MXR Micro Chorus not working & new Hong Kong SAD512D chip

[kdude34567]

I realize there are a few threads on the MXR Micro Chorus that are a couple years old, and I've watched these threads on this site for a long time, so I posted comments on those threads, but also wanted to post a new thread, in the hopes that it will be seen quicker. As I'm against the wall on this. It always seems that great dialog, awesome tech info, & good people jump in on these posts to help out & support guys like us who have a ton of old pedals and do all their own tech work, mods, repairs, builds, etc. So my hope is the same will happen with my post

Anyway, I've got a 1978 MXR Micro Chorus I've had since 1978 when I bought as a teen rock guitarist, along with my MXR Phase 90 (1977), Phase 100 (1975), Micro Amp (1978), Flanger (1977), 6-band EQ (can't remember what year I picked it up), and Noise Gate (1978). The Chorus broke down years ago when the the SAD512D BBD chip failed. Been on the hunt for a new chip for many years, but alas, they're either totally unavailable, or some scheister is asking $50-$100 for one, and 9X's out of ten, what you receive doesn't work anyway, because either A) the guy's a scheister, or B) in the process of removing and/or packaging, the guy fried the chip.

So I had resigned myself to convert the pedal to the MN3007 version posted elsewhere on this site, a very good topic/post, great info & a lot of details/dialog, that I've been following for years now. If fact, the schematic and other info from that thread were the backbone of support for this latest repair effort for this pedal. Just couldn't bring myself to modify the old girl from the stock circuit w/o hopes of a SAD512D falling out of the sky someday to solve my problem, so I could resurrect the old girl.

http://www.wizardinside.it/foto/schemi/micro_c-sch.pdf

So I've went down the path along these lines and ended up with my current problem:

1) I found a source in Hong Kong claiming to be making & selling brand new made SAD512D & bought (3) @ $6 each to see if I could resurrect the chorus. A company called DigChip, who sells a ton of all kinds of ICs & related, claiming ED*GE is the company manufacturing these new SAD512D chips. The problem is these are not real Reticon chips & I have no idea is these guys are making a true quality knock off, or simply a weak facsimile. Has anybody else heard of this or tried these chips? Are they any good? Because I'm not having great results with them, and I need a sanity check to see if its just me & my repair effort failing, or is it these dang chips!

DIgChip.hk SAD512D Product Page: http:/ ... D512D.html

NOTE: Keep in mind the last time I was under the hood looking at the circuit & components on the pedal was probably 20 years or so ago, I don't remember if other components were bad besides the known bad SAD chip, when I started this new effort. Needless to say, I've been deep under the hood on it now for a couple of days straight, starting at ground zero going through everything, & just cannot get the dang pedal to work completely. It's in a weird 'almost working' state, but not really. That said, I'll break down where I'm at now & why I need some assistance, or at least a sanity check & someone to bounce things off of.

2) I've gone though the entire circuit. I replaced the old failed Reticon SAD512D w/the new DigChip SAD512D, the TL061 & TL062 op-amps w/new ones, as well as all (4) ZTX109 transistors with 2N3904, along with a bunch of caps, mostly in the output section post SAD chip, as well as changed the (2) 10uf Tantalum caps to quality electrolytics. But still can't get the pedal working really. Its basically passing through clean signal with a very slight bit from the effect, that's has a bit of LFO noise and possibly a very very VERY faint chorus, but I don't think so (Its wishful thinking I think)! Its driving me nuts & I've hit a wall...

NOTE: I'm using an old BK o-scope, hand made audio probe, & decent quality DVMMs (Fluke 87 & Protech 506), plus have all the tools of the trade, a nice solder station w/fine tip, hand tools, PCB supplies, solvents, flux, cleaners, compressed air, etc. A fairly decent stash of components, connectors, hardware, etc (except a dang real Reticon SAD512D chip!).

