Cadet 4 - Ramp Generator Troubleshooting

Hey folks, quick question. Building a Cadet IV Ramp Generator and the horizontal section appears to be functioning correctly. However the vertical section looks like I goofed something up. The top jack outputs a flat color without a gradient, the middle jack outputs nothing or black and the bottom outputs something identical to the top. I have been checking my board and I can’t seem to locate a problem. Thanks in advance.

Can you post a hi res shot of the back of the board as well?

Great photo - excellent resolution!

The horizontal and vertical circuits are basically separate and essentially identical. If one works and not the other then there’s likely just a component or solder point in the horizontal circuit that’s at fault. Could you please post a photo of the solder side of your board so we can peer at it and critique your work? :slight_smile:

Check the soldering on R20, R29, D4, U3. Also D2 looks ugly (but is probably fine). Some of your other solder points look like they could be heated a bit more to flow the solder better. You might have a dry joint?

Also try reseating the ICs - those sockets aren’t the best (the ones with rounded holes are better).

If nothing’s obvious from the photos then the next step is to start checking voltages. Do you have an oscilloscope?

Thanks, I will check those. I don’t have an oscilloscope but that be something I will be needing in the near future. Here is the back of my board.

I can’t see any obvious dry joints from that side, which is a good start.
In terms of reseating chips, U12 is the main one to check for the horizontal ramps.
Try swapping U12 and U13 (the two TL074s) to see if the problem moves to the vertical (in which case the chip is at fault) or stays on the horizontal.

Check you didn’t mix up the transistor (BS170 Q4, 2N3906 Q2) and the voltage references (TL431 U9). They all look the same.

Is there a missing bit of track near the base (middle pin) of Q2? It looks like it’s missing in the photo, but it could just be shadow. That would probably cause the problem you’re seeing.

It is the shadow on the photo unfortunately, I have gone back through and re-flowed solder as many places as I could think of and the places you have recommended. I tried switching the IC’s and it’s still the same. I tested the sockets and they are working (I used my multimeter to test that legs from the IC’s were connected to the solder joints).

Just to clarify, I am having issues with the vertical section, not the horizontal.

I was wondering if the the tantalum capacitors I used for C2 and C6 could be the problem? The BOM said to use radial. I know tantalum should be an acceptable sub for radial, but I don’t know if maybe this project may require a more specific match.

Also because the top vertical jack is putting out a full color screen, the middle jack is all black and the bottom is a full color screen I feel like it is related the trim pot or the nearby resistors. This is a real head scratcher. Here’s a new photo of the back of the board with more even lighting.

Is it possible that another module is making contact with your Cadet 4 inside the case? The 4 hp modules can get a bit tight sometimes and there could be unexpected shorts that way.

I wish…but I have been testing it outside of my rack, to calibrate the trim pot.

clean your pcb. some pads seem to have tin residue between them
in general, use a bit less tin and snip off your wire leads shorter.

all pads should look like this:

parts placement seem to be ok, as far as I can tell.
try replacing c2 and c6 with electrolythics, just to be sure .

Thanks for all your suggestions. I reflowed solder, and thoroughly cleaned the board.

I don’t think the issue is those tantalum capacitors. I looked over the schematic and they are part of the Eurorack power header, and I feel like if those weren’t working properly, the horizontal section wouldn’t be working.

When I re-seated the IC’s I checked voltages and all of the voltages for the IC’s appeared to be in their functioning range. I will keeping looking through the schematic to see what could be the issue. I feel like everything I check makes it seem like it should be functioning correctly.

I have noticed one thing that may be an issue, but I am not sure. When I checked voltages, going into U3, it’s reading +11 on the input side and the schematic says it should be +12. Could that be related to the problem? Or is that normal?

If you mean the power supply pin V+ then that’s fine - the eurorack supply is ~+12V, but the 1N4004 diodes have an 0.7V drop leaving you with ~+11.3V, which is plenty.

Thanks, aladan.

So, I tried replacing C2 and C6 with electrolytic capacitors, and the same result. Still getting a full color screen on the top vramp, black on the middle and full color on the bottom. This is so strange.

nothing happens when adjusting the trimpot?

That is correct. Nothing happens when I adjust the trim pot (the lower one for vertical).

My reading of the schematic is that U9/U13.1/Q2 provides a constant current to linearly charge C18 and form a rising ramp voltage at the +input to U13.2. Q4 shorts the capacitor to ground on sync. If it’s full colour (high voltage) then maybe the reset isn’t working so try replacing Q4, R27 and C18. With the module unplugged and unpowered, use a multimeter to check connectivity (< 0.2 ohms) between the pins of every component that should be connected in that area and see if there’s a missing connection.

Do you see any sort of rising ramp at the junction of Q2, Q4, C18 and U13.2 +in? Input it into an RGB encoder input if you have no other option, but you might need to use a voltage processor (i.e. attenuator and offset controls) to get it into a useful range. If that’s OK then check U13.2 out (before the output voltage divider pot). If that’s OK then check U13.3 +in.

Check the negative feedback connections between each of the U13.2 and U13.3 -in and outputs: if they’re open circuit the op-amps might be hardlining on +V which would also cause what you’re seeing.

Thanks again, aladan. That is plenty for me to work on! Though I may need to order more parts. I will keep you posted.

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Just an update. I had a replacement for C18 so I did that. I removed R27 and Q4. On a whim I tried it without the parts that were missing and no change. Therefore I think it is possible my resistor, capacitor or BS170 was a dud. But that is probably being optimistic. Keep you posted, I am waiting for parts to replace R27 and Q4.

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Ok, so I replaced C18, Q4 and R27 and still no dice. Is it possible that it is the trim pot? Also, forgive my ignorance, how would one check the negative feedback on 13.2 and 13.3?