Project/One Solid State Stereo Receiver help.

Yeah thats the hard part. that board is buried under the front panel  making it hard to probe and test. 

But I re capped it anyhow since I had all the caps, except for some 10uf i went with 22uf as I have a crap load of those.

The .47uf caps had 45 ohm ESR !! And most caps were all way higher than the ones I replaced and the values were all almost doubled or 1.5 times more than what they
were listed as.

I also sprayed WD-40 into the all the pots and turned them about 25-40 times each .

So At this point I'll solder the board back in maybe with jumper wires so If I do need to hook up my scope and
a 1 khz signal ( my one LCR meter does that) I can look for the noise then.

I'll report back once I get it rigged up.

Nashou

Oh one thing i noticed and forgot to mention in previous post.


The two transistors are 2SC900's . I pulled one and tested it and saw the highest hFE i have ever seen on a transistor. is this normal for these?

Sounds much better !!! Still some back ground noise with no input and volume turned up. Might be those 2SC900's or maybe that other board is in play . I'll need to do some tracing of the
inputs . But the re cap helped a lot or it may have been the pots being cleaned and worked more than I did before .
I did not change any of the ceramic caps, I wonder if that might make any improvement, if I choose the right ones to
replace. I have some of the 101's ( 100 pf) in silver micas I think. and maybe some others in film.

Oh and the HUM is still there but maybe not as loud. 


Nashou

Gotcha , Well I have the lights coming for this and i decided to get some nicer coupling caps for the two outputs and the main power cap, the 2200uf 63 volt. I went up to 4700uf and 80 volts Nichicon LKG1K472MESCBK
and on the output caps I went to 50 volts and 3300 uf Nichicon LKG1H332MESABK . Might help a bit maybe?

Been playing for about 20 minutes and sounding better as it settles in I think.

Nashou

One thing I realized, now looking back, that may have been an issue was using my iPhone for testing. I can't remember if I had the volume all
the way up on the phone when I noticed the distortion. Because after the re cap I had the volume at mid point on my iPhone. I turned it up to full and i got distortion again.
So I think the input limit on this amp is not tolerant to high voltage input. I need to look up max output voltage on the iPhone.
And I wonder what the max input voltage is on the pre amp?

But I think I might change those two 2sc900 transistors that drive the LD 3120's .

because even with no input turning volume up I still get noise, just down about 80% than before the re cap.

What is a good replacement I think i read some where the KSC1845's?

Nashou

Just looking around and the Ic is too low with the KSA1845, here is the 2SC900 Data sheet the 1845 has 50mA Ic the c900 has 100mA , but the package is reversed so I'd just have to put it in backwards if I was going to use it.

the BC550 I think would work better but then id have to watch leg placement. These are said to be low noise. I started a thread on AK about what small transistors to keep in stock .

What else would be good? Looking on mouser now.

I want to place an order with a crap load of different transistors and more caps in one order so I'm not constantly wasting money on shipping. 

plus I hate waiting for parts, it be nice to have most common parts on hand if I'm going to be doing more vintage repairs.

nashou

Yeah, i saw that thats why i was looking for others, and here I think Id like to use a lower noise part. The way the through holes are laid out on the board adjusting the legs to their correct locations will not be hard.
Also the existing c900 have a insulator on one leg I can move over incase the legs get too close for comfort.

Nashou