Jim -
I understand that the concern is high frequency "noise" getting out of
the regulators, but I'm saying that by using tantalum caps they were
able to use larger values while still keeping the low inductance of the
ceramic style cap. Electrolytic caps inductance and AC impedance go up
rapidly with increases in capacitance, due to their internal
construction, but tantalums do not, as they are a "solid" pellet with
very low inductance. Just like a ceramic multi-layer cap.
Yes, as I said before, there's lotsa wideband gain in those chips, so
any transient occurrence, (or Zener noise,) is going to be amplified and
maintain a very fast rise-time, i.e., HF (and even low VHF) "noise".
I don't have a problem with caps on the three terminal regulators,
although I believe the tantalums address that concern adequately, but I
am concerned about hanging caps on multiple 723 pins. Depending upon
just where those pins are in the regulator circuit, adding capacity
could seriously impact the ability of the chip to maintain it's terrific
low noise performance.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Jim Shorney wrote:
Hi Gary,
The concern here is RF bypassing. Not to keep RF out of the device, but to keep
RF from GETTING out of the device. Which means smaller value ceramic capacitors
in addition to any electrolytics used. Diode junctions can produce wideband RF
noise that could theoretically impact receiver performance. If you look at the
manufacturer data sheets for the 3-terminal devices, they recommend low value
bypass caps on both input and output, as physically close to the device as
possible. I figger that on the device legs is pretty close... This is the logic
behind bypassing the Zeners. There's a repro of the QST note that the OP
referenced on the Drake Museum web site.
Otherwise, I saw an LM-317 based home built supply years ago that the builder
had failed to include the manufacturer's recommended bypass caps in. It was
very spikey, going from TX to RX on a 5 watt transciever caused the lamps in
the radio to get real bright for a second. Installing one critical cap fixed
that. Ever since then, I've been almost evangelical about following the data
sheet recommendations for bypass capacitors.
73
-Jim
On Sat, 24 Apr 2010 09:22:15 -0400, Garey Barrell wrote:
Ron& Jim -
Maybe I'm missing something, but according to the documentation I have,
all of the regulators and Zeners seem to have input and output bypass
capacitors.
Bypassing multiple pins of a 723 might be counterproductive, actually
degrading noise reduction on the output. There is fast, high gain
amplification going on in there! This is still one of the best
regulators around.
Most caps seem to be larger than datasheet values, but based on other
Drake gear I assume they are tantalum and so work as well as lower value
caps at the higher frequencies.
I don't have any numbers, but I suspect that a 1 uF tantalum is just as
good (low dissipation and inductance) as a multilayer ceramic.
I haven't worked on a lot of TR-7s, usually find that reseating the
cards "fixes" them! :-) Perhaps early units didn't have these caps?
Doesn't seem likely.
Certainly added caps wouldn't make things worse, with the possible
exception of the 723 "Christmas tree", but I question the value.....
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
Jim Shorney wrote:
I did do some of the Zener bypassing, but did not measure the results. It
didn't make things any worse, I will say that, and I have pretty good MDS
numbers on my 7. In addition, I also bypassed the 3-terminal regulator ICs to
manufacturer's spec with some surface mount caps on the legs of the IC right
next to the body. Again, no measurements, but it didn't hurt, and I've learned
over the years that they tell you to put those caps there for some very good
reasons.
I did find it interesting that all of the TR-7s that I have worked on so far
have had the IF gain control maxed out already anyway, in some cases apparently
by the good folks at factory service.
73
-Jim
On Fri, 23 Apr 2010 17:30:55 -0700 (PDT), Ron wrote:
Has anyone actually done and measured the results of bypassing the zener diodes and
regulator chips as discussed on QST Feb 1989? The author had "anecdotal" data
only. Was just wondering if there was any instrument measured data floating around out
there. Not really planning on doing this, just more of a curiosity.
73,
Ron WD8SBB
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