e-licktronic Forum

So I'm having a hell of a time getting tm4 in noise section to 130mv. I'm trying the noise rev in the service manual but have a few questions.
I've put a jumper in r 129
I've changed r131 to 100k
These 2 things are pretty straight forward however I'm not clear on the last 2 things that need to be done.
I have a 10uf electrolytic cap to put in R127 but am unsure which way to put put it given there is positive and negative leads.
I'm am not sure where to put the 22p cap. It says to put it in C202 but I can't find it anywhere on the board. It also says to take it from 0---->22p. I don't understand the reference to 0. Very confusing.
I've searched the forums and haven't been able to find a clear answer. I found one user who said it goes parallel with a resistor and even showed a picture but made no mention of where.

Perhaps do they mean put the 22p cap in parallel with R202?

Yes the cap is parallel, look at the schematics in the service manual. I soldered it on the back of the board. See my post here: viewtopic.php?f=16&t=155&start=70#p2757
The other cap i don't remember, but i think it should be negative to ground.
Maybe this helps:

Thanks. I found that post with the schematic and figured it out. It did the trick in allowing me the range to get to 130mv. But now I have another issue. I'm no longer hearing the low Tom /Congo or hi tom but can hear the conga in this section. The only thing I hear is the noise...nothing else. I'm also noticing the caps in the power section above the toms are getting very hot and generating considerable heat. I've checked and can't find anything that looks unuasual. Any ideas how to proceed from here?

I have successfully built my 808.

However, no matter what I try I can't get the noise section output down to 130mV.
I've tried the original (Yocto) circuit with R127 of 4K7 and ended up with a minimum of 420mV noise. (Measured on Fluke 87V and 10M 2445B Tektronix scope)

I've tried increasing R127 to 47K (and even higher) but still had 290mV noise and it didn't look too nice on the scope (DC offset).

I've also tried building the revised noise circuit as per the service manual (10uF instead of R127, 0R for R129 and 100K for R131, 22pF in parallel with R130). That gives beautiful white noise up to 6V (!) rms. But no matter what I do I get 500mV of white noise as a minimum. You can't reduce gain any further as it's a non-inverting amp so gain = (1+rf/rg)

I've also tried decreasing R131 to 1K which brings down the noise but it's then lopsided (negative excursions > positive ones).

I've increased R128 to 10M to try to increase the input impedance but that didn't work either.

I'm starting to think I've got an exceptionally noisy 2Sc828 but I don't have a spare for that.

Measuring direct on the emitter of Q35 (10mOhm scope) shows 10V DC offset and 300mV raw noise even here (before being subject to any gain).

Suggestions?

MeToo wrote:
I'm starting to think I've got an exceptionally noisy 2Sc828 but I don't have a spare for that.

Measuring direct on the emitter of Q35 (10mOhm scope) shows 10V DC offset and 300mV raw noise even here (before being subject to any gain).

Suggestions?

So I swapped out the 2sc828 for another small signal NPN after testing with 1Mohm to +15V to the emitter and base connected to 0V to measure the basic noise voltage out of circuit.

My 2sc828 was producing a DC offset of 10V and 300mV of noise.

The replacement (2sc945) had a DC offset of 8.4V and 50mV of noise.

So I can confirm the original problem was a "too noisy" 2sc828 supplied with the kit.

Now I can trim the total noise output to 130mV with ease.

The revised noise circuit from the service manual works really well and I can recommend it: less DC offset and no +/- bias on the noise.

Hello, I am having the exact same problem!

I did the service manual mods too, still got 300mV. I have the exact same problem!
I'm going to order some more 2sc828 and 2sc945 and do some tests, I believe these transistors are supposed to be selected for the hFe value.

How are you measuring the noise level? Keep in mind that it's in RMS Volts, not peak to peak volts

After trying at least 3 different 2SC945 transistors I was able to turn in the 130mV range. I am measuring with a Fluke 175 true rms multimeter and the DPSCOPE II about 40mV of difference between the two! I trusted in the Fluke for now.

hmm, I'm not sure how well a meter will work for measuring noise. You'll know soon enough, everything will sound wrong if that is off