3) Here's the troubleshooting breakdown:

*The clean signal is fine & being passed thru all the way thru to pin 7 (output of U1b) of the TL062, so I know the chip is working, especially U1a, though I have no signal I can see on the scope or hear with the audio probe on pin 6 (input of U1b) of the TL062, yet the clean signal is strong at pin 7, as well as at the output jack, so it must be making it through the resistors/capacitors above U1b on the schematic (R11, R15, C6, & C8), because I don't see any input at pin 6, which would be needed to be able to output on pin 7, whether from the clean signal coming thru R16 (270k) or the effect signal coming thru R17 (240k). Plus I replaced the TL062 w/a new one with same result. So I'm wondering why pin 6 has nothing on it. Clearly the input signal is present on the other side of the 270k resistor, so why nothing on pin 6. Now the SAD512D output is a more convoluted story. I'll get to that in detail in the next bullet point...

*Basically, there appears to be a true summed BBD output, where I can see on the scope, what appears to be a modulated signal riding on top of a bunch of clock pulses, where the odd/even output are summed together, and there's no dropout between clock pulses, but its very weak at only .4-.5V, and appears to die off by the time it reaches the base of Q2. Using the audio probe, I confirmed exactly that. I can hear a weak modulated output w/clock artifacts when probing pins 3 & 4 of the SAD chip, nut don't hear anything coming out of Q2. So what could be killing the signal at pin 6 of the TL062? And what could cause the new SAD512D summed output (pins 3 & 4) to be so weak and at such a low voltage?

I changed most of the caps post SAD chip to eliminate a bad cap taking out the effect signal coming out of the SAD chip, but to no avail. Same result after all those caps were replaced w/new ones. I also tried 2 of the 3 new SAD512D chips from DigChip. Same exact results with both. I haven't replaced the resistors yet post SAD chip, though unless destroyed, burnt, shorted, or opened, resistors aren't usually the culprit in this type of scenario. The pedal is running off of 9VDC battery, so no AC earth ground involved, and I can't find a short anywhere in that part of the circuit. So is there something else I can look for that would cause this, or is my fear confirmed, these DigChip.hk SAD512D chips are just no good, and have a really low quality BBD output w/a very low level. If not the circuit itself cause the issue, could it be something with the clock signal causing this? Which leads me to my next point...

*The TL061 & clock/LFO circuit appear to be working, though the actual clock arriving at pin 1 of the SAD chip looks pretty dirty to me w/a lot of slope & rounding in the square wave. The output on pin 6 of the TL061 however, is very strong and swings from 0V to 9V (Battery V+, battery voltage was actually around 8.3V - 8.6V throughout the effort ), around every 50msec, or about 20Hz, and the square wave is a lot cleaner & has very little slope or rounding of the pulses. The clock signal after going through Q3 & Q4 w/related RC networks, at pin 1 of the SAD chip, is also swinging between 0V & 9V, about every 14usec, or 71KHz.

Frequency wise, that's probably about normal since the clock has to be 3-4 times that of the input signal for the SAD chip do its BBD thing. TR1 & RV1 both seem to be working and behave roughly as expected. RV1 changes the speed of things, which I can hear with the audio probe and see with the scope, and TR1 changes the bias voltage as expected, which I can also hear & see in the weak output of the SAD chip and read with my DVMM. The problem is the bias trimmer never really finds a point (voltage), where the SAD chip kick starts the modulation process into full motion, and the SAD512D weak output never makes it the pedal output. Could it be the clock? I read somewhere that the clock is not supposed to go below 5V, and it definitely is in my case. What could cause that? Also, the clock doesn't look like a clean square wave, its kind of a blend between a saw-tooth and square wave, where the valleys between the pulses drop below 5V.

