I built one of these with the RS-232
option many decades ago. It a three channel HF receiver (5,
10
& 15 MHz) that receives either the U.S. or Hawaii WWVB time
and
frequency broadcasts and displays the time to a tenth of a
second. It also has a discipline function that tweaks a 3.6
MHz
crystal oscillator. So it could be called an HFDO, which was
available decades prior to any GPSDO like the Thunderbolt or Stanford Research PRS-10.
Operation
In the photo at the top of the page
the
clock has just been powered up and is searching for the signal
using
only the 53" whip antenna. When the speaker is turned on you
can
hear the signal but the clock is not capturing the data
stream.
Maybe the signal is too weak, or the receiver needs to be aligned?
Manuals
1983 Manual p/n:595-3050? or p/n: GCA-1000-4? which includes
the
Illustration Booklet
that has the 1:1 scale PCB X-ray views
Using the HP
8648A signal generator, with the 1E2 modulation option, it's
possible to generate an RF
signal at 5, 10, or 20 MHz with either 100 or 1000 Hz modulation
(and a
choice of sine, square and other 100% AM waveforms).
There is an audio file that could be used to AM modulate the
signal
generator and should cause the clock to display the encoded
time.
A better way would be to have a PIC micro controller clock that
outputs
the 100 & 1000 time code data as audio to feed the AM input of
the
sig gen.
15 MHz RF Problem as received
The GC-1000 will respond to the 100 Hz modulation at 5 or 10 MHz
but
not 15 MHz.
Using the Rigol scope on the LO
transistor, Q305 shows strong oscillation at 5 & 10 Mhz but
very
weak oscillation at 15 MHz.
DC checks of the crystal selection circuitry does not show any
obvious
problems.
Guessing that the problem might be: (1) crystal that's not too
active
or (2) a weak Q305 (2N5770) transistor, (3) too close to the
bandwidth
of the scope or some combination of these.
1 kHz Modulation
It does not respond to the 1000 Hz modulation at any RF frequency.
After swapping the two 567 tone decoder ICs (U402, U403), no tone
can
be detected, even tried adjusting the pots.
BUT . . after returning them to the original locations
testing at
5 MHz showed the yellow data LED will light with 100 Hz modulation
and
the capture LED with 1 kHz modulation. But after trying 10
Mhz,
no luck on either 100 or 1000 Hz modulation, and coming back to 5
Mhz
now only the 100 Hz tone decoder is working.
At 5 Mhz after re-capping
the
red AM LED is off at -132 dBm and on solid at -120 dBm (in TEST
mode).
This required a slight tweak of the 1000 Hz pot. This may be
a
better way to set the tone board pots than using the internal
calibration which probably has a strong audio signal.
Note: the green Capture LED has a very long time constant.
It
takes 12 seconds after applying -110 dBm @ 5 MHz to turn on, and
12
seconds to turn off after the RF is turned off.
At 10 Mhz after re-capping
it
takes 15 seconds for the green Capture LED to turn on after
applying
-100 dBm andjsut a few seconds to turn off. But the turn off
time
seems to vary between a few seconds and almost a minute.
At 15 Mhz after re-capping
it
takes -90 dBm to get the red AM light to come on in TEST
mode.
BUT, after switching to normal mode the green Capture LED does not
turn
on.
100 Hz Modulation
At 5 Mhz the yellow data LED is out at -111 dBm, starts to flicker
at
-103 dBm, and is on solid at -101 dBm.
At 5 Mhz After re-capping yellow data LEC is
out
at -111 dBm, and on solid at -109 dBm
At 10 Mhz it takes -50 dBm to turn on the yellow light, after
tweaking
the 100 Hz pot at 15 Mhz it's now -109 dBm
At 15 Mhz it takes -30 dBm to turn on the yellow light, after
tweaking
the 100 Hz pot -94 dBm.
IF Bandwidth
Measured by feed in a signal modulated with 100 Hz square wave at
enough power to have a solid yellow LED at nominal band center.
It looks like the LO crystals for 10 & 15 Mhz are not
centering the
signal in the IF passband. This may mean that it matters
which
channel is used to tweak the tone board pots. For now I'll
leave
it with the 5 MHz channel well centered since that's the channel
that
works best at night.
The total IF bandwidth should be the same for all the
channels.
That it's not indicates that my method of using a low flash rate
for
the yellow LED could be inproved.
