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Author Topic: HW-8  (Read 2306 times)
WB2JNA
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« on: February 24, 2013, 08:00:43 AM »

I am working on an old Heathkit HW-8. It seems I need a 5 picofarad capacitor connected to the source of Q2 in the VFO section. Will a larger capacitor work (maybe a few hundred picofarads)? Thanks.
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N4NYY
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« Reply #1 on: February 24, 2013, 08:13:55 AM »

The engineers here can answer your question. However, if you decide to get the small cap, here is Mouser's listing on 500V and 1KV ceramic discs:

http://www.mouser.com/Passive-Components/Capacitors/Ceramic-Capacitors/_/N-5g8m?P=1z0wpt6Z1z0x86rZ1z0wxeoZ1z0z7l5&Keyword=capacitors&FS=True

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AC5UP
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« Reply #2 on: February 24, 2013, 09:32:02 AM »

Without looking at the schematic, I have a hunch the key phrase in the original question was: "...to the source of Q2 in the VFO section"

If the condenser is there only as a DC block the value isn't critical. But, as you're aware, larger condensers will pass lower frequencies more readily and 5 mmf is a very, very small amount of capacitance.  Which leads me to suspect the 5 mmf value was chosen to act like a high pass filter in a circuit relying on the overtone of an oscillator.

I wouldn't stray much more than 50% and that takes you all the way up to 7.5 mmf.  Which isn't very far, and if Heathshkit could have subbed a more common value (like 68 mmf) I suspect they would have.......   Tongue
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AA4PB
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« Reply #3 on: February 24, 2013, 10:50:15 AM »

Capacitor C54 (5pfd), on the source of the Q2 (VFO) is switched into the circuit during transmit in order to provide the required transmit frequency offset. This capacitor must be EXACTLY 5pfd if you want the transmit frequency to match the receive frequency when the signal is properly tuned in.
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WB2JNA
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« Reply #4 on: February 25, 2013, 06:45:02 AM »

Thanks to everyone. I'll keep working at it!
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AA4PB
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« Reply #5 on: February 25, 2013, 08:40:58 AM »

Mouser Electronics part number 598-CD5CC050DO3F
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N4NYY
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« Reply #6 on: February 25, 2013, 09:36:54 AM »

Mouser Electronics part number 598-CD5CC050DO3F


Why can't he get away with a 4.7pf? He is going to pay out the nose for an offbeat value.
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KE3WD
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« Reply #7 on: February 25, 2013, 10:05:24 AM »

Capacitor C54 (5pfd), on the source of the Q2 (VFO) is switched into the circuit during transmit in order to provide the required transmit frequency offset. This capacitor must be EXACTLY 5pfd if you want the transmit frequency to match the receive frequency when the signal is properly tuned in.


That's why.


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AA4PB
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« Reply #8 on: February 25, 2013, 10:11:46 AM »

The silver-mica cap from Mouser costs $3.68 and there are lots of 5pf capacitors available - not an offbeat value.

By the way, Mouser shows 405 **different** capacitors with a value of 5pf.
« Last Edit: February 25, 2013, 10:57:20 AM by AA4PB » Logged
WB6BYU
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« Reply #9 on: February 25, 2013, 11:29:47 AM »

Or put in a small air-spaced trimmer and adjust it for the desired offset.
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AA4PB
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« Reply #10 on: February 25, 2013, 11:56:27 AM »

Or put in a small air-spaced trimmer and adjust it for the desired offset.

Not sure if there is room. If you do that, make sure the capacitor is physically secure. Remember, this is the VFO - if things move around then the  frequency will change.
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N4NYY
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« Reply #11 on: February 25, 2013, 02:12:38 PM »

Capacitor C54 (5pfd), on the source of the Q2 (VFO) is switched into the circuit during transmit in order to provide the required transmit frequency offset. This capacitor must be EXACTLY 5pfd if you want the transmit frequency to match the receive frequency when the signal is properly tuned in.


That's why.




OK, I am not going to get into a pissing contest. But you are talking about .3pf difference. Most 4.7 values I have measured have have been somewhere between 4.7 and 5. Not to mention your are talking about Heathkit, that lived on 20% tolerances and routinely substituted listed parts with subs or crosses. Also, the Mica he listed from mouser is a 10% tolerance. If you truly need the 5pf, I would have to agree about the trimmer cap to get that precise value.
« Last Edit: February 25, 2013, 02:30:01 PM by N4NYY » Logged
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