VCA build (classic euro V2) issues

[thesingingarc]

Hello All.

First post here, first Oakley build. I've attached pics

This is the classic VCA euro V2 (no longer produced)

I got the board as my first ever Eurorack build and whilst it wasn't actually the first thing I did I chuffed away at it.

Having finished (obviously not) I plugged in and failed to get anything through it all from either AC or DC.

Clearly I'm a novice at this so have not much idea of how to proceed.

I've attached some pics for info. Any assistance gratefully received. It won't be the end of the world if it doesn't work but I dislike leaving things.

Ps. I reflowed a few points I thought looked weak.


PPS - bet I have a fake THAT...but I don't know which one...

[thesingingarc]

Addendum: I used 4007 not 4004 as I think they're compatible?
Power is from an FC small power board. It's within tolerances for mA on both rails.

[Synthbuilder]

I used 4007 not 4004 as I think they're compatible?

Yes, they should be fine.

The first thing to do is test that power is getting into the board properly. Measure the voltage at pin 7 of U2. Measure between pin 7 U2 and 0V. 0V can be found at the striped end of D8. It should be +12V. Now measure the voltage at pin 4 of U2, again with respect to 0V. This should be -12V. It doesn't have to be exactly 12V, 11.5V or 12.5V is fine.

Now check the voltage across C4, ie. connect the voltmeter probes across the pins of C4. It should be around 1.8V. Do the same for C2, it should be 1.2V.

[Synthbuilder]

Some more voltages for you to measure:

The voltage between pin 2 (red lead of your meter) and pin 3 (black lead of your meter) of U1.

The voltage between pin 5 (red lead of your meter) and pin 6 (black lead of your meter) of U3.

Both should be around +0.6V. Note the + as the voltage should be positive.

[thesingingarc]

Thank you Tony I'll get the meter out and probe and report back.

[thesingingarc]

Hi Tony

Pin 4 of U2 (opa) measures -12.xx pin 7 measures -560!

The caps mentioned are with tolerances

U1 pin 2-3 is 0.622
U3 pin 5-6 is 0.64

Is it the OPA?

[Synthbuilder]

Is it the OPA?

It could be but check that D8 is around the correct way first.

If it is, then take out U2, and measure the voltage on pin 4 again. If it is back up to -12V then it is likely that OPA is not working.

[thesingingarc]

Checked and double checked d8 and it's correct. I measured the 4007s on the way in as well to match as close as possible.

Will try again tomorrow. Thanks for the input

[Synthbuilder]

Will try again tomorrow. Thanks for the input

Sorry, I made an error in my previous post. I misread your post and reversed the pins. I'm dyslexic and sadly this often happens.

Anyway, you may have a problem with the +12V supply on the board. So what you need to do is check D7 is around the right way.

Assuming that D7 is in correctly, what you should do now is take out U2, and measure the voltage on pin 7 again. If it is back up to +12V then it is likely that the OPA134 is not working.

What is confusing is that the voltage on C4 was correct. If this was +1.8V then this suggests that the +12V is getting to the board correctly. This may suggest that pin 7 of U2 is not soldered correctly, or that the chip's pin isn't making contact with the IC socket.

[thesingingarc]

Hi Tony

I checked both D7 and D8 are the correct way round
Checked voltages without U2

All was well. Remeasured diodes and both seemed fine

Pulled OPA and reflowed the joints for the DIL

Reseated OPA , now I have sound from AC and DC but it's very low

The gains for each channel seem to act more as LPF

The exponential and linear inputs don't seem to respond to gate signals very much.

So overall ...well. Not sure !

[thesingingarc]

Replaced OPA and same issue occurs so either both bad or its something else.

[Synthbuilder]

Replaced OPA and same issue occurs so either both bad or its something else.

It's probably not the op-amp as to have both dead would be very unlucky.

Power the module up with U2 installed and measure the voltages on pin 2, 3 and 6 of U2. Measure each voltage with respect to 0V, ie. striped end of D8. All three voltages should be very close to 0.00V.

The exponential and linear inputs don't seem to respond to gate signals very much.

What do they do? Gate signals should turn the audio output on and off.

Ignoring the CV inputs for the time being, what does the gain pot do? With the input signal going to the AC input and the AC input turned up, the gain pot should act like a volume control. No audio should be heard from the output with the gain pot down. If you can hear a signal, it is possible there is an error with the wiring to the sockets, or that not all six pins on the power header are connected to 0V at the power supply.

Do you have an oscilloscope?

[thesingingarc]

Hi Tony

Will measure and recheck what's occuring

In the meantime I've looked closely at the 340 chips and now suspect one to be fake. The dot is at the wrong end and there are other notable physical differences

I've attached the image

[Synthbuilder]

One thing to try, if you suspect the THAT chips, is to replace them with standard BC5XX transistors. Look at the VCA schematic and you'll see that each of the THAT transistors, there's two NPNs and two PNPs in each package, are wired so that they can be replaced with individual transistors. These can simply be slid into the IC socket one at a time, using the top three pins on each side as the NPNs and bottom three pins on each side as the PNPs. Make sure you get the pin out of the individual transistors to match the pin out on the schematic. I use BC549 for the NPNs and BC559 for the PNPs.

For the best sonic results the transistors need to be matched. However, to just see if the rest of the circuit is working, it should be fine to use the transistors as they come out of bag.

The middle pins of the THAT chip are connected to ground, but this isn't required when you are replacing the THATs with discrete transistors, so pin 4 and 11 should be left empty.

I've not heard of fake THAT chips before, although it wouldn't surprise me.

[eggpie]

Hi.
Thought it would be handy to have a pic of a known genuine THAT340P from my 5U VRG to compare yours with. If one is a fake or iffy this may help determine which one.
Tony's idea of the discrete transistors is definitely worth a shot too.

THAT340P

[thesingingarc]

Thank you chaps

The pic is v helpful. I'll take another look in the daylight and report back.

I have some bc5xx knocking about Tony so will give that a shot

Thank you for the help

[thesingingarc]

Spot the fake?

[Synthbuilder]

Spot the fake?

I reckon the one on the left of the bottom picture is fake.

Where did you buy them? Anything other than Thonk, Mouser, Digikey, or Farnell/Newark, could well be dodgy.

[thesingingarc]

Farnell for RHS one, LHS ebay before I understood the issues with chips. Since then I've found a decent (I think) uk seller. Profusion have them in cheap and they have links to THAT Corp so would hope they're good. Im gonna order some at months end.

In the meantime will try the transistor trick

[Synthbuilder]

Profusion have them in cheap and they have links to THAT Corp so would hope they're good.

Profusion are indeed good. They are the UK distributor for THAT Corp. and it's where I used to get my THAT parts from when I bought them in large quantities. I had forgotten that they sell in small quantities too now.

[thesingingarc]

I wish I had known before. The hateful ebay one cost me 14 quid 0_o