Note that
AGC
When it's back toghther use the HP
8648A signal generator to vary the input power and watch:
the AGC voltage on Rx PCB at Q313E = connector X301-13
Tone Decoder PCB at U404C or connector X401-10
control bits on
Tone
Decoder PCB at X401-12, X401-14, X401-16 If U404C
output is OK
then these must be OK
Receiver Board
Photo just prior to testing
AGC
electrolytic caps C348 & C359
Green
is after re-caping.
Using the ESR-Cap meter to
check the electrolytic caps
Compare to new Capacitors
C#
Cap
uF
Nominal
ESR
Meas
ESR
Meas
Cap
C301
100
0.7
4 0.22
3.6 156
C348
4.7
na
20 4
3.58 4.9
C353
10
6
7 4.2
11.9 11.17
C354
10
6
58 4.2
6.85 9.29
C355
220
.5
3 0.4
122 211
C357
220
.5
1 1.1
180 177
C359
4.7
na
14.7 4.3
3.87 4.67
There was a question about
different
versions of receiver board in relation to the crystal
loading
capacitors.
GC-1000 Receiver board p/n:
85-2795-1
You can see the three
crystal
cans at the bottom center.
The loading caps have been installed on the boack
There are three caps on the
back. They are not the correct values: Measured
Nominal
Error 5,455,620
-
5,455,000 = 620 Hz 10,455,610 -
10,455,000 =
610 Hz 15,455,770 -
15,455,000 =
770 Hz
Local Oscillators
After some discussion about the loading caps for the three LO
crystals
I checked my GC-1000 LO frequenices by connecting the antenna and
one
of the rear panel BNC connectors ground to the 4395A in spectrum
analyzer mode.
As long as the LO keeps the WWV carrier and sidebands inside the
IF
bandwidth it should not matter. The tone frequency that's
output
will depend only on the AM input signals.
Local Crystal Oscillator
schematic (click on image for larger version)
Q305 is an 2N5770
NPN
Transistor
5.455 MHz LO Common anodes
of
D302, D303 & D304, base of Q305
10.455 MHz LO Common anodes
of
D302, D303 & D304, base of Q305
15.455 MHz LO Common anodes
of
D302, D303 & D304, base of Q305
5.455 MHz LO
Between the time the signal was centered and the graphics
were saved
the LO drifted a couple of Hz.
10.455 MHz LO
15.455 MHz LO as received
not
present
after recapping OK
3.6 Mhz master Oscillator
Since the bandwidth of the 100 Hz PLL is narrow it may be that
using
the built-in tone generator for tuning is a mistake. It may
be
better to use a signal generator or an off the air signal to peak
the
tone board decoders.
Micro Controller Upgrade
I think the micro controller is the
Mostek MK3870/22, i.e. it has 2048 bytes of ROM and 64 bytes of
RAM. The /44 part has twice the ROM so might be one way to
upgrade the uC.
Note the 3870 (either the clock or the RS-232 chip) use a 3.6 MHz
clock.
As received circa 1985 the 3.6 MHz
output was off by about 70 Hz using p/n 444-200. Heathkit
sent a
new micro controller p/n 444-293 and that fixed the problem, i.e.
the
3.6 MHz output measured 3,600,000.0 Hz. I wrote a letter to Heathkit thanking them
and
pointing out that the DST/Standard Time switch was in error.
A modern micro controller, like one of the PIC uCs could be
programmed
with a better algorighim, like the PST
1020
or an even better one. This could be done by making a PCB
that
would plug into the existing U205 socket. Note the existing
3870
is a 40 pin IC and the replacement PIC will probably need to have
that
many pins.
A Mil-max DIP
or SIP
header can be used to make the interconnection.
Pin
U203 Main
U401 RS-232
1
2
3.6 MHz in
3
Display
Digit
Mode
Mode
4
Display
Digit
Mode
Mode
5
Display
Digit
Mode
Mode
6
Display
Digit
Mode
Mode
Test Tone output (TP1-33k-TP2)
7
8
DIP sw
RS-232
Baud
9
DIP sw
RS-232
Baud
10
DIP sw
RS-232
Baud
11
DIP sw
Year
12
DIP sw
Year
13
DIP sw
Year
14
DIP sw
Year
15
DIP sw
Year
16
Display
Add
Osc Trim
Mode
17
Display
Add
Osc Trim
Mode
18
Display
Add
Osc Trim
Mode
19
Display
Add
Osc Trim
Mode
20
Ground
21
22
Display
Segment
23
Display
Segment
24
Display
Segment
25
Audio
on/off
26
Ground
(clock)
+5
(RS-232)
27
Test L
28
1 kHz
tone
29
/Stop
30
AGC
31
Band
Switch
32
Band
Switch
33
Band
Switch
RS-232
#
Stop bits
34
Display
Segment
35
Display
Segment
36
Display
Segment
Ext Int
input
37
Display
Segment
RS-232
TxD
(output)
38
100 Hz
tone
RS-232
RxD
(input)
39
/Reset
40
+5
Tone Board
This clock has the optional RS-232
parts. At the top of the tone decoder board there's a 4-pin
header where one pin has been cut away and a pin is inserted into
the
mating cable plug so they can only mate in one orientation.
