Uh-oh newbie and SMR4 MK2 collision

I’ve begun building the kit I bought for the Shruthi with SMR4 MK2. I don’t think my soldering is hideous and I’m careful to make sure that I don’t overheat components. I followed all instructions on the site, I soldered the 7805 and 7905 into the correct places and the diode and caps are all in the correct polarity and in the right places. I also added the single C1 capacitor at the right polarity and a wire to cross the two ‘switch’ solder points, exactly the way it is written on the website. The IC is also in its holder in the correct way. Unfortunately when I inserted a DC adapter set to 9v (which says neither 'regulated or ‘unregulated’ on it so I assumed it was unregulated) and checked with my multimeter the results were not promising:

C5 = 10.92
C14 = -4.98
C7 = 10.92
C16 = -4.98
C8 = 3.6 the first time I checked and 18.7 the 2nd time (it seems to be continuously rising in voltage at a slow rate which worried me)
C17 = 30.2
C9 = 0
C18 = 10.98
C24 = 10.98
C31 = -4.98
C26 = -4.98
C33 = 11.01
C28 = 11.02
C35 = -4.98

I also checked the two voltage regulators:


  • = 0
    1 = -11.21
    0 = -4.98

1 = 11.59

  • = 10.98
    0 = 10.82

I’m completely clueless about what has gone wrong so any input would be gratefully received. I’m not going to continue with the build until I’ve sorted this. I’ve put two photos below of my build so far.

Thanks in advance for advice

I have never observed that with the regulators that come with the kit, but the incorrect voltage might be due to the lack of load on the rails. Try temporarily putting a 10k resistor between the GND and +5V pins and check the measurements again.

I am a complete noob also, however when I got lots of inaccurate readings I found that things changed after I switched the polarity of my power supply… This wasn’t the only thing I had wrong but it definitely had an effect. Is your power supply definitely centrepin-positive?

The last 20 or so Shruthis i built were center positive…

what i mean is that I was getting strange readings because I had centre negative. started getting more sensible readings when I switched to centre positive

Due to the protective Diode you shouldn’t had any readings when reversing polarity. Are you shure your Wallwart is running on DC (direct current, Gleichstrom, the one Thomas Alva Edison preferred…)? If its a really cheap Wallwart the current emitted isn’t rectified sufficient and may have big bad ugly ripples - indicated by the lower dotted line on the = symbol. Id suggest you try another PSU.

thanks for this info! my unit is actually working great now - but maybe I’ll get a new PSU soon.
I feel like I’ve taken the attention away from the thread so I’m going to shut up now until I can come up with something beneficial to AdmiralBinkatron

No problem jackmck it’s all useful! I had my PSU set up as centre positive.

Do I connect the 10k resistor between the gnd and +5 pins on the far left of the top-side of the board pichenettes? I’m just making sure before I do something daft that only a noob could do! On my photos above is it the two pins at the far left of the first photo? And resistors aren’t polarised are they?

Thanks for the help everyone

Yes, the GND and +5V pins on the far left. Resistors are not polarized. Do not solder it, just keep it there so as to make a connection - it’s temporary.

OK, I’ve now taken readings with a 10k resisitor between the GND and +5V pins. I have now realised a daft error - I hadn’t noticed that the reading on C8 and C17 is actually mV and not V when I did the readings the last time.

These are the new readings:

C5 = 8.73V
C14 = -5V
C7 = 8.77V
C16 = -4.98V
C8 = slowly rising up to and beyond 120mV
C17 = also slowly rising up to and beyond 120mV
C9 = 0
C18 = 8.75V
C24 = 8.75V
C31 = -4.98V
C26 = -4.98V
C33 = 8.78V
C28 = 8.72V
C35 = -4.98V

The two Voltage Regulators:


  • = 0
    I = -11.22v
    0 = -4.98v

I = 11.55V

  • = 9.64V
    0 = 8.78V

Again, any thoughts or input would be gratefully received!


are you shure the regulators arent swappped/ rotated 180degree?

oh, and readings from the caps are rather pointless until we dont know on what pin you measure. please take some readings from the colored points on the picture in the building instructions (step 5)

You can see on my photos at the top that the regulators are correctly placed so that’s not the problem.

I am taking the readings just like in step 5 - with the black probe on GND and the red probe testing the caps at the exact points that the coloured dots are suggesting.

Not sure why you check the voltages at “C8”, “C9”, “C17” - these points have nothing to do with the power supply. There are capacitors adjacent to the power supply pins of the (yet to be soldered) ICs ; but what you are testing is unrelated to the capacitors themselves.

