Author Topic: Repairing the Humidity Sensor on a HeathKit ID-5001  (Read 139 times)

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Offline HCMP

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Repairing the Humidity Sensor on a HeathKit ID-5001
« on: November 16, 2021, 04:01:55 PM »
All, This is my first public post to the forum. I was encouraged to share the following by one of your esteemed senior members. So, for what it's worth...

I am in the process of trying to resurrect a 1990-era ID-5001. The outdoor humidity sensor after getting dislodged and hanging fully expose to the weather for at least ten years was in a very sorry state and certainly didn't work. Visually, apart from rust and dirt, the only obviously dead component was D701 (see attached circuit diagrams) the body of which was in two pieces. Replacing this was fairly straightforward the only challenge being getting clean surfaces to solder to without destroying the foils which in many place showed signs of lifting, Very fine emery paper and liquid flux were very helpful.

The circuit still did not work. I then spotted and purchased a working 1980s-era  Heathkit Relative Humidity Indicator ID-2295.  This had sensor enclosures which looked identical to mine. I hoped that the sensor units, if not the same, would be close enough that the variation in calibration ranges might allow me to get it to work with my ID-5001. Unfortunately, I could not get it to work.

With an oscilloscope I was able to see that the reference oscillator in the original (weathered) sensor was running nicely at about 10kHz. However the oscillator containing the humidity sensor (A701) was not. However putting either the 110pF calibration capacitor across pins E & F or adding the 42pF to pins H and G woke things up nicely. Adjusting the trimpot R704 did nothing. Replacing the trimpot was straightforward. Fortunately, the capacitative humidity sensor  used in both ID-5001-1 and ID-2295 is identical (Heath p/n 473-32). Thus, with care, I was able to remove the sensor from one of the ID-2295 circuits and fit it to the one feeding  the ID-5001, It is working very nicely at the moment!

I have attached copies of the circuits for the sensors for both the older id-2295 and for the id-5001-1. it is interesting to see how the circuits have evolved although the basic operating principle for each remains the same. Also, a picture of the defunct sensor removed from my unit.

i hope that this may be of some interest and would be happy to answer questions or elaborate.

Offline dupreezd

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Re: Repairing the Humidity Sensor on a HeathKit ID-5001
« Reply #1 on: November 16, 2021, 06:18:17 PM »
Replacing this was fairly straightforward the only challenge being getting clean surfaces to solder to without destroying the foils which in many place showed signs of lifting, Very fine emery paper and liquid flux were very helpful.

To clean edge connectors, especially gold plated one, use an eraser. The white ones are very delicate and will not remove or scratch the gold plating, while the ones on a pencil (pink ones) are a bit more abrasive.
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Offline WA7FWF

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Re: Repairing the Humidity Sensor on a HeathKit ID-5001
« Reply #2 on: November 16, 2021, 09:47:27 PM »
Well Done!
  AH the good old Phillips H1 humidity sensor. Several years back I was buying up ID-5001s and 4001s to repair and sell on eBay, I even made a circuit board and sold a few humidity kits to replace the guts in failing ones. There was a company in England that still had the H1 sensors and I bought a batch from them.

 There is also a later version Heathkit did that used a different sensor, pretty rare as I have only seen pictures of them.
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Glad to see someone else keeping the old Heathkits running.
Kevin

 

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