The
three wires are Ground, TxD and RxD. The two pots at the
upper
right are the 1000 (or 1200) Hz and 100 Hz tone center frequency
adjustments. The micro controller used on this board is
identical
to the one on the main board. Pin 26 determines if the uC is
the
main clock (gnd) or the RS-232 interface and audio test tone
generator
(+5).
Note this board has the 444-293 micro controller,
i.e. the new one that's more accurate.
ESR-Cap measurements on
Tone
Board electrolitic caps
C#
Application
Cap
uF
Nominal
ESR
Meas
ESR
Meas
Cap
C401
5V
PS
100
1
174 0.30
44 108
C404 Tant
U401
de-coup
3.3
na
3.6 4.5
68 70.5
C405
U401
de-coup
10
6
7.3 1.57
8.4 10.2
C412
U402
PLL
1
na
86 8.9
0.69 0.99
C414
U402
PLL
10
6
97 4.2
4.7 10.02
C421 Tant
U403
PLL
2.2
1
9.0 3.9
3.34 2.92
C422
U404D
Op
Amp
0.33
na
nr 29
nr 0.33
C424
R451
de-coup
150
1
30 0.37
41.5 217
C425 Tant
Q403
AGC
2.2
1
10.5 9.3
3.56 2.17
Note: My manual Tone Baord parts list (pg 21) is
missing C401 and
C404.
Display Board
The display board plugs into the
motherboard and has the Tone Board connected to its back side.
Motherboard
The micro controller is the newer 444-293 that's better
at disciplining the 3.6 MHz oscillator.
Electrolytic Caps
C#
Cap
uF
Nominal
ESR
Meas
ESR
Meas
Cap
C203
2200
0.1
0.7 0.04
1840 2920
C205
1000
0.2
2.2 0.12
1458 2414
C206
1000
0.2
2.1 0.13
458 1022
C208
220
0.5
9.5 0.15
165 222
C212
22
4
---- 0.85
0.05 24.9
C213
3.3
na
2.1 0.16
470 1018
Mystery
Cap
on Bottom of Mother
board
The cap with the sleeving
(C224) is in the manual, but the small cap marked
"47" is not.
Note the open connection is in a straight line with the
transformer
mounting hole and a ventilation hole in the PCB. If
you look at
the overall photo just above you can see these holes and
Q205 collector.
I have left the "47" cap disconnected, just as it is in
the photo.
It's connected between the collector of Q205 (the same as
the bases of
Q206 & Q207 which are the totem pole outputs for the
3.6 MHz
signal). Most likley there was an oscillation and
the cap is
there to stop it. Note the capactive reactance is
about 970 Ohms
at 3.6 MHz so will not effect that signal but would effect
signals
above 36 MHz.
This (C225) cap is only found in factory assembled clocks,
not in the
kits
hence it's not in the assembly manual.
Re-Capping
The following are power supply or
decoupling caps where low ESR electrolytics would be good:
C102, C203, C205, C206, C208, C212, C213, C301, C354, C357, C401,
C404,
C405, C424.
The following are caps where low leakage is important and so
either a
plastic or low leakage electrolytic would be good:
C348, C353, C355, C359, C412, C414, C421, C422,C425.
Some 3-terminal linear voltage regulators might oscillate if the
capacitors used with them have too high or too low ESR values.
U201 +8 V out is a ST 78M08 "No
output
cap needed for stability, but does improve transient response"
U202 +5 V out is a Fairchild
UA7805 "No output cap needed for stability, but does improve
transient
response"
But in this case there are no cap restrictions.
To remove the caps one lead was heated with a fine point soldering
iron
and the case tilted to pull that lead out of the PCB. Then
the
other lead was heated and the case tipped in the opposite
direction,
pulling that lead out a little. The process was repeated a
few
times to completly remove the cap. Then solder wick and
liquid
flux were used to suck the solder out of the hole. Sometimes
it
was necessary to add new solder and flux both the PCB and solder
wick
to get it to work well.
