1979-1980 2 Meter Transceiver EPROMS

 Although the following describes changing an early design to a newer implementation using EPROMs the general information might be useful for suggesting other applications. I could have gone further and used a microcontroller such as a PIC24 series chip which would further simplify the design at the expense of needing to learn how to use it.

 An annoying fault during the overhaul of the old 2m rig was that the numeric display above the dial intermittently showed some (but not all) numbers correctly but often just a series of red dots. The device is an ancient HP5082-7300 which requires a 5-bit code to display a particular digit with or without a decimal point. I've reconstructed the codes for the diode matrix in which these are stored below using the Avago datasheet. Oddly, despite the high standing of Avago, they reproduced incorrect codes in their datasheet causing me a little puzzlement. The datasheet is puzzling anyway because they specify the device usage by three different standards viz. chip pin numbers (1), function (INPUT 2) and an alpha-numeric (X2).

In the table below the display echoes the 4th digit (0-9) as shown X8=Pin 3, X4=Pin 2, X2=Pin 1, X1=Pin 8, DP=Pin 4. Vcc is Pin 7 with Ground as Pin 6. Latch Enable (E) is Pin 5. DP is OFF at logic high (=1)

Latch enable is set to logic low (=0) to turn on the display and logic high (=1) to blank the display.

The supply voltage needs to be 4.5 to 5.5 volts and to not exceed 7.0 volts.

Logic 1 has to be greater than 2 volts (@250uA) and logic 0 less than 0.8 volts (@-1.6mA).

Besides the digits 0 to 9 there are a number of other displays which may appear if the wiring isn't quite right (as in my case). These are (in octal) 22 lots of dots, blank for 27, 31, 35 and 37 plus a line of dots for 33. All-in-all a fault in the wiring or a bad diode is tricky to diagnose.

HP5082-7300 Display codes 

 

FREQUENCY MHz

X8

X4

X2

X1

DP

OCTAL

144.000

0

0

0

0

1

 01

144.100

0

0

0

1

1

 03

144.200

0

0

1

0

1

 05

144.300

0

0

1

1

1

 07

144.400

0

1

0

0

1

 11

144.500

0

1

0

1

1

 13

144.600

0

1

1

0

1

 15

144.700

0

1

1

1

1

 17

144.800

1

0

0

0

1

 21

144.900

1

0

0

1

1

 23

145.000

0

0

0

0

1

 01
 

FREQUENCY MHz

X8

X4

X2

X1

DP

 OCTAL

145.000

0

0

0

0

1

 01

145.100

0

0

0

1

1

 03

145.200

0

0

1

0

1

 05

145.300

0

0

1

1

1

 07

145.400

0

1

0

0

1

 11

145.500

0

1

0

1

1

 13

145.600

0

1

1

0

1

 15

145.700

0

1

1

1

1

 17

145.800

1

0

0

0

1

 21

145.900

1

0

0

1

1

 23

146.000

0

0

0

0

1

 01

 The current draw from the display will dictate the value of the resistors used in the diode circuits. This is liable to be different to those used for equivalent pull-ups used in the PLL code requirement. From the picture of the Diode Matrix the pull-up resistors look like 620 ohms for the display pins. Maybe it's all academic because I found the display chip had failed. The pins had come adrift from the substrate and much to my surprise Pin 2 had bent when I'd plugged the chip into a DIL holder in 1980. Because of that, Pin 2 was a continuous Logic 1 and the numerical sequence had never worked except when the bent pin was (just) in contact with the socket in the holder. I suppose I could try soldering extremely thin wires to the pads on the substrate or look in my junk box for a replacement. Soldering wires to the pads proved impossible and a new display was far too expensive so I decided to modify the design.

After wading through lots of specs relating to the display devices in my junk box it was obvious that the various options to the failed device would mean adding a special chip to handle the recoding from the simple binary (0, 1, 2 etc) held in the diode matrix to seven-segment drive (a, b, c etc). The old HP device I was using has this recoding (BCD to numeric code) built-in but apparently, after searching for a replacement, this design wasn't commonly carried forward in the industry. The reason being that displays often have more than one digit making the inclusion of the BCD decoder awkward to handle. My easiest option is to use one of my plentiful supply of 27C256 EPROMs. Although this device is huge compared with my data requirement, the address range of 15 bits is very useful as it considerably simplifies its use. I'm using 13 bits (A0 to A12) and I'll have 2 bits left over for any future changes in design.

