[MRCA] DAV IF issues

[Ray Fantini]

I have been working on a DAV 2 man pack radio and have been having some
issues with sensitivity. First let me review the DAV for those who are
not failure. The DAV and its evil twin sister the MAB both use a four
tube AM only receiver. This is a crystal controlled one channel radio in
the 3 to 4 MHz range. The receiver uses a four tube lineup a 1R5
converter, a 1T4 IF amplifier, a 1S5 detector and first audio and a 3S4
second audio. There are two power sources for the receiver with 67 Vdc
for the screens and IF amplifier plate and 135 Vdc for the converter
plate and second audio plate.
The issue I have is that the radio requires about 500 uv to produce
about 2 volts of output in the headphones, in compression a modern
receiver requires around 15 to 20 uv for the same result. And full size
radios from around the same time period will do this with 20 to 40 uv,
perhaps 500 uv is normal for this type of radio?
Getting the radio operational required replacing about six leaky
capacitors that were pulling down screen voltages and bleeding off the
screen and B+ supply but they have been changed, also the last owner
must not know how to wire the batteries and burnt out all the tubes in
the receiver so they have all been replace and are known good. So at
this point all capacitors have been checked and bad ones replaced. After
this I did an IF alignment. There are two IF transformers, an input
transformer between the 1R5 and 1T4 and an output transformer between
the 1T4 and the 1S5 the detector and first audio. The output IF
transformer peaks with both top and bottom slug right in the center just
about where they started out but the input transformer had to have both
top and bottom slug turned all the way in and the transformer peak
output is greatest when both slugs are this way. No peak just maximum
output with full in. When I started the slugs were just about the same
location as on the output transformer. I suspect something is not right
with this transformer but have limited experience with battery powered
IF stages. I looked at the resistance of the primary windings of the
transformer in question and it read around 10 K in compression with the
output transformer that has a primary or plate side resistance of around
50 ohms. The 1R5 pentograde converter only draws about 0.7 to 1.4 Ma
total plate current so maybe the high resistance is normal for the
transformer. Also it has been proposed that I open the IF transformer
and check the capacitors across the windings. Although have to wonder if
they ever fail? So if anyone has any ideas on what the normal
sensitivity of a four tube radio should be, or experience of IF
transformers that appear not to be resonate, or ideas of just what the
resistance of the first IF transformer should be please don’t hastate
to respond.
Ray Fantini KA3EKH