The AM Forum

Hey guys...

I'm in the process of troubleshooting a problem with my 75A3 BFO.  A couple of months ago the BFO simply quit working one day... all other functions of the receiver were unaffected. A day later the BFO was working again as though nothing had happened.  It continued to work until about a week or so ago when without warning it died again. As before, no other functions of the receiver are affected. Before I march ahead with my repairs, I thought I'd get some opinions from the receiver gurus here on the board.  I have attached a picture of the schematic so you guys can take a look.  Here is what I have checked so far...

According to the manual, the voltages around V12 (6BA6) with the BFO turned on should be as follows....   and what I've measured....


Should be....                              Measured....

Pin 1  (G1)     -7.5vdc                  -3.0vdc

Pin 2  (G3)     GND                       GND

Pin 3  (H)       GND                       GND

Pin 4  (H)       6.3vac                    6.3vac

Pin 5  (Plate)  32vdc                     10.0vdc (+/-  2vdc)

Pin 6  (G2)     70vdc                      65vdc

Pin 7   K         0                            0

As you can see there are significant differences in plate voltages and (G1) voltages.  I noticed that when the BFO is turned on, there is a very weak and unstable bfo signal that warbles and crackles in the background. I've checked resistors R-51, R-52, R-53 and R-54.  All check within tolerance. If I remember, the B+ at the intersection of R-52 and R-53 was around 140vdc so I don't think I have a issue with the B+...  I'm really starting to suspect that the two padder caps ( C-116 and C-88) inside the BFO can may be the most likely culprit. Before I remove the can and open it up, I thought I'd get some opinions here... Am I on track or do you guys have some other ideas?

Thanks!
Brian - W8KHZ

Either the tube or the silver micas.

Get on it with a scope, it may be oscillating somewhere way off frequency - in which case I'd expect the capacitors.

The time has come for many a solver mica recently.  It's getting so that's one of the first things I suspect when something funky's going on.

Also check the bias. A number of tubes in that radio are part of the cathode bias supply.
I would think an oscillator would be most sensitive to a bias problem.
I had a 75A3 once but it was too clean to modify and didn't like the performance.

I agree with John,
It's either Silver micas or the coil is intermittent
Fred

Thanks for the suggestions guys... good to know I was on track.  I did swap out the 6BA6 with a known good one with no difference.  I'll get the silver micas ordered up and I'll let you know how it turns out... In the mean time, I'm still open to any other suggestions.

73,
Brian - W8KHZ

Didjya get on it with a scope and see what freq it may be oscillating at?

Check C-90.  Those little rectangular red silver micas in Collins gear are notorious for flaking out.  My 75A4 quit working on 20m.  I tried a new xtal (have two extras on hand) with no success.  Then I replaced the 120 pf mica that  resonates the coil in the xtal oscillator stage, and it came back to full life.

The micas inside the BFO can may not be the same  thing.  I'd leave them alone until all other possibilities are exhausted, beyond a reasonable doubt.  They are part of the frequency-determining circuit.  Unless the replacement is precisely the same value as the  original, the frequency range, and perhaps the linearity of the BFO will be off a little.

You can compensate for any frequency change due to capacitor replacement with the slug.  The BFO is in no way precision calibrated on these RXs/.

You're right.  I was thinking of the 75A-4.  If the linearity or tuning rate in the BFO gets messed up, the mechanical passband tuning doesn't work right.  It can be realigned, but is more involved than just retuning the slug. The A3 and earlier A series do not have the passband tuning feature.

The R-390  series receivers use a tiny sealed BFO unit that looks like a miniature PTO. I don't remember if it in inductively or capacitively tuned.

Never mind !

Well guys, looks like we nailed it...  Replaced the mica caps C116 and C118 inside the BFO can and replaced C-90 as well for good measure and the BFO is working perfectly now... thanks for the ideas and advise!

73,
Brian - W8KHZ