V-Register Diagnosis & Repair

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Principles of the 2-word ROM

Testing suggests that the V-Register may not be storing data. The 2-word ROM can be used in this situation to generate a single microinstruction that exercises the V-register in a known way and loops to itself, allowing the result to be easily viewed with the logic analyzer. A second microinstruction can be set up and it is surprising how complex a test code can be designed with just two microinstructions. In the present case however, just one microinstruction is required:

0000: 0 + KK -> U, next=0000

This loads the value set in the KK field of the microinstruction into the V register

0000: 0 + KK -> T, next=0000
0000: 0 + KK -> V, next=0000

These variants are easily set in the ROM and can be compared with the V-register version.

Testing the U, T and V registers

With the microinstructions above available on the ROM, the logic analyzer was set up to monitor the KK register value and the data present in the T, U and V registers. These registers are held in individual 4-bit shift register chips, the data is read, processed in the ALU and written in serial form and the paralled output of the shift registers is used for addressing RAM.

Looking at the serial data flowing through each shift register it can be seen that T0 and V0 have data that has been copied from the KK field (grey boxes) but U0, the U-register serial data stream, is always zero (yellow box).

This directs attention to the logic that circulates data in the U-register shift register and controls if current data is circulated or if replacement data is written. This logic is implemented on the 6195 PCB with IC L17, a SN7451 dual AND-NOR select chip. AND-NOR is a logic construction that was common in the manner that logic equations were constructed in the 1960s, it is used here to select between new and recirculated data. Probing with and old-school HP545A logic probe showed no activity at the AND-NOR output when new data was written by the microinstruction: this looked like the cause of the problem. Briefly stimulating this otherwise silent output with an HP546 logic pulser forced some data into the shift register and this data was correctly circulated and preserved, showing that the shift register chip was OK and that the fault was most likely in the SN7451.

V register Repair

The SN7451 was removed and replaced without difficulty. Wang fibreglass PCBs of this era are quite vulnerable to heat, the tracks lift easily and the board material softens and degrades with any excess heat. In this case the replacement was done without any damage.

Testing after replacement showed that the T, U and V registers could now be loaded and would hold values.

Test with Calculator Microcode

Testing with 4K and 1K microcodes failed with the same symptoms of the left-hand + sign illuminating after a variable short delay. As noted before, this is a non-specific symptom that is best interpreted as “partial function of logic engine”. Further investigation requires more work with diagnostic microcode.