I misplaced the 4-40 screw that connects the telescoping antenna
to the
mounting bracket on the receiver and the first time I tried the
receiver used a longer screw. This had the effect of lifting
the
receiver board up out of it's socket. After cutting off the
screw
and
re-tapping the threads in the bottom of the antenna everything fit
toghther properly. Also there was a problem in getting the
display
board to seat fully that was fixed by using a screwdriver to press
directly on the socket.
Hi Spec
Once the above board seating
problems
were overcome the clock was allowed to run and on my way to bed
noticed
that there were two green LED lit (Capture and Hi Spec) and the
yellow
Data LED was blinking. The time shown had the correct minute
and
second values but was set for some random time zone. Note
this is
with the clock indoors using the telescoping antenna.
The improvement is solely due to the recapping. Although I
have
spare 567 Phase Lock Loop and Op Amp ICs they have not been
installed.
The Hi Spec light seems to come on shortly after sunset at 5
MHz.
Every day during the night it's in Hi Spec mode for a number of
hours.
The next thing to do is repeat the functional test
at 5, 10 & 15
MHz, then see if the IF bandwidth can be determined by changing
the
carrier frequency up and down while plotting the power level for
threshold sensitivity.
Battery Backup
The DC Power plug has a positive
polarity center. It's 5.5 mm OD and has a 0.082" central
hole
(2.08 mm). (Measuring)
This is commonly called a 2.1x5.5mm DC Power Plug.
I'd like to move the clock but not have
it loose time or the 3.6 MHz VCO trim setting so will modiby the
backup
cable to have Power
Pole connectors. First I checked the max input voltage
spec
for U202 and it's 35 Volts. That way I can use one of the 257477BA-PP
battery adapters that holds 10 "D" batteries.
For backup Alkaline cells are better than rechargable cells.
3.6 MHz Output
This is a WWV Disiplined Oscillator
(WWVDO). The motherboard micro controller drives a D/A
converter
that drives a crystal oscillator.
When the Hi Sped LED is on the tuning voltage is set.
There is a lot of distortion on the 3.6 Mhz output. Don't
know if
that's normal or jsut the way it is.
Looked out to 500 MHz and did not see any spurious oscillations,
so the
mystery cap (C225) is not needed to supress
them.
Below is a summary of the
modifications I have done to the Heathkit GC-1000 Most Accurate
Clock to improve its performance:
1.) I replaced all of the IC sockets with machine tool pin
sockets. Having to re-seat some of the IC's every now and
then is unacceptable. I can't believe that HEATH used such
CHEAP IC sockets! (Well, maybe I can...)
2.) Re-designed the clock drive to the microprocessors. The
470pF AC coupled 3.600Mhz clock drive relied on the fact that the
input to the F8 uP's had a diode input clamp (probably the
substrate of the uP) and the input voltage would swing from -0.6V
to +3.5V. I replaced the cap with a 74HC14 CMOS driver with
am 82 Ohm series resistor (to match the impedance of the driver to
that of the PC board to reduce ringing). The voltage now
swings from ~ 0.2V to ~ 4.8V. Risetime of the clock remained about
the same.
3.) I also replaced the output transistor push-pull circuit that
drives the external 3.6000 Mhz output reference with a 74HC14
driver. I actually tied 3 of the inverter gates on the same
chip in parallel (prop delays are almost identical, so you can
usually get away with this) to drive this output. Created a nicer
looking waveform. This was the other half of the chip used
for the clock driver section (I used 1 gate of the other half for
the CPU clock driver, 2 gates remain unused (and tied off)).
4.) I added a second +5V regulator (78L05) and separated the D/A
Latch, R/2R resistor ladder and all associated circuitry that runs
the Colpits Oscillator / Varactor Diode / Clock Driver circuitry.
One cut on the top side of the PCB by the input inductor
(and the feed-thru hole) isolated the clock section +5V very
nicely. The problem I noticed is that when you turn on the
displays, the main +5V regulator would droop about 100mv causing
the D/A voltage to the varactor diode to droop. This was
enough to shift the 3.6000 Mhz frequency by ~ 15-20 Hz. Still
within the spec of the clock for output freq accuracy, but by
adding this second regulator the frequency now shifts less than
0.1 Hz (thats the resolution that my freq counter can measure to).
I also bypassed the clock section with a few 10uF Tantalum
caps to reduce switching noise.
