Questions about Boss CS-1

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
gigelmargel
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Post by gigelmargel »

Hello all!

Before starting my story, here is the schematic of Boss CS-1 (there were 2 variants - mine is V2, with the red values):
Boss CS-1 variants.jpg
I've bought a Boss CS-1 from '80s at a very good price, but it didn't work: almost no compression and a big boost of signal. This looks very strange to me, because CS-1 is famous for its squishy sound (https://www.youtube.com/watch?v=ArBb0rTSwBM) and a huge volume drop (https://www.youtube.com/watch?v=wjYL8WA4vVg). After opening it, I saw that it has a couple of components damaged due to a battery drain, so I decided to replace them and to test other components. I found that the second optocoupler PH2 is dead, so I replaced it with a generic cheap Chinese VTL. The original optocouplers were HTV P873G35-380, which has a maximum on-resistance of 1K, a minimum off-resistance of 1M and a rise/fall time of 20ms (https://www.datasheetarchive.com/pdf/do ... P873G35380). My VTL seems to be closed in terms of on/off resistance, but I have no idea about its rise/fall time. After that, it worked almost OK: it compress the sound (but not so strong as in the videoclip aforementioned) and the volume drop were diminished.

Anyway, I didn't like it too much, so I decided to mod it to V1 specifications (the original values in the schematic posted above). Also, I replaced the first optocoupler PH1 with other cheap Chinese VTL, having similar specifications. Now, it acts stranger than before: it has a huge volume drop, almost the same insufficient compression and the sustain pot has only 2 states (when is at minimum the compression is completely off and after it sounds almost the same until maximum).

Because I don't want to damage it making other mods, I tried to understand the differences between V2 (red values) and V1 (original schematic) variants and to try to improve it a little. Because I don't have serious electronically knowledges, that's what I think:
1. the input cap was decreased from 100nF to 47nF, so the bass was lowered (I like it more with 47nF)
2. the biasing resistor of Q1 were increased from 220K to 470K, so the input impedance were increased (I have no idea if it's better or not)
3. the treble resistor (it has a normal/treble switch) were increased from 560 ohms to 1.2K (I like it more with 1.2K, because with 560 ohms it's too shrilling)
4. the output resistor were increased from 47K to 150K (I have no idea why, because I lowered it to 10K and it still has a huge volume drop)

My first question is which is the role of PH2 optocoupler (from input 2 of TA7136 IC to ground)??? Excepting PH2, it seems to be similar to EQD Warden/BJFE Pale Green Compressor or, even, DOD280. My guess is that will ground almost the all signal when its resistance goes to 0, leaving only some highs to pass (due to the 470nF cap), but I am almost sure that I'm wrong!

The main differences are in the envelope detector part (similar to Dynacomp and many other compressors), but my knowledges aren't sufficient to understand them:
1. the cap before Q3 were drastically lowered from 1uF to 47nF (maybe to keep only some mids and highs for envelope detection, but I have serious doubts about that)
2. the 220 ohms resistor from Q4/Q5 emitters to ground appears only in BJFE Pale Green (it's 110 ohms), otherwise it doesn't exists, so it's 0 (looking at other schematics, it seems to me that it controls the attack and it can be replaced with a 5-10K pot)
3. the cap between collectors of Q4/Q5 and ground was increased from 1uF to 4.7uF, so the attack time was increased (I'm almost sure of that...)
4. the 330K resistor between collectors of Q4/Q5 and ground was increased to 560K (maybe it increased the released time, but I have serious doubts about it)
5. the 4.7K resistor between LEDs of optocouplers PH1/PH2 to V+ was increased to 10K (maybe it decreased the maximum intensity of the LEDs, so it decreased the release time, but I have serious doubts about it)
6. the 5.1V Zenner after the optocouplers LEDS was totally eliminated (I have no idea why)
7. the resistors from Q4/Q5 bases are only 100K, while in many other schematics they are 1M (no idea why)
8. in many other schematics, there are 2 diodes from Q4/Q5 bases to ground (no idea why they are missing)
9. the 47nF capacitors between Q3 and Q4/Q5 seems to be normally, as in other schematics there are from 10nF to 100nF (no idea what are for...maybe a high pass filter with 100K resistors)

So, I need a patient person to clarify me these doubts and to tell me how I can improve it to have the squishy sound from the videoclip (I like it a lot...I'm not a fan of transparent compressors) and more volume. Maybe the secret is in the optocouplers? I read the VTLs and NSL32 specifications, but I didn't find anything similar in terms of rise/fall time.

Thank you a lot!!!

