EMRFD Message Archive 13237

Message Date From Subject
13237 2016-10-31 17:34:56 wb8yyy_curt HYCAS behaving strange
I am back to debugging the HYCAS, built on the through-hole version of the board, and built to the proper list of parts.  But I am getting some strange behavior. 

The amp seems to work with the AGC turned off.  With the AGC on, I get only about 0,3 volts difference between no input and a large signal input. 

I have some strange bias voltages:

I did adjust the regulator for 9 volts out, but it required a 1446 ohm resistance on the middle pin to get there. 

Strange I am getting around 10 volts on each collector (yes higher than the 9 volt regulated voltage) and around 2.6 volts on each drain,  The AGC voltage with no input is 3,3 volts. 

I did check the diode stack - it has 2 volts on it, which may be just a little high.

Suggestions on how to debug this thing are appreciated. 

73 Curt

13238 2016-10-31 23:25:34 Dana Myers Re: HYCAS behaving strange
13239 2016-11-01 00:03:52 kg4rll Re: HYCAS behaving strange
Hi Curt,
Could be oscillating, maybe.
Also , does the if gain control function with the AGC on and with it off?
And is the amp input being driven with a 50 ohm source?
13240 2016-11-01 09:25:34 vasilyivanenko Re: HYCAS behaving strange

A few builders suffered from the hycas IF amp working as a HF oscillator. Especially if the front end matching network sports a high Q inductor. 

See my thread

12329 RE: [emrfd] Re: Hycas-Cascode amp