TTSH NOISE MISTAKE? R2 missing

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KSS
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TTSH NOISE MISTAKE? R2 missing

Post by KSS » Tue Jun 16, 2020 12:57 am

While looking at noise circuits for another project, I noticed something again which I don't remember ever got addressed. I remembered amir hav ing mentioned some problem with the noise circuit and also that there was a kludge of another transistor to TTSH circuits.

The original ARP 2600 service manual has a mistake in the Schematic of the noise circuit. The TTSH V2 schematics also have this mistake. I didn't check V1, V3 or V4, and I'm posting a new topic here rather than trying to get this into several TTSH build threads. It may or may not have been fixed, and it may have been addressed in one or more of those 100 page plus build threads.

R2, 100K is missing from the schematic as can be seen in the attached drawing. It's clearly there on the PCB (marked with red arrows in the drawing) and I can confirm this drawing is correct for original 4022 submodules

R2 should go from pin 2 of the op amp to GND.
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Re: TTSH NOISE MISTAKE? R2 missing

Post by roglok » Tue Jun 16, 2020 3:05 am

thanks for this. it's missing on the v1 schematics, too. someone proposed to fix the issue by adding a 10K resistor between the emitter of Q1 and the 1uF cap rather than from pin 2 of Z1 to ground... that's what my notes say, at least.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by fuzzbass » Tue Jun 16, 2020 6:45 am

I would describe the problem with the noise circuit like this: it contains random frequencies, but has less than random gain values. It is clipped such that the output voltage spends extra time at either the positive or negative extremity of its range. This is easy to see on my old analog scope. It looks like two bright lines at around +10V and -10V, and a gray area between. I had in the past applied the previously suggested fix and found it only worked somewhat. Another suggestion was to use the low gain variant of a BC558 (IIRC). Again, I only found this improved things, but did not eliminate the problem.

On my next commission build, I am going to test this new information. Thank you KSS.
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Re: TTSH NOISE MISTAKE? R2 missing

Post by nickbtx » Tue Jun 16, 2020 6:49 am

KSS, thanks for putting this out there.
roglok wrote:
Tue Jun 16, 2020 3:05 am
thanks for this. it's missing on the v1 schematics, too. someone proposed to fix the issue by adding a 10K resistor between the emitter of Q1 and the 1uF cap rather than from pin 2 of Z1 to ground... that's what my notes say, at least.
Roglok, is your fix for v4?

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Re: TTSH NOISE MISTAKE? R2 missing

Post by roglok » Tue Jun 16, 2020 6:52 am

nickbtx wrote:
Tue Jun 16, 2020 6:49 am
KSS, thanks for putting this out there.
roglok wrote:
Tue Jun 16, 2020 3:05 am
thanks for this. it's missing on the v1 schematics, too. someone proposed to fix the issue by adding a 10K resistor between the emitter of Q1 and the 1uF cap rather than from pin 2 of Z1 to ground... that's what my notes say, at least.
Roglok, is your fix for v4?
It has been proposed for V1 and IIRC it's how I built mine. I have not looked at any subsequent revisions...

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Tue Jun 16, 2020 10:08 am

I just saw this post, I posted in the other thread also but there is an old post on here from "don t" that says the output is supposed to be 20vpp. He said that if it was the actual 10vpp as listed on the original schematics that it would not be strong enough to run the S&H circuit. Have you guys measured your outputs? I'd like to stop pulling my hair out if my 20vpp coming out of the jack is normal behavior. I noticed the fix you have listed here as well but assumed it was applied on the v3 schematic. I'll double check this. At 20vpp I am not seeing any clipping on my scope. I have tried both Patricks transistor fix as well as a random 2n5172 and no clipping with either but I am outputting 20vpp.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Tue Jun 16, 2020 2:30 pm

Easy enough to get a resistor on there. Going to get it on the scope in a second. Anything specific I should check for?
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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Tue Jun 16, 2020 2:50 pm

Initial tests, everything looks the same on the scope except now my 2n5172 is clipping. looks nice and flat at the edges. Still outputting same ~20vpp. I will try swapping the transistor later but I need to fix my oscillators before I can do valid tests. One thing I did notice was that my oscillators are now scanning the full frequency range. Before they were acting funny and would go up to about 5khz and then drop down as I increased the slider. They seem much more stable now but still outputting higher than normal vpp. But I don't want to confuse the issue here so Ill carry that discussion back in the original thread once I get my oscillator parts in.

Can anyone verify on a scope that their noise is outputting exactly 10vpp at the jack? I believe this to be false and should be around 20vpp.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by KSS » Tue Jun 16, 2020 3:30 pm

@amir: At the jack, it's 10V p-p on all color settings with level slider full up. Ck components and soldering round the A36 buffer OPA 'near' the jack. 20V p-p at the white end of the color slider, and at the entrance to the LF filter, 0.02 cap C90. Not sure what Don T was getting at as the noise is used as signal input to S/H, not for sampling.

Agree this needs to move back to your build thread.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Wed Jun 17, 2020 9:04 pm

.
Last edited by amir on Thu Jun 18, 2020 10:48 am, edited 1 time in total.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Wed Jun 17, 2020 10:21 pm

amir wrote:
Wed Jun 17, 2020 9:04 pm
KSS wrote:
Tue Jun 16, 2020 3:30 pm
@amir: At the jack, it's 10V p-p on all color settings with level slider full up. Ck components and soldering round the A36 buffer OPA 'near' the jack. 20V p-p at the white end of the color slider, and at the entrance to the LF filter, 0.02 cap C90. Not sure what Don T was getting at as the noise is used as signal input to S/H, not for sampling.

