide sound stage! Although the highs is slightly harsher than the C70- MX55, and vocals are less focused, but it's been a long time since I had this kind of sound stage in my room.

Now that I need to reflow the bulb circuit, and let it shine! Can't wait to see the lights! I just love the lights of these vintage receivers. Last one I got was the Marantz 2270, and I have not been using vintage for a while until the C70, and it drew mt back to the vintage world :music: I just wish I knew more about electronics to diagnose more problems with these vintage gears myself (like the A-1 sitting on my desk now waiting to be fixed:tears:)

I will report back if the light issues were fixed by reflowing (hopefully!). Thanks everyone for the tips!

Ps. I am familiar with deoxit, just need to be careful using it more sparingly... Had used 2 cans now for all my vintage gears :D
 
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