[Boatanchors] Collins 75A2 PTO issue

[Bill Cotter]

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Some of the caps have been replaced by a previous owner.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
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