[SignalOne] AW: Signal/One Service and Repairs

[Graf Ulrich Ulm]

Hi Paul,

since I didn't know which beta range would be required, I ordered the MRF
422 series with high beta (MRF 422 HB MP). That was obviously not correct.
But if you don't get the correct types of transistors at times you might get
anyway into the situation, that you have to use the transistiors available.
For this case, the negative feedback solution is a way to regain full stable
operation.

Of course there is no direct connection of a resistor allowed between
collector and base of these pa transistors. So - just to clarify my process
mentioned yesterday - I connected one resistor between the base of the
"upper" transistor (in the schematic) and the junction between C6 and the
primary of T3 and the other resistor between the base of the "lower" MRF 422
to the junction of C7 and the other end of T3's primary winding. Works very
well. The negative feed back reduces excessive gain, stops oscillation and
improves intermod distortion. It has several benefits.

Best 73 Uli, DK4SX


>Usually the MRF422 beta range up to 45 works well in the CX11A final
circuit, 
>the red range of 20 - 32 (now beta code "C") seems to work best for the
stock 
>circuit.  What was the beta code for the transistors where the 70 ohm/2watt

>swamping resistor works?  

>This is a very ingenious solution to a difficult problem.  Thank you !

>73,
>-Paul   W8ZO

>>In a message dated 1/15/02 2:52:45 AM, Ulrich.Graf@ulm.siemens.de writes:
>>just some information I want to let you know...
>>Helmut Hengstenberg, a friend of mine living in Stuttgart/South Germany,
>>owns two CX11A and two Milspecs 1030. One of the CX11A had a blown PA and
>>I ordered transistors from RF Parts. When installed, these more modern
series
>>of transistors exhibited a much greater gain and the PA started to
oscillate
><whenever activated. The easiest way to stop the oscillation was  by
>>introducing a negative feedback. So I inserted some low inductance
resistors
>>with short leads between collector and base of both final transistors with
a
>>final value of about 70 Ohms and 2 W. This measure reduced excessive gain
>>and stopped oscillation completely. Gain was readjusted and the radio was
>>working as well as before.
>>Since this might happen quite often with moderns transitor substitutes,
>>it could be of interest to you as well.
>>Best 73
>>Uli, DK4SX