27C256 EPROM for a 7 Segment display 
 

 DIGIT

a DQ6

b DQ5

c DQ4

d DQ3

e DQ2

f DQ1

g DQ0

 HEX
 ADDRESS

0

0

0

0

0

0

0

1

 01

 7FFE

1

1

0

0

1

1

1

1

 4F

 7FFD

2

0

0

1

0

0

1

0

 12

 7FFB

3

0

0

0

0

1

1

0

 06

 7FF7

4

1

0

0

1

1

0

0

 4C

 7FEF

5

0

1

0

0

1

0

0

 24

 7FDF

6

0

1

0

0

0

0

0

 20

 7FBF

7

0

0

0

1

1

1

1

 0F

 7F7F

8

0

0

0

0

0

0

0

 00

 7EFF

9

0

0

0

0

1

0

0

 04

 7DFF

 10

 0

0

0

0

0

0

1

 01

 7BFF

 Here, I've used my preferred definitions of the 7 segments in a numeric indicator, ignoring a decimal point and selecting the codes by using one of eleven address bits per each of the seven codes (see the schematic further down). Interestingly a potential option available to me is to use the old diode matrix to directly drive a new 7 segment device. The initial truth table above has 5 columns but the decimal point code is hard-wired to "1". I'd need to swap diodes around and add a couple more columns to the diode matrix (but I had only one spare) and as you can see the columns are not logical so lots of rewiring of diodes would be needed.

After investigating the specs for available displays I found most are common anode so the EPROM needs to produce a ground for a segment to light (ie. a logic 1 relates to an unlit segment). My junk box yielded an "Archer 0.4" DIL style display supplied by Tandy in one of their sales, probably around 1978, with a peak rating of 25mA per segment.

Having now decided to replace the cumbersome diode matrix with a pair of 27C256 EPROMs, I now have to work out the contents of the one to be used for driving the PLL.

Whilst working out the method of EPROM data access I initially assumed that sequential addresses would be OK but in fact that would need a decoder to select sequences 0 to 10 for 144.0 to 145.0 inclusive. The chosen EPROM actually has no less than 32,768 x 8 bit stored codes and to access these requires a simple uncomplicated method of selecting the addresses in which our data is stored. The old method coupled with the diode matrix merely grounded one pattern of diodes from the set of 22. This method is still possible except that the data won't be stored sequentially.

Using my twin 12-way rotary switches will give me maybe 23 or 24 possible addresses but the EPROM has only 14 address lines if we use a single switchable ground; however by using address lines A0 to A10 for 144.0 to 145.0 then selecting address line 12 from the first switch when fully clockwise and at the same time providing a ground to the wiper of the second 12-way switch, also wired to address lines A0 to A11 I can access all the codes I need.

In fact less wiring and much simplification can be gained by merely substituting a toggle switch in place of the second rotary switch as in the drawing below. The tables below show how I worked out the data layout for the PLL EPROM.

As with the 7-segment device, I need to select the codes for driving the SP8922 chip using my original single-pole 12-way rotary switch. The solution as with the 7-segment display is to ground each of the eleven EPROM address bits in turn. This places the codes in different areas of the EPROM (not sequentially) as shown in the drawing below.

 

Here's a rough schematic of the way I'm using two EPROMs in place of the diode matrix board and the old HP display.

The PLL receives codes from the top EPROM which are stored at addresses selected by one of the original 12-way rotary switches. Because the switch handles 100KHz channels for 144-145MHz and 145-146MHz producing both PLL codes and seven segment display codes to produce numbers 0-9, I'll be illuminating one of three LEDs for 144, 145 and 146MHz to indicate where the 10 channels (0-9) are in the 2m band.

I've also incorporated an additional feature for generating a 600KHz downward frequency shift for certain channels used for repeater operation. Grounding address bit A12 changes the code sent to the PLL. I had a choice of also changing the display number but decided to leave this to show the receive channel together with an LED showing the repeater feature is selected. The repeater frequency shift is only operative when transmit is selected by permitting the ground to the repeater switch.

In the EPROM version here I need only a single 12-way switch to select all twenty-two100KHz channels in the 2MHz band (145MHz is duplicated for convenience).
 

