> I got hold of a Lafayette HA-225 communications receiver a couple
> of years ago at a flea market. It was probably one of the last
> vacuum-tube sets, back in the late 60s or so. It has pretty good
> reception, and I like playing around with it.
> It has one problem: It keeps drifting off the station. Now, I expect
> some drifting with a vacuum-tube set for the first half hour or so
> after it's turned on. But this one keeps drifting long after that,
> and I have to re-tune every few minutes or so. And I'm not talking
> about weak stations, I'm talking about the BBC World Service.
> Is there anything I can do about this? I was thinking of giving it
> to my nephew at some point (He's 8 years old right now.), but I doubt
> it'll get him interested in shortwave radio if it keeps drifting like
> this.
You can start off by re-tubing the receiver, if the existing tubes
either have not been changed or it cannot be determined how long they
have been in there. My guess is that this radio is an "All-American
Five" derivative, and the tube probably most responsible for the drift
problem will be a 12BE6 pentagrid converter. It may be a 6BE6 if the
receiver has a transformer-based power supply. Radios with separately
e***d mixers typically use a 6C4, but not having a circuit diagram
in front of me I can only "swag" at it. Look for the local oscillator
in any event; it will either be combined with the mixer in one bottle
or a separate tube, depending on circuit complexity.
If this does not do the trick, the next thing to look for is a thermal
intermittent chassis component, probably a fixed carbon resistor or
wrapped capacitor (ceramic discs seem to be less susceptible to heat-
related drift problems, but anyone who has experienced otherwise can
feel free to correct me here). By "wrapped" capacitor I would refer
to either mylar or paper (depending on the age of the radio) caps in
the range usually of 0.001uF to 0.5uF. A component causing the radio
to drift would have to be either in the local oscillator or interme-
diate frequency (IF) circuits. The heat generated by the tubes causes
the component to change either its resistance or capacitance value,
and if located in a resonant circuit, the resonant frequency changes,
causing the radio to drift.
It will be a challenge to locate the problem; this is not one of the
easier repair jobs out there. Good luck...
C.K.
by