[PETERg0rsq]

The function of the PROM is quite complex, but easy to check.

On each band it monitors the counter outputs (A1 to A8 NOT including A0 to confuse matters!). It then triggers the outputs Q0 to Q5 depending on the band edge detection and if in High Octave or Low octave.

The band edges are also dependent on the channel spacing, and this information is fed into the multiplexer IC11 which selects the correct outputs from the ROM (depending on channel spacing selection) and feeds them into the Level converters (Q18, Q19 and Q22).

The PROM and IC11 supplies and outputs are floating due to the PROM supply requirements, so be careful when measuring what is going on!

Once level shifted the outputs can then be used to determine if the frequency selection is in band or not, and if it is too high, or too low:-

Q0 is used for band edge detection on 5, 10, 20 kHz channels
Q1 is used for band edge detection on 6.25, 12.5, 25, 50 kHz channels
Q2 is used for band edge detection on 15, 30, 60kHz channels

Output is LOW when in band, high when outside of band.


Q3 is used for high octave indication for 5, 10, 20 kHz channels
Q4 is used for high octave indication for 6.25, 12.5, 25, 50 kHz channels
Q5 is used for high octave indication for 15, 30, 60kHz channels

Outputs are high when in HIGH half of octave.


The high octave output is used to control some oscillators in the 9081 AND used to determine if the band edge detection is the LOW edge or the high edge using IC17.

Bands 2-5 are identical in operation, but band 1 has a different set of parameters in the ROM and is detected by A0

With the above information, and a logic analyser it is straightforward to determine if the PROM is bad.

If it is bad I have some "repair boards" made that can use an EPROM to replace the PROM, though it is not difficult to fabricate something.


However the PROM supplies are prone to failing, so check the voltages and Q20 and Q21.