AU-505 recap questions

eedork

Super Member
I'm finally getting around to recapping the AU-505 my uncle gave me a few years ago. It is in absolutely incredible condition, and I can't wait to hear how it sounds after the recap.

So far I've replaced all of the problematic 2SC871 transistors with new KSA1845s. I've also replaced the mica insulators and grease on the outputs with new high quality sil-pads. Elna Simic IIs are on the way for all electrolytics minus the main filter cap and the two output caps, and that's what my questions relate to:

1. The main filter is 2200uF / 63V / 35mm diameter; should I bump this up? I know many do just because it is easy and cheap, but I'm worried about the initial surge current and switch contacts if I go much higher than what was originally spec'ed. Fortunately Mouser has a large selection of 35mm replacements, although the 2200uF / 63V replacement is only 20mm high (hah!! just barely larger than the clamp itself).

2. The output caps are 1500uF / 63V / 35mm. Should these be bumped up? I believe that increasing the capacitance here lowers the low frequency cutoff point of the amp, which I don't think is a big deal in this particular case.

Thanks!

-Matt
 
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If you can get the original spec main filter caps tat fit in the clamps, just go with that.

Bumping up the output caps is good in my book. I like the "fatter" bottom end of a cap coupled amp.
 
Thanks!

If you can get the original spec main filter caps tat fit in the clamps, just go with that.

Bumping up the output caps is good in my book. I like the "fatter" bottom end of a cap coupled amp.

How much would you bump the output caps? Is there a general rule of thumb for this, or should I just go as big as I can here?

I appreciate the feedback!

-Matt
 
Hi Matt, iam running my 505 with 4700 on the outputs, some uses 3300 and others 5600. I don't think it matters much - except to say that bumping up the value does as mentioned add some muscle at the lower end.

As for the smoothing cap , I'd recommend 4700 as well. I think this one is more important to upgrade. Aside from inrush current, - more reservoir and filtering is generally better until a point whete it becomes silly. Keep in mind this is a single rail amp - so everything downstream suck on that one source for clean DC. Had this been a dual rail amp you would most likely see 2 x 2200 for this weight class of amp. Regarding inrush current, 4700 is a tried and tested upgrade for the 505 and 555(a).
 
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Thanks guys.

For those of you that have recapped one of these, can you tell me what the lead spacing is like for the main output caps? I haven't taken one out yet, but all of the pictures I've seen seem to indicate that the stock caps have leads that are not in the center of the case. This means I will most likely *not* be able to fit a new 35mm part in there and that I'll have to look for something with a smaller diameter (not a problem - Mouser has plenty to choose from).

-Matt
 
22 mm. I wasn't as careful as you - on both my 555 and 505 I ended up drilling new holes - this worked well too, albeit not the optimal solution.
 
Thanks stereofun .. I'd like to avoid drilling so I'll just go with a smaller diameter part when I place the order.

-Matt
 
That's the best way to go, although the drilling can be an alternative if your are set on a specific type or brand of cap. It can be done nicely. The original caps have 4 solder points, can be a bit tricky to get loose. A desolder iron really helps here then followed by use of thread to clean the joints before gently rocking cap loose. Good luck !
 
I'm getting ready to place an order for the last few parts and have a few questions for you guys:

1. Should I replace the working rectifier (just because I can) with something modern like a pair of UF4007s?

2. Should I bypass the main filter and output caps? If so, what value do people recommend? I've read various things here, and not sure if it's worth the effort ...

Thanks again guys!!

The silmics shipped today so I should have an update on at least that part of the rebuild later this week.

-Matt
 
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1 ) It is not customary to replace the rectifiers unless you significantly beef up the smoothing cap. The 10D-1 already in, are rated for 1.5 A average forward current and 48A peak. Your suggested replacements have far higher voltage specs but actually lower current handling both for the average (1.0A) and peak (30A). I would leave the originals in.

