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AU-719 on the bench

Hyperion said:
Here is a picture of the finished and reinstalled Control Amp. (sorry - no 'before' picture).

View attachment 2120525

Component changes are as follows:-
C25, C26, C27, C28 - 10µF 50V, replaced with 10µF 63V Nichicon PW.
C10, C11 - 10µF 25V, replaced with 10µF, 25V Nichicon KL.
C17, C19 & C18, C20 each combination replaced with 4.7µF 63V WIMA film, with vacant position jumpered.
TR01, TR03, & TR02, TR04 - 2SC1313 each combination replaced with a matched pair of KSC1845.
Click to expand...

I still enjoy seeing your "after" pics.

The matching TU-719 is stuck in Austria :(. It was supposed to be a welcome home present for the AU-719 but the seller has just informed me that he's unable to find a courier brave enough to tackle the walls of customs red tape and import duties that we've built around ourselves.


upload_2021-2-5_13-13-22.png

Does anyone know of a courier company that's managing to get stuff through the blockade ? If so, should I make sure the seller declares it as a "returned repair" ?

Fortunately I have a backup plan, as I can always have it sent to friends in Belgium, then I'll have to pop over on the Chunnel when it's safe to travel again. Maybe in summer (not sure which summer yet). A bit of a pain really. I was hoping I could get you to run your well-trained eyes over it when I come to pick up the AU-719 but that's not likely now, unless you progress incredibly slowly.

Seriously, keep up the great work :whip:
 

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Here is a 'before' picture of the regulated PSU/Protector board - I had to remove the two largest electrolytics to make it easier to get the board out.

IMG_3366a.jpg

And the completed board.
IMG_3367a.jpg

PSU
C606 & C607 - 470µF 63V, replaced with 680µF 63V Nichicon PM.
C609 - 100µF 25V, replaced with 100µF 50V Panasonic FR.
C612 & C613 - 1µF 50V Electrolytic, replaced with 1µF 63V WIMA film.
C614 & C615 - 220µF 35V, replaced with 470µF 63V Nichicon PM.
ZD602 -13V 400mW zener, replaced with 13V 500mW zener.
TR612, TR608, TR609 - 2SC1313, replaced with KSC1845.
TR607, TR610, TR611 - 2SA726, replaced with KSA992.

Protection
C601 - 47µF 50V, replaced with 100µF 63V Nichicon PM.
C603 - 470µF 10V, replaced with 470µF 16V Nichicon PW.
C605 & C606 - 4.7µF 50V, replaced with 4.7µF 63V Nichicon PW.
C17 & C18 - 220µF 6.3V BP, replaced with 220µF 16V BP Nichicon VP.
SCR601 - 2SF656, replaced with STMicroelectronics 'P0102DA 1AA3', 400V 0.8A

I removed and tested the speaker relay and it was perfect, so I put it back in.

When I powered up the amp to test the PSU (on DBT) the LED flashed and the protection relay engaged, but I had very low voltage out of both regulators - about +/-1.5V. Upon investigation I found TR608 although it tested OK, had reduced in hfe from around 350 (as selected) to ~265, so I changed it for another ~350 hfe KSC1845 and the voltages came up OK at ~ +/-32V. ;)
 
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Hyperion said:
Here is a 'before' picture of the regulated PSU/Protector board - I had to remove the two largest electrolytics to make it easier to get the board out.

When I powered up the amp to test the PSU (on DBT) the LED flashed and the protection relay engaged, but I had very low voltage out of both regulators - about +/-1.5V. Upon investigation I found TR608 although it tested OK, had reduced in hfe from around 350 (as selected) to ~265, so I changed it for another ~350 hfe KSC1845 and the voltages came up OK at ~ +/-32V. ;)
Click to expand...

That must have been a heart stopper but I'm relieved you found the problem and rectified it. I'm glad you're proceeding very cautiously while I'm enjoying the narrative. Keep posting
 
A before picture of the Head Amp - sorry, got carried away removing capacitors - then realised I hadn't taken a before picture. :)

IMG_3368a.jpg

And the Head Amp finished.
IMG_3369a.jpg

C03, C04 - 22pF Black Flags, replaced with Murata 22pF C0G/NPO.
C09, C10, C13, C14 - 10pF ceramic, replaced with 10pF Murata C0G/NPO.
C15, C16, C17, C18 - 33pF ceramic, replaced with 33pF Murata C0G/NPO.
C07, C08 - 47µF 16V, replaced with 47µF 25V Nichicon PW.
C23, C24 -1µF 50V, replaced with 1µF 63V WIMA Film.
C29 & C27, C28 & C30 - 47µF 25V, each combination replaced with a single 22µF 50V Nichicon ES.
C35, C37, C36, C38 - 100µF 35V, replaced with 220µF 50V Nichicon PW.
C39, C41, C40, C42 - 47µF 50V, replaced with 47µF 50V Nichicon PW.
VR01 & VR02 - 470Ω Trimmer, replaced with Bourns 500Ω 25 Turn trimmer
TR03 & TR05, TR04 & TR06 - 2SC1313, each pair replaced with hfe matched KSC1845 pairs.