VOLTAGES (DC) as read by DVMM:

V+ Battery: ~8.3V - 8.6V during testing
Vbias: 3.2V
Vbias range: 2.2V - 4.4V
Vdx: 7.9V

SAD512D
pin 1: 4.2V - 8.2V, 5.4V - 7.2V clock swings (meter cycled between these ranges)
pin 2: 0V
pin 3: .4-.5V
pin 4: .4-.5V
pin 5: 8.3V
pin 6: 1V
pin 7: 0V
pin 8: 8.3V

TL062
pin 1: 3.3V
pin 2: 3.3V
pin 3: 2.9V
pin 4: 0V
pin 5: 3.2V
pin 6: 3.3V
pin 7: 3.3V
pin 8: 8.2V

TL061
pin 1: n/a
pin 2: 2.6V - 8.2V swings
pin 3: 2.2V to 8.2V swings
pin 4: 0V
pin 5: n/a
pin 6: 0V - 8V clock swings
pin 7: 7.9V (Vdx output from R25)
pin 8: NC

Q1
E: 1.2V
B: 1.7V
C: 8.2V

Q2
E: 25-45mV
B: .4-.5V
C: 8.2V

Q3
E: 0V
B: .4V
C: 4-8V clock swings

Q4
E: 0V
B: .4V
C: 4-8V clock swings

TR1
(+): 4V
wiper: 3.2V
(-): 2V

RV1
grey: 2.2V - 5.5V swings
orange: 1.29V - 6.6V swings


So ideas or suggestions on what could be causing these issues?

1) No signal at PIN 6 of the TL62 (0V, flat line on scope)
2) Weak SAD512D output on pins 3 & 4 ( .4-.5V, can barely hear it w/audio probe)
3) Dirty clock signal at pin 1 of SAD512D with alot of slope & rounding, with valleys between pulses below 5V
4) And ultimately, no chorus effect coming, only dry clean signal w/minor clock artifacts coming through.

Are these Hong Kong SAD512D bogus? Or are there things in the circuit that could be the problem?

I have enough components to replace every component in the pedal, except a real Reticon SAD512D, but before I do, I wanted a sanity check on what I'm seeing.

Any help would be greatly appreciated!

[Dirk_Hendrik]

Are these Hong Kong SAD512D bogus?

This is where I'd bet my money. A "remake" suddenly shows up, only here, from an unknown manufacturer? Bullocks. You got ripped off. Sorry to hear.

[kdude34567]

That's what I've been thinking, however DigChip is pretty big & been around awhile & sells all kind of ICs all of the world.

Plus the fact that I AM GETTING some kind of BBD output from their 'new SAD512D' chip, just very weak, leads me to believe there's more to the story...???

[kdude34567]

Can you confirm anything about my circuit observations or voltage read outs that would give me more insight to the matter?

[Dirk_Hendrik]

I am unfair.
Usually one needs to ask questions to gain more information. In this case it's the other way around. You provide so much info it's overwhelming. That means that it's me who is to blame for not reading closely enough.

You say you see a signal "riding" the BBD's output on your scope? (You have a scope! That's a nice change! ). Usually seeing a weak signal at that point means a misbiased BBD. Can you change the BBD output signal level by altering the bias voltage?

[kdude34567]

Thanks for the follow up.

When changing the bias voltage via TR1, there is marginal change on the BBD output. I can dial it in to get a tad stronger output, meaning the signal riding on top, gets a little clearer, and it appears the modulation is a little better, but overall its still very weak & doesn't make it past Q2. It's still very low at the base of Q2, less than .4V - .5V, and is in the low mV range at the output of Q2. So it would seem that a significant circuit change would have to happen between the SAD output on pins 3 & 4 and the base of Q2. Something that would boost the signal and make the circuit more sensitive.

But is that possible? And what changes would be needed? And since the signal is so weak, even if I got the signal boosted, it would probably be very noisy & thin, since the signal isn't that far off the noise floor.

[Fender3D]

.4 ~.5V @ BBD's output means there's no output mosfets on chip...
You should get ~1/2 Vcc there, you may try bypassing one or more diodes (1,2,3) and check if you get a better bias point, (it might be a BBD afterall...)

Otherwise you might look for Reticons R5106s
They're basically the same BBD as SAD512D, except they work with a different bias voltage.

As a side note, these BBDs don't need a "perfect" square wave clock since they have a D-type flip-flop inside the chip providing the required dual phase clock

You may also lift R16 on my schematic to easily check delayed signal

[kdude34567]

Thanks. Very helpful info! I'll give it a try!