2) This is debated back and forth. A standard used by many AK'er is 2.2uf 250V - I have tried with and without and cannot not tell the difference. I did bypass mine as mentioned - believing it probably doesn't hurt and may help, even if my ears cant pick it up. Many of Sansuis higher models used bypassing, but again, perhaps they had to because Electrolytes in the 1970'ties actually needed it....

Regarding the outputs, I bypassed with 20uf 250V polypropylene, because it was highly recommended by fellow AK'er Transfrm who restores professionally (qrxrestore.com) - I did some comparisons and came to the conclusion that the bypass acts as a very gentle filter, effective at cleaning up digital "edges" from the highest registrars in older digital recordings - it makes for a more analog sound in these instances with less digital "sizzle" I ended up keeping the bypasses as I mostly listens to cd's and streaming 128K.
 
Thank you stereofun!

I'll order a 2.2uF / 250V bypass cap for the main filter and a pair of 20uF / 250V bypass caps for the outputs.

-Matt

EDIT: 20uF / 250V caps are hard to find! Mouser has a 10uF / 250V Panasonic film but it is huge at 31x26mm .. that should still fit, but I can't find much in the 15 to 20uF range as recommended that has leads suitable for this kind of work.
 
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I restored an au555a once and I did put in 6 amps diodes.
Overkill? I bought a defective au666 it did have its rectifier bridge shorted while rest of the amp was working okay....

The main cap of the 555a I changed for something like 6800 or so (original 3300).
Loudspeaker caps I did put in 2200 or 3300, I can not remember anymore.

Bypassing, yes people debating.....

BTW beefing up the caps will NOT really beef up the output power, it will only get you slightly better peak power, being the difference (on a certain volume) of the lowest peak of the ripple on the cap compared to the old cap.

This is because average power voltage will go down instantly with a larger volume. The bottleneck being the transformer. At least on the AU555A this was a rather small one.

Anyway, in the end, having done a total recap of my AU555A ,when listening again I clearly did have the impression the amp sounded better than before.
So also in my case it definately did leave a good feeling about it (leading to more amp refurbishments afterwards and a Sansui/Luxman/Tandberg addiction)
 
Thanks guys! I'll try this out and let you know what I find.

I just recapped the tone board, the power supply, and the main amplifier board. I also replaced two more 2SC871 transistors in the power supply with KSA1845s. This is a very easy amp to work on.

In any event, I fired it up slowly on a variac between the recapping of each board. It will play music and it sounds very good, but I'm having one problem. There is some static when I first power it up and the power bulb flickers (in unison with the static which seems to be cyclic). With my meter across the main filter cap I'm seeing a slow fluctuation in the DC supply voltage from 39V down to maybe 15V and then back up. It takes a second or two for this fluctuation to happen, but it seems fairly consistent. This was observed without an input signal.

This is the same issue I was having prior to the recap. I've cleaned the switches and pots with D5 and F5, respectively (twice) so I don't think that's it.

Could this be a weak or failing main filter cap? That cap along with the two outputs caps are the only electrolytics in this entire unit that have not been replaced yet. I find it weird that the bulb flickers too - I originally figured it was just a bad bulb, but now I'm thinking there must be something else going on.

Thanks guys!

-Matt

EDIT: Looking at the service manual for the AU-505 here on AK, there is a ceramic cap across the main filter (C002, 0.01uF / 500V ceramic). Mine doesn't have this, so I assume it was an addition to later production units by Sansui.

EDIT #2: Looks like a faulty power switch is the problem. Jumpering the switch resolves the issue; the amp sounds wonderful! Now I need to figure out what to do about the switch ...
 
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Today I disassembled the switch and sure enough, the contacts were charred and there was a sizable amount of back dust inside the enclosure. I burnished the contacts and the amp now works perfectly!

I'll post some pictures at some point when I finish things up.

-Matt
 
Excellent - look forward to pics, and let us know what you think of it when all the caps have been replaced. Cheers, tom
 
thanks for posting your story. it's the little things like your switch issue that are always interesting for me.

looking forward to any further pics/posts.
 
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