 

PLL Codes

 

FREQUENCY MHz

F

E

D

C
 B

144

145
 146

 HEX

144.000

0

1

0

1

 1

0

 1

 1

5B

144.100

0

1

1

0

 0

0

 1

 1

63

144.200

0

1

1

0

 1

0

 1

 1

6B

144.300

0

1

1

1

 0

0

1

 1

73

144.400

0

1

1

1

 1

0

1

 1

7B

144.500

1

0

0

0

 0

0

1

 1

83

144.600

1

0

0

0

 1

0

1

 1

8B

144.700

1

0

0

1

 0

0

1

 1

93

144.800

1

0

0

1

 1

0

1

 1

9B

144.900

1

0

1

0

 0

0

1

 1

A3

145.000

1

0

1

0

 1

1

0

 1

AD

 145.100

 1

0

1

1

 0

1

0

1

B5 

 145.200

1

0

1

1

 1

1

0

1

BD

 145.300

 1

 1

 0

 0

 0

 1

 0

 1

C5

 145.400

 1

 1

 0

 0

 1

 1

 0

 1

CD

 145.500

 1

 1

 0

 1

 0

 1

 0

 1

D5 

 145.600

 1

 1

 0

 1

 1

 1

 0

 1

DD

 145.700

 1

 1

 1

 0

 0

 1

 0

 1

E5 

 145.800

 1

 1

 1

 0

 1

 1

 0

 1

ED

 145.900

 1

 1

 1

 1

 0

 1

 0

 1

F5 

 146.000

 1

 1

 1

 1

 1

 1

 1

 0

FE

 This table carries the codes for the PLL and the LEDs for 144, 145 and 146MHz.

As I recall the PLL requires logic ones for selecting the correct channels whilst the LEDs I've connected to Vcc so that LED current is drawn when the EPROM ground is selected.

If I made a mistake I can always reprogram the EPROM.

...

I now have two programmed EPROMs, one for setting the PLL plus driving the three LEDs for indicating 144-146MHz and a second for driving a common anode 7-segment display.

I'll need to fit these to a pair of 28-pin sockets soldered to a piece of Veroboard to replace the diode matrix board.

I've selected a pair of Texas TMS27C256-25 EPROMs from my junk box and suitably erased before programming with my TL866-II Plus programmer.

