Sansui capacitor recognition ?????

[Jes]

Hi there, I am new to AK, and hoping someone can help me. I am some way through restoring a sansui 1000a, I have come across a capacitor that I cant find on the manual, its a Bianchi 100uf 350v polar, as I can make out it is running into r179 and via a resistor to leg 6 on v4 & v5.

also on the pre-amp section ( at least I think it is) with the 30uf 10v caps it looks like the earth side goes live ???
I'm afraid I only have 2 numbers from the serial number (84).
any help would be greatly appreciated.
thanks
Jeremy,

Sansui 1000a, sansui au-317, Technics sl-ql1--- 205mk4 cart, sansui tu-5900, nakamichi bx300, oppo bdp-95eu. ditton 44x2, ditton 15.

 

[Dandy]

Hi Jeremy and welcome to AK. I can't be much help, but I hope this reply gives your query a bump.

It's a hard to answer your first question from the schematic, as there seem to be two places marked R179.

The second caps you ask about, the 30uF at 10V ones, are in the phono stage, which is transistorised rather than tubed. Sorry I can't tell the polarity from the schematic either.

 

[larryderouin]

BIANCHI caps were never used in the 1000A as OEM parts. From the description it sounds like one of the voltage doubler caps crapped out and a 100uf350v was installed to bypass that cap. Pictures from Multiple angles of the area in question would help us nail it down to which section of the DUAL power resistor the cap is connected to.

 

[Jes]

Thank you Larry and dandy, yes after a further inspection you are right it looks as though 156 0r160 has gone down so they have by-passed. I have replaced 113,156,160 and all is heading in the right direction.
Thanks again.
Jeremy

 

[Jes]

Hi Larry,

Just one more question if I may, I'm now about to attack the phono stage, there are 5 caps in each side which are elna's, they have a arrow down one side that looks to be pointing to the load side, question is are they polarized and if so are they running + ground ? I'm ery new to all this so its more of a guestimation.....
Just this to go and await the variac and its all go.
Have to admit that I'm getting quite excited. my first tube amp.....
Regards
Jeremy

 

[Audiovet]

Jes,

Regarding polarity the input 30µF 10V capacitor will have its negative end going to the input transistor base, or junction of the 100K and 170K resistors. (My schematic does not show component annotations.) With the second cap (30µF 12V) the collector of the first transistror runs slightly more negative than the following base, this the negative there.

But I have two more points to make. Firstly I feel slightly uneasy to use capacitors with a voltage rating lower than the applied rail voltage of a stage. It is undefined what peak voltages might be reached on power-on or fault conditions. In this case it is -20V, thus there cannot be much quibble about using 25V caps there; size small enough these days.

BUT THEN: The high values of these caps. Spice analysis will show that the input impedance of the relevant stage cannot be below some 20K at 40 Hz, increasing with frequency as NFB increases. Thus the -3dB frequency with an input cap of 10µF will be all of .... < 1Hz! Using electrolytic caps here?

Simulation will show that using as low as a 1µF cap (which can then be polycarbonate or such) still gives a -3dB frequency of about 11 Hz; low enough I would guess without any audible influence on a phono input. Similarly for the second (coupling) 30µF -12V cap. (This is easily verified by practical measurements. By the way, the same arguments are also relevant to the more popular AU-111 amplifier.)

Not criticising a very good design; it was occasionally habit in those days to use rediculously high coupling cap values
to get their poles 'out of range' as it were. Only when that approach forces the use of electrolytics however, there is a problem.

 

[Jes]

Thanks Audiovet,

I think some of that makes sense. increase volts incase of surge, possibly decrease farads !!!!
I'm going to presume that they are still polarized...
regards
Jeremy

 

[Dandy]

Jeremy, you're fortunate to have input from Audiovet, as he is very knowledgeable.

Audiovet, could you please confirm what values, voltages, cap type and polarity you'd recommend Jeremy to use? If he goes for 1uF at say 100V, could this be a non-polarised polyester one?

 

[Jes]

Thank you Dandy.
I am very greatful for all the help as I think it would be a totally different challenge..
Thanks again for all the information..
Jeremy

 

[Dandy]

Can you post a closeup of those Elna caps, showing the arrow down the side? Is the board in your picture a PCB, or hard wired? If a PCB, there is usually a polarity mark printed on the board which is visible when you desolder the cap. This may be shaded and mark the negative.

 

[Jes]

Thank you dandy. I will have a look tomorrow and post some pictures. I believe it is a pcb.

 

[Audiovet]

Thanks for the inferred compliment Dandy; one hopes to help.

Locally (South Africa) the small 'Wima' 1µF 63V polyester caps are freely available (I have not noticed lower voltages, but these are convenient enough). They easily fit in the 10µF electrolytics' spaces. I believe they also come in other brands in other countries.

 

[Jes]

Thank you Audiovet,

I will look for some of those, are they non-polarized ?? sorry if its a obvious question but when I say I'm a novice I mean it in the truest sence..
Many thanks again

Jeremy

 

[Jes]

Can you post a closeup of those Elna caps, showing the arrow down the side? Is the board in your picture a PCB, or hard wired? If a PCB, there is usually a polarity mark printed on the board which is visible when you desolder the cap. This may be shaded and mark the negative.

Here you go Dandy, hope these are what you ment....

 

[Dandy]

Thanks Jeremy for the pics. Replace both the 30uf electrolytic caps with those recommended by Audiovet ('Wima' 1µF 63V polyester caps). If these dont't work to your satisfaction (unlikely), you can always install 30uF electrolytic caps with say a 50V rating.

 

[Jes]

Thanks Dandy, I have ordered both but will go for the wima, looks to be a lovely amp, fingers crossed the transformers and tubes are ok..

 

[Dandy]

We look forward to hearing how it sounds, maybe with a couple of 'glamour shots'.

 

[Jes]

Hi Dandy, well ordered a few 2 12ax7 tung-so tubes and 1 6an8, works great but only on 1 channel, it would appear that I need the other 6an8 aswell, so I am going to replace the lot, sounds amazing even on 1 channel and on a baby ditton 15, I was happy with my au-317, but after hearing this, cant wait to get my 44's rigged up..
It really is a lovely amplifier.......
Photos to come in due course.
Just need to polish the plate metal... Any recommendations ??

Thanks
Jeremy

 

[Dandy]

Have you moved the 6AN8 to the other side to check if that tube is the problem? There could be other simple causes, e.g. dirt on switches (rotary and slide) and pots. Have you given them all a good cleaning with Deoxit?

 

[Jes]

Hi Dandy, Yes I swapped sides and it worked fine on both with the good tube, so deffinately a bad tube, which make me quite happy to be honest.
I was thinking of replacing all of the tubes with tung-sol ?!?! good idea or bad ????
Oh I forgot, I have cleaned all the connections, switches plugs twiddley thingys. everything with a conections... FM seems abit ropey and the stereo light only eluminates as I increase the voltage but then dies off.