AU-517 Recap/Restore with a little sting in the tail

Hi,
A friend recently acquired an AU517 and asked me to check the unit over and do a recap/clean etc, and I thought I’d share the work that was done. And as the title suggests, there was a little hiccup in the work that will be revealed towards the end.
The work to be done was as follows –
Full clean, replace all electrolytic caps (Panasonic FC and Nichicon HE) except the main filter caps, replace all fuse resistors, replace the VD1212 diodes (2X1N4148) and the Bias pots with 10 turn versions.
The unit as received





A quick operational check was done (the owner advised that the unit was working) and all appeared fine, it came out of protection, bias and offset checked OK, and a quick listen showed it worked OK (or at least appeared to…. more about that later)

First off were the driver boards

Driver boards were removed from the amp and separated from their heatsinks and caps removed

The dreaded glue




All caps removed and glue cleaned




New 10 Turn Bias pots with leads modified to fit pcb hole spacing




New caps and bias pots fitted and boards reassembled to the heatsinks (Panasonics Stacked films were used for the 1uf 50v electros)




These were then put aside until final re-assembly.

Next in line was the Power Supply board
Partially removed from the chassis




More of the glue







With some of the caps removed




Glue removed, caps replaced, ready to refit to the chassis




And back in place





One of the things that was discovered when the Power supply board was done was a previous repair, and the substituting of some component values in the protector circuit.





C601 changed from 47uf to 100uf, R601 changed from 390 ohm to 180 ohm, R608 change to 22K and TR603 changed to a 2SC2320. Looked like there had been some previous problem with the protector circuit, and I decided to change back to the original components as per the schematic ( I also checked against a 517 I have, and it agreed with the values shown in the schematic).
Next was the Tone Control and Equaliser boards

Here is the Equaliser board recapped, and the 82 ohm fuse resistors and VD1212 diode pairs replaced




I didn’t take any pics of the Tone Control board, it is a pretty simple board to recap in the 517

The front panel and knobs were given a good clean, and I also cleaned/polished the heads of all the screws that were removed.










Once it was all back together, it was time to put it on the dim bulb tester and see how she behaved with power applied…… now to the sting in the tail……

Hooked her up, power on….. bright then dim…. All looking good…… and the protector light just kept on flashing, and flashing, and flashing….. hmmmmm.
Checked offset…. Both channels were ok. Checked base drive to relay transistor – OK, checked rail voltage of protection circuit, was fluctuating around with the flashing for the protector LED – smacked of not enough rail voltage, and my thoughts went back to the mods that had been done the protector circuit, lower series feed resistor for the + rail, larger filter cap, reduced time constant resistor…. Looks like this had been a problem that the previous person that worked on the unit had circumvented with the component changes that I mentioned in an earlier post.

OK, lets do some cold checks on the left channel power supply (the protector circuit gets its power from the left channel transformer). A resistance check of the main fuses showed that F03 was open, even though it appeared to look fine - always check fuses with a meter, don’t rely on a visual check





This unit is a universal voltage model, and has fuses in both halves of the secondary outputs for each channel. A new fuse was installed, and the unit turned on…… dim bulb on full brilliance… there is a short somewhere…… Main rectifiers checked out OK, so I disconnected the output of the left channel supply to the filter caps at the power supply board… short still there…. So I removed the power supply board for further investigation. First thing to look at was the two rectifier diodes for the protector supply (D601 and D602), and guess what, D602 was short circuit








A new diode was installed, the power supply board reinstalled and reconnected, and the unit was powered up again….. Bingo, dim bulb indication was normal, and the unit came out of protection as expected.
It appears that D602 failed short sometime in the past, taking out the main fuse F03, but not visually

Whoever repaired it did not notice (or check) that the fuse had gone, and this would have led to the low rail voltage for the protector circuit. He substituted the components in the protection circuit in order to get the relay operational again, and even with this main fuse blown, the amp will still operate OK on lower volume settings, as it would be getting half wave rectification via the one half of the transformer secondary winding that was still in circuit. Once he got the relay to click in and it sounded “OK”, it must have appeared to him that the fault had been fixed……….
Well it is now!!

Cheers

John

good work Sir, and what a sleuth you are....:thmbsp:

Nice work, on both the restore and the repair.

Sometimes tracking down the bad parts can be very time consuming, but is worth it in the end.

Rob

Its like a detective story - who is the crook........thanks for sharing and well done.

Well done John !

Caution, knowledge, experience, and a DBT - a perfect combination :thmbsp:

And to cap it all off (pun intended) a nice restore.

Nice work.
I have a 517 on the bench that I would like to get at this winter. Pics like these help a lot.

Nice work and pictures. What did you use to remove the glue?

Lee.

excellent work John

Thanks for the comments gents!!

Always makes it interesting when a previous tech leaves a puzzle to solve

ricohman said:

Nice work.
I have a 517 on the bench that I would like to get at this winter. Pics like these help a lot.

Click to expand...

I can advise that there are no differences in the schematic to what is fitted regarding Capacitor values ( I can post up a cap list if that would be of help to anyone)

leesonic said:

Nice work and pictures. What did you use to remove the glue?

Lee.

Click to expand...

Hi Lee,

I use a scraper made out of a thin strip (about 8mm wide) of FR4 PCB material with the end filed to a chisel shape. Some has posted elsewhere that they had good success with a sturdy plastic spoon with the end similarly shaped





Cheers

John

That's what I call a fine procedure.
Whoever invests so much time in keeping a Sansui alive, deserves our compliment.

Nice resto and detective work as always Dad!

skippy124 said:

I can advise that there are no differences in the schematic to what is fitted regarding Capacitor values ( I can post up a cap list if that would be of help to anyone)

Click to expand...

That would be much appreciated.

And.. thank you for sharing this informative and interresting story.

-John-

Thanks for the detailed report. I read all of these I can because this is a way to learn what to look for when I run into this stuff. I learned a lot from this one.
Rob