 27C256 EPROM TABLE A
 

FREQUENCY MHz

F DQ7

E DQ6

D DQ5

C DQ4

 B DQ3

144 DQ2

145 DQ1

 146 DQ0

 HEX

 S1

 S2/A11

 EPROM ADDRESS

144.000

0

1

0

1

 1

0

 1

 1

5B

 A0

 OFF

 7FFE

144.100

0

1

1

0

 0

0

 1

 1

63

 A1

 OFF

 7FFD

144.200

0

1

1

0

 1

0

 1

 1

6B

 A2

 OFF

 7FFB

144.300

0

1

1

1

 0

0

1

 1

73

 A3

 OFF

 7FF7

144.400

0

1

1

1

 1

0

1

 1

7B

 A4

 OFF

 7FEF

144.500

1

0

0

0

 0

0

1

 1

83

A5 

 OFF

 7FDF

144.600

1

0

0

0

 1

0

1

 1

8B

 A6

 OFF

 7FBF

144.700

1

0

0

1

 0

0

1

 1

93

 A7

 OFF

 7F7F

144.800

1

0

0

1

 1

0

1

 1

9B

 A8

 OFF

 7EFF

144.900

1

0

1

0

 0

0

1

 1

A3

 A9

 OFF

 7DFF

 145.000

 1

0

1

0

1

1

0

1

AD

A10

OFF

7BFF

145.000

1

0

1

0

 1

1

0

 1

AD

 A0

 ON

 77FE

 145.100

 1

0

1

1

 0

1

0

1

B5 

 A1

 ON

77FD 

 145.200

1

0

1

1

 1

1

0

1

BD

 A2

 ON

 77FB

 145.300

 1

 1

 0

 0

 0

 1

 0

 1

C5

 A3

 ON

 77F7

 145.400

 1

 1

 0

 0

 1

 1

 0

 1

CD

 A4

 ON

 77EF

 145.500

 1

 1

 0

 1

 0

 1

 0

 1

D5 

 A5

 ON

 77DF

 145.600

 1

 1

 0

 1

 1

 1

 0

 1

DD

 A6

 ON

 77BF

 145.700

 1

 1

 1

 0

 0

 1

 0

 1

E5 

 A7

 ON

 777F

 145.800

 1

 1

 1

 0

 1

 1

 0

 1

ED

 A8

 ON

 76FF

 145.900

 1

 1

 1

 1

 0

 1

 0

 1

F5 

 A9

 ON

 75FF

 146.000

 1

 1

 1

 1

 1

 1

 1

 0

FE

 A10

 ON

 73FF

 This table shows the layout of the EPROM using single address bits for 144 to 145MHz and then adding the next address bit A11 via a toggle switch marked 144-145MHz in the off state and 145-146MHz in the on state.

The data (DQ7-DQ0) is not held in sequential bytes making it inconvenient to program, but considerably simplifying selection of channels.

As a recap.. columns F,E....B contain either a logic 0 or 1 to drive the appropriate address bit on the SP8922 device in the PLL.

144, 145 and 146 carry a logic 0 which turns on the appropriate LED which together with the 0-9 display gives the frequency of the 100KHz channel within the 2m band. This matches the output from the PLL.

 Then I worked out how to insert repeater channels. I used address bit A12 operated from a further toggle switch (or a grounding contact on the mode switch) to select transmit frequencies.

 

 27C256 EPROM TABLE B
 

 RX FREQ MHz

 RX EPROM MHz

 TX FREQ MHz

 TX EPROM MHz

 S3 A12

 F DQ7

 E DQ6

 D DQ5

 C DQ4

 B DQ3

 144 DQ2

 145 DQ1

 146 DQ0
 HEX

 EPROM ADDRESS

 145.6000

 145.6

 145.0000

 145.0

 0

 1

0

1

0

1

1

0

1

 AD

 67BF

 145.6125

 145.6

 145.0125

 145.0

 0

 1

0

1

0

1

1

0

1

 AD

 67BF

 145.6250

 145.6

 145.0250

 145.0

 0

 1

0

1

0

1

1

0

1

 AD

 67BF

 145.6375

 145.6

 145.0375

 145.0

 0

 1

0

1

0

1

1

0

1

 AD

 67BF

 145.6500

 145.6

 145.0500

 145.0

 0

 1

0

1

0

1

1

0

1

 AD

 67BF

 145.6625

 145.7

 145.0625

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.6750

 145.7

 145.0750

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.6875

 145.7

 145.0875

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7000

 145.7

 145.1000

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7125

 145.7

 145.1125

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7250

 145.7

 145.1250

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7375

 145.7

 145.7375

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7500

 145.7

 145.1500

 145.1

 0

 1

0

1

1

0

1

0

1

 B5

 677F

 145.7625

 145.8

 145.1625

 145.2

 0

 1

0

1

1

1

1

0

1

 BD

 66FF

 145.7750

 145.8

 145.1750

 145.2

 0

 1

0

1

1

1

1

0

1

 BD

 66FF

 145.7875

 145.8

 145.1875

 145.2

 0

 1

0

1

1

1

1

0

1

 BD

 66FF

This table shows standard UK 2m repeater frequencies together with additional EPROM contents for driving the PLL to derive a downwards shift on the received frequency of 600KHz for repeater operation.

Because the receiver is tuned to an offset determined by the setting of the VXO a change in the address bits applied to the PLL will automatically select a transmit frequency directly related to the received frequency.

The repeater Tx frequencies are selected by setting the EPROM address bit A12 to logic 0 with a toggle switch or contacts on the mode switch (as is the case in the original design where the diode matrix carries extra switching diodes for the purpose).

The PLL EPROM is programmed with data from a combination of the two tables; 21 hex codes from EPROM Table A and 3 hex codes from EPROM Table B..

 

 Just to recap for a specific example.. Listening on 145.600MHz the toggle switch is set to 145MHz placing a ground on address bit A11 and the grounded wiper of the 12 way rotary switch would be at position 7, grounding EPROM address line A6. The address bits when unselected are held at logic 1 so the selected address for 145.600MHz is 111011110111111 which can be written as Hex 77BF. Stored at address 77BF is the 8 bit code 11011101 which is Hex DD. The first five bits represent address lines FEDCB feeding the PLL chip SP8922, selecting a basic frequency of (145.600-10.7MHz) divided by 5=26.98MHz. The other 3 bits of the 8 bit code are 101. These bits feed three LEDs connected to Vcc (= +5 volts) so that the centre one marked 145MHz seeing a ground signal will illuminate.

