*UPDATE* Re: Need help designing attenuator pad to help signal tracing an HW-12A

[VE3FWF]

Just an update to thank all of you who replied.

I received some excellent suggestions and will update the list regarding my
experiences
getting this old radio going.

Based on the various suggestions here are the things I am going to do in
order:
1. Clean the copper edges of the circuit board that  faces the chassis and
tighten things down nice and snug.
2. Re-solder all the joints on the board.
3. Check all resistors for changes due to aging.
4.  If still no joy, then try the signal tracing idea from Neil Morrison
(see below):
==================================================
Don't make it too complicated. Stick to volts, not dB.

15000 uV = 15 mV so use a 5K resistor followed by a 50 ohm resistor.
1500 uV = 1.5 mV so use a 50K resistor followed by a 50 ohm resistor.
50 uV use a 1500K resistor followed by a 50 ohm resistor.

Close enough.

Neil



      .-----.
      |  G  |
      |_-_-_|----------|  <--- 1.5 VAC
      |     |          |
      '-----'         .-.
                      | |  (Varies)
                      | |
                      '-'
                       |
                       |
                      .-.
                      | |  50 Ohm
                      | |
                      '-'
                       |
                       |
                      ===
                      GND
(created by Andy´s ASCII-Circuit v1.26 beta www.tech-chat.de)
======================================================

73,  Bernie


----- Original Message -----
From: "VE3FWF" <ve3fwf at sympatico.ca>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Sunday, October 03, 2004 4:16 PM
Subject: Need help designing attenuator pad to help signal tracing an HW-12A


> The HW-12A that I purchased a while ago needs some signal tracing to find
> out why there is significant sensitivity loss in the receiver. The S meter
> reads S-1 on signals that should be many db over S-9. All resistance
> checks and static DC voltages seem OK. I did find a bad tube socket and
> that has now been replaced. I suspect the problem is in the first or
> second IF stage.
>
> According to the Heathkit manual,  I need to inject  2305 KHz at 15000 uV
> at test point R2, 1500 uV at test point R3 and 50 uV at test point R4 with
> an impedance of 50 Ohms.  My old Heatkit signal generator is a rather
> primitive device and is not calibrated.
>
> My thought was to calibrate my signal generator with my scope at the 1.5
> volt level and then build an attenuator pad to reduce the generator signal
> voltage level to the required level for testing.
>
> 15000 uV is  1.5 x 10**(-3) volts or a reduction of 1000   or db = 20 log
> (1000/1)   = 60 db
> 1500   uV is  1.5 x 10**(-4) volts or a reduction of 10000 or db = 20 log
> (10000/1) = 80 db
>
> If some EE type of this list could verify the above calculations, that
> would be great.  Based on the above
> calculations (if they are correct)  I would need to build a 60 and 80 db
> pad with a 50 ohm input and output impedance for test points R2 and R3.
>
> I've browsed with Google but have not found anything too useful on this
> topic.  If anyone could point me to a good reference on the design of a 50
> ohm to 50 ohm attenuator pads, it would be greatly appreciated. If anyone
> in the Ottawa area has a calibrated signal generator that I could borrow,
> that would even be better.
>
> 73, de Bernie, VE3FWF, Ottawa, Canada.
>
>
>

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