Thursday, December 9, 2021

Another Three Hour Tour?

 Current project On The Bench is a Heathkit IM-5248 Intermodulation Distortion Analyzer, seen here in it's eBay listing photo:

It was listed as "Working", but with caveat "Unit may require some maintenance due to age".

All the capacitors looked and checked OK, so I began by cleaning it from top to bottom. The black bezel around the meter had a broken screw boss on the backside, and since that's how they mount the meter, the meter was shaky and wobbly. It also had a nasty scratch on the face that I polished off. I wound up pulling the front panel off to get the meter loose, so I scrubbed the bare panel. Three of the six knobs are split, and one is a mismatched knob from another Heath product. ALL the hardware on the rear panel was rusty, so it was replaced with new stainless hardware, as will be the screws holding the cabinet panels to the frame. during reassembly. All the other hardware was tightened up as most of it was loose, and the switches and controls cleaned. The AC line cord had seen better days, and was replaced.

And after doing some preliminary checks, I proceeded to begin the calibration procedure. It failed on Step 2, calibrating the AC Voltmeter. Something is messed up with voltmeter's Range switch, and when I switch to the 1 Volt range, the sensitivity disappears, and I can't get it to calibrate to match the 1 Volt RMS at 1000Hz that I'm feeding it. I've been going over the multiplier resistors soldered on the switch, and had one fall off when I wicked off the solder. That's a new one on me! Anyway, all the resistors are in the correct position, and are of the correct value. It's beginning to look like a flaky switch contact, so I'll flush the daylights out of them, and treat them again with DeoxIT.


On the bright side of things, SLW and I replaced all the grout on the kitchen counter tops and backsplash. It was really groaty looking, had chunks popping out, and was discolored everywhere. I used a 1/16" carbide tile cutting / grout removing bit in my Dremel, and a lot of patience and steady hands to grind out all the old grout that was loose and flaking away. Did several clean-up passes over all the seams, vacuumed out all the debris, and turned it over to SLW to apply the new grout. She let it cure 48 hours, and then sealed it, so we hope it will look nice for some time to come.


10 comments:

  1. Good your fine motor skills still function. Put my car in the shop this morning for some minor work in years past I would have done.

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    Replies
    1. It's helped my manual dexterity quite a bit, and most of my skills at bench work are coming back.

      Delete
  2. Deoxit has saved my bacon many times over the last 50 years. Still keep a can on hand. Sure is expensive these days though.

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    1. I've also got some in a little squeeze bottle with a needle-point applicator. Works great for putting just ONE little drop on accessible switch contacts and pot tracks.

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  3. Deoxit works! And glad you got the grout done... I HATE doing that stuff!

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    Replies
    1. Grinding the old stuff out takes some patience, and you can't force the tool. Let it cut at it's own rate, or you WILL snap off the $15 carbide bit.

      Power tools and SLW don't get along very well.

      Delete
  4. Reminds me of my Naval Aviation days fixing old ARC-27 VHF/UHF transceivers. So much bad workmanship in low-bidder avionics kept us techs in the shop busy.

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    Replies
    1. Never worked on any dotmil gear from that era, except maybe to strip for parts.

      Heathkits are a dice roll as far as build quality. The SB Series I have were very nicely built, as are the two audio signal generators I have. This one is so-so, and the FM Stereo Multiplex generator was at best a "C-" project by somebody long ago.

      I'm still spinning my wheels trying to figure out why the metering circuitry won't pass calibration. Some of the power supply voltages are off, so I'm going to replace all the filter caps and all of the electrolytics Heath used for coupling caps in the metering circuitry. The individual stages are DC coupled within the stage, but capacitively coupled between stages. Might have a leaky one in there throwing off the bias voltages.

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    2. Using electrolytic caps for coupling was pretty much a no-no when I was working at Hughes and Northrop. Even back then they were famously leaky and threw off biases. That may well be your problem with the metering stuff.

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    3. I agree 100%! Ceramic, Mylar, Polystyrene, Mica, just about anything BUT an electrolytic. But, it's audio, and to couple the stages takes more capacitance than RF.

      The funny thing is that all the voltages are within specs, and the signal flow goes as expected until I get to this one stage, and then POOF! It just goes all wonky.

      Heath is also running some of the parts AT their ratings. A 15 Volt cap with 15.5 Volts on it? That's just begging for trouble. I'm ordering a bunch of replacement caps today for this unit and the Harmonic Distortion Analyzer I'll be rebuilding. Since they were produced at the same time, I'm expecting that one to be a nightmare, too.

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