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Old 22nd May 2017, 3:01 pm   #65
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 advice

Part of the audio problem was that the Aurora was set with an audio gain 9 dB too low.
I set it so it clipped occasionally on the ordinary signal and almost always on the very
strongest (from a digital set top box).

While working on the audio I investigated the bias voltages on the apparently
unrestored TRF chassis. They were within 20% of what I would expect with a 10
MOhm meter instead of 100K-250K as specified.

But ... that explained why the "sensitivity" control does little. A very large 15K
resistor has been put between the top of the pot and the bottom of the rest of
its divider chain (between R30 and VR10). This means it can't reduce the first
TRF stage screen almost to zero. It is the large resistor seen just at the bottom
of the top holding mechanism in the TRF photo.

This does not look like a recent (post 1975) change. Its mounted on the tag board
in the expected place with the usual looking solder posts holding it, but the
wires to it are not the stiff varnished ones mostly used but rather very old
cloth covered ones.

Also, R2 is 60K.

Changing R2 to 5K and the new resistor to 2K by paralleling did result in
a larger sensitivity control range, but I did not leave that change in
since the sensitivity control itself, while period correct, looks replacement
and possibly too low wattage. Changing R2 only to 5K made little difference
as the screen current is very low; the measured voltage was still in correct range.

Has anybody else seen such changes to the TRF chassis?

Since audio is now working OK I will install the back and TRF side panel,
awaiting a good MHD4 for the other side.

Last edited by dtvmcdonald; 22nd May 2017 at 3:06 pm.
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