Tuesday, January 8, 2019

Heathkit AR-15 Progress

Been chugging along on the Capacitor Replacement Train here for the last week, and as of tonight, all fifty-four of the printed circuit board mounted electrolytic capacitors have been replaced.

These boards are (from left-to-right) the Multiplex Board, which handles the special process of reconstructing separate left and right channels out of  what's broadcast, then two identical Power Amplifier Driver Boards, which take the selected audio stream and amplify it enough to drive the four power transistors mounted on the heat sinks. Above those two boards is the Power Supply Board. This board provides mounting space for the main power rectifiers, as well as the circuitry for several other regulated supplies.

The circuit board with all the control shafts jutting into free space at the top of the picture is the Control/Preamplifier Board, which selects the input you want (Phono, Tuner, Tape, Aux), processes the audio for Bass, Treble, Balance, and Volume, and then sends it to the Power Amplifier Driver Boards.




In the picture below you can see how all the pots (potentiometer) the shafts are connected to are soldered onto the Control/Preamplifier Board. I wound up having to take this thing a lot further apart than I wanted to, but it was the only way to get enough access to the Control/Preamplifier Board. At least it gives me a chance to clean both sides of the dial glass!



This is the "top" of the component side of that board, as seen looking towards where the front panel normally is, with the old caps in place.



And this is the "bottom" of the component side with the new caps installed.


Besides replacing the caps, I cleaned all seventeen controls, the Input Select rotary switch, three slide switches, and seven rocker switches. Some of the controls I could only get at from the top, and the rest I could only get from the bottom, so cleaning them was a multi-day affair, depending on what side of the chassis I was working on.

On the Tuner Board, I replaced all the electrolytic caps, and cleaned and lubed the bearings in the tuning capacitors. The two rectangular grey boxes are 10.7MHz crystal filters. These give this receiver razor-sharp tuning, even in crowded Metro areas with 50kW stations all over the place.



And I disassembled the heatsinks, cleaned them to get all the old silicone grease off, and them reassembled them with new thermal pads instead of using mica washers and grease. That's a new 3-wire grounded AC cord temporarily strapped to the chassis.




After doing these heinous things to this old war horse of a receiver, I rounded up the usual suspects to interrogate them.



As expected, they knew nothing....Off to the landfill with them! I've seen used parts like this for sale on eBay. Seems people are looking to reproduce "Vintage Tone", and as all the Audiofools know, you need Vintage Parts to get Vintage Tone! Seriously.......

Now I'm In Work coming up with some mounting bracket to use with the replacement power supply filter. The OEM cap was 8,000uF @ 90VDC, and even though the new one is 10,000uF @ 100VDC, it's considerably small in diameter; 2" vs 3".



So even though she's a stripped-down hulk right now, work is progressing, and I'm looking forward to doing the alignment on it with my new test gear.



Have fun, be safe, and Carry On!

9 comments:

  1. I only saw the title: Heathkit AR-15. I said to myself, "since when have they made rifles and why do I need a kit..." Only to find out that it was a bait and switch for the gun nuts in the blogosphere.

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  2. But I thought Heathkit made lots of receiver kits? Just think of how beautifully clear the construction manual would be.

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  3. yuk-yuk!

    A day after I bought the receiver, eBay pulled *EVERYTHING* with "AR-15" in the title as (I think) the Las Vegas Shooting happened, and "AR-15" = BAD. The listing completely disappeared, and the seller and I didn't know what was going on, and eBay wouldn't respond to either of us.

    So you aren't the only two people that think "AR-15" is *only* a firearms related term.

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  4. I’ve done cap replacements in various “vintage” rigs, like I’m sure you have. Sometimes electrolytics, sometimes paper caps with orange drops. It can be a relaxing way to spend a day.

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    Replies
    1. When I'm done with the AR-15, I can start on my Drake 4-Line Twins, and then my Hallicrafters SX-117 and Ht-44!

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  5. I see you possess more than x grams of pot. I hope it's medicinal.

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  6. You've been BUSY! I'm sure it will be a definite improvement when you get it back together...

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Keep it civil, please....