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
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