Agree this needs to move back to your build thread.
I’ve been checking all that stuff and have heated up every pad. I’ve ordered some more opamps and transistors to test. It’s definitely impacting my oscillators because I tried some lf411 opamps that I had and they started oscillating which changed the vpp and freq range of the oscillators for some reason.

I’ll update about my noise section once I get some parts in. The schematics obviously have issues in this section, if you notice the first opamp is even missing it’s compensation cap in the schematics. I wonder if the traces are missing also (haven’t looked). Has anyone compared all the ttsh schematics to the original arp manual to make sure other stuff isn’t missing? I assume someone has but I looked at them a lot myself and never noticed this resistor missing until recently as I was trying to figure out my output issues.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Sat Jul 25, 2020 10:33 pm

fuzzbass wrote:
Tue Jun 16, 2020 6:45 am
I would describe the problem with the noise circuit like this: it contains random frequencies, but has less than random gain values. It is clipped such that the output voltage spends extra time at either the positive or negative extremity of its range. This is easy to see on my old analog scope. It looks like two bright lines at around +10V and -10V, and a gray area between. I had in the past applied the previously suggested fix and found it only worked somewhat. Another suggestion was to use the low gain variant of a BC558 (IIRC). Again, I only found this improved things, but did not eliminate the problem.

On my next commission build, I am going to test this new information. Thank you KSS.
Fuzzbass,

I just re-read this and want to see if you can clarify something. Your post seems to say that your noise is outputting between -10v to +10v. Is this at the jack? Is that not 20vpp then? I get the same -10-10v. Also wanted to point out that with the resistor mod the values do move more. Before it was almost always at the peaks but now it’s more 33% peak values and randomness in between.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by KSS » Sun Jul 26, 2020 9:31 am

amir,
Can you take five readings or scope shots?
With Color and Level slidere full up, full down and centered. That's three.
Then Level full up, and color full down. Finally, Color full up and level full down. These are two more.

With the sliders centered, you have 50K contribution from each slider -effectively 100K- going into A31 with a 47K feedback resistor.
Roughly a gain of 0.5. 20V becomes 10V.

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Sun Jul 26, 2020 12:04 pm

KSS wrote:
Sun Jul 26, 2020 9:31 am
amir,
Can you take five readings or scope shots?
With Color and Level slidere full up, full down and centered. That's three.
Then Level full up, and color full down. Finally, Color full up and level full down. These are two more.

With the sliders centered, you have 50K contribution from each slider -effectively 100K- going into A31 with a 47K feedback resistor.
Roughly a gain of 0.5. 20V becomes 10V.
Attached. Sorry about camera shots. I can’t find the memory card for my scope to take screen shots. The sliders seem to be changing the values as they should.

edit: I forgot to mention, I am using the bc337 reversed transistor fix in addition to the resistor from ground to pin 2.
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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Sun Jul 26, 2020 12:20 pm

Forgotten 6th pic. Color all the way down, level up to show freq change.
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Re: TTSH NOISE MISTAKE? R2 missing

Post by fuzzbass » Mon Jul 27, 2020 5:04 pm

fuzzbass wrote:
Tue Jun 16, 2020 6:45 am
I would describe the problem with the noise circuit like this: it contains random frequencies, but has less than random gain values. It is clipped such that the output voltage spends extra time at either the positive or negative extremity of its range. This is easy to see on my old analog scope. It looks like two bright lines at around +10V and -10V, and a gray area between. I had in the past applied the previously suggested fix and found it only worked somewhat. Another suggestion was to use the low gain variant of a BC558 (IIRC). Again, I only found this improved things, but did not eliminate the problem.

On my next commission build, I am going to test this new information. Thank you KSS.
I tried this fix on the build currently on my bench. I did not notice any difference in the result. YMMV FWIW.
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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Tue Jul 28, 2020 3:42 pm

fuzzbass wrote:
Mon Jul 27, 2020 5:04 pm
fuzzbass wrote:
Tue Jun 16, 2020 6:45 am
I would describe the problem with the noise circuit like this: it contains random frequencies, but has less than random gain values. It is clipped such that the output voltage spends extra time at either the positive or negative extremity of its range. This is easy to see on my old analog scope. It looks like two bright lines at around +10V and -10V, and a gray area between. I had in the past applied the previously suggested fix and found it only worked somewhat. Another suggestion was to use the low gain variant of a BC558 (IIRC). Again, I only found this improved things, but did not eliminate the problem.

On my next commission build, I am going to test this new information. Thank you KSS.
I tried this fix on the build currently on my bench. I did not notice any difference in the result. YMMV FWIW.
Which transistor are you using and can you answer my question above? Where did you measure -10v to 10v as the outer limits on your scope?

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Re: TTSH NOISE MISTAKE? R2 missing

Post by amir » Tue Aug 04, 2020 4:39 pm

Since fuzzbass isn’t answering, I wanted to make clear that the noise output is supposed to be 10vp not 10vpp which are two different things. I confirmed this by testing the noise going into the S&H circuit. When both slider are raised and outputting 10vp / 20vpp the S&H correctly outputs between 5v and -5v but as I reduce the noise output to 10vpp or 5vp, the S&H gets cut in half also outputting between 2.5 and -2.5. Therefore, the noise jack should be outputting 20vpp on your scope or 10vp but not 10vpp.

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