[Milsurplus] Re: [MRCA] DAV IF issues

[Al Klase]

Hi Ray,

It does sound like your RX is a more than a little deaf, and 10K strikes 
me a lot for an IF can winding.  When I have a problem like this, I put 
a scope across the winding, and excite the winding from a signal 
generator through, oh, a 100K resistor.  You should be able to slew the 
generator, observe a resonate peak, and sort things out.  When I get to 
storage, I'll grab my MAB-DAV box and make measurements, if you're not 
out of the woods.  I may also have a spare chassis.

Regards,
Al

Ray Fantini wrote:
> 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
>
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>
>
>   

-- 
Al Klase - N3FRQ
Flemington, NJ
http://www.skywaves.ar88.net/


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