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Post by Manfred »

Please be patient, this is a lot of stuff to read and understand.

gigelmargel
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Post by gigelmargel »

I am very patient, no problem :)

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Post by bossman »

I think that the secret is really on the correct optocouplers

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Post by gigelmargel »

I think that the Chinese VTLs that I bought are really a big shit. I've finished an EQD Warden clone using another Chinese VTL and it sounds like crap, mainly it has a too big attack. I will in the next day to replace the VTL with a LDR+LED and to see what it will happen. I suspect that's the problem with The Warden because I built one sometime ago using same PCB with a LDR+LED combination and it sounded so good that a very good friend "stole" it :)

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Post by Manfred »

I am currently still researching the circuit and making circuit simulations with the detector circuit.
One thing I can say in advance, the current through the LEDs of the optocouplers differs significantly between the original circuit and the modified circuit.
The detector circuit has to be designed exactly for the used optocoupler, so that everything works as it should.
You do well to restore the original circuit and use the original optocoupler.
It is a big effort to adapt the detector circuit to an optocoupler.
I had already had such negative experience with components ordered from China because it was not what was printed on the package, since then I do not buy semiconductors there anymore.

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Post by gigelmargel »

Hello, Manfred!
I have no chance to use the original circuit, because one optocoupler (PH2) is completely dead (an infinite resistance of LDR all the time) and PH1 is almost dead (an intermittent infinite resistance of LDR). Both have the LED voltages around 1.7-1.8 volts.
Anyway, can you tell me which is the role of PH2? This is my greatest perplexity :)

Thank you very much!

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Post by Manfred »

PH2 together with the series resistor with a value of 100 kiloohm forms a voltage divider, i.e. an attenuator.
PH2 and PH1 form the amplification characteristic of the OP-Amp stage and together with the detector the compression behaviour.
To show the difference between a fixed and a variable resistor I have recorded the transmission curves and plotted them on a diagram.
More about the detector circuit soon.
gianstage versus PH2.jpg

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Post by gigelmargel »

Very interesting, Manfred! It's clear that I was totally wrong about PH2. I will try to understand much better what you said/plot :)

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Post by Manfred »

Here the diagram with the curves for the original detector circuit and the modified circuit.
The curve shows the LED current as a function of the peak input voltage @ 1kHz
Detector_ILed_versus_Uin_peak.jpg

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Post by gigelmargel »

Very interesting, Manfred!
The plot shows that the modded detector ensures a stronger compression?

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Post by Manfred »

I calculated from the resistance curve of the VTL5C3 from the datasheet, the obtained current values from the simulation, and the original circuit,
the gain curve as a function of the input peak voltage for amplifier potentiometer settings from 0% to 100% and shown it in the diagram below.
Could you measure the resistance of your LDR optocoupler in function of the LED current?

Gain versus input voltage.bmp
Gain versus input voltage.bmp (968.98 KiB) Viewed 341 times

gigelmargel
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Post by gigelmargel »

"Could you measure the resistance of your LDR optocoupler in function of the LED current?"
I have no ideea how to do that, Manfred :(
I know how to vary a voltage (using a pot in series with a battery), but I have no idea how to vary the current.
If you explain me, of course that I will do :)

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Post by The G »

One option is this (adapted from imajeenyus.com):

Change the resistors' values before using it.
Change the resistors' values before using it.

Read the article before building it.
According to my calculations, R1 = 22kΩ, R2 = 330Ω, and R3 = 100kΩ would give you current intensity between ∼0.06mA and ∼17.3mA. Of course, measure it before burning your optocoupler's LED :wink:
To extract the graphs, you'd need to have your meter in the circuit anyway.

I'd use a multi turn variable resistor for R3.

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Post by Manfred »

gigelmargel wrote:
28 May 2020, 08:07
"Could you measure the resistance of your LDR optocoupler in function of the LED current?"
I have no ideea how to do that, Manfred :(
I know how to vary a voltage (using a pot in series with a battery), but I have no idea how to vary the current.
If you explain me, of course that I will do :)
You can also use fixed resistors in series with a 9V battery.
Please measure the LDR resistance with an ohmmeter for the following series resistors in front of the LED:
33k, 22k, 15k, 10k, 6k8, 3k9, 3k3

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Post by gigelmargel »

Hello, Manfred!
I measured the optocouplers using fixed resistors as you said and the results are in this Excel file:
LDRs.xlsx
(9.79 KiB) Downloaded 5 times
.

After I have finished measuring the Chinese VTLs, I thought I'd try to measure the original HTVs optocouplers and I have many surprises:
During this operation, I have a lot of surprises:
1. both original HTVs seems to work!!!
2. the Chinese VTLs are far away from the HTVs
3. the HTVs are very different between them, even on both is written the same thing!!!

