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Author Topic: Heathkit AR-3 Receiver reflex BFO issue  (Read 2025 times)
N6AF
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Posts: 86




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« on: October 11, 2018, 02:41:05 PM »

I picked up a swap meet Heathkit AR-3 receiver in very good physical shape.  It works fine in AM mode but when I try CW mode (whereby the reflex BFO oscillator is allowed to run), I get a huge AC hum out of the speaker.  I stuck a scope on the plate of the 12AV6 (pin 7) and it's clean DC in AM mode but has about an 80 Volt p-p 60 Hz AC signal superimposed on the DC when in the CW mode.

I tried the following with no improvement seen:

1) changed out each tube, especially the 12AV6
2) checked for wiring errors in the reflex BFO circuitry
3) checked the BFO tapped inductor and all associated caps and resistors for proper value
4) checked all tube voltages per the suggested troubleshooting section in the manual

I am really at a dead end with this little gremlin.  Fortunately my intent was to just use it with my Viking Ranger on 75 and 40 meter AM nets.  But I'd sure like to see that BFO circuit play right just for completeness.  I repaired my SB-220, HT-37, Viking Ranger and a homebrew tube AM transmitter so this seems extremely trivial but I've run out of ideas.  Anybody ever seen this issue in the good old Heathkit AR-3 receiver?
Thanks!  73  N6AF  Chuck
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W8RLC
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Posts: 127




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« Reply #1 on: October 11, 2018, 03:14:13 PM »

What does the power supply look like? Is it clean? Are you sure the capacitors are not breaking down when powered up?
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N6AF
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« Reply #2 on: October 11, 2018, 03:29:12 PM »

The power supply is clean and confirmed not the issue. Thanks
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W8RLC
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« Reply #3 on: October 11, 2018, 03:56:31 PM »

Have you checked the other tubes? What about the 12BA6 any filament to cathode leakage?
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AC2EU
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« Reply #4 on: October 11, 2018, 04:01:07 PM »

bypass and resonace caps OK?
paper probably leaky
micas can short
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KM1H
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Posts: 5078




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« Reply #5 on: October 11, 2018, 04:38:24 PM »

Heath used a lot of WW2 surplus parts in those early days so even mica caps can be leaky or change value.

I thought the AR-3 used a regenerative IF for CW such as in the S-38B and later?

Carl
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N6AF
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« Reply #6 on: October 11, 2018, 04:45:48 PM »

Tubes all confirmed good.
Regarding caps and other components, the crazy thing is that the 12AV6 tube works fine in the AM mode and it is the same tube in use when in CW mode so the tube and its bias is not the problem. If you Google "Heathkit AR-3 manual" you will see the source of my frustration. The
circuit description explains how the reflex BFO is enabled in the 12AV6. Keep in mind that it works great in AM mode. Thanks.

.
  
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N6AF
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« Reply #7 on: October 11, 2018, 04:53:05 PM »

It uses a reflex oscillator in the 12AV6 detector tube when in CW mode. The osc function is disabled in AM mode. It's a true BFO implementation-not regenerative.
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KM1H
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« Reply #8 on: October 11, 2018, 05:00:18 PM »

Thanks, I learned something new! Never bothered with the AR receivers so didnt realize they were ahead of Hallicrafters where only the original S-38 hada real BFO.

Carl
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W8RLC
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« Reply #9 on: October 11, 2018, 05:10:18 PM »

If you turn the RF gain control down do you still have the oscillation? If so why do you try replacing some of the capacitors. As mentioned above even disc ceramic and mica can fail.
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N6AF
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« Reply #10 on: October 11, 2018, 05:32:24 PM »

RF gain setting doesn't help. Nearly every cap is in use when it's working in AM mode.
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N6AF
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« Reply #11 on: October 11, 2018, 05:54:14 PM »

OK on the cap replacement suggestion.  The only two caps that are brought into use when switching to CW mode are a 33 pF and a 150 pF in the 455 kHz BFO resonant circuit.  I temporarily replaced each one and it didn't help.  Very strange.
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W8RLC
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« Reply #12 on: October 11, 2018, 08:37:23 PM »

You stated you have AC riding on the DC buss when in AM mode, so where is it coming from? Also in the AR-3 manual advises if you have noise and squealing to check the wiring contained in the spiral shielding.
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N6AF
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« Reply #13 on: October 12, 2018, 06:19:38 AM »

Strange because the 60 Hz AC superimposed on the plate of the 12AV6 is only present when the reflex oscillator function of that tube is enabled (during CW mode), yet that same tube is in use in AM mode at which time it works perfectly. Pretty sure it's related to the few passive components that come into play when in CW mode. The schematic shows the situation. The saving grace here is I picked up the AR-3 only for AM receiving anyway but for me I can't stand it when something doesn't fully work. And until I solve it, it's almost an obsession. I know there are far worse afflictions but the XYL sure starts to wonder after I've been hunched over my work bench for hours on end, days in a row.
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W8RLC
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« Reply #14 on: October 12, 2018, 06:27:41 AM »

Out of curiosity have you done an alignment on the radio?
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