[Boatanchors] Collins 75A2 PTO issue

[Tree]

Following up in this - I found the root cause of the problem.

Bill, N4LG gets to take a bow as he pretty much nailed it.

There is a 1.5K Ohm resistor in series with the B+ lead coming from the
radio chassis.  This resistor had cracked and was basically an open circuit
(measuring over 3 Meg Ohms).

It is a wonder this circuit could oscillate at all.  It was a 1/2 watt
resistor - and I replaced it with a 1W version just because it was the
first thing I found in the junk box and it fit.  It's tucked
way down near the "bottom" of the chassis.

Works fine now across the whole range of tuning.

Now - to put this back together.  The last time I had it apart - I think I
did something wrong and couldn't turn the dial indicator all the way to the
right - needs another rotation of the circular dial
which might have actually resulted in the band pass tuning being off by 100
kHz.

Thanks for the suggestions.

73 Tree N6TR

On Tue, Mar 16, 2021 at 7:27 AM Bill Cotter <n4lg at qx.net> wrote:

> Hi Tree,
>
> I have found in repairing some older wooden radios (Philco, Zenith,
> Scott, etc) that when the receiver stops working in the higher end
> of the bands, that the culprit is often the plate load resistor in
> the oscillator circuit going high in value.
>
> This may not be the case for your PTO, but you might be able to
> measure the plate load resistor and screen resistor from the PTO
> tube socket to the B+ point without having to open up the PTO.
>
> Best wishes,
>
> 73 Bill N4LG
>
> At 11:06 PM 3/15/2021, Tree wrote:
> >Have an interesting problem with the PTO in my 75A2.
> >
> >I did have to replace the feedback capacitor about a year ago.  It
> >worked
> >fine until recently.  Now, I have a problem where it only wants to
> >oscillate nearest the lowest frequency (down between 1.8 and 1.9
> >MHz).  As
> >I tune up - the output gradually starts declining and then it totally
> >stops.  This might be getting worse over time.  Currently, I can't
> >even go
> >50 kHz without it pretty much stopping.  It is very gradual - so
> >not like
> >something is suddenly getting shorted.
> >
> >Some of the caps have been replaced by a previous owner.  I will
> >probably just rebuild the whole circuit with new components if I can't
> >figure anything else out.
> >
> >However, I thought I would ask here if anyone else has seen this
> >and offer
> >any advice on fixing it.
> >
> >73 Tree N6TR
> >tree at kkn.net
> >______________________________________________________________
> >Boatanchors mailing list
> >Home: http://mailman.qth.net/mailman/listinfo/boatanchors
> >Help: http://mailman.qth.net/mmfaq.htm
> >Post: mailto:Boatanchors at mailman.qth.net
> >
> >List Administrator: Gary Harmon, K5JWK
> >** For Assistance: gharmon at idworld.net **
> >
> >
> >This list hosted by: http://www.qsl.net
> >Please help support this email list: http://www.qsl.net/donate.html
> >Message delivered to n4lg at qx.net
>
>