5.) Eliminated the display ghosting (display shut off but the
5/10/10MHZ indicators still glow a little). This was accomplished
by simply grounding the unused side of the display enable switch
(labeled 3 + 6 on the schematic). You also need to cut the
power to the decoder IC U101 (pins 1 and 16) and take these two
pins directly to the power connector pins 1+2 (+5) on the display
board (before the switch). If you don't, when you shut the
display off IC U101 will load down the uP lines to it and the D/A
latch will always get loaded with 00H, thus screwing up the
ability of the clock to tweek its own oscillator frequency.
6.) Replace all of the caps associated with the 100Hz and 1Khz
tone decoder circuits with polypropolene or stacked foil caps.
This reduced the clocks sensitivity to temperature drift
(and thus lousy performance) during times when the display is on
and the insides of the clock heat up. I also replaced the 2
- 5K Ohm open face pots with 20 turn adjustable pots - much easier
to adjust accurately. The 2 phase-locked loop adjustments
are made much easier by just tying a high-impedance probe on pin 5
of the 567 PLL chip and adjusting the frequency to either 100.0 Hz
or 1000.0 hz. Much easier to adjust than Heath's method. (In
talking to the techs at Heath, they recommend this method over
that in the manual). Caps and everything were ordered from Digikey
for about $10 total.
7.) I added a MOV and a .001uF 1KV cap on the AC input to help
line noise rejection and spike suppression. I personally run
the clock off of a 12V 8AH GEL-CEL (I had sitting around for a
while, figured I better use it or loose it) and then use a
float-voltage charger for the battery (not a cycle- voltage
charger). I also added a 0.1 uF ceramic disc cap on the
output of the transformer (input to the full-wave bridge) for
added noise suppression.
8.) The transformer that HEATH supplies in the GC-1000 is just as
bad as the Radio-Shack transformers: They skimp on the wire
size AND the # of turns on the primary thus giving LOUSY line
regulation and they run HOT! DUMB! DUMB! DUMB! I only plug
my unit in when I move it (in case the 12VDC connector unplugs and
wipes out the previous months of clock oscillator tweeking).
If you use the transformer, replace it with a real one that
can handle the 800ma load without sagging so badly (and getting so
blasted HOT!).
9.) I also added a computer interface to directly look at the
5-10-15 Mhz band indication, HI-Spec LED and the 100Hz and 1Khz
tone decoder outputs. (The interface is nothing more than a 74HC14
inverter tied to the appropriate lines on the F8 uP). I have
a CMOS Z80 system monitoring these lines and when the clock goes
into or out of Hi-Spec, I kick the clock's serial interface and
store the time and band info in an EEPROM. I'm still writing
the code, but have the basic system working now (capturing data).
It will basically give me information on when the bands are
'open' to Ft Collins, CO. (which is 1240 miles west of me). I plan
on using the 100Hz and 1Khz data for a later project - To be able
to decode the WWV data stream myself (probably using a 68HC11 uP)
and create a real serial interface that tells you what time it IS,
not what time it WAS 1-2 seconds ago (the HEATH serial 'bit
banger' interface STINKS!) I'm still debating on whether to attack
the receiver section of the clock - It works ok, but it COULD be a
WHOLE lot better... hmmmmm...... anyone else
tweeked the receiver yet???
[It's a fun little project that keeps me out of trouble....]
--
John Gibbons
N8OBJ
Macedonia, Ohio
Internet Address: gibbonsj%iccgcc.dec...@consrt.rok.com
"Welcome My Son, Welcome
To The Machine" - Pink Floyd
Heathkit ID Cards
These are two different cards.
The one on the left is just an ID card with my cutomer number.
The Master Builder card on the right has a different number and
was
issued after I submitted a list of about three dozen Heathkits
that I
had built.
Some of the earliest ones being the HiFi stereo AM/FM tuner and
seperate power amplifier. To get stereo you needed to have
one
channel on AM and the other on FM. Later added the stereo
subcarrier
kit which allowed receiving stereo from a single FM station.
Related
GC-1005 Electronic Clock
This is a line powered and
timed
clock
that reads out to seconds.
GCW-1001 Slave Clock
This is a slave clock that
needs
a GCW-1001 Master clock with the power line interface.
It uses a plug-in transformer (not a wall wart power
supply) and gets
the time over the
A.C. mains at 120 kHz using a system like X-10. The
NE5050N IC
has many features
that minimize power line noise.
Note the GCW-1001 is a WWV receiver like the GC-1001 Most
Accurate
Clock that has
an optional PCB to transmit the X-10 like time signal to
slave clocks.
