[Drake] RE: [drakelist] TR-7 24V Power Supply

Fri, 1 Mar 2002 20:13:18 -0500

I have heard a similar complaint in the past. The answer as I recall was
to increase the value of C2108, 220uf/16V electrolytic. Guess you could
probably fit 1,000uf nowadays. Capacitors have gotten a lot smaller
these last 25 years. What surprised me was the extremely small size of
RFC2101. I know I measured it but can't locate the paperwork right now.
I noticed a whine coming from the AF stage when the gain was turned up.
It was most obvious on AM. I thought it came from the converter so I
tried a number of experiments. I tried larger values of chokes and even
tried a new type of organic electrolytic capacitor called "Oscon" at
C2108. Very expensive and sole-sourced by Sanyo even though Vishay likes
to advertise they have them too. They're made by Sanyo. No change. I
then disconnected the converter and supplied +24 and -5 from an external
power supply. Again no change. To make a long story short, I ended up
redesigning the AF stage and tacking on a TDA2003 audio chip on a small
board. The noise is still there, but barely detectable. The only time I
hear it is with the RF gain at minimum and AF gain at maximum. Not
necessarily a real world situation but a worse case scenario.

As for the bypass caps at U2102, MC7805 5 volt regulator and U2103,
MC78L24C 24 volt regulator, they are more than adequate. At 20 - 30 khz,
1uf on the input is just right. The output of ANY 3 pin regulator never
needs anymore than 1uf. Unless something else is out of whack, 0.1uf is
all you should ever need. If the bypass capacitors are not physically
located close to the IC leads or the leads are too long, then you have
problems. This is a universal rule. I noticed that there were several
"liberties" taken with regards to grounding and bypassing throughout the
radio. Which means when you have mixed-signals (digital and analog) all
using the same ground return lead, noise always ensues. One more word
about bypass capacitors. It is always good practice to use at least 2
types of capacitors for this service. A large electrolytic may show
enough inductance at higher frequencies to become ineffective so it's a
good idea to also use a smaller value, say 0.1uf right across it.

The -5 volts always seems more like -3.5 volts. I thought mine was out
of spec until I saw the same thing (approximately) being said by one of
the former service technicians at Drake. By the way, varying that
voltage from about -2.5 to -5 volts didn't seem to make any difference
in operation.

In concluding I want to say the Drake Engineers did a good job designing
the TR-7. They used technology which was then available and pushed the
state-of-the-art. Let's not forget the TR-7 design is around 25 years
old. We've made a lot of progress since then, thanks to their efforts.  

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Joe Mann
Sent: Friday, March 01, 2002 1:30 AM
To: Drake List; DRAKE List
Subject: [drakelist] TR-7 24V Power Supply 

Sirs,

1 - 2 years ago I posted to the list(s) a problem that I was having with
the harmonics of the DC to DC inverter circuit when listening in the 0 -
500 kHz band segment.  At least 1 head crash ago I lost the fix for this
problem. Its now time to actually do something about it.  As an aside,
the TR-7 circuit description states the multivibrator runs @ 23 kHz;
mine seems to be much closer to 30 kHz.

I recall it involved the addition of an electrolytic across the ? volt
output of the PS board.  The PS board is the one with the
CW/LSB/USB/RTTY centering pots located at the front/right of the radio.
The connections from the PS to the rest of the radio are accessible when
the bottom plate is removed and seems like the best place to put this
additional bypass.

I also recall that the 3 legged voltage regulators on the PS board could
use additional bypassing.  Does anyone know what symptom that is fix
attempting to address?

I someone remembers the specifics, please send them along.

Joe, K9HDE
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