So technically, I should see about 4.5V on the BBD output lines?

Btw, out of curiosity, whats the purpose of the 3 diodes in series? And what am I accomplishing by removing 1 or more of them from the circuit?

So it sounds like I shouldn't worry about the clock signal, since its working, but just looks a little dirty on the scope...

By lifting R16 I would essentially be disconnecting the clean signal so there is only the wet effect signal present, correct? Thats a great idea

You wouldn't happen to know a good economical source for the R5106s chip with a quick turnaround? I take it that the R5106s is 1-1 compatible pinout as the SAD512D, and only the bias voltage is different....?

[kdude34567]

By the way, thank you very much Fender3D! Not only for your reply, but it was your schematic of the original SAD512D based Micro Chorus that has been my guide throughout my repair effort & circuit/component research. I can't tell you how much it has helped me!

[kdude34567]

I just finished trying a few of your suggestions & this is what I observed:

1) Lifting R16 was a great idea. I could scope & listen to just the BBD output, though its super weak.

2) Bypassing 1 or 2 of the diodes didn't do enough to effect the bias or range in any big way. Just small increments. Little effect on the BBD.

3) What did have an effect & increased the bias voltage range significantly was changing the bias circuit resistors, R27 from 510K to 100K & R36 from 270K to 100K. This gave TR1 a bunch more range, where I could sweep the bias from nearly 8V, down to around 2V, allowing me to find a bias voltage that had the most effect & improvement on the BBD output, which turned out to be around 5.5V. Whereas before max bias was around 4V. At 5.5V I got the strongest & best looking signal out of this weird Honk Kong SAD512D chip. At over 6V & approaching 7V, the BBD started to overdrive, and the delayed/modulated sine wave, started to look like broken uneven cloud shapes all in the positive direction. So I stuck with 5.5V for bias.

4) With R16 lifted & biased improved, I could now hear the BBD output on the output jack for the first time, whereas before the BBD output never made it thru Q2 & you couldn't hear it at all at the output. You could only hear it with an audio problem placed directly on the BBD output on pins 3 & 4 up to the base of Q2. Problem is, even with these adjustments, it just isn't enough & it is really faint, and completely drowned out by the clean dry signal when R16 is put back in the circuit. So the odd thing is the HK version SAD does appear to be performing some level of BBD function, but it has no drive, no balls...alas, a very thin & faint output. Is there any way for me to easily boost this output, without seriously modifying this 1978 MXR pedal? Something easy & benign?

5) The BBD signal seems to be having trouble at Q2. At the Base, I got the BBD output to get as high as .6V, Collector was of course at V+, but the Emitter was nearly nothing, with barely any signal getting thru, with a voltage only around 35mV, which is significantly less than what's coming in at the Base.

Question:

Isn't the Emitter supposed to be the amplified/boosted/driven BBD output being pushed thru the output section into U1B for final drive to the output jack? There is no amplification happening at Q2, if fact the signal is coming out less than input. Why wouldn't Q2 be amplifying the BBD output? Which has been one of the issues all along, even though I've replaced Q2 2X's just to make sure it wasn't the transistor. Each 2N3904 I put in behaves the exact same. Why isn't Q2 driving the BBD output, even if it is only .6V? Any kind of amplification would only improve matters significantly. How can I make Q2 drive the BBD signal? Or said another way, why isn't Q2 doing its job?

Below are some voltage readings that may shed some light.

DC VOLTAGES MEASURED w/black lead on DC common ground.

Q2 - 2N3904
B: .6V
E: .035V
C: 9.5V (new battery)

C9 .047uf/47nf (tried replacing the stock .05uf/50nf cap, made no difference)
Q2 side: .035V
U1b side: 5.5V

U1b - TL062
pin 5 (bias): 5.5V
pin 6 (IN): 5.5V (flat line, no audio signal) <--- I would expect to see the audio signal I see on the Q2 side of C9 here, but I don't...?
pin 7 (OUT): 5.5V (with audio signal, dry signal)

Seems like I've got some kind of trouble happening between Q2 Emitter & pin 6 of U1b, that killing the little BBD output I do have coming out of this Hong Kong SAD knock off chip... Any ideas on that?