The address bits A0 to A10 of the second EPROM are wired in parallel to A0 to A10 of the first EPROM whilst A11 to A14 are wired to logic Vcc (= +5 volts). EPROM 2 therefore is addressed at Hex 7FBF (because we're only interested in the 0.6MHz part of the frequency we don't bother with A11). In address 7FBF is stored 00100000 or Hex 20. The most significant bit isn't used but the other 7 bits carry the code equating to display segments "a" to "g" and here you can see that only the bit driving segment "b" is set to logic 1, the others being logic 0. The EPROM therefore delivers a code which illuminates a "6" on the display. That means the PLL plus the 10.7MHz crystal oscillator produce a frequency of 145.600MHz whilst the LED for "145MHz" is lit next to "6" on the display. If we flip the toggle switch to the 144MHz position the basic PLL frequency flips to (144.600-10.7MHz) divided by 5= 26.78MHz, the 145MHz LED goes out and the 144MHz LED comes on, with the display remaining at "6".

For repeater operation we need to modify the PLL address bits so that the rig generates a transmit frequency 600KHz less than that for receive. That means the EPROM needs to hold a second set of codes which are accessed in repeater mode. As I've only used EPROM address bits A0 to A11, I used the next bit A12 set to ground when in repeater mode transmit. Selecting A12 with a ground changes the address for 145.600MHz from Hex 77BF to 67BF and in the new address is the code 10101101 or Hex AD. The last three bits are unchanged, still pointing to the 145MHz LED but the PLL code is now 10101 which is the code which generates 145.000MHz.

 

 Lots of scope to make an error but I programmed the two EPROMs and now have to wire a pair of 28-pin sockets on Veroboard and find a suitable 7 segment common anode display. Then untangle the mass of wiring around the diode matrix board which will be removed.

These are early examples of EPROMs and have a centre quartz window for erasing the contents with UV light. Ideally the cover should be 100% lightproof to prevent degradation of the contents.

Programmers are a lot better than they used to be but still loads more room for improvement. This is the TL866II Pro.

 

 

 A note about EPROMs... I have a largish quantity of quite early types of devices, made in the 1990s by various manufacturers. They all include, buried inside, a couple of codes indicating the manufacturer and type. When selecting the chip type before reading or writing, you may notice (at least on my example of programmer) a box that can be unticked so that the operation will not be aborted if these two codes do not match the selected device. In fact the majority of my EPROMs, outcasts from HDRS, were made by Toshiba and are TC57256AD-20 or 25. The former have Vpp 12.5V and the latter 21V so are not fully compatible in write mode. The TC57256AD and therefore the two relevant pairs of codes are not in the database for my programmer so the box has to be unticked, and also the type of chip needs to be either 12.5 or 21 volt. Several show up as faulty and it's quite possible this is so because 21V was used for Vpp in error in HDRS days.

I found a feature in my TL866II Pro that lets me add new EPROM types not already in the library so I added TC57256-20 and TC57256-25, adding the manufacturer's name, Toshiba together with their codes 98-C4 and 98-04 respectively. The latter's Vpp can be stepped up to 18V so that now both types can be read and written to without error messages.

 

  Right, is the new EPROM board tracking layout. This is drawn to make optimum use of the horizontal tracking on the Veroboard. Many tracks go across the board except where circles representing drillings to cut the tracks are shown down the centre of the two chips. Pin 1 is top left with Pin 28 opposite. There are minor differences between this layout and the the layout of the Veroboard pictures below mostly to allow for the addition of pull-up resistors on the address lines.

The Veroboard measures 70mm x 110mm) and replaces the old diode matrix pcb. White insulated wires are ground connections; the top socket is for the PLL EPROM and the lower socket is for the display EPROM. Using EPROMs allows for changes to be made more easily than before.

I suppose it would have been possible to use EPROMs back in 1979 as I have a large number of C1702A chips dating from 1972. These are 2Kx8bit but have only 8 address bits making them slightly more difficult to use. I could select 9 sets of data by using address FF but it wouldn't be enough even for use with a 7-segment display without extra circuitry. The two 27C256 (256Kx8bit) chips I'm using are certainly not new devices being dated 1989 and 1990.
 

