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 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.
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://www.digchip.hk/stock-SAD512D.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!