Boils down to:

1) Can I boost the BBD output somehow?
2) Why isn't Q2 amplifying the BBD output?

[Fender3D]

Btw, out of curiosity, whats the purpose of the 3 diodes in series? And what am I accomplishing by removing 1 or more of them from the circuit?

I'm not really sure if they're there for a thermic reason whatsoever, surely they provide correct bias voltage drop @ BBD, bypassing 1 of them will allow 0.6V more @ bias point...

You wouldn't happen to know a good economical source for the R5106s chip with a quick turnaround? I take it that the R5106s is 1-1 compatible pinout as the SAD512D, and only the bias voltage is different....?

... a forumite here promised me one, once...

5) The BBD signal seems to be having trouble at Q2. At the Base, I got the BBD output to get as high as .6V, Collector was of course at V+, but the Emitter was nearly nothing, with barely any signal getting thru, with a voltage only around 35mV, which is significantly less than what's coming in at the Base.

1) Can I boost the BBD output somehow?
2) Why isn't Q2 amplifying the BBD output?

I'd say Q2 has trouble with BBD, since he's not receiving a proper bias voltage, you may try with a 47K to +Vcc connected to BBD's outputs.

Sadly I can't give you a proper support without knowing what is the chip you got... any datasheet?
Supposedly it's a BBD since is delaying signal (isn't it?)
If it is a R5106 relabeled, it may be worth trying lifting BBD's pin 5, since on Rxxx chips that's Vbb OUT

[kdude34567]

Thanks again Fender3D!

Tried the 47K to +Vcc connected to BBD pins 3/4. Slight increase in voltage of the BBD outputs by 50-75mV only, and no effect on Q2 output really. Q2 still isn't amplifying the BBD output. At pin 6 of U1b TL062, its still flat-line. Though a tiny bit of residual BBD output is making thru to the output jack, but its very very low level & faint. Just not enough to be useful. All testing was done w/R16 lifted so only the BBD output could be observed thruout the circuit.

Tried lifting pin 5 of the BBD. No effect. And no signal/voltage is present on that pin. Doesn't appear to be a Vbb out.

I've been hounding DigChip Hong Kong for a datasheet on this poor replica of a Reticon SAD512D BBD chip. They keep saying follow the original Reticon datasheet, which is complete BS. I'm trying to get them to at least provide the specs or datasheet on whatever this chip really is, so I have some chance in hell to get it to work, because it is outputting some kind of BBD signal on pins 3 & 4, which is saying alot, because if it wasn't even a BBD chip at all or pinned out like a real SAD512D, it wouldn't even fire up or output anything close to what's expected, or even support an audio signal. And it is fire up, receiving the input on pin 6, and is outputting what appears to a BBD signal on pins 3 & 4. At least as far as I can tell on the scope. I mean I see an audio signal riding on top of a sea of clock signals at the output on pins 3 & 4, where odd & even outputs appear to be overlapped across the duty cycle, to make a complete signal, just as the original SAD512D does, and that signal changes modulation speed, as you vary the rate/speed knob on the pedal. Its just that the BBD output has no drive and/or doesn't match the circuit bias needs for Q2 to drive the BBD output forward to the rest of the circuit. And if I put the original failed Reticon SAD512D chip, there is zero output on pins 3 & 4. This HK SAD is at least outputting an audio signal looking very much like what would be expected. So that's saying alot.

So that is too damn close to be simply a random bogus chip or rip-off by this company in Hong Kong, who is actually a fairly large scale chip supplier. It appears to me that someone over there recognized the market demand & need for an SAD512D/R5106 replacement, tried to design & manufacture it, but missed the mark on putting BBD output drive within the chip & don't have the bias needs addressed properly, but are selling it anyways.

Yes, that's BS & ultimately a rip off or false advertising, but it tells me there should be a way to make the BBD chip work and play ball with the rest of the circuit. Meaning there is yet hope for this chip...

