[DEMI] 144-28K problems

[Shawn Tayler]

On Thu, 28 Jun 2001 19:14:32 -0700, George Fremin III - K5TR wrote:

>
>
>I want to thank everyone who offered advice on my sick transverter.
>
>It is still sick.
>
>To recap the problems:
>
>It is deaf on receive - sort of like if a 20 or 30 or more db pad
>was in place.  When truned on the RX noise floor comes up only 
>very very slightly.
>
>It does not put out much power - like less than 1 watt.
>
>All of the (there are four in the manual) RX voltage test points
>check out ok.
>
>All of the TX test points check out ok.
>
>The T/R relay seems to be working.
>
>I called DEMI today and got some other places to check.
>the suggestions were:
>
>1. PIN diodes CR3 and CR4.
>   I am not sure how to check them but the look and act the same
>   as the ones in my 222-28 transverter when checked with my volt meter
>   when the transverter is not powered.

With the unit powered and in TX mode with no drive and a dummy load on
the output, your should have approx 1.4-1.5VDC at the junction of
CR4/L1/C32  and approx 0.7VDC at the junction of CR3/C5/L10

>2. Check the inductors in the LO signal path - they all seem
>   to conduct - I might need to double check them.
>
>3. R28 - near F1 - should see a votage drop across this - I do.
>   It has 13v on the 13v side and about 6-7 volts on the other side.
>   The only thing that I notice is that this resistor is getting 
>   almost too hot to touch.  I powered up the 222 transveter and 
>   the same resistor gets warm as well but I think the one in the 
>   144 box is getting a bit warmer.
>   Could this mean that IC7/MAV11 be drawing too much current?

Well he is trying to run the MAV-11 into saturation for maximum drive
level to feed the mixer after the filter.  It's probably fine.  What is
the voltage at R15?
>
>4. The junction at R15 / C22 peak the voltage with C24.
>   I can do this and the voltage does change and I can peak it up
>   just fine.

And it is?  Do you have a freq counter?  Or a broadband Rcvr to see if
it is running on 116Mhz?

>5. And then something in my notes that says R3 - TX.
>   I am not sure what he wanted me to check here but maybe it 
>   was to see the voltage drop on TX across this resistor and I 
>   think it is doing the right thing.
>
This another test point for the PIN diodes.  13V on one side, 1.5 on
the other....In TX mode....

>I have also heated up some of the solder joints in the LO chain 
>that looked like they might have problems.
>
>Comments?

Do you have both C22 and C19 installed?  IAre they in series carrying
the LO energy to IC-7, MAV-11.  If so and you can short the point
between the caps (no DC this way) to gnd, killing the LO, see if the
voltage at trhe junction of R28/L19/C57 goes up slightly.  That'll tell
you that the MAV-11 is getting close to sat and you have LO drive....

What if any test equipment do you have access to?

Shawn
N7LQ