Heathkit GC-1000 Most Accurate Clock

© Brooke Clarke 2011

Heathkit GC-1000
        Most Accurate Clock just after power up
Background
Operation
Manuals
Functional Test
    15 MHz RF Problem as received
    1 kHz Modulation
    100 Hz Modulation
    IF Bandwidth
    AGC
Receiver Board
    Local Oscillators
Micro Controller Upgrade
Tone Board
Display Board
Mother Board
Re-Capping
Hi Spec
Battery Backup
3.6 MHz Output
Video
Comments by John Gibbons
Heathkit ID Cards
Related
    GC-1005
    GCW-1001 Slave
   
Links

Background

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 WWVH 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.  The PST 1020 is a newer WWV/WWVH clock which is faster to acquire the time.

There are other radio clocks that work with the 60 kHz transmissions from WWVB/WWVBH that are often called atomic clocks.

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?  It's also possible that the "8" digits are an artifact of the camera exposure.

Manuals

Functional Test

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.
Band
Power
dBm
IF low
kHz
IF Hi
Khz
Total IF
BW kHz
LO Offset
kHz
5
-116
-3.7
+3.0
6.7
-0.633
10
-110
-3.5
+1.5
5.0
-2.195
15
-93
-4.5
+0.4
4.9
-3.050

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:

Receiver Board


Photo just prior to testing AGC electrolytic caps C348 & C359
Heathkit
                  GC-1000 Most Accurate Clock Receiver board top

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
Heathkit
                GC-1000 Receiver board Front

Heathkit
                GC-1000 Receiver board

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
Heathkit
                  GC-1000 Most Accurate Clcok Local Crystal Oscillator
                  schematic
5.455 MHz LO Common anodes of D302, D303 & D304, base of Q305
Heathkit
                GC-1000 Most Accurate Clock 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
Heathkit
                GC-1000 Most Accurate Clock 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
Heathkit
                GC-1000 Most Accurate Clock 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.
Heathkit
                GC-1000 5 MHz LO coming out antenna
10.455 MHz LO
Heathkit
                GC-1000 10.455 MHz LO coming out antenna
15.455 MHz LO as received not present
after recapping OK
Heathkit
                GC-1000 15 MHz LO coming out antenna
3.6 Mhz master Oscillator
Heathkit
                GC-1000 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).

Heathkit
                  GC-1000 Tone Decoder board

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.

Heathkit
                  GC-1000 Most Accurate Clock Tone Decoder shcematic

Display Board

The display board plugs into the motherboard and has the Tone Board connected to its back side.
Display Board Front
Heathkit
                  GC-1000 Most Accurate Clock Display Board Front
C102 (1000 uF)
meas 0.15 Ohms & 1152 uF
meas 0.14 Ohms & 1039 uF

Mother Board


Motherboard
Heathkit
                  GC-1000 Most Accurate Clock Mother Board Front
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
Mystery Cap
                  on Bottom of Heathkit GC-1000 Most Accurate Clock
                  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.
Heathkit GC-1000 Most
          Accurate Clock in Hi Spec mode
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.
Heathkit GC-1000 Most
          Accurate Clock Backup Battery

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.
Heathkit GC-1000
        Most Accurate Clock 3.6 MHz Output spectrum
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.
















Video

Youtube

Video

Comments by John Gibbons (GC-1000 pg1, pg2, pg3 1999-2009)

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.

Heathkit ID
                Card
Heathkig
                Master Builder Card




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Links

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