Nice work Mark! You're making progress. It will be interesting to see where you end up.
73
-Jim
NU0C
On Fri, 29 Mar 2024 12:17:03 -0700
"atlasstuff" <g4fph@...> wrote:
Jim,
I found the article from Feb. 1989 by AE7K, entitled 'Zener Diode Noise'. I won't be undertaking 'carpet-bombing' bypassing all of the regulators in my radio!
I had previously added the additional bond to the corner of the HPF board to fend off the 40 m oscillation issue. I've now added a couple of additional bonds (well, there were empty holes, now filled with bolts and solder tags!), so that all three boards comprising the HPF assembly are bonded to nearby metalwork (the centre board having a bond at either end). Interestingly, even after doing this, by waggling the piece of metal around the HPF behind the S-meter lamp, I could still provoke the low-level spurs in the receiver to show themselves. At some point, I will replace the sheet metal screws that fix the HPF 'shield' to the sidewall with bolts and nuts, as these screws clearly do not clamp the two metal parts together sufficiently. I see more sheet metal screws holding the screen between HPF and LPF assemblies. I might do those to.
I played around with the 7805 regulator that feeds the display part of the DR7. I added a solder tag it's nearby bolt to give a more local ground. Installing capacitors between input and output pins of 7805 to this local ground has reduced the level of the spikes on the DC rails both sides of the regulator. Output-side fast spikes have dropped from 80 mV p-p (when I turned the brightness on the 'scope up!) to 20 mV p-p. Input-side now has no fast spikes, just a 50 mV p-p 1 kHz ripple. Capacitors used were a 470 uF electrolytic on the input side and a 100 uF tantalum, plus a 150 nF (probably not needed) on the output side. I'll play with these values some more, before deciding whether to keep the mod.
On TX, the effect of the additional bypassing has dropped the level of the 1 kHz modulation by 10 dB, or so, 2 kHz modulation is at about the same level as the 1k. I had another go at the carrier balance and achieved between -62 and -65 dB, so now in spec:-)
There's quite a few 0.22 uF capacitors on the Transmit Exciter board. I need to order some of these in.
Regards,
Mark, G4FPH.
--
73
-Jim
NU0C
Jim Shorney