Collins 75A-4 Receiver

[aa5wg]

Hello to all.

I have two questions regarding my 75A4 receiver.  

1.  When adjusting the passband control the CW note changes pitch.  It is acting slightly like a BFO.  Is this normal?

2.  On 160 meters the passband control has to be set to the USB position in order to receive LSB signals.  Is this normal?

Thank you.
73,
Chuck

[w4bfs]

I'm not sure what I'm sharing is apropos to this problem but is a potential cause ... watch what happens to the pto as the passband control is varied .... if there is any binding of this action then uniform actuation may not happen ... loosen mounts to relieve stresses, retighten mounts and try again...

as always if there are any rubber mounts check to see if still supple ... if not may need to replace

[aa5wg]

W4BFS

Sir, are you saying the PTO is suppose to move uniformly left or right in correspondence to the left or right movement of the passband control?

The PTO does move a little when the passband control is moved, initially.  After initial PTO movement is wont move anymore unless I move the PTO to a new position. 

Is the PTO suppose to move back and forth when the passband knob is moved back and forth?

Chuck

[w3jn]

The PTO *assembly* moves in concert with the passband tuning.  The * tuning knob* should not.

Look at the mechanism and clean old lube out and re-lube where necessary.  It's a pretty simple affair, but the BFO pitch should not change appreciably when the passband tuning is adjusted.

[aa5wg]

Let me review and see if I got this right.

1.  When the PTO knob moves the PassBand tuning knob should not move.  

2.  When the PassBand Tuning knob moves the PTO assembly (not the PTO knob) also moves.

Okay?

Chuck

[w8khk]

You are correct.  

On the shaft that varies the BFO frequency capacitor, which resides hidden in the BFO coil can, there is also a pulley on the shaft, connected to a metal strap belt, that rotates the entire PTO assembly when the BFO frequency is changed.

By changing the BFO and the PTO frequencies in sync, the BFO pitch should remain almost constant, because the actual BFO frequency and the mixed frequency to the low frequency IF move in unison.  So while the heterodyne between the IF and the BFO is constant, the IF frequency is moved up or down in relation to the mechanical filter frequency, thus moving the passband.

If you are experiencing a change in the beat note when turning the passband tuning control, the most likely cause is a loose metal strap belt, or weak spring holding the belt tight, or friction/resistance in the movement of the PTO.  The PTO should turn freely in concert with the BFO control shaft.  Hope this helps.....

[aa5wg]

Rick,
It does help. 
I need to figure out how to get everything working as it should.
Again, thank you.
Chuck

[w3jn]

ANd per your second question, that is normal behavior (having to receive LSB signals using the USB part of the passband tuning) due to the mixing scheme on 160.

[aa5wg]

Thank you for the 160 meter updated information.
This is good news.