But without a real datasheet or true spec on this HK knockoff SAD512D, its just guess work, which you have been most gracious in helping me with.

*Any other ideas on how I could amplify the BBD output or get Q2 to start amplifying?

*Any other ideas on finding out what the bias needs are for pins 3 & 4 of the BBD, to get it to play ball w/Q2?

*Is there another transistor I could try for Q2? I have a decent inventory of NPN, PNP, Darlingtons, J-fets, Mosets, switchers, Triac, Diac, regulators, bridges, with most of these, I have hi & lo voltage, current, & frequency versions, plus I'm near a Fry's, RadioShack, Marvac, Orvac, and similar that sell electronic components to some degree or another.

*Theoretically, what would need to happen to get Q2 to amplify, given these voltages? Could I change the reference on the Collector by adding a resistor in line to +Vcc? Would that help?

Q2
B: .4V -.6V, nominal .5V
E: 3.7 - 6V, nominal 5.5V
C: 8.5V - 9.5V, +Vcc nominal 9V

I'll post photos & videos of the signals as seen on my scope shortly, in case it helps shed some light.

[Scruffie]

Put a 47nF cap in series before R14/after R18 and then add a 100k resistor from the connection between R14 & the new cap to Vbias. That way the transistor will no longer rely on the BBD output voltage to bias it.

[kdude34567]

Thanks Scruffie! I'll give those mods a try & report back! Much appreciated!

To all on this thread --> Here are video clips of the signals observed during troubleshooting & testing:

1) Signal @ input jack. 0VDC bias. 1VAC peak-peak audio signal @ 1KHz source from a tone generator. R16 lifted.
https://youtu.be/sOSLxmZz7S4?list=PLH9_ ... io48GQ6QgJ

2) HK SAD512D BBD chip output signal on pins 3 & 4 @ .4-.5VDC bias & .1-2VAC peak to peak, delayed & modulated audio signal. Input source audio from tone generator at 1KHz, 1VAC peak-peak. R16 lifted.
[/url][/url]

3) BBD output signal @ C9/Q2 side @ Q2 collector output, .036VDC bias, .045VAC delayed & modulated audio signal @ 1KHz. R16 lifted. Note DC bias voltage is extremely low @ .036V, as well as the AC audio signal @ only .045V. Q2 is not amplifying & its output is less than its input.
[/url][/url]

4) BBD output @ C9/U1b side,@ Q2 collector, but before R17. 5.26VDC bias .0025VAC delayed & modulated audio signal @ 1KHz (almost completely flat lined, meaning Q2 is not amplifying). R16 lifted.
[/url][/url]

5) BBD output @ pin 6 of U1b TL062 after R17. 5.26VDC bias .0025VAC delayed & modulated audio signal @ 1KHz. R16 lifted.
[/url][/url]

6) BBD output signal @ output jack. 0VDC bias .045VAC delayed & modulated audio tone signal only @ 1KHz w/R16 lifted.
[/url][/url]

Play all 6 video clips in a playlist in order:

[youtube]]

[kdude34567]

Photo of the pseudo fake SAD512D BBD chip made in Hong Kong.

In the pic is 1 real Reticon SAD512D chip (failed) & 3 of the Hong Kong knock-off version.

Note the chip labeling is nearly identical to the real Reticon chip.

[kdude34567]

I tried your suggestion Scruffie. It improved things noticeably! Decoupling the BBD bias voltage from Q2 & inserting bias through the 100k resistor on the other side of the cap was a great idea!

It finally allowed Q2 to operate a bit & let the BBD output through, rather than hold it back as before. Now I can see the full BBD output on the emitter. Though Q2 doesn't appear to be amplifying really. The BBD output is only .1-.2VAC to begin with, which is going into the base of Q2. The BBD signal coming out the emitter still roughly .1-.2VAC, and not really being amplified. However, the improvement of have the BBD signal come thru Q2 in full form is a big improvement, because before, there was only a few mV of BBD at the emitter. Now the full signal is getting through and is approaching a couple hundred mV & a clean looking signal.

