Help identifying holes in Noisecom NC1105A PCB assembly - Page 1
Forum section: electronics / beginners

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 08, 2023, 07:08:59 am
content: I received a non-functional NC1105A noise source. I've attached an image of the internal assembly. A bench power supply providing the 15 V goes into current limit above the specified max 160 mA current draw. I also measured the zener (near the lower right) in-circuit with an HP 4145A curve tracer (with a compliance set to 200u). It reached compliance in the reverse direction below 1 V (instead of the 7-10 in the spec). But, this could be something in parallel with the diode (eg that resistor).What are those holes to the right of the diode (and also further to the left). Is there a component on the other side?I believe something has to be shorting out that line other than the zener since the resistor between the 15 V line and the zener measures almost 8k.I'd prefer to hold off on desoldering the connectors and lifting the PCB out of the enclosure until necessary.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 08, 2023, 09:05:53 am
content: Despite you writing about the zener in lower right, I think you meant lower left. The only diode that is at 45 degrees to the case sides.I don't understand what "200u" means. If you meant 200V for 200 Volts, why would you set the compliance voltage so high? And what do you mean by "reverse direction"?Where on the PCB is the power input point?

author: u666sa
author_rank: Frequent Contributor
timestamp: September 08, 2023, 11:25:34 am
content: With this design something definitely would be shorting out.I'd look carefully at those blue caps and also all the legs of everything else.Then I'd pop out ohm meter and measure, where ever short shows less is the place to look.

author: CaptDon
author_rank: Super Contributor
timestamp: September 08, 2023, 01:04:29 pm
content: I find it amazing how very expensive things in military grade enclosures look like an absolute kludge inside!!! That thing looks like the circuit board was once an inline amplifier with input on one end and output on the other and they kludged it to amplify a noise diode. Look at the evidence of traces that were stripped from the FR4 material. Looks like that circuit board was designed to be 'something else' originally. I suspect there are TO-5 cased wideband amplifiers on the backside of that board and one or more may be blown due to 'operator error', most likely over voltage. Reminds me of the RHG line of I.F. amplifiers and video detector building blocks for radar systems.

author: wasedadoc
author_rank: Super Contributor
timestamp: September 08, 2023, 01:12:30 pm
content: Quote from: CaptDon on September 08, 2023, 01:04:29 pmThose 3 sets of 8 or 9 holes in circular patterns with wires coming through from the undeneath give a lot of weight to that suspicion.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 08, 2023, 02:45:15 pm
content: Quote from: wasedadoc on September 08, 2023, 09:05:53 amYeah that looks like it's a diode too (maybe a zener, maybe a 4148, I would have to give it a closer look).But I'm referring to the noise-generating Noisecom zener diode that's basically the heart of the circuit. Unless I'm very much mistaken, that's the component in the lower right. I've attached another picture showing a closeup of this package. Hopefully that helps identify it.Quote from: wasedadoc on September 08, 2023, 09:05:53 amI probably could have been a bit clearer here. 200u means 200uA. If you look at the NC201 datasheet (https://noisecom.com/Portals/0/Datasheets/nc100200300400web.pdf) (I suspect this zener is an NC201), it gives a breakdown voltage of 7-10V and operating current of 200-500uA. In retrospect though, this test doesn't mean all that much because of a possibility of a short in parallel, and that resistor in parallel too.Quote from: wasedadoc on September 08, 2023, 09:05:53 amThat's in the upper right, above the SMA output (for the noise signal). There's a feedthrough filter for the +15 V and below it (toward the table) though you can't see it, a ground turret.

author: anmony
author_rank: Newbie
timestamp: September 08, 2023, 04:47:15 pm
content: The zener diode in the bottom left is the primary noise generator and there are likely several amplification stages before reaching the output port as the others have pointed out. The feed on the left side of the zener coming from the top provides the bias current to generate noise from the zener diode, and the noise signal goes off to the right. What you are looking at in your close-up photo is probably a trim capacitor (something like:https://www.knowlescapacitors.com/getattachment/566753a2-3214-45b2-b2fc-664ddb27a9a8/Thin-Trim) to adjust the output spectrum of the noise.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 08, 2023, 04:49:06 pm
content: Quote from: anmony on September 08, 2023, 04:47:15 pmAh, well I'm very glad you and others pointed that out. Thanks!

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 08, 2023, 05:33:33 pm
content: Here's a curve trace of the noise diode in the lower left corner. This seems to be working just fine, which is great news. I'll pull out the assembly and start checking other components. If one or more of the amplifiers is at fault, hopefully they're not too hard to source. I'll keep this thread posted as I make progress.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 08, 2023, 05:52:41 pm
content: Also I should mention that if I measure the DC resistance between the power rail and ground I see about 23kOhm. That probably absolves the power rail caps from being short. Maybe one of the amplifiers is dead.

author: CaptDon
author_rank: Super Contributor
timestamp: September 09, 2023, 12:56:58 am
content: How much current does it actually want to draw? Your readings don't indicate a hard short anywhere. If you start at zero volts and work up toward 15vdc how much current is it trying to draw? "Goes into current limit above specified 160ma. maximum current draw"? Your statement confuses me. Did you set your supply current limiter at 160ma.?? Maybe the thing wants 200ma. at startup?

author: matthuszagh
author_rank: Frequent Contributor
timestamp: September 09, 2023, 01:04:55 am
content: Quote from: CaptDon on September 09, 2023, 12:56:58 amI initially set the current limit to 200 mA. I then tried with more, up to almost 400 I think and it still went into current limit. When I first turn it on at 200 mA current limit, it gets up to about 7 or 8 V for a couple seconds and then drops down to maybe 2 or 3 V. It doesn't behave like a hard short.

author: croma641
author_rank: Regular Contributor
timestamp: September 09, 2023, 02:15:33 pm
content: Noise source data: