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skippy124

Sansui AU-5500 on the bench

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Hi,

 

 

I was able to help out a new member who recently joined in search of someone to have a look at his Sansui AU-5500 integrated amp that had developed some problems.

 

The AU-5500 was manufactured from 1974 to 1976, and this series (AU5500, AU6600 and AU770) were some of Sansui's first direct coupled amplifier designs. The later AU5900 through AU7900 (1976 to 1978) used the same physical design that was developed for this earlier series. The 5500 is rated at 32W per channel, and features tone defeat, high and low cut filters, muting and loudness controls, plus two AUX and two Phono inputs.

 

This particular unit had developed intermittent channel drop out and distortion. One the bench, it performed perfectly, and all controls operated correctly - the faults could not be reproduced. so time to have a look inside. The design of this amp has the main amplifier/heatsink assembly "up side down" at the rear of the amp. To get access the bottom cover is removed, and I also removed the top cover for access to the tone control and phono boards. Turned the amp on again, and one channel had dropped out. Some tapping on the main amp board caused both channels to drop in and out, so the most likely cause was dry solder joints, a poor connection (plug, PCB mounted fuses, or speaker relay). All the protection fuses were removed, cleaned and fuse holders retensioned, but this had no effect on the fault.

 

The main amp module was then removed and the soldering inspected, and some dry joins on the audio input connector posts were found. Resoldering the dry joints improved the drop out problem, but it was still evident. The Protection relay was then removed (this was the original suspect) and inspected for pitting and then cleaned. Ohm Meter checks before and after showed that the contacts had high resistance before the cleaning, and zero (or close enough) resistance after cleaning. The relay was refitted, and the intermittent drop out problems were fixed. A general service was done, including checking the output transistor bias an DC offset.  Offset was good, but the bias needed adjustment. This was done, and the amp put back together for an extended leisting test to make sure that the problem has been completely fixed

 

 

The main amplifier board removed

 

 

AU5500%20main%20amp%20module_zpss19f08wv

 

Dry Joints

 

 

AU5500%20cold%20solder%20joint%201_zpsdt

 

 

AU5500%20cold%20solder%20joint_zpsehrebx

 

 

Relay removed for cleaning  NOTE: do not use "wet and dry" paper, or any type of sand paper to clean relay contacts. Doing this will remove the special plating on the contact surface, and it will only be a matter of time before the relay fails again. Use strips of plain copy paper sprayed with de-oxit, and pass the strip through the relay contacts several times (you have to actuate the relay by hand to do this - put the paper in, actuate the relay, and drag the paper out - this cleans both contacts at the same time). then set the realy aside for 10-15 mins to allow the deoxit to do its job, and then repeat the paper strip cleaning, but with just the paper strip - no deoxit.

 

AU5500%20relay_zpsiz1ykufu.jpg

 

 

The bias check for this amp (and many Sansui amps) require the current into the amplifier board to be measured and set for each channel. This requires the fuse to be removed and a meter clipped to the fuse holder to measure the current. This can be dangerous - the meter clips can slip off, and if they contact the chassis or other parts of the circuit magic smoke can be released.... I use a modified methd of checking and setting the current at the fuse holder.

 

Blown fuses fitted with 1 ohm resistors

 

AU5500%20bias%20current%20test%20jigs_zp

 

These are fitted in place of the fuses where the measurement is required

 

AU5500%20bias%20current%20test%20jigs%20

 

 

and the meters clipped on set to measure volts

 

 

AU5500%20bias%20current%20test%20leads%2

 

 

the initial readings (mv = ma), one channel is just out of spec (25ma +/- 10ma)

 

 

AU5500%20bias%20current%20measurement%20

 

 

After adjustment

 

 

AU5500%20bias%20current%20measurement%20

 

some pics of the overall unit

 

 

AU5500%20top%20inside_zpsiv81y090.jpg

 

 

AU5500%20front%20side_zps34pbmuif.jpg

 

 

AU5500%20_zpsttwlrzci.jpg

 

 

AU5500%20top_zpsum6jcuqv.jpg

 

 

AU5500%20rear_zpsiwwsiow6.jpg

 

 

Cheers

 

John

 

 

 

 

 

 

 

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Nice work as usual John :thumb:

 

Very clever to solder the 1 ohm resistors to a blown fuse with a loop for the multi-meter probes to clamp onto!

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Awesome history and info.

A much neater layout than the few 'Suis I've seen.

Is it just me or do others get all teary when they see a very well looked after bit of gear. ;)

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Amazing stuff. Looks very well looked after. I wish I had your talent John. 

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Needed a trip back to Johns for a faulty capacitor at the start up earlier in the year.

 

otherwise still going strong 

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On 08/10/2018 at 3:51 AM, blakey72 said:

Amazing stuff. Looks very well looked after. I wish I had your talent John. 

Nice work ,John!!

 

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