Has anyone DIY'd the Cadet II clipper circuit?

Category: Unknown · Tags: cadet · Posts: 9

#1 — VisibleSignals · 2019-04-01

I’ve built the voltage ref plus a single channel of the Cadet II circuit on veroboard (U1, U2, U9) and I’m seeing my U2 getting extremely hot (after about 45 seconds it starts smoking) when I power it up. Has anyone DIY’d this circuit (based off the schematics, not the LZX PCB) and seen similar issues?

I’ve checked my layout/wiring several times (especially the supply pins), visually inspected AND checked for shorts with a multimeter, checked every net for connectivity and metered passive component connections between pins (with the chip out of circuit). I can’t find anything wrong. I tried two different LM6172s (purchased from Mouser) - same result both times.

I know it’s most likely still an issue with my layout, but I’m asking just in case someone has been down this road and had this problem before me

#2 — creatorlars · 2019-04-02

Are you using all the same resistor values as the schematic and 1n5711 schottky diodes?

What are you using to generate your voltage reference? Is it from a buffered source?

The schematic is correct; it matches the PCB layout.

#3 — VisibleSignals · 2019-04-02

I am using 1N5711, LM6172 and TL431, all purchased from a reliable source (Mouser). I am using the voltage regulator circuit from the cadet II schematic (U11, U9). In my build I have shared the one LM6172 for both U9.2 and U1.1, in case that matters.

For the resistors, almost all are as specified in the schematic, excepting three that I substituted for values I had on-hand:

In the voltage reference circuit, R38 4.99K I used 5.7K and R40 3.48K I used 3.6K. By my calculations, 3.48/4.99 = 0.69 and 3.6/5.7 = 0.63 which is about 10% off, so my voltage reference won’t be quite right, but I wouldn’t think that would be the cause of my LM6172 overheat issues.

Between the U1.1 output and the U2.1 input, R13 1.4K I used 1.5K. A higher value resistor should attenuate the input signal slightly, but again I can’t see how this might cause the overheat issue.

Do you think the substituted resistor values are at fault? I do have the correct value resistors on order.

I’m using a DIY FCUK power supply, and the rails look clean and I have used it to power other circuits just fine.

Thanks again for your help.

#4 — creatorlars · 2019-04-02

Did you place 100nF bypass caps to GND right at the +V and -V pins of your opamps? The caps are in the schematic, but maybe easy to miss.

#5 — VisibleSignals · 2019-04-02

You’re right - I missed them. I will remedy this tonight (and try a new LM6172).

#6 — creatorlars · 2019-04-02

Hopefully that’ll do! Bypassing becomes more critical when you’re working with these wideband parts.

#7 — VisibleSignals · 2019-04-02

Cool, thank you. I’ve only done audio frequency electronics up until recently. Things do seem to be a lot more exacting with video!

#8 — VisibleSignals · 2019-04-02

Thanks Lars, I can confirm that installing 100nF power conditioning caps to ground on all LM6172 supply pins in solved the overheating LM6172 issue. You are a legend, thank you!

Without them I guess there must have been undamped high frequency oscillation taking place, beyond the chip’s maximum operating specifications, causing it to heat up.

#9 — creatorlars · 2019-04-04

That sounds like the correct diagnosis to me. Good work!