Re: 7A26 edge distortion


 

On Wed, Oct 21, 2020 at 07:52 AM, Roger Evans wrote:


The picture suggests that the signal might be switching from the full
bandwidth to the 20MHz version at around the 40% level. Have you looked at the
biassing of Q860, Q880 and also for leakage in Q860, Q880, CR860 CR880? It
seems to me that the problem must be related to the semiconductors since a
purely linear circuit should affect positive and negative edges equally.
Hi Roger,
Since these components constitute most of the common parts, I had done some testing in that area. I checked Vbe across Q820 / Q840, Q860 / Q880 and they were correct. I pulled Q860 and Q880 and lifted one end of CR860. No difference.
Next, I bridged some of the trimmer caps with small fixed caps each at a time and found a significant influence. I'm going to check and replace trimmer caps!

Raymond

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