That completes the component changes for this amplifier. ;)
(apart from the main filter capacitors ;))
 
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I have a problem with this AU-719 which I hope you can help me with.

Here is a snip of the power supply

upload_2021-2-8_18-56-34.png

The actual transformer setup is like this:-

upload_2021-2-8_19-24-29.png

Between point 02 and point 04 (the lower transformer 44-0-44V winding) I am seeing 82V AC, on the good channel (between points 01 and 03) this is 95V AC. (Note the rear panel voltage selector is set for 240V, and increases the voltage output on both transformers if temporarily set to 220V - with 240V applied).

The rectified and smoothed main rails are +/-52V DC for the suspect channel and +/-63V DC for the good channel.

The regulated PSU pass transistors do not run hot, and I know for this model they normally run very hot. I noticed this on the same day the amplifier arrived, but I didn't investigate further at the time.

I am thinking this is a very rare (for Sansui) bad transformer - but would welcome any input.

So, what do you reckon guys - any thoughts as to why this is happening?
 
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Yep, I was thinking something like that.

If I unplug the driver module, the voltage goes up from +/-52v to +/-57v - a bit too sensitive to loading I am thinking. :idea:
 
DougBrewster said:
Given the availability of fuses it shouldn't be difficult to compare current draw from the two circuits?
Click to expand...
The spoiler here is that the regulated supplies are powered from the suspected faulty transformer. But the AC voltages - eliminating any loading issues seem to point to something e.g. 82V AC - just looking into a DMM - straight off the transformer, no load at all, and the good channel says 95V AC.
 
Hyperion said:
The spoiler here is that the regulated supplies are powered from the suspected faulty transformer. But the AC voltages - eliminating any loading issues seem to point to something e.g. 82V AC - just looking into a DMM - straight off the transformer, no load at all, and the good channel says 95V AC.
Click to expand...
Perhaps someone lost count of the turns when winding the transformer. They might have had too much Saki the night before. :p A resistance check might confirm (or deny) it. Check both sides from the center tap. And as mentioned before, the internal thermal fuse might have a bad connection too.
 
Hyperion said:
TR03 & TR05, TR04 & TR06 - 2SC1313, each pair replaced with hfe matched KSC1845 pairs.
Click to expand...
First, I am following this thread with a lot of interest. Did recap / restore my au 719 with some help from you. Seeing your choices I am luckely not that far of. What you did here I also considered because is always recommended to replace the c1313. Then I noticed that Sansui used the c1845 here in other functions and still used the c1313 for the differential pair, was just questioning the reason they had to do that? Also, if I should replace the double caps on - + + - ( mimicking bipolar) with 2 low leakage or 1 bipolar. I recently opened an au 417 and noticed that where they used 2 polar capacitor to mimick a bipolar in the au 517/717 but they used a bipolar here in the 'cheaper model'( almost identical boards). Again wondering what the reasoning was behind these choices Sansui made. Did they consider in that time that 2 low leakage ( noise ) where better then the bipolar they had in that time. Mhh just contemplating.
Hope you find a solution for the different output voltages from the transformers.
Thanks for this interesting thread.
 
DougBrewster said:
That's not so good. How about looking into the primaries from a resistance perspective?
Click to expand...
Yes, I attempted to do this but with the primaries in parallel, and going through the input voltage selector (and not wishing to disturb those connections), I didn't come out with anything conclusive. Other than the thought that possibly the 220V & 240V taps were swapped on one transformer (wires colour coded wrong perhaps?), I am fairly certain I proved that they were not swapped and were wired correctly. Such resistance readings that I did get seemed to make sense.

Thanks for all the input guys, I think we are pretty close to saying the transformer is defective.

Any other input on this still very welcome....
 
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GrisPato said:
Did they consider in that time that 2 low leakage ( noise ) where better then the bipolar they had in that time
Click to expand...
As far as I know quality Bipolar capacitors are a fairly recent thing, they had bipolars back then, but I think I am correct to say that 2 polar capacitors beat the available bipolars they had when these units were built. Or at least 2 polars were cheaper than a bipolar? - some weird decisions are based on price.
 
DougBrewster said:
That's not so good. How about looking into the primaries from a resistance perspective?
Click to expand...
The individual secondaries measure the same at 1.2Ω, the primaries measure 13.4Ω for the 240V setting and 12.7Ω for the 220V setting, I'm not keen on unwiring the voltage selector switch to check the primaries any further.
 
Apart from asking if you have an RCD on your ring main - which would/should reveal a primary ground leak - only thing I can think of is a secondary winding high voltage test with a metrohm or something to identify a secondary earth leak.

Edit: but it's a two wire supply, yes?
 
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Yes it is a 2 wire supply, no earth wire, we do have RCD's and they are very sensitive. :)
 
John, my AU-719 is similar to what you have observed - I noted before recapping, Transformer PT701 (nearest the back panel) measured 44-0-44V, while PT702 measured 41-0-41V (giving approx +/-63VDC and +/-58VDC)

Like you I am thinking maybe the transformer primary taps for PT702 are swapped / mis-wired , and like you did not want to mess with the 'Mains' input selector switch to prove.

I wonder if any other AU-719 (EU / BS models) owners with the 220V / 240V 'Mains' input selector have similar Transformers discrepancies
 
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