Problem w/low-power AM, using af output transformer...

Joel Govostes jwg6 at CORNELL.EDU
Sun Feb 15 08:14:04 EST 1998

Hi Elmers, I could use a little help:

I have a Lafayette VHF am ham trcvr, ~1964 vintage, and can't seem to get
any modulation to the plate of the final amp.

The way the thing is designed, the af output transformer is used both to
feed the speaker and (primary only) to connect the mod. tube to the final
amp.  I guess this was fairly common practice with these low-power rigs.

Anyway, I get a fine carrier but hardly any modulation.  Tubes check out
OK, voltages are what they should be.  Question is, is the af transformer
likely to be the problem?  Do they tend to go bad?

One thing I have noticed is that the af output tube (for the speaker audio)
gets very hot.  Could this be due to a poor load on it, if the transformer
is indeed bad?  I do get audio from the speaker, but don't know if it is
what it should be.

The same tube (7868) is also the modulator tube, by the way.  Even if I
don't do any transmitting, the tube gets very hot in a very short time, to
the point I can smell it (or nearby components).  No smoke or anything, but
I picked the rig up at a hamfest and don't know what abuse it took before I
ever saw it.

The af transformer is only labeled "T1017-2", so don't know what a proper
replacement would be(?).

FINALLY, I don't quite understand this modulation system, i.e., just using
the secondary of an audio output transformer to plate-modulate the final.
In some similar HANDBOOK circuits, this is referred to as serving as a
"1:1" transformer, but -- what is it transforming?  How does the single
winding of the transformer match the modulator output to the plate of the
final?  Is the value crucial, or would any old af output transformer be

Please pardon my ignorance on this, as I'm just getting my boatanchor feet
wet...!  Thanks to all for the information on the listserv


Joel Govostes

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