Bias voltages after mod:

BBD side of cap: .485V
R14 side of cap: 4.85V

Audio signal (BBD output only) after mod:

Q2 base: .1-.2VAC
Q2 emitter: .1-.2VAC
@ output jack: .1-.2VAC

The good news is there's enough BBD signal coming through now, that for the first time, I can hear & see some actual chorusing happening, when I put R16 back into the circuit. So this Hong Kong knockoff SAD512D chip is actually indeed a BBD chip, which is surprising.

The bad news is the BBD signal is still too weak & the chorus effect is simply not strong enough to be useful. It would need to be at least doubled, tripled, or more to be a usable chorus.

I did get a little more BBD signal by changing R20 from 47K to 100K. But even with that, the BBD signal is still too weak, and the chorus effect is just not strong enough.

Any thoughts on how i can boost the BBD signal 3X-5X's? Or get Q2 to amplify the BBD signal more?

And something that's been bothering me throughout this effort, is why pin 6 on U1b TL062 had nearly zero signal on it. It's DC bias is around 5VDC, but the AC audio signal is nearly zero. The BBD signal appears to be getting to the output jack thru R11, C6, C8 network above U1b. Any thoughts on why this is?

[Fender3D]

Good news then k...

maybe BBD doesn't push enough current...
you may try with a current mirror (check SAD's datasheet, last page)
(of course you'll need the extra cap and resistor Scruffie suggested)
Q2 is an emitter follower, it won't amplify...

[kdude34567]

Another great idea Fender3D!

I am somewhat familiar with Current Mirrors, but certainly not an expert. Usually it means cascading 2 identical transistors, where one of them has the base-collector tied together on 1 transistor, which is cascaded which a 2nd identical transistor. Sometimes requiring that the transistors be matched. There are also Current Mirrors using extra resistors & a diode on the transistor base, as well as cascaded MOSFETs arranged like I mention above. So I'm not sure which type or the specific arrangement you're inferring, but looking at the last page of the Reticon SAD512D datasheet, figure 10 shows a 'Summing Amplifier', which looks to me to be a Current Mirror.

So I'm guessing a bit here, but take a look at the attached photo of my sketched drawing adding a current mirror transistor to Q2 in the MXR MC circuit. Is this what you mean? Have I got the circuit right? Or am I missing something?

Please forgive any mistakes, as I'm not a component level design engineer, though I have an EE, by trade i design systems at the application level, not down to the component level. So while I am familiar with the theories, physics, and math, I don't practice component level design every day. So it's a stretch at times

Being a career guitar player for around 35 years, coming up in the early 80s in the Hollywood strip with all the 80s big hair bands, many of which are still good friends of mine, playing in the band & recording an album w/Black Sheep in 84-85 with Paul Gilbert, touring & MTV video w/Michael Bolton in '85 when he was a rock guy (me replacing Bruce Kulick who went to KISS), playing in bands with original drummers from Ratt & Quiet Riot, as well as several other bands & albums recorded in my own projects, I've always worked on, repaired, modded, upgraded, restored, & serviced all my own gear, and sometimes for others, as my musician friends & family think I'm a mad scientist, who can figure out & repair anything, from iPhones, to Marshall stacks, to TV/stereos, to computers/laptops, to XBox/PlayStations, to cars/motorcycles, to appliances/machinery, to building/home electrical.

That said, it doesn't make me a component level design engineer by any mean. Just an engineer who's been a musician his entire adult life, and been under the hood with this stuff a lot more than most musicians over the years. And am a tech geek/tinkerer/would-be inventor/half-baked wanna be scientist/physicist.

So please forgive my ignorance

[kdude34567]

Note: the emitter connections should be shown coming off the transistor emitter arrows, not the transistor cases

[Fender3D]

I don't know if CM will work properly with that R-C net...
but you may use it instead of R18 and the R (3K3?) to Vcc should give you a bias for Q2.

As a side note why your scope is upside down?