All the green points are connected with each other anyway.

So you need to figure out why your 7805 is not generating +5V. It looks like the problem is due to its "" pin (the middle one) not being grounded it should read 0V normally. Try resoldering his middle pin.

The colored points + the Text clearly indicate testing the IC pads so you should put your Probes exactly where the Dot on the Pic is, the way you describe it you could also be testing the Cap near the Dots.
All ICs are numbered counter clock wise beginning with Pin 1 being on the IC Label (upper left) so the green dot on the Tl072P is correctly identified as IC2 Pin 4.
It would help a lot if you would use this worldwide common convention.

Nonetheles the reference readings for the 7905 are OK (and you have a nice negative Voltage at the right spots, the 7805s reference readings should be something like
0 = 5V (yours is 8,78V)

  • = 0V (yours is 9,64V)
    I = 8,25V (or whatever your PSU delivers, yours is 11,55V which seems reasonable)

First you should switch your PSU to 7,5V if possible,
The - pin on the 7805 is supposed to be connected to GND, so (on my board theres 0R resistance to the GND pad on the Stacking Connector, so i wonder how you can make any readings at all in respect to GND. Please Check this by removing the Power Plug from your Board, switching your Meter to the any “Ohm” “Ohmsymbol” or “R” setting and putting the Black Probe on any GND pin (J4 Pin 7 labeled “GND”) ) and the Red Probe to the - PIN on the 7805 (correctly IC9 PIN2). If you Should read anything else than 0R. While you are at it you could check the connection of - (IC9 PIN 3) to +5V being also 0R-

I also wonder about your “C9” Reading, IC4 PIN 11 being at 0V would mean this Pin is connected to GND instead of -5V. If this would be the case you shouldn’t read any negative voltages whatsoever and smell a bit of the Burned LT1054 as this one isn’t protected against shorting. Please measure with Power unplugged if this Pin has really a resistance of 0 Ohms to GND, you do this described as before. YOu should read no resistance whatsoever (same result as with the Red Probe in the Air or in an 28day due Mozzarella). Check if this Pin is connected to the 0 PIN of the 7905 (IC5 PIN3).

If these Tracing have not the correct result it either means you have the first faulty Mutable Instruments PCB ever or you need some advice Measuring.

OK so in order to be clear I’ve taken more photos. I tested pin 11 on IC4 - it is indeed at -5V. I hadn’t followed that photo instructions closely enough to see where the pins are to be tested. I’m sure I followed the guidelines for the build well though and have just re-checked to make sure that everything is in its right place. It’s all exactly as the instructions state it should be.

Before I took these readings I removed the 10K resistor between GND and +5V and also removed the power cable. Then I placed the black probe on the GND (or +5V) and the red probe went to the IC Pins or Voltage Regulator’s pins. For the voltage regulators I touched the red probe against the metal wire that has been soldered to the board (making sure to only touch the one that I was testing) - I assumed this is the right way to do it.

13.8 KOhms at IC4 Pin 11
GND to IC9 Pin 2 is 9.6 MOhms
+5 to IC9 Pin 2 is 14.88 Ohms
+5 to IC9 ‘0’ (Pin3? - think this is perhaps the one you meant fcd72?) is 0 Ohms but does sometimes show a reading of 0.1 or 0.2 Ohms

I then re-did these readings while taking the photos at the same time and the readings were slightly different (but not much). You can see the multimeter well enough in the photos to see the readings. I even took a photo of the multimeter’s red probe catching some air to show that it’s not gone haywire (and all the mozzarella was already in my belly)…


The - pin of the 7805 (middle pin) should be grounded (0V and continuous with any point marked GND).

You read a non-null voltage there, which indicates that either the 7805 has been damaged, or that this middle pin is incorrectly soldered (your reading is consistent with this second option). Please post a photo of the soldering of the 7805. Check for bad solder blobs for the middle pin.

Yups, Pichenettes is right (as always, this gets boring ;) ). Resolder the middle Pin of the 7805. If this doesn’t work cut all 3 Pins, desolder one by one and replace it with a new one its a 29ct Item.

I’ll try the re-soldering of the middle pin, rather than post a photo of my soldering :frowning:
It’s not worthy of a Shruthi hall of fame, let me put it that way. I’ll see how it goes…

The middle pin needs a little longer as it is connected to the ground plane which absorbs some of the heat. Have your iron tip well tinned to make it easier.