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Old 19th May 2017, 9:34 pm   #53
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 227
Default Re: Marconi 702 advice

I measured the EHT and focus and they are 3.75 and 0.8 kV.
Not great, but I did notice that after an hour running the
EHT transformer was just barely warm.

Other effects make really improving this moot. First and most important,
there is 60 Hz interference from magnetic fields from the two big transformers.
This results in "wiggle" in the picture which shows in blurring on the
posted picture which was over 1/10 second.

My "test" permanent magnet has visible effects from four feet away!
The focus has a severe astigmatism, visible on that test card.

I looked at the frequency response using Digital Video Essentials
chirp pattern and it is clearly visible "in person" at 2.5 MHz, which
would be "big signal" tuning back in the day.

Since I didn't do the restoration and am afraid to remove the tube,
its good enough for me. Remember ... the previous owner never
turned it on. Had he done so, he would have got a picture pretty
much the same except that the frequency response was really
really bad.

I'm more interested in getting the sound working. I'll ask again ...
does anybody know the speaker impedance? It measures 3.7 ohms DC.
Is this an ordinary radio speaker of the time? Five volts RMS peak
(on a Simpson meter) into it does not result in "loud". That's
seven watts into the resistance, and the tube in the TV can't
put out much over one watt.

How are the knobs supposed to work? The correct ones are there
but I disbelieve that the holding mechanism is original. The controls are
slotted. The is a screw hole in the knobs, but no threads. The knobs are
held on by putting a sheet metal screw in the hole and mildly
jamming it in the slot in the shaft.
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