Re: {Collins} 75A-4 Issues



Paul.....Fee Fi Fo fum, I smell an Signal Article

Bill

-----Original Message-----
From: Collins [mailto:collins-bounces@xxxxxxxxxxxxx] On Behalf Of Paul
Christensen
Sent: Wednesday, December 05, 2012 3:06 PM
To: Bob Sullivan; CCAref
Subject: Re: {Collins} 75A-4 Issues

Bob,

Have you been peeking behind my back the last week?

I've been concentrating on 75A-4 PD hum as well as residual hum/buzz when 
using modern headphones and the AF control is full CCW.   Since there's far 
too much audio gain for modern headphones, I've changed to a Switchcraft
tip/ring/sleeve (i.e., stereo) headphone jack with an extra set of N.C. 
utility contacts.  A series resistor limits audio level to the headphones
such that normal listing occurs at a point just a bit higher than normal. 
You'll find a similar adaptation of this idea in the 75S.

On the A4, with just the crack of the AF Gain control, headphone level is
much too loud.  Now, when the headphone plug is pulled from the jack, the
new resistor is automatically shorted by an extra N.C. contacts on the
Switchcraft jack.  This allows full power to reach a table speaker while
limiting only headphone audio.  By limiting audio this way, the headphone
signal-to-hum ratio is substantially better.  Now, with AF Gain at full CCW,
headphone hum/buzz is gone and there's still ample reserve before getting
anywhere near clipping.  Modern stereo headphones can be used without an
adapter (owing to the use of a stereo 1/4-inch) and the front panel
nut/washer are still the identical nickel-plated brass type.

After the holidays, I'm next going to finish up on the PD hum issue.  My
75S-3C had significant PD hum/buzz until I re-routed some components.  Now,
whether the AF Gain is full CCW or full CW, there's zero trace of hum/buzz. 
Even at full CW it's so quiet you can hear the microphonic sound of the
filaments sing.  I'm hoping for similar results with the A4.  Had balanced
A.C. filament strings been used in much of this era's gear (like 1950s
vintage Hi-Fi), hum reduction would be a lot easier to tame.  When the
filaments use the chassis as a return, it can be pretty tough to get hum
levels down, but not impossible.  Although sometimes, the re-grounding work
involves such a massive chassis facelift that one must decide to live with a
certain amount of hum, or live with ugly modifications.

Finally, I ordered one of the German 75A-4 filter caps that plugs into an
octal socket.  Darn, I thought it was a Twist-Lok and did not realize it
plugged in until opening the box.  That scares me.  It's one thing to pull a
rectifier tube when the power is on, but I would hate to even briefly run
the A4 with that triple-section cap accidentally unplugged.  Anyone share my
concerns?

Paul, W9AC

----- Original Message -----
From: "Bob Sullivan" <robert@xxxxxxxxxxx>
To: "CCAref" <collins@xxxxxxxxxxxxx>
Sent: Wednesday, December 05, 2012 3:38 PM
Subject: {Collins} 75A-4 Issues


Listmates,

For those of you with 75A-4's ... do this little "test;"

Turn RF Gain full CCW and turn up AF Gain to listening level. Switch back 
and forth between AM and CW-SSB. Do you note a significant increase in hum 
in the CW-SSB position (and nearly none in AM)? I have done with with five 
75A-4's and they all hum in the CW-SSB position.

I realize we seldom use the receiver with RF gain full off (hi) but it's an 
interesting test. Why the hum? Where is it coming from?

73, Bob
WØYVA
_______________________________________________
SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
http://www.collinsradio.org/?page_id=37

 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
  SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
  1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)

The Collins Reflector reserves the right to refuse access to anyone.

LIST HELP?  ADMIN EMAIL:  collins-owner@xxxxxxxxxxxxx
This ONLY goes to the list admins and does not post to the group! 

_______________________________________________
SHARE THE FUN: JOIN THE COLLINS COLLECTORS ASSOCIATION TODAY !
http://www.collinsradio.org/?page_id=37

 NETS: Tues & Thur: 3.805 MHz-2000 Central + Fri: 3.895 MHz-2000 Pacific
  SUNDAYS: 14.263 MHz-2000 UTC  +  AM Net: 29.050 MHz-1200 Central
  1st Wednesday AM Net 3.880 MHz-2000 local (ET, CT, MT, PT)

The Collins Reflector reserves the right to refuse access to anyone.

LIST HELP?  ADMIN EMAIL:  collins-owner@xxxxxxxxxxxxx
This ONLY goes to the list admins and does not post to the group!


-----
No virus found in this message.
Checked by AVG - www.avg.com
Version: 10.0.1427 / Virus Database: 2634/5438 - Release Date: 12/05/12






This archive was generated by a fusion of Pipermail (Mailman edition) and MHonArc.