 

 

 

 The next task was to fit four cables to the new board for address and data lines plus power and to fit a couple of decoupling capacitors. One data cable drives the 7-segment display whilst a second connects to the PLL via a small plug and socket and a third is wired to the three LEDs for 144/145/146MHz. Eleven address lines A0 to A10 go to the remaining 12-way rotary switch with A11 and A12 go to a pair of toggle switches (one selects 144-145MHz or 145-146MHz with the second selecting Normal or Repeater operation). Power for the EPROMs is supplied by the 5 volt rail.

The 8 SIL plug connections to the PLL pcb are very confusing.

 SP8922

 Pin10

Pin11

Pin12

Pin13

Pin14

Pin15
 GND

Pin16

 EPROM

 DQ6

DQ4

GND

DQ3

DQ5

DQ7
 GND

GND

 REF

 E

C

A

B

D

F
 GND

G

 SIL

 1

2

3

4

5

6

7

8

 
 

 Now the messy task of removing all the old wiring to the diode matrix, PLL, LEDs and various switches then wiring in the new assembly with plenty of scope for making errors. The first error I made was confusing the connections to the SP8922 because the PLL circuit board carries an 8-way SIL socket wired to the chip pins in the table above. Two of the selection bits are not used so need to be grounded viz. "A" for 50KHz and "G" for 25KHz settings and to facilitate this I added a ground wire in the cable between the EPROM board and the PLL mating plug. The connections to the 7-segment display are made from only 7 of the 8 EPROM data bits, with DQ7 not used and with a 5 volt feed for the display chip common anode.

Below is the completed assembly with connectors for the PLL and the display, 12-way rotary switch and leads for the three LEDs and two toggle switches.
 

 I tested the new EPROM pcb today to check the performance of the display. It worked OK at 3 volts but oddly 0-7 and 9 worked but "8" failed to illuminate. A programming error despite being ultra-careful. I'd used EPROM address 7E77 instead of 7EFF for storing code "00" for "8" so the default of "FF" which represents a blank display came up when I turned the 100KHz switch to position 9. I'd also checked the pcb cable wiring for the third time and had to rewire the PLL connections. The picture above gave the game away because I couldn't see the three LED wires. I'd forgotten them and wired the PLL connections to the wrong EPROM date outlets. After checking the display I added an 82ohm resistor in the common anode supply voltage so it could be driven safely at a sensible brightness and without exceeding the spec of the device. I should really fit a 2.7 zener as well to reduce the brightness of "1" which because it only uses two segments, draws less current, and therefore sees a slightly higher voltage. Maybe a better solution is to just add a series resistor in each of the 7 segment connections and connect the common anode directly to Vcc (=5 volts)

I removed lots of transistors and resistors from the three LEDs that's no longer required now because they're driven from the EPROM and no longer need external logic circuitry. After sorting out the new wiring to the LEDs I'll test the operation of the PLL to check the EPROM has correct data. I've already proved it works on one frequency, with the old diode matrix, so hopefully with the new EPROM it should be more reliable. Checking the front panel LEDs for polarity I found three (144/145 and PLL Lock) were OK but the 4th representing 146MHz was open circuit so will be replaced. I'd noticed during intial tests with the old diode matrix board that only one setting of the 12-way rotary switches put the PLL lock LED out. Whether that was due to a fault in the diode matrix connections or a latent problem with either the PLL pcb or the VHF local oscillator pcb remains to be seen.

Further tests were a bit inconclusive.The old wiring around the chassis was letting me down with intermittent mode switch settings affecting progress but I entually found a couple of problems. One was a missing pull up resistor which allowed address bit A015 to be floating. I corrected this but found the spec of the EPROMs didn't allow sufficient current to illuminate the original green LEDs for 144/145MHz indication. I'd changed the one for 146MHz and this lit quite well so I temporarily fitted a pair of modern low current types which lit OK. I'd swapped the PLL 27C264 for an early version, a TC57264AD-20 without the "C" but if anything it was worse. I then decided to strip out all the original wiring and replace this with decent stuff and properly labelled. I also removed the SSB unit so I could test this independently on the bench. This job, I'll tackle in parallel with the testing of the EPROM task.
 
 

 pending

 

 Return to the overhaul

 Return to description of the 2 Meter Transceiver

See the SSB refurbishment

 Return to Reception