[SignalOne] Oscillating Q4

[jerry]

OK,

   I'm always challenged by oscillators.  Whether on purpose, or 
accidental.  Because the signal goes round & round.  Is the problem at 
the OUTPUT of the oscillator, or the INPUT?  Poo, it's bad all over at 
the same time - you can't use linear troubleshooting.

   I'm going to remove the RF driver board.  Check all the resistors and 
caps in the Q4 circuit.  Beef up the bypassing.
  There's a negative bias feedback path from R26 to R12.  That's bypassed 
by the factory, and an extra .1 uF capacitor
was added for the CX7A.

   Also, the 34V line could use some more bypassing.  And the ALC line.  
The ALC out line has a 10uF electrolytic.  And the ALC In line has a 
.01.  And another .01 at the control pin of the CA3028A.  So clearly 
Signal/One was trying to
make those lines clean.  Well, it's 2024, and we have better caps now 
than they had then.

   OK, out it comes.  Starting by making sure every single wire is 
labelled.  There's one wire that's not listed on the board schematic or 
pictorial.  #381.  It goes to the R/H front corner of the board.  It's 
in the wire list going to
A3( the power supply )-106, signal T/R.

   It's out.  Gee, it looks like I'm not the first person to remove it.  
The screw under the Front End board is missing.  So is the screw hiding 
under the Counter board.

   Well, it's getting late & I'm tired.  Taking that QR circuit apart 
will have to wait till tomorrow.  So far, I can
say that R28 & R29 - the emitter resistors - are a little high, but 
within reason at 12.4 ohms.  That wouldn't make it
oscillate, just the opposite.  R27, the collector-base negative feedback 
resistor - is a little low: 195 ohms vs 220.  But again, that wouldn't 
make it oscillate.

Moving back one to Q3, it's emitter resistors are perfect at 6 ohms.  
The third - HF only emitter resistor is a little
high at 4.6 ohms, but that wouldn't make it oscillate either.

I think the answer to this riddle will be in the capacitors.  I saw 
about a volt p-p of RF hash on the ALC line.  It should be pure DC.

To start with, I think I'll replace the three electrolytics, and tack an 
SMD .1 across each one on the bottom of the board.

And speaking of caps, I have a 3kV replacement for C30, the output 
capacitor.

The big transistors are 2N5641's.  I note that RF parts has them, but 
only a package style with big fat low-inductance leads.  The ones with 
the small leads are all over Ebay, but mostly in China.  I'm thinking 
maybe I should get a few,
just in case.

                 - Jerry, KF6VB