All the measurements that I did today where taken 3-5 times each, until I was sure that are OK!!!

Very intrigued, I have measured all 4 optocouplers in the dark (no voltage). The Chinese ones are very different (>2M first and around 500 K second). But the HTVs give me a serious headache: my cheap multimeter (with autocalibration) said that both have over 10 M (maximum it can measure). Even in the datasheet (see the link from my first post) said that must be minimum 1M, I didn't imagine that can be so big. Thus, I take my old multimeter (which have no autocalibration and it can measure up to 200M) and I tested it. It measured OK a resistance around 50M (I put 5 resistors of 10M each in series) and for the Chinese VTLs it give me the same results as the first multimeter. When I have tried to measure the HTVs, it said that both are over 200M (overflow)!!! I'm in a thick fog now: the HTVs are working or not?!!! What is your adivice: put them back in CS-1 or not? I decided that are not working due to the fact that I can't measure the off-resistance, but now I think that I was wrong :(

Anyway, the story goes on: I put back first Chinese VTL in CS-1 and I replaced PH2 only with a 5mm red LED (around 1.6 V) to see what happens. It worked relatively OK: it has a strong compression (not as strong as in the videoclip, but enough to hear it very clear) and no volume drop (to be more specific, I boosted the signal a lot...the unity gain was around 10-11 o'clock). The red LED flashes very dimly and for very short times (let say it has a fast attack and and a fast release, but only for very strong picking, even the humbuckers from my Ibanez have a big output), which confirms what I heard: it has a strong compression which is not very sensitive to the picking (the sustain pot has 3 different settings...over 12 o'clock it seems to sound the same)! Moreover, when I put it after a Boss DS-1 (bypassed), LED seems not to flash anymore (maybe the buffer of DS-1 is the cause). I built a couple of optical compressors and in all the LED lit very strong. For example, in EQD Warden I used a transparent LED at 2.5-2.8 V which lights up my while room :)

So, I am completely confused now. I am very curious about what you will say :)

Thank you very much!

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Post by Manfred »

That the Chinese optocouplers have different values is what I suspected, but I was wrong too.
I looked again at your first post on this topic and I noticed that the optocouplers are HTV P873G35-380 and not VTL5C2. I had a schematic somewhere on the internet where PH1 and PH2 were listed as VTL5X, but now I don't know what the original couplers really are.
Now I have to rebuild the gain curves using the HTVP873G35-380.
What surprises me is that both HTV P873A you measured have different values.
The question is whether both should have the same values or if they are specially selected for the application?

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Post by gigelmargel »

I measured both HTVs for 3 to 5 times, because I was very intrigued and I know that a optocoupler needs some time to accommodate with the light. So, i think that the measured values are OK. But, taking into account that it's a 40 years old stompbox, it's hard to tell if the HTVs was selected or are aged.
I've just searched on Internet and I found this HTV P873 datasheet: https://www.datasheets360.com/pdf/-3465715029550131844. Maybe it could be useful to you.
May I help you with something?

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Post by gigelmargel »

Today, I had some free time and I put back the HTVs. It sounds better than the Chinese VTLs, but still is not so strong as in the videoclips. Now, it has enough volume with the output resistor lowered from 150K to 10K, even with the same resistor and the Chinese VTLs it had a huge volume drop.
So, I think that the problem could be the fact that the LED from HTVs has not enough current/voltage to light strong. As I said before, yesterday I replaced PH2 with a 5 mm red LED and it lit up very anemic :)
To clarify, now my CS-1 has the values from the original schematic, but the HTVs are from the second version (the red ones).

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Post by Manfred »

Thanks, for the link.
I have found these CS-1 service notes and a picture of the circuit board which shows me that the P873 is the original photocoupler,
but the P873A is the type on the circuit board and the P873G35-380 in the parts list of the service manual.
I also found a useful picture with a table with measured values for the P873-G25-380, from which one can see that there are significant product variations.
But from this I can see that the values you measured are in the range you found there.
Since there is no data sheet for the P873A and there are these product variations, I assume that these were selected for Boss, since the other components are always the same and there are no adjustment possibilities.
Since there were very few pickups with high output levels in the period 1978 - 1982 when the CS-1 was sold, I assume that the pedal was designed for pickups with the conventional output levels and that the compression effect with higher output levels is more responsive.
Now that you know that both P873A are working, I would bring the pedal back to its original condition if I were you.
Here is another modification that has received positive feedback from other users:
https://ironman28.wordpress.com/2009/06/27/boss-cs-1/
boss_cs-1_compression_sustainer.pdf
(1.83 MiB) Downloaded 3 times
CS-1 Componets.jpg
